JOURNAL OF GEOPHYSICAL RESEARCH, VOL JOURNAL GEOPHYSICAL RESEARCH, VOL. 106, 106,NO. NO. Cl, C1,PAGES PAGES899-915, 899-915,JANUARY JANUARY 15, 15,2001 2001 Satellite-measured chlorophyll chlorophylland andtemperature temperature variability variability off off Satellite-measured northern northern Chile Chile during duringthe the1996-1998 1996-1998 La La Nina Nia and and El El Niño Nio A. Thomas,'1J. A. C. C. Thomas, J.L. L. Blanco,2 Blanco, 2 M. M. E. E.Carr,3 Carr,3 P. P.T. T.Strub,4 Strub, 4 and andJ.J.Osses2 Osses 2 Abstract. ofofsatellite are patterns of of surface surface Abstract.Time Timeseries series satellitemeasurements measurements areused usedto to describe describepatterns temperature and chlorophyll chlorophyll associated associated with with the the 1996 cold La La Nifia Nifia phase phase and and the the 1997-1998 1997-1998 temperature and 1996cold warm Niflo phase phase of of the the El El NifioNiflo - Southern Southern Oscillation Oscillation cycle cycle in in the the upwelling upwelling region region off off warmEl El Nifio northern Chile. Surface data northernChile. Surfacetemperature temperature dataare areavailable availablethrough throughthe theentire entirestudy studyperiod. period.SeaSeaviewing Field-of-view Sensor Sensor (SeaWiFS) (SeaWiFS) data data first first became became available available in in September September viewingWide Wide Field-of-view 1997 during aa relaxation relaxation in in El El Nifio Nifloconditions conditionsidentified identifiedby byin insitu situhydrographic hydrographicdata. data. Over 1997during Over the time time period period of of coincident coincident satellite satellite data, data, chlorophyll patterns closely the chlorophyll patterns closelytrack tracksurface surface temperature patterns. Increases both chlorophyll concentration and temperature patterns. increases bothin innearshore nearshore chlorophyll concentration andin incrosscrossshelf extension of elevated concentrations are associated with decreased coastal temperatures shelfextensionof elevatedconcentrations are associated with decreasedcoastaltemperatures during in in 1997 duringboth boththe therelaxation relaxation inEl ElNiflo Nifio conditions conditions in September-November September-November 1997and andthe the recovery from E1 El Nifio Niflo conditions conditions after after March March 1998. 1998. Between recoveryfrom Betweenthese thesetwo twoperiods periodsduring during austral (December 1997 to March March 1998) and maximum maximum El El Nifio Niño temperature temperature australsummer summer(December 1997to 1998) and anomalies, temperature patterns normally associated with upwelling were absent and and anomalies, temperature patternsnormallyassociated with upwellingwereabsent chlorophyll concentrations were minimal. Cross-shelf chlorophyll distributions appear to be be chlorophyllconcentrations wereminimal. Cross-shelf chlorophylldistributions appearto modulated by surface temperature frontal frontal zones zones and and are are positively positively correlated correlated with with a a satellitesatellitemodulated by surfacetemperature derived upwelling index. index. Frontal and the upwelling index in in 1996 imply an an derivedupwelling Frontalzone zonepatterns patterns andthe upwellingindex 1996imply austral summer nearshore chlorophyll maximum, consistent with SeaWiFS data from 1998australsummernearshore chlorophyllmaximum, consistent with SeaWiFSdatafrom 19981999, after after the the El ElNifio. Niflo. SeaWiFS SeaWiFS retrievals retrievals in in the the data data set set used used here here are are higher higher than than in in situ situ 1999, measurements by aa factor 2-4; however, however, consistency consistency in in the the offset offset suggests suggests relative relative patterns patterns measurements by factorof of 2-4; are valid. valid. 1. Introduction 1. Introduction heterogeneous with with concentrations concentrations> at heterogeneous > 1.0 1.0mg mgm3 m'3extending extending at least 100 km offshore. In spring, cross-shelf gradients are least 100 km offshore. In spring, cross-shelf gradients are Equatorward wind stress stress along along the the northern Equatorward wind northerncoast coastof of Chile Chile enhanced, and the region of increased biological activity (Figure 1) upwelling, surface (Figure 1) produces produces upwelling,relatively relativelycold coldcoastal coastal surface enhanced,and the region of increasedbiological activity (chlorophyll>>1.0 to km of of the the 1.0mg mgm3) m'3)isisrestricted restricted towithin within37 37km temperatures, and a biologically productive region, temperatures, and a biologicallyproductiveregion,which which (chlorophyll coast, considerably narrower than upwelling regions off supports commercial fishery. supportsaa strong strong commercial fishery. A A climatology climatologyof of coast, considerablynarrower than upwelling regions off California and Baja. seasonal seasonalhydrographic hydrographicpatterns patterns for for the the region region recently recently occurs aL [2000] compiled by compiled byBlanco Blancoet et al. [2000]shows showsthat thatupwelling upwelling occurs Fonseca year round, consistent with the assessment of year round,consistent with the assessment of Fonsecaand and Rojas and and Silva Si/va [[1996]. shown Farias Farias[1987] [1987]and andpatterns patterns shownby by Rojas 1996]. Climatological isotherms and and isohalines isohalines tilt tilt upward Climatological isotherms upwardtoward toward the coast by surface the coastaccompanied accompanied by equatorward equatorward surfacegeostrophic geostrophic flow throughout the the year. year. Strongest upwelling and and cross-shelf cross-shelf flow throughout Strongest upwelling gradients occur in austral summer (January, February, gradientsoccurin australsummer(January,February,and and March) in concert maximum March) in concertwith with the the local localseasonal seasonal maximumin in both both equatorward alongshorewind windstress stress and and solar solar heating equatorwardalongshore heatingof of wind gradients, surface surfacewater. water. Weakest Weakestalongshore alongshore wind stress, stress, gradients, Synoptic occur in austral winter. and and upwelling upwelling occur in austral winter. Synoptic phytoplankton patterns in in the the region region are are less well known, phytoplankton patterns lesswell known,but but previous work suggests aa general with previouswork suggests generaltrend trendconsistent consistent withthe the Using field hydrographic patterns. hydrographic patterns. Using field data datafrom fromtwo twoseasons, seasons, chlorophyll Morales et aL Morales et al. [1996] [1996] show showwinter wintercross-shelf cross-shelf chlorophyll temperature patterns patterns are concentrations concentrationsand and temperature are relatively relatively CaliforniaandBaja. The Pacific The Pacific coast coastof of South SouthAmerica, America,however, however,is issubjected subjected to to the the direct direct effects effects of of the theElElNiflo-Southem Nifio-Southern Oscillation Oscillation (ENSO) cycle cycle [e.g., EnfIeld, 1989; (ENSO) [e.g., see seeEnfield, 1989; and and Philander, Philander,1990], 1990], and local al., 1987, 1987, 1991] and and localhydrographic hydrographic[Huyer [Huyer et et al., 1991] and biological [Barber [Barber and patterns exhibit biological and Chavez, Chavez,1983, 1983, 1986] 1986] patterns exhibit strong interannual strong interannualvariability. variability.Strub Strub et et al. al. [1998] [1998] review reviewEl El Niflo off the the coast Nifio conditions conditions off coast of of South SouthAmerica, America, which which include strengthening strengtheningof of poleward flow along include poleward flow along the the coast, coast, deepening of of the the thermocline, shiftingin in the the location of deepening thermocline,shifting location of maximum upwelling upwelling favorable favorablewinds, winds, and and changing changing of of the the maximum land-sea temperature gradient. These last two factors at least land-seatemperaturegradient.These last two factorsat least partially offset offset each each other other and and result of partially resultin in the thepersistence persistence of local local upwelling favorable upwelling favorablewinds. winds. Off Off Chile, Chile, satellite satellitedata datashow show surface interannual interannual variability variability is is manifest as positive surface manifestas positiveand and negative sea surface temperature (SST) anomalies during the the negativeseasurfacetemperature(SST) anomaliesduring Niflo (warm) (warm) and and La Nifia Nina (cold) (cold) phases El Nifio phasesof of the thecycle cycle[Yanez [Yanez et aL, Additional nonseasonal and direct et al., 1995]. 1995]. Additional nonseasonalvariability variability and direct 'School of Sciences, University of Orono, Maine. equatorial connections connections result result from •School ofMarine Marine Sciences, University ofMaine, Maine, Orono, Maine. equatorial from equatorial equatorialwaves, waves,which which 2lnstituto de de Fomento Fomento Pesquero, Pesquero, Valparaiso, Chile. 2Instituto Valparaiso, Chile. [Enfield et propagate into the region propagate intothe regionas ascoastal coastaltrapped trappedwaves waves[Enfield et 3Jet Propulsion Propulsion Laboratory, Laboratory, Pasadena, Pasadena, California. 3jet California. aL, 1987; Shaffer et aL, 1997]. al., 1987; Shaffer et al., 1997]. 4College of Oceanic and Sciences, Oregon State 4College of Oceanic andAtmospheric Atmospheric Sciences, Oregon State 1996 to 1998 the From to early University, Corvallis, Oregon. From 1996 early 1998 the eastern easternPacific Pacificprogressed progressed University,Corvallis, Oregon. from relatively cold La to warm from relativelycold La Nifla Nifia conditions conditionsto warmEl El Niflo Nifio 2001 by Copyright Union. In an accompanying paper, conditions [Chavez et aL, 1998]. Copyright2001 by the theAmerican AmericanGeophysical Geophysical Union. conditions[Chavezet al., 1998]. In an accompanying paper, J.L. Blanco Blanco et et al. (Hydrographic (Hydrographic conditions conditions off northern northern Chile J.L. Chile Paper 999JC000052 during Papernumber numberI1999JC000052 duringthe the 1996-1998 1996-1998 La La Nifta Nifia and and El El Niflo, Nifio, submitted submittedto to referred Journal of Geophysical Geophysical Research, 0148-0227/01/1 999JC000052$09.00 0148-0227/01/1999JC000052509.00 Journalof Research,2000, 2000, hereinafter hereinafter referred 899 899 AND EL EL NIl•IO Nl1O THOMAS ET AL.: NORTHERN CHILE LA NI1JA NI•A AND 900 900 73 73 17 72 Longitude W 71 70 I I 69 18 18 ARICA 19 19 • 20 20 * IQUIQUE • 21 22 22 23 23 ANTOFAGASTA 24 I I Figure 1. area ofofnorthern Chile showing the Peru) Figure 1.Map Mapofofthe thestudy study area northern Chile(18°-24°S) (18ø-24øS) showing theregion regionfrom from17° 17ø(southern (southern Peru)to to 24°S. 24øS. The monitoring Thestar starindicates indicatesthe thelocation locationof of the thehydrographic hydrographic monitoringstation. station. to as manuscript, 2000) the coevolution coevolution in in time. time. The continuous to asBlanco Blancoet etal., al.,submitted submitted manuscript, 2000) present presentthe Theimagery imageryprovides providesaa more morecontinuous temporal evolution of hydrographic conditions in the northern examination of evolving patterns than that possible temporalevolutionof hydrographic conditionsin the northern examinationof evolving patternsthan that possiblefrom from cruise data data and and an of the the extent Chile an estimation estimation of extent to to which which the the Chile coastal coastalregion region over over this this ENSO ENSO cycle. cycle.The The El El Niflo Nifio cruise conditions of 1997-1998 local annual cycle cycle of of cross-shelf cross-shelfpatterns patternsmay may have havebeen beenaffected affected conditionsof 1997-1998include includesome someof of the thestrongest strongest local annual hydrographic anomalies anomalies on by the In section Briefly, hydrographic the El El Niflo. Nifio. In section2 2 we we present presentthe the data, data,processing, processing, hydrographic on record. record. Briefly, hydrographic by data reduction, and analysis methodology. In conditions followed a normal [see Blanco et al. 2000] section3, 3, time time conditionsfollowed a normal [see Blanco et al. 2000] datareduction,and analysismethodology.In section series of of SST seasonal cycle in in 1996 and up SST and andSeaWiFS SeaWiFSchlorophyll chlorophyllare are presented presentedboth bothas as seasonalcycle 1996 and up to to March March1997 1997 but butwith with series images during during specific specific periods periods and and as time negative sea sea level with the the La La images asmore morecontinuous continuous time negative level and and SST SST anomalies anomaliesassociated associatedwith series of of cross-shelf profiles. These These are Nifia conditions. Beginning and cross-shelfprofiles. are placed placedin in temporal temporal Nifia conditions. Beginningin inMarch March1997, 1997,temperature temperature and series context using the evolution of vertical thermal sea level anomalies became positive. Maximum anomalies structure sea level anomaliesbecame positive. Maximum anomalies context using the evolution of vertical thermalstructure measured at at aa hydrographic hydrographic station. station. In occurred during during two two sea In section section44 comparisons comparisons occurred sealevel levelpeaks peaksin in May May 1997 1997(austral (australfall) fall) measured biological of satellite-measured and satellite-measured biological and andphysical physical variability, variability, andin in December December1997 1997(early (earlyaustral australsummer) summer)when whensurface surface of contrasting La La Nifia Nifla and and El flow El Niflo Nifio periods, periods,are areprovided. provided. flow was wasstrongly stronglypoleward, poleward,disrupting disruptingthe thenormal normalseasonal seasonal contrasting Concurrent wind wind and are cycle. Strong positive and surface surfacechlorophyll chlorophyllmeasurements measurements are cycle. Strong positive surface surfaceanomalies anomaliescontinued continueduntil until Concurrent presented and compared to the satellite data time series. In March 1998. By May 1998, SST had returned to nearpresented and compared to the satellite data time series. In March 1998. By May 1998, SST had returnedto nearsection 5 5 we we present present aa summary. climatological conditions. Readers summary. climatologicalconditions. Readersare arereferred referredto to Blanco Blancoet et section al. 2000) for for more more details details of of the al. (submitted (submittedmanuscript, manuscript,2000) the horizontal associated with horizontaland andvertical verticalstructure structure associated withthis thistemporal temporal 2. Data and 2. Data and Methods Methods evolution. evolution. The SeaWiFS mission began began producing global images of Satellite data data allow allow aa more of the Satellite more detailed detailed examination examination of The SeaWiFSmission producingglobal imagesof ocean chlorophyll chlorophyll in in September 1997. Daily Daily level temporal and and spatial September1997. level 2 2 global global temporal spatialevolution evolutionof of patterns patternsfrom fromthe therelatively relatively ocean area coverage cold present in coverageimages imagesof of the theSouth SouthAmerican AmericanPacific Pacificcoast coast cold regime regimepresent in 1996 1996 through throughthe the arrival arrivalof of El El Niflo Nifio area Center Flight were received from the Goddard Space conditions in 1997 and the recovery to near-climatological were received from the Goddard Space Flight Center conditionsin 1997 and the recoveryto near-climatological DistributedActive ActiveArchive ArchiveCenter Centerand and remapped remappedto to aa conditions by mid-1998. mid-1998. The conditionsby Thelaunch launchof of NASA's NASA'sSea-viewing Sea-viewing Distributed projection covering covering the Wide standard projection thestudy studyarea area(Figure (Figure1) 1) at at44 km km Wide Field-of-view Field-of-view Sensor Sensor(SeaWiFS) (SeaWiFS) mission missionin in mid-1997 mid-1997 standard resolution. All with initiated of chlorophyll All scenes scenes withvalid validdata datawithin withinthe thestudy studyregion, region, initiatedoperational operationalmeasurements measurements of mesoscale mesoscale chlorophyll resolution. in 1997, to form patterns in in the In this beginning in September September 1997,were wereused usedto formaa time timeseries series patterns the global globalocean. ocean. In this paper paperwe we present presentaa beginning et composites over over 88 day general overview to of temporal temporalcomposites dayperiods periods[Campbell [Campbell etal., al., general overviewof of biological biologicalresponses responses to the thehydrographic hydrographic of 1995]. Chlorophyll retrievals are (1998) patterns presented presented by by Blanco manuscript, Chlorophyll retrievals arethose thoseof of the thesecond second (1998) patterns Blanco et et al. al.(submitted (submitted manuscript, 1995]. and National Oceanic Full resolution reprocessing. 2000). Patterns from concurrent ocean color Patterns extracted extracted from concurrentocean color and and reprocessing. Full resolutionNational Oceanic and Atmospheric Administration (NOAA) advanced very high SST satellite data of the upwelling region of northern Chile, Administration (NOAA) advancedvery high SST satellitedata of the upwellingregionof northernChile, Atmospheric collected coincident with with the the hydrographic data of of Blanco resolution radiometer radiometer(AVHRR) (AVHRR) images images are are collected coincident hydrographicdata Blanco et et al. al. resolution the Centro (submitted manuscript, manuscript, 2000), 2000), are are examined to show routinely at routinely at the Centrode dePercepciOn Percepci6nRemota Remotade de Ia !a (submitted examinedto showtheir their THOMAS THOMAS ET AL.: AL.' NORTHERN NORTHERN CHILE CHILE LA LA NI1A NIl•IA AND AND EL ELNl14O NllqO 901 901 Universidad Cat61ica Católica de All scenes scenes of of the the study Universidad deChile. Chile. All studyarea area from January January 1996 to May into from 1996 to May 1998 1998were wereprocessed processed intoimages images of SST of SST using usingstandard standardNOAA NOAA coefficients, coefficients, multichannel multichannel algorithms, and then algorithms, and cloud-screening cloud-screening techniques techniques and and then remapped to a standard projection at 1 km resolution. These remappedto a standardprojectionat 1 km resolution. These data were were formed formed into into a a time time series series of of 77 day using data daycomposites composites using all available available passes. passes. Composites with all Composites with over over50% 50% missing missingdata data over the the study over study region region were were not not included includedin in the theanalysis. analysis. Differencesare are expected expectedin in the the extent extent of of cloud Differences cloud cover, cover,and and hence missing data, both seasonally and interannually. hencemissingdata, both seasonallyand interannually.The The extent to to which which this this missing missing data data might might bias bias coverage by the the extent coverageby AVHRR is is unknown unknown but but is is reduced by the AVHRR reducedby the formation formationof of the the temporal temporalcomposites. composites. Surface temperature and and chlorophyll Surfacetemperature chlorophyllpatterns patternsmeasured measuredby by the from each each image the satellite satellitedata datawere were extracted extractedfrom imagein in the the time time series as as six profiles, of latitude series six cross-shelf cross-shelf profiles,spaced spaced10 1ø of latitudeapart apart over extent of of the the study study area. area. Each overthe the latitudinal latitudinalextent Eachcross-shelf cross-shelf profile was formed formed as as aa spatial profile was spatial mean mean of of pixel pixel values values equidistant from the the coast equidistantfrom coastover over aa latitudinal latitudinalrange rangeof of 100 100km km centered at each each of 200 km centeredat of the the six six latitudes latitudesand andprogressing progressing 200 km offshore, preserving the the maximum offshore,preserving maximum spatial spatialresolution resolutionof of the the satellite imagery imagery in in the the cross-shelf cross-shelf direction. direction. satellite Wind data Wind data are are monthly monthlymeans meanscalculated calculatedfrom from daily daily measurements made at at the the meteorological stations at at the measurementsmade meteorologicalstations the airports of of Arica, Iquique, and and Antofagasta. airports Arica, lquique, Antofagasta. The The measurements at at 1500 hours were were used, the measurements 1500hours used,as asthese theserepresent represent the maximum in both wind intensity and stability of direction and maximumin bothwindintensityandstabilityof directionand speed over over the the daily values used speed dailycycle. cycle. Climatological Climatological values usedfor for comparison to study study period period conditions conditions were were calculated calculated from from aa comparison to 27 over the the period 27 year yeartime timeseries seriesover period1970-1997 1970-1997[Blanco [Blancoel et al. al. 2000]. The predominantly north-south orientation orientation of of the 2000]. The predominantlynorth-south the coast use of v component coast allows allows use of the the v componentto toapproximate approximate alongshore wind. alongshore wind. pushing the below 300 300 m positive pushing the13°C 13øCisotherm isotherm below m and andcreating creating positive anomalies down to to 250 anomalies>> 3°C 3øCdown 250 m. m. This Thissecond secondpulse pulseis isboth both longer-lived and and stronger stronger than than the longer-lived the first. first.Surface Surfaceanomalies anomalies> > 4°C are present present through through March subsurface in 4øCare March1998. 1998.Beginning Beginning subsurface in February February1998, 1998, however, however, El El Niflo Nifio conditions conditionsbegin begin to to deeper than 250 deeperthan 250 m, m, and andpositive positiveanomalies anomalies>> 2°C 2øCextend extend below 400 m. Surface anomalies remain > 4°C July below 400 m. Surfaceanomaliesremain> 4øC ininJulyAugust 1997 In the period August 1997 (winter). (winter). In the SeptemberNovember September-November period (austral winter-spring), winterspring), isotherms isotherms shoal (austral shoal and and anomalies anomalies In begin to to drop 200 In April April(fall), (fall),SSTs SSTsbegin dropover overthe theentire entire 200km km weaken, and by by May, May, ship weaken,and shipdata datashow showthat thatboth bothsurface surfaceand and subsurface temperature conditions off northern subsurfacetemperatureconditionsoff northernChile Chile are are approaching climatological means means (Blanco (Blanco et et al., approachingclimatological al., submitted submitted manuscript, 2000). 2000). manuscript, Satellite SST SST patterns concurrent with with the the eight Satellite patternsconcurrent eight cruise cruise periods by Blanco manuscript, periodspresented presentedby Blancoet etal. al.(submitted (submitted manuscript, 2000) 2000) are are shown shownas asmonthly monthlycomposites compositesin in Plate Plate2. 2. These These provide provideaa contrast contrastbetween betweenthe the spatial spatialpatterns patternsduring duringeach each season, by La season,affected affectedby La Nifla Nifia conditions conditionsin in 1996 1996 and andEl El Niflo Nifio conditions in 1997-1998. conditionsin 1997-1998. May May 1996 1996through throughMarch March 1997 1997 represent an annual cycle from fall, through winter, representan annual cycle from fall, throughwinter, spring, spring, and in and summer. summer.Coldest ColdestSST SSTnearshore nearshore ineach eachimage imageindicates indicates coastal consistent with with the coastalupwelling upwellingof of subsurface subsurfacewater, water, consistent the climatological associated with climatological hydrographic hydrographic fields fields associated with these these surface flow seasons [Blanco seasons [Blancoet et al. al. 2000]. 2000]. Large-scale Large-scalesurface flow is is equatorward throughout this period (Blanco et al., submitted equatorwardthroughoutthis period (Blanco et al., submitted manuscript, Mesoscale recirculation recirculation patterns patterns in in the manuscript,2000). 2000). Mesoscale the southern portion portion of of the the study study area area in in May May 1996, evident in in the the southern 1996, evident geopotential anomaly fields fields and and as geopotentialanomaly asnegative negativeanomalies anomaliesin in surface temperature temperature and and salinity salinity in in the the cruise cruise data data (Blanco (Blanco et et surface al., submitted submitted manuscript, manuscript, 2000), 2000), appear appear as as jets jets and al., andeddies eddiesin in the the satellite satellite imagery imagery shown shownhere. here.Negative Negativetemperature temperature anomalies the coastal anomaliesin in the coastalupwelling upwellingzone zonein in spring springand andsummer summer (December 1996 and and March (December 1996 March 1997), 1997), especially especiallyoff off Iquique lquique (2l.5°S) manuscript, 2000), (21.5øS)(Blanco (Blancoet etal., al.,submitted submitted manuscript, 2000),indicate indicate that that the the upwelling upwellingevident evidentin in the thesatellite satelliteimagery imageryis is stronger stronger than normal. and than normal. In InMay May1997, 1997,temperature temperature andsalinity salinitysurface surface anomalies anomalies switch switch to to positive positive (Blanco (Blanco et et al., al., submitted submitted manuscript, 2000), most most strongly strongly in in the the southern portion of of manuscript,2000), southernportion 3. Results Results 3. the study area, and are associated with anomalous poleward the studyarea, and are associatedwith anomalouspoleward A time structure taken 10 surfaceflow. flow. The The satellite satelliteimage imagefor for this thisperiod periodconfirms confirmsthis this A time series seriesof of vertical verticaltemperature temperature structuretaken 10 km km surface warming, with strongest differences between the 2 years offshore at Iquique (20.5°S, see Figure 1) during ship surveys warming, with strongest differences between the 2 years offshore at lquique(20.5øS, seeFigurel) duringshipsurveys in the carried out Instituto de out by (IFOP) evident in the south southand andalong alongthe thecoastal coastalupwelling upwellingregion. region. carried by Instituto de Fomento FomentoPesquero Pesquero (IFOP) evident This overall SST difference between years, characterizes the temporal evolution especiallywithin within characterizesthe temporal evolution of of hydrographic hydrographic This overall SST differencebetweenyears,especially the upwelling conditions through the the relatively cold period period of upwellingregion, region,is is present presentthrough throughwinter, winter,spring, spring,and and conditionsthrough relativelycold of 1996, 1996,the the the in the arrival of of El El Nifio Niflo conditions conditions in in 1997, and recovery in 1998 summerin the satellite satellitedata data(August (August1997, 1997,December December1997, 1997, arrival 1997,and recoveryin 1998 summer (Plate and March surface conditions and March 1998), 1998), coincident coincidentwith with the thestrongest strongest surface (Plate1). 1). Detailed Detailedanalysis analysisof of hydrographic hydrographic conditions and and temperature and and salinity all anomalies associated with with this this period period is salinityanomalies. anomalies.Warm WarmSST SSTencroaches encroaches all anomalies associated is given givenby by Blanco Blancoet etal. al. temperature the way to the coast during December 1997 and March 1998, (submitted manuscript, 2000), and only a brief description (submitted manuscript, 2000), and only a brief description the way to the coastduringDecember1997 and March 1998, the surface will largely eliminating eliminatingthe surfacesignature signatureof of upwelling upwellingalong along will be be given givenhere hereto to illustrate illustratethe the timing timing of of events eventsand andto to largely coast. the coast. place patternsinto intotemporal temporalcontext. context. The placesatellite-measured satellite-measured patterns The the hydrographic time series Annual SST structure hydrographictime series(Plate (Plate 1) l) shows showsrelatively relativelyweak weak Annual cycles cyclesof of cross-shelf cross-shelf SST structureduring during the the vertical stratification (the (the 13°C isotherm is is within within the the upper 1996-1998 study verticalstratification 13øCisotherm upper 1996-1998 studyperiod periodare areshown shownin inFigure Figure22 at atsix sixlocations locations 11 Om)and andcold coldsurface surfaceanomalies anomaliesduring during the the period period from from late the study 110m) late spanning spanningthe studyarea. area.Data Data from from 1996 1996(Figure (Figure2a) 2a) illustrate illustrate 1995 1997. Cold temporal evolution evolution of of patterns during the relatively 1995 through throughearly early 1997. Cold surface surfaceanomalies anomaliesare are temporal patternsduringthe relativelycold coldLa La strongest in the conditions. SST (> (> strongestin the (austral) (austral)summers summersof of 1995-1996 1995-1996 and and19961996- Nifla Nifia conditions. SSTisishighest highest (>22°C) 22øC)offshore offshore (> 50 50km) km) 1997. Increased and warmer 1997. Increased stratification stratification and warmer surface surface water water during austral austral summer summer (JanuaryMarch) with during (January-March) withaamaximum maximumin in appear in the February-March period in 1997, similar February. Within appearin the February-Marchperiodin 1997, similarto to that that February. Within50 50 km kmof ofshore, shore,coastal coastalupwelling upwelling[Blanco [Blanco of In el al. 2000] submitted manuscript 2000; Blanco al., submitted of 1995, 1995,effectively effectivelyending endingthe theLa LaNifla Nifia conditions. conditions. In May May et al. 2000; Blanco et et al., manuscript,2000] 1997, however, warm warm surface reduces SSTs cross-shelf strong cross-shelf 1997, however, surfaceisotherms isothermsdeepen deepenrapidly rapidlywith with reduces SSTs and and creates creates relatively relatively strong the arrival of the gradients. Within Within 30 30 km the arrivalof the first firstEl El Niflo Nifio pulse. pulse.The The 13°C 13øCisotherm isothermis is gradients. km of of shore, shore,SSTs SSTsare aregenerally generally<< 19°C. 19øC. weaken to to 1°-2°C during aa relaxation relaxation in in El El Nifio Niflo conditions. conditions. A A weaken 1ø-2øC during second pulse arrives with great rapidity in December 1997, secondpulsearriveswith greatrapidityin December1997, wide sampled region, most most strongly strongly in regions (> (> 50 wide sampledregion, in offshore offshoreregions 50 km). By surface km). ByMay Maymost mostcross-shelf cross-shelf surfacethermal thermalstructure structureis is gone, and dominate the the region gone, andtemperatures temperatures < 17°C 17øCdominate regionfrom from0 to to 200 km 200 km offshore offshorethroughout throughoutwinter. winter. Beginning Beginningin in October October (austral spring) at the lowest latitudes and November (austral spring) at the lowest latitudesand November at at the the ET AL.: AL.:NORTHERN NORTHERNCHILE CHILELA LANIl•IA NI1AAND ANDEL ELNIl•10 NflO THOMAS ET 902 902 A) 1996 A) 1996 Arica Atica 18.5S 18.5øS 19.5°S 19.5øS 200 200 •. 150 150 200 200 ß'-::. '. ß '. 150 150 .... 0 0 .0 100 100 "• 100 ... a 50 0 J FMAMJMonth J J F M A M J J A .... '. 50 5{) S OND 0 S O J N Iquique 20.5°S lquique 20.5øS 200 2O0 FMAMJ JASON F M A M J J A S O N D D Month Month Month 21.5S 21.5øS 200 ' 200 . . 150 150 •.• 150 0 lOO •'•100 :" 10o E . E 50 50 0o 50 50 0 0 J FMAMJ JASON D J F M A M J J A S O N J FMAMJ JASON D D j Month Month F M 200 '' '..... ;•.................................. •""""•'• ';? ........... M J J S O N D Month Month Antofagasta Antofagasta23.5°S 23.5øS 22.5°S 22.5øS 200 A :i.'. 150 200 2O0 150 I!100 a B 50 5O 0 J FMAMJ JA SOND J .:-. ............... • .•,••:" • 50 F M A M J J Month Month A S O N D 0 J FMAMJMonthJ A SOND J F M A M J J A S O N D Month Chile in (a) 1996, (b) 1997, and (c) the first 5 months Figure 2. seasonal distribution of (°C) off northern Figure 2.The The seasonal distribution ofSST SST (øC) offdistance northem in(a)1996, (b)1997, and (c) the first 5months cross-shelf atChile six latitudes spanning the study area. Data are crossof 1998, plotted as contours in time and of1998, plotted ascontours intimeandcross-shelf distance at six latitudes spanning the study area. Data are crosscomposite NOAA AVHRR data. Each cross-shelf transect is shelf transects subset from the time series of 7 day shelf transects subset fromthetimeseries of7 daycomposite NOAAAVHRRdata.Each cross-shelf transect isaa each latitude. mean over 100 km in in latitude, latitude, centered centered at at eachlatitude. meanover 100km offshore temperatures temperatures increase, increase, and and by higher latitudes, higher latitudes, offshore by NovemberDecember, summer November-December, summerconditions conditionsare arereestablished. reestablished. Latitudinal variability within the study area Latitudinalvariability within the study area is is present, present,but but is stronger. stronger. Lower variability in in the direction variability thecross-shelf cross-shelf direction is Lower latitudes experience longer latitudesexperience longersummer summerperiods periodsof ofwarm warmoffshore offshore water and shorter shorter winter winter periods periods of of cooler cooler offshore offshore water water than than waterand higher latitudes. Colder coastal SST at lquique (20.5°S), higherlatitudes. ColdercoastalSST at lquique(20.5øS),aa the cold known upwelling center, is Despite the known upwelling center, is evident. evident. Despite cold anomalies of this this period period (Plate (Plate 1) I) (Blanco anomaliesof (Blancoet et al., al., submitted submitted manuscript, 2000) the the annual annual cycle cycle presented presented here here is is very manuscript,2000) very similar to to the Chile similar thenormal normalcycle cyclefor forthe thenorthern northern Chileupwelling upwelling [1995] and the climatology of of Barbieri et al. region shown by regionshownbyBarbieriet al. [1995]andtheclimatology The SST patterns of austral summer Blanco el al. [2000]. Blancoet al. [2000]. The SST patternsof australsummerearly fall (JanuaryApril) early fall (January-April)1997 1997 (Figure (Figure2b) 2b) are aresimilar similarto to those of cross-shelf gradients within tho•;e of 1996, 1996,with withstrong strong cross-shelf gradients withinthe the first < 19°C within 25 25 km km of first50 50 km, km, temperatures temperatures < 19øCwithin of shore, shore,and and temperatures > 22°C offshore. After April, however, temperatures > 22øCoffshore.After April, however,distinct distinct differences become apparent. differencesbecome apparent.Warm Warmoffshore offshoreconditions conditions persist fall-winter persistlonger longerinto intothe theaustral austral fall-winterperiod periodthan thanthey theydid did in 1996. in 1996. Winter Winter (July-September) (July-September)temperatures temperaturesare are warmer, warmer, ANDEL ELNIIqIO NI1O THOMAS ET AL.: AL.' NORTHERN CHILE LA LA N111A NIIqIA AND 903 903 B) 1997 Arica 18.5S Arica 18.5øS 19.5°S 200 r Jt 200 200 ......... '..... "................ •........... •-•>:•?•-"• .... '.......... •/i......... '•'•-'• •"• ...... '-'•-i[--i 200 150 6I 150 a a 100 10o . E 50 50 0 J FMAMJ J A SOND J F M A M J J A S O N D J FMAMJ J J Month Month F M A M J A Month Month Iquique lquique 20.5'S 20.5øS 200 2OO J 2- 0 50 50 S OND S O N D 21.5°S 21.5os 200 200 150 150 100 100 0 E 50 0 50 F M A M J J Month Month A S O N J FMAMJ J A SOND D 22.5°S 22.5øS 200 2OO 150 150 0 J J FMAMJ J A SOND J .•'•/• F M M J J Month A S O N D Antofagasta 23.5'S Antofagasta 23.5øS 200 , . • A ........ '-.... '"•....... ',i ............ '•<%•'"?"•'*•"'i'"'•'-'•" ..... '• .... •................ •: """"•'• ' ' • •,.,'".•.-• '•'•'"• • •a •5o .•••' 150 .:•:.. .. a 100 100 E 0 50 50 50 0 0 J FMAMJ J F M A M J J Month Month A A S S OND 0 N D 0 J J FMAMJMonthJ F M A M J J Month A S OND 0 N D S Figure Figure2. 2. (continued) (continued) decreasingbelow below 17øC 17°Conly only at at the latitude in the regions. SST decreasing thetwo twohighest highest latitude in theoffshore offshore regions. By By May May 1998, 1998,cross-shelf cross-shelf SST transects. in spring is patterns resemble resemble those those of of May May 1996. transects. Warming Warmingin spring(NovemberDecember) (November-December) is patterns 1996. stronger than was SeaWiFS data first became 1997, strongerthan was observed observedin in 1996, 1996,and andwarm warmoffshore offshore SeaWiFSdatafirst becameavailable availablein in September September 1997, temperatures (21°C) intrude all all the the way way to to the the coast, coast, resulting the relaxation period between the two major El temperatures (2 IøC)intrude resultingduring duringthe relaxation periodbetween thetwo majorEl Niflo Nifio in thermal in very verylittle littlecross-shelf cross-shelf thermalstructure. structure. This trend trend is is pulses This provides four images pulses(Plate (Plate 1). l). This provides fourchlorophyll chlorophyll images carried through to to early with fields by et carriedthrough early1998 1998(Figure (Figure2c), 2c),where wheresummer summer coincident coincident withhydrographic hydrographic fieldsdiscussed discussed byBlanco Blanco etal. al. (JanuaryFebruary) cross-shelf patterns are dramatically (submitted manuscript, 2000). 2000). These in (January-February) cross-shelf patternsare dramatically (submitted manuscript, Theseare arepresented presented inPlate Plate33 different from from those those in in both of (using 1997 different bothearly early1996 1996and andearly early1997. 1997. as asaa sequence sequence of monthly monthlycomposites composites (usingSeptember September 1997 Temperatures are are>> 21°C to to represent the Temperatures 21øCadjacent adjacent to the thecoast, coast,and andthe theentire entire to represent theAugust August1997 1997(spring) (spring)cruise cruiseperiod). period).Each Each offshore region region from from 25 25 to to 200 200 km by SST concentrations in the offshore kmis isdominated dominated by SST>> image image shows showselevated elevatedchlorophyll chlorophyll concentrations in the 22°C. in cooler upwelling region region nearshore 22øC.Beginning Beginning in March, March,however, however, coolerSST SSTis isobserved observed upwelling and lower concentrations nearshore and lower concentrations within and to offshore. In (winter), concentrations in within25 25 km kmof ofshore, shore, andin inApril, April,SST SSTbegins begins todecrease decrease offshore. In September September (winter),chlorophyll chlorophyll concentrations in AL.: NORTHERN NORTHERN CHILE CHILELA LANII•IA NllA AND THOMAS ET AL.' AND EL EL NI1lO NII•IO 904 904 0- -0 24 24 a) a) 23 23 22 22 100lOO -100 -lOO 21 -21 20 20 19 -19 18 -18 E 200-- -200 20o -1717 -1616 15 14 14 11 13 13 11 300- i \ 300 / i 12 -12 11 10 lO Temperature (øC) 9 g 400- I J M M M 3J 1995 1995 0- M 3 M I I M S 1996 S N iJ M M1996 J S 3 S -400 I I I I I i I I I I I I -•0-----8 N .1 M M 3 M M1997 J S N ]J M M 1997 1998J 8 S -0 o b) b) 7 7 6 -5 5 100-lOO , o,,.,-.,l•./!••ø -100 100 -4 4 -3 3 E 0. 200- - -200 200 0 / • •j / -2 2 0 -00 300 300- -300 l, ' i ' , Temperature anomaly (°C) 0 Tempera•:ure anomaly (øC)o 0 400-4oo-J 'J3 M M• 'M• j I 'SS t J 1995 1995 M N ij I 'M •M' M N J 3 J • S IS 1996 1996 N N 3 ' J I O -2 o M M 3 S 199 1997 Ni j I 'M IM I'M MI Iji -1 --3-3 'S S -400 4o0 199 1998 Plate of temperature structure and anomalies from 1995 Plate1. 1.Time Timeseries series of(a) (a)vertical vertical temperature structure and(b) (b)temperature temperature anomalies fromJanuary January 1995to toMay May 1998 measured measured 10 10km km offshore offshoreat at20.5øS 20.5°S during during approximately approximatelymonthly monthlycruises. cruises. Anomalies are from from aa 30 1998 Anomaliesare 30 year year climatology the same discussed in detail by by Blanco Blanco et et al. al. [2000]. climatologyat at the samelocation, location,discussed in detail [2000]. THOMAS ET El AL.: CHILELA LANIl•IA NI1A AND EL Nll•lO NI1O THOMAS AL.: NORTHERN NORTHERN CHILE May May 1996 1996 .18 -18 905 905 Sep 1996 Sep 1996 Dec Dec 1996 1996 Mar 1997 Mar 1997 Aug Aug 1997 1997 Dec 1997 Dec 1997 Mar 1998 Mar 1998 -19 -19 -20 -2O -21 -21 -22 -22 0 -23 -23 -24 -24 May May 1997 1997 -18 -19 -19 -20 -2O -21 -21 -22 -22 -23 -23 -24 .74 -74' -73 -73 -72 -70 -74 -74 -73 -70 -71 -73 -72 -70 -74 -72 -71 -70 -71 -72 -71 '-'70-74 -73 -72 -71 -71 -70 -74 -73 -73 -72 -71 -70 -74 -73 -72 -72 -71 -70 SST SST I 100 10.0 15.0 15.0 25.0 25.0 20.0 20.0 30.0 30.0 Plate 2. 2. Monthly composites of representativeof ofaustral australfall fall (May), (May), winter winter Plate Monthly composites ofsatellite satelliteSST SST(°C) (øC)for forthe thestudy studyregion, region,representative (September andAugust), August), spring spring (December), (December),and andsummer summer(March) (March)conditions conditionsduring duringLa LaNifia Nifia (1996) (1996) and and El El Nifio Niño (Septemberand to the the hydrographic fields by Blanco manuscript (1997-1998). (1997-1998). Months Monthscorrespond correspond to hydrographic fieldspresented presentedby Blancoet et al. al.(submitted (submitted manuscript, 2000). 2000). -18 Dec Dec 1997 1997 -18 --- Sept 1997 Sept 1997 1' -18 -18';.......................... '..' ........ -18.......j........ ,....... 51•1.•1• ;.....-18 ' -18 -18 -19 -19 -19 -20 -20 -20 -21 -21 -22 -22 -22 -22 -22 ' -22 -22 -23 -23 -23 -23 -23 -23 . -,• Mar Mar 1998 1998 -19 • % -19 I • ' May 1998 May 1998 'd -19 -19 -20 •• -20 -20 -20 - -21 ' • -23 . -24 -24 -74 -74 -73 -73 -72 -72 ' -24 -24 -71 -71 -70 -74 -70 -74 -21 ' . -21 .• -73 -73 ' -72 -72 -21 -21 -24 -24 -24 -24 -70 -70 -74 -74 SeaWiFS SeaWiFS [CIIL] [CHL] -73 -73 -71 -71 -72 -72 -71 -71 -72 -70 -70 -74 -74 -73 -73 -72 -71 -70 -70 l l 0.04 0.04 0.13 0.13 0.40 1.26 4.00 4.00 12.65 12.65 40.00 40.00 Plate composites of chlorophyll concentration (mg region, representative of Plate3. 3.Monthly Monthly composites ofSeaWiFS SeaWiFS chlorophyll concentration (mgm3) m-3)for forthe thestudy study region, representative of winter spring (December), (December),summer summer(March), (March),and andfall fall(May) (May) conditions conditions during during El El Nifio Niño (1997-1998). winter(September), (September), spring (1997-1998). Months to hydrographic hydrographic fields fields presented presentedby byBlanco Blancoet etal. al.(submitted (submittedmanuscript, manuscript 2000), 2000), beginning with Monthscorrespond correspond to beginning with the start start of of the the the SeaWiFS SeaWiFS mission. mission. THOMAS THOMAS El ETAL.: AL.:NORTHERN NORTHERNCHILE CHILELA LANINA NI]qAAND AND EL EL NIRO NI]qO 906 906 C) 1998 Arica 18.5'S Arica 18.5øS 200. 19.5°S 19.5os 200 - 150 150 150 150 100 E 50 50 •0 0 J FMAMJ JAS J F M A M J J A S Month Month OND O N Iquique 20.5°S lquique 20.5øS 200 0 J FMAMJ J A SOND D J F M A M 150 150 100 11: 50 50 0 o J FMAMJ JASOND F M A M J J A S O N 0 J F M A M I 150 150 10050 50 - 150 100 50 J FMAMJMonthJ ASOND M A M J J O N D I J J A S O N D Antofagasta 23.5°S Antofagasta 23.5øS 22.5'S 22.5øS F S Month Month 200 J A J FMAMJ J A SOND D Month Month 0 J 21.5°S 21.5oS 200 200 150 150 J J Month Month A S O N D Month 0 iF MAM JMonthJ A SO ND F Figure Figure2. 2. (continued) (continued) M A M J J A S O N D Month concentrations remain relatively relatively low, ofof7171øW ow concentrations remain low, but butinshore inshore both of both the the width widthand andconcentration concentration of coastal coastalchlorophyll chlorophyllare are greater than those and the greaterthan thoseof of December December andMarch Marchthroughout throughout the latitudinal latitudinalrange rangeof ofthe thestudy studyarea. area. cross-shelf chlorophyll of cross-shelf The The temporal temporal evolution evolution of chlorophyll concentrations from the beginning of to concentrations fromthebeginningof the theSeaWiFS SeaWiFSmission mission to In general, the end of May 1998 is shown in Figure 3. the-end of May 1998 is shownin Figure3. In general, elevated concentrations within elevatedchlorophyll chlorophyll concentrations within50 50 km kmof ofthe thecoast coast are evident conditions offshore and and coastal .and coastal trends trends continue continue into into March March 1998 1998 when when surface surface and more moreoligotrophic oligotrophicconditions offshore are evident the study area. In September, 1.0 thermal anomalies anomalies have have begun begun to subside but but salinity throughout the study area. Inearly early September, 1.0mg mgm3 m'3isis thermal to subside salinity throughout is present and the maximum chlorophyll concentration anomalies are still strong et al., submitted (Blanco anomalies are still strong (Blanco et al., submitted the maximum chlorophyllconcentrationpresentand is to within and 1998 (fall), In May May 1998 (fall), offshore manuscript restrictedto within 15 15 km km of of shore. shore.Later Laterin inSeptember September and manuscript, 2000). 2000). In offshore restricted the zone (> theoffshore offshore zone(west (westof of 71°W) 71øW)are arethe thehighest highest (> 0.5 0.5 mg mgm m' 3) observed over over the the study period, apparently associated with 3)observed study period, apparently associated with eddy-like eddy-likestructures. structures.In In December, December,at at the thetime timeof ofmaximum maximum positive poleward positiveEl El Niño Nifio anomalies anomaliesand andanomalous anomalous polewardflow flow these offshore concentrations drop drop to to <<0.3 in these offshore concentrations 0.3mg mgm3, m'3,except except in the the most mostnortherly northerlyregion, region,evidence evidenceof of offshore offshoreeddy eddystructure structure largely and both largely disappears, disappears,and both the the width widthand andchlorophyll chlorophyll concentration of the coastal concentration of coastal zone zone decreases. decreases. These These offshore offshore 907 907 THOMAS NORTHERN CHILE LA NIl•lA N1IA AND THOMAS ET AL.: NORTHERN AND EL ELND4O Nll•lO In May ot1hore to through October, nearshore nearshore concentrations concentrationsincrease increasetoto> extend offshore to distances distancesof of 40-60 40-60 km. km. In May at at the the throughOctober, > 2.0 2.0 extend lowest latitude studied (l8.5°S), concentrations> 0.5 mg m3 mg m3, and concentrations> 1.0 mg m3 expand offshore to lowest latitude studied (18.5øS), concentrations > 0.5 mg m -3 mgm-3,andconcentrations > 1.0mgm-3expand offshore to extend distances of 30-50 distances of 30-50 km. km. Both Both concentrations concentrations and and cross-shelf cross-shelf extend150 150 km offshore, offshore,similar similar to those thoseof of early earlySeptember September 1997. extension concentration collapse extension of of elevated elevated concentration collapse during during 1997. November at all except (18.5°S). November at all latitudes latitudes exceptthe themost mostnortherly northerly (18.5øS). 4. Discussion Discussion By within By December, December,maximum maximumconcentrations concentrations within 10 10km km of of the the 4. coast are 0.5 mg m3, except at the two lowest latitudes, where coast are0.5mgm-3,except atthetwolowest latitudes, where 4.1. Comparisons of Temperature and Chlorophyll Comparisonsof Temperature and Chlorophyll concentrations> Oligotrophic offshore concentrations > 1.0 1.0mg mgm3 m-3remain. remain. Oligotrophic offshore 4.1. Patterns Patterns water (< 0.25 0.25 mg mg m m3) within 50 km km of of the water -3)is ispresent present within thecoast coast Earlier beginning in January January 1998 this pattern Earlier work work with with Coastal Coastal Zone Zone Color Color Scanner Scanner(CZCS) (CZCS) beginningin 1998 and and maintains maintainsthis patternuntil until imagely illustrates the value of concurrent SST and ocean March. Offshore concentrations begin to increase in March (> imagery illustrates the value of concurrent SST and ocean March. Offshoreconcentrations beginto increasein March (> color satellite satellite imagery imagery in in understanding understandingevolving evolvingbiologicalbiological 0.25 mg at the latitudes, while (< 0.25 mgm3) m-3)at thethree threehigher higher latitudes, whilenearshore nearshore (< color patterns. Arnone Arnone and and LaViolette [1986] show show a a close close 10 km) km) concentrations surpass i.0 1.0 mg mg m m3 physical patterns. LaViolette[1986] 10 concentrations surpass -3and andall alllatitudes latitudes physical relationship between thermal and color features in eddies show the 0.5 mg m3 isoline expanding offshore. In April, showthe0.5 mgm-3isolineexpanding offshore. In April, relationshipbetweenthermaland color featuresin eddies along boundary there is is an increase near the the coast coast at at all there increase in in concentrations concentrations near all along the the North NorthAfrican Africancoast. coast.Within Withineastern eastern boundary between cold latitudes to>> 1.0 up to to 0.5 0.5 mg currentsthe the relationship relationship betweensatellite-measured satellite-measured cold latitudes to 1.0mg mgm3 m-3and andconcentrations concentrations up mgm3 m-3 currents Arlca 18.5'S Arica 18.5øS 200 •..:..•. . .-•:?...-... .• . •. ...-.•. ... . . ... .. . .•••.....•...:.•:•.• .. 19.5'S 19.5os ....•:• .... .•. .:. .._....:.:...-•:. 150 150 150 150 ,%100 100 ..:::.,,..,.-., ...... 100 50 0 200 SON D 0 N D JJ 0 S F MA Mi F Month Month M A M S ON 0 N D D J Iquique 20.5'S Iquique 20.5øS 200 . ':".-::....::.: ' ..... - 500 ,--....::.,. ;... S .-,, ::::............... :•: F MA Mi J F J Month Month M A M J 21.5'S 21.5øS 200 .... ::-•:.>>•.'-:-..: ..... 150 100 1•-: ........ ' % 0 : . 50 50 SON Di F MA Mi 00 "' ":5'[' S O ............. • 100 lO0 "•"-"":t':: .............................................. :;,:->' '"'-......................... '-': ';::.'": '%" :,';.::":', :•" •'"'"'::::::" 0 N D J F M A M J S S ON D Month Month 22.5'S 22.5øS 200 200 150 O N D ,J J F MA Mi F M A M J Month Month Antofagasta 23.5'S Antofagasta 23.5øS [•J•t ........ >"' " . ' "•••••i 150 .2 100 . 0 SON D J F MA Mi Month Month E 50 0 SON D D i J F MA Mi .............................. F M A M J Month Month Figure 3. evolution of cross-shelf chlorophyll concentration concentration (mg (mg m m3) off northern Figure 3.The Thetemporal temporal evolution ofsatellite-measured satellite-measured cross-shelf chlorophyll -3)Off northern Chile transects are Chile in 1997-1998 1997-1998 at at the thesix sixlatitudes latitudesused usedin Plate Plate2. Cross-shelf Cross-shelf transects aresubset subsetfrom from the the time time series seriesof 8 day day SeaWiFS images at 44 km km resolution resolution and and are are a a latitudinal latitudinal mean mean over over 100 km at at each each location. location. SeaWiFScomposite composite imagesat 100km 908 908 THOMAS ET AL.' AL.: NORTHERN NORTHERN CHILE LA LA NIgIA NI14AAND ANDEL ELNIl•10 NPO THOMAS upwelled water water and pigment concentrations well upwelled and pigment concentrationsis is well documented [e.g., [e.g., Abbott Abbott and and Zion, Zion, 1985; documented 1985; Van Van Camp Camp et et al., al., 1991]. Here Here we relationship between 1991]. we examine examine the the relationship between the the temporal evolution evolution of of SST temporal SST and andchlorophyll chlorophyllpatterns patternswith with two two approaches; first, first, using approaches; usingthe the strength strengthand andposition positionof of surface surface thermal fronts and, second, using an thermal fronts and, second, using an index index of of coastal coastal upwelling intensity. upwelling intensity. Previous work in in the the California California Current Current [Strub [Strubet etal., Previouswork al., 1991; 1991; 1991] Hood et al., has shown that elevated pigment Hood et al., 1991] has shown that elevated pigment concentrations resulting resulting from from coastal coastal upwelling are found concentrations upwellingare foundon on the inshore the inshoreside side of of the themain maincurrent currentjet jetand andhydrographic hydrographic Cross-shelf extensions extensions of of higher frontal frontal zone. zone. Cross-shelf higherchlorophyll chlorophyll concentrations characteristic of the upwelling zone are are thus thus concentrations characteristicof the upwelling zone modulated by the cross-shelf position of the jet and frontal modulatedby the cross-shelfpositionof the jet and frontal zone. If dynamics zone. If similar similarbiological-physical biological-physical dynamicsoperate operatewithin within the northern the northernChilean Chilean upwelling upwelling system, system,we we would wouldexpect expect cross-shelf patterns patterns of of chlorophyll to be with cross-shelf chlorophyllto beassociated associated with the the position and strength of the frontal zone, which position and strength of the frontal zone, which we we can can characterize in in the the satellite data by characterize satellitedata by the theSST SSTspatial spatialgradient. gradient. The time The time series series of of 77day daycomposite compositeSST SSTimages imageswere were transformed into into SST gradient images, transformed SST gradient images,defined definedat at each eachspatial spatial point as gradient point asthe theunweighted unweightedtwo-dimensional two-dimensional gradientmagnitude magnitude given by T(x,y)I==l/(2Ah){[T(x-Ah,y)I /(2h){ [T(x-&i,y) - T(x+My)]2 given byIIT(x,y)l T(x+Ah,y)] 2++[T(x.y[T(x,y- Latitudinal Latitudinal Mean Mean -- 1996 1996 A)200 A)oo ........ 150[ 150 together to gradient pattern together to form formaa single singlecross-shelf cross-shelf gradient patternfor foreach each time representative of time period period(weekly (weeklycomposite), composite), representative of the themean mean over the the latitudinal range of of the over latitudinalrange the study studyarea. area. Figure Figure4 4 shows shows the annual cycle of SST frontal strength, contoured as aa the annualcycle of SST frontal strength,contouredas function of of time time and position, function andcross-shelf cross-shelf position,over overthe the1996-1998 ! 996-1998 study period. period. The study Thetime timeseries seriesfor for1996-early 1996-early1997 1997(Figures (Figures4a 4a and and 4b) 4b) shows showspatterns patternsduring duringLa LaNifia Nifia conditions conditionsand and establishes the seasonal establishesthe seasonalcycle cycle during duringaanon-El non-ElNiflo Nifio year. year. 0 0 50 0 .I J FMAMJMonthJ J F M A M J J H A S OND S O N D Month B)2® B)2oo Latitudinal Latitudinal Mean Mean -- 1997 1997 ß ß ß ß ß ß ß . ß ß A S O N 150 150 I- C 100 50 where TT is the at aa Ah) - T(x,y+ih)]2}"2, T(x,y+Ah)]2} •/2,where thetemperature temperature at separation. particularx,y x,y location locationand and hh is particular is aahorizontal horizontal separation. Cross-shelf transects were then subsampled from Cross-shelf transects were then subsampledfrom these these gradient images images at at the the six six latitudes 2 and gradient latitudesused usedin in Figures Figures2 and 3. 3. Cross-shelf SST SST gradient gradient structure structure evident evident in in these Cross-shelf these six six transects (and (and in transects in Figure Figure2) 2) indicated indicatedthat thatwhile whilelatitudinal latitudinal variability is present, similar temporal patterns variability is present, similartemporalpatternsexist existat at each each of the six were of the six latitudes. latitudes.For For brevity brevitythe the six six transects transects wereaveraged averaged -1 o J FMAMJMonth J J F M A M J J D Month Latitudinal Latitudinal Mean Mean -- 1998 1998 C)200 C)2oo ß ß ß ß ß ß ß 150 150 C 100 50 i i i A S O J FMAMJMonth J AS OND J F M A M J J N D Month Fronts and Frontswithin within the the study studyarea areaare arestrongest strongest andhave havethe thelargest largest Figure 4. Seasonal cycles of frontal strength and position Figure 4. Seasonalcyclesof frontal strengthand position cross-shelf extension in in austral cross-shelfextension australsummer-early summer-earlyfall fall (January(January- calculated calculated as as the the two-dimensional two-dimensional gradient gradient magnitude magnitude of of SST SST April). In May, fronts stronger the 0.05°C km' disappear, April). In May,frontsstronger the0.05øCkm'l disappear,in (a) 1996, (b) 1997, and (c) the first 5 months of 1998. in (a) 1996, (b) 1997, and (c) the first 5 monthsof 1998. and In and weaker weaker fronts fronts are are closer closer inshore. inshore. In late late austral austral fallfallValues are are means means over over the the latitudinal Values latitudinalextent extentof of the thestudy studyarea area winter SST plotted as as contours contours in in time time and and cross-shelf cross-shelf distance. distance. Data winter (June-September), (June-September), SSTfronts frontsare areminimum, minimum,and andthose those plotted Data are are present are relatively relatively close calculatedfrom fromtime time series series of of 77 day shore (within to shore dayNOAA NOAA AVHRR AVHRR present are close to (within 60 60 km). km). calculated composite images. Beginning images. Beginningin in spring spring(October), (October),frontal frontalposition positionbegins beginsto to shift shift composite farther offshore. This continues to increase through farther offshore. This continues to increase through December-January, when fronts stronger than km''! May, May, patterns patternsin in 1998 1998 resemble resemblethose thoseof of 1996 ! 996 and and1997. 1997. December-January, when fronts stronger than 0.05 0.05°C øCkm are within 50 50 km are again again present presentwithin km of of shore shoreand andweaker weakerfronts fronts These Thesegradient gradientdata datasuggest suggestthat thatthe thestrong strongSST SSTanomalies anomalies and 2b) extend with the the May associatedwith May 1997 1997 pulse pulse(Plates (Plates !1 and 2b) extend200 200 km km offshore. offshore. SST SST frontal frontal patterns patternsin in the the early early associated portion of 1997 (Figure 4b) are similar to that of 1996, with occurred at a time when the normal seasonal pattern of portionof 1997 (Figure 4b) are similar to that of 1996, with occurredat a time whenthe normalseasonal patternof frontal frontal maxima in in both both strength maxima strengthand and cross-shelf cross-shelfposition positionin in summersummer- activity activity is is weakening weakeningand and shifting shifting inshore. inshore. Differences Differences early winter (July-September) (July-September) 1997 1997and and 1996 1996 are are not early fall fall (January-April), (January-April),although althoughfronts frontsin inearly earlyfaIl fall (April) (April) between betweenwinter not do not do not extend extend as as far far offshore. offshore.Thereafter, Thereafter, in in 1997, 1997, however, however, obvious. obvious.The The second secondpulse, pulse,however however(December (December1997, 1997,Plate Plate distinct differences are apparent. weaken and and are occurs in in late summer, during distinct differencesare apparent.Gradients Gradientsweaken are 1), 1), occurs latespring-early spring-early summer,when whengradients gradients during present closer closer to to shore non-El Niflo Nifio conditions conditions (1996) (1996) are are maximum maximum in in both both present shorein in late latefall-winter fall-winter(June), (June),earlier earlierin inthe the non-El year than weaker extension, creating strong year than those those of of 1997 1997and andmuch much weakernearshore nearshore magnitude magnitude and and cross-shelf cross-shelf extension, creating strong (gradients 0.03°C km km''l are between the the 2 2 years. years. These are (gradients >> 0.03øC arenot notpresent). present).They Theybecome become differences differences between Thesedifferences differences arecarried carried stronger and are farther offshore in September-early October with strongerand are fartheroffshorein September-early October through throughthe thesummer summerinto into1998 1998(January-March) (January-March) withweaker weaker but again and and are located closer closer to to shore but then then weaken weaken again are closer closer to to shore shorein in spring spring gradients gradientslocated shorethan thanthose thosepresent presentin in early early (November-December).In In 1998 1997. (November-December). 1998 (Figure (Figure 4c) 4c) the therelatively relatively 1997. Comparisons with with the weak gradients gradients are are still still present present in in January. January. In Comparisons the chlorophyll chlorophyll patterns patternsin in Figure Figure33 weak In February, February,fronts fronts concentrations chlorophyll show that elevated strengthen and begin to progress offshore, such that by Aprilthe that strengthen andbeginto progressoffshore,suchthat by April- show that the elevated chlorophyll concentrationsthat THOMAS ET AL.: AL.: NORTHERN NORTHERN CHILE CHILE LA LA NIl•IA NINA AND AND EL EL NII•10 NINO THOMAS develop 1997 developin in late lateSeptember-October September-October 1997 are are associated associatedwith with the brief of SST the brief strengthening strengtheningof SST gradients gradients(Figure (Figure 4b) 4b) and and increasing that occurred occurred in in the the increasing cross-shelf cross-shelf extension extension that relaxation period period between between the the major El Niño relaxation major El Nifio pulses pulses(Plate (Plate1). 1). This is This is consistent consistentwith with the theconceptual conceptualmodel modelthat thatupwelling upwelling produces strengthened frontal frontal gradients gradients and and increased nutrient producesstrengthened increasednutrient availability, leading to to higher biomass. availability,leading highercoastal coastalphytoplankton phytoplankton biomass. Relatively low coastal and reduced Relatively low coastalconcentrations concentrations and reducedoffshore offshore extension of chlorophyll in November 1997-February extensionof chlorophyllin November 1997-February1998 1998 are with are associated associated with weak weakgradients gradientsrestricted restrictedto to the thenearshore nearshore region at at the region the time time of, of, and andimmediately immediatelyfollowing, following,the thearrival arrival of the second pulse ............. pulseof of strong anomalies (Plate I). Data from 1996 suggest suggest that that SST SST (Figure (Figure 22 and 1996 and Plate Plate 3) 3) and andfrontal frontal structure (Figure 4) during this El Niflo period are structure (Figure 4) during this El Nifio period are dramatically different than than nodal normal austral austral summer dramaticallydi•erent summerconditions. conditions. In increase In fall •all (April-May) (April-May) 1998, 1998,chlorophyll chlorophyllconcentrations concentrations increde nearshore and and expand expand in direction nearshore in aa cross-shelf cross-shel• directionin in association association with the the strengthening and with strengthening andoffshore o•shoremigration migrationof o• SST SSTfronts. •ronts. Comparisons fronts and Comparisons between be•een SST SST •onts and chlorophyll chlorophyll time/space patterns simplified by time/space pa•e•s are are simplified by decomposing decomposingthe the variance •sociated associated with with each variance eachusing usingempirical empiricalorthogonal o•hogonal functions The cross-sheff cross-shelf transect transect of both chlorophyll •unctions(EOF5). (EOFs). The o•both chlorophyll and gradient at 20.5°S is presented presented as and gradientmagnitude magnitude at 20.5øSis asan anexample; example; other other profiles profiles had had similar similarvariance variancestructure. structure.The The dominant dominant mode variability (Figure (Figure 5a) 5a) explains explains 81% of mode of o• chlorophyll chlorophyllvariabili• the variance the El the varianceand and captures capturesthe El Niflo Nifio signal. signal.The The spatial spatial pattern associated with this mode shows a simple cross-shelf pa•e• •sociated with this modeshowsa simplecross-sheff gradient with maximum restricted gradientwith maximumconcentrations concentrations restrictedto to within within20 20 km km of oCshore. shore.The The amplitude amplitudeof o• this this pattern pa•e• is is strongest strongestin in September-October, withaa peak peak in in early September-October, with earlyOctober, October,becomes becomes weak until March, weak from •rom mid-November mid-Novemberuntil March, and and then thenstrengthens strengthens to of the to aa peak peakininlate lateApril. April.The TheEOF EOFdecomposition decompositiono• the gradient series coincident gradient magnitude magnitude time time series coincident with with the the chlorophyll time series series is is shown chlorophylltime shownin in Figure Figure5b. 5b. The Thedominant dominant mode 82% o• of the mode explains explains82% the variance, variance,with with aa spatial spatialpattern pa•e• showing located showingmaximum m•imum gradients gradients located20 20 km kmoffshore, o•shore,spatially spatially Spatial Pattern Spatial Pattern 4 Variance =•1% A) A) 4 a= ß 3 coincident with the the offshore coincidentwith offshoreedge edgeof of elevated elevatedchlorophyll chlorophyll concentrations. The amplitude concentrations. The amplitudetime time series series indicates indicatesthis this pattern increases to to intermediate strength in in late patternincreases intermediate strength lateOctober, October,is is weakest in December (coincidentwith with the the arrival arrival of of the weakest in December (coincident the second (Plate secondpeak peakof of SST SSTanomalies anomalies (Plate1)) 1)) and andthen thenbecomes becomes late showing April, in March and again strong strong again in March and late April, showingmany many similarities to the similaritiesto thechlorophyll chlorophyllamplitude amplitudetime timeseries. series. These These data control pigment structure by by the datasuggest suggest controlof of cross-shelf cross-shelf pigmentstructure the SST frontal zone and any current jet associated with SST frontal zone and any currentjet associatedwith it, it, consistent with observations consistentwith observationsin in the the California California Current Current during during the et al., the coastal coastaltransition transitionzone zone experiment experiment[Sirub [Strub et al., 1991]. 1991]. Although SeaWiFS data are are not not available from the the 1996 ^ •k .... k k.•.,,(•l, c•xxz;•c data available c•,•,• 1996 VV ix • lintIll Z'-•kll,!!•../U•:•11 period for period for comparison, comparison, EOF EOF analysis analysisof of the thegradient gradient magnitude time series for 1996 (Figure 5c) provides aa contrast magnitude timeseriesfor 1996(Figure5c) provides contrast of the between of thefrontal frontalpatterns patterns betweenEl El Niflo Nifio and andLa LaNifia Nifiayears. years. This contrast contrast can can be be used This usedto to imply implydifferences differencesthat thatmight mightbe be expected in in chlorophyll pattern expected chlorophyll patternduring during1996. 1996. Months Monthsfrom from 1996 have have been been rearranged rearranged to to emulate the monthly 1996 emulatethe monthlysequence sequence coincident with with the the availability coincident availabilityof of SeaWiFS SeaWiFSdata datafrom from1997 ! 997to to 1998 for for direct direct comparison. The dominant 1998 comparison. The dominantmode mode(Figure (Figure5c) 5c) explains 57% of of the considerably less than than that that of explains57% thevariance, variance, considerably less of the 1997-1998 series, indicating more complex space/time the 1997-1998 series,indicatingmore complexspace/time frontal patterns variance associated with patternsand andincreased increased variance associated with frontal patterns spread across across higher higher modes. modes. The The spatial spatial pattern patterns spread patternin in this year shows differences from from that that of of thisLa La Nifia Nifia year showssignificant significant differences 1997-1998. Strong fronts extend farther offshore, out to -75 1997-1998.Strongfrontsextendfartheroffshore,out to-•75 total cross-shelf width of increased km. The total cross-shelfwidth increasedfrontal frontalactivity activityis is stretched over over -50 -50 km, stretched km, wider wider than thanthat thatevident evidentin in 1997-1998, 1997-1998, with two two distinct distinct peaks peaks located located 25 25 and and 60 60 km km offshore. offshore. The with The amplitude time series also shows a very different seasonal amplitude time seriesalsoshowsa very different seasonal is weak cycle than than that The cycle thatof of1997-1998. 1997-1998. Thepattern patternis weakin in September-October (late winter-early winter-early spring), spring), aa time September-October (late timewhen when the 1997-1998 time series the 1997-1998 time series indicated indicated a a local local maximum. maximum. In In 1996 the the pattern pattern strengthens strengthens in in November November to to aa peak peak in 1996 in early early February (midsummer), times when the pattern in 1997-1998 February(midsummer),times when the patternin 1997-1998 was weakest. weakest. In In late the pattern pattern weakens was late summer summer(March) (March) the weakensto to aa Spatial Pattern Pattern Spatial B) B) 3 , 909 909 Spatial Pattern Spatial Pattern C) c) 3 Variance = 82% ß Variance =57% Variance = •7% • 2 1 I • ; o • o -1 150 100 50 150 100 50 km Offshore Offshore km 200 200 2.0 2.0 200 200 0 0 0.05 0.05 1.5 ' 2OO Amplitude Time Amplitude Time Series Series .... 0.05 0 0.05 ........ 0.04 0.04 0.02 • 0.02 000 0.00 000 0.00 0.01 0.01 100 50 100 50 km km Offshore Offshore 150 150 Amplitude Amplitude Time Time Series Series 0.03 0.03 0.0 Month Month 0 0.03 0.03 0.02 ONDJJ FMAMJ FMAMJ 50 50 0.04 a 0.04 0.5 S OND 100 100 km km Offshore Offshore Amplitude Amplitude Time Time Series Series 1.0 150 150 .... SONDJJ FMA FMAMJ MJ S OND Month Month 0.02 . 0.01 0.01 SONDJ S 0 N D J FMAMJ F M A M J Month Month Figure 5. of SeaWiFS distributions over over the the 1997-1998 Figure 5. EOF EOFdecompositions decompositions of (a) (a) cross-shelf cross-shelf SeaWiFSchlorophyll chlorophylldistributions 1997-1998 study study period, (b) cross-shelf cross-shelf SST SST gradient gradient magnitude magnitude for the the same same period, period, and and (c) cross-shelf SST for period, cross-shelf SST gradient gradientmagnitude magnitude for 1996, prior prior to to El El Nifio Nifloconditions. conditions. The The largest largest mode mode and and its its associated associated variance variance are are presented presented in in each each case, case, showing showing 1996, both the the spatial pattern and and an an amplitude time series. Months in in 1996 both spatialpattern amplitudetime series.Months 1996 (Figure (FigureSc) 5c) were wererearranged rearrangedinto into the thesame same order as as the the SeaWiFS order SeaWiFSdata dataperiod periodto to facilitate facilitatecomparison. comparison. THOMAS ET AL.: AL.: NORTHERN NORTHERN CHILE CHILE LA LA NIIqA NI1A AND Nl1O THOMAS AND EL NIIqO 910 910 local minimum minimum in in mid-April, mid-April, opposite opposite to to that that associated with local associated with the of 1997-1998. the El El Niflo Nifio conditions conditionsof 1997-1998. The Theamplitude amplitudetime time series with seriesfor for 1996 1996isisconsistent consistent withfrontal frontalactivity activityexpected expected from annual favorable from the theclimatological climatological annualcycle cycleof ofupwelling upwelling favorable wind by windforcing forcingpresented presented by Blanco Blancoet etal. al. [2000]. [2000]. Elevated coastal chlorophyll concentrations Elevated coastal chlorophyll concentrationsin in eastern eastern boundary currents result from the upwelling of cold, boundarycurrentsresultfrom the upwellingof cold,nutrientnutrient- rich rich water, water, and and time time series seriesof of nearshore nearshoreSST SSTmeasurements measurements should signal The shouldcontain containany anyseasonal seasonal signalof of upwelling upwellingintensity. intensity. The upwelling on, upwellingsignal, signal,however, however,is issuperimposed superimposed on,among amongother other factors, factors,both boththe theannual annualcycle cycleof of surface surfacesolar solarheating, heating,which which at is considerable, and any at the thelatitudes latitudesexamined examinedhere here is considerable,and any horizontal influencing horizontaladvective advectiveprocesses processes influencingSST. SST. For For this this reason reasonthe the seasonal seasonalcycle cycleof of upwelling upwellingstrength strength(cold (coldwater water near the coast) is poorly captured in a simple time sequence of nearthe coast)is poorlycapturedin a simpletime sequence of coastal coastalSST. SST. A A partial partialcorrection correctioncan canbe bemade madefor forthese theseeffects effects by that the the annual by assuming assumingthat annualcycle cycleof of solar solarheating heatingis is the thesame same at at offshore offshorelocations locationsof of equal equal latitude latitude and and that thatsurface surface advective processes influencing advectiveprocesses influencingSST SST are aresmall smallrelative relativeto to the the annual cycle and differences caused by upwelling. An index annualcycle and differencescausedby upwelling.An index of of coastal coastalupwelling, upwelling,which which we we call call the the"coastal "coastalSST SSTdeficit" deficit" [Hi!! the [Hill el et al., al., 1998; 1998; Thomas, Thomas,1999], 1999],is is formed formedby by subtracting subtracting the coastal coastalSST SST (the (the 1I km km nearest nearestshore) shore)in in each eachtime timeperiod periodfrom from the SST the SST 200 200 km km offshore offshore at at the the same same latitude. latitude. This This coastal coastal deficit one-dimensional (cross-shelf) deficit captures capturesaalarge-scale large-scale one-dimensional (cross-shelf) aspect the gradient as aa localized aspectof of the gradientstructure structurepresented presentedabove aboveas localized small-scale two-dimensional calculation calculation in in Figure Figure4. 4. A small-scaletwo-dimensional A mean mean coastal deficit over the study region for each time period coastaldeficit over the studyregionfor eachtime periodwas was formed by averaging averaging in in latitude. latitude. The formedby Theannual annualcycle cycleover overthe the14 14 month 1997 monthperiod periodbeginning beginningin in January January1996 1996and andin inJanuary January 1997 is is shown shownin in Figures Figures6a 6a and and6b 6b to to contrast contrastupwelling upwellingsignals signalsin in conditions, respectively. the La Nina and El Nub the La Nifia and El Nifio conditions,respectively. In In 1996 1996early deficit (> early1997 1997the thecoastal coastal deficitisismaximum maximum (>44°C) øC)in insummer summer (JanuaryFebruary), indicative (January-February), indicativeof ofrelatively relativelycold coldcoastal coastalSST, SST, when annual when annual maxima maxima in in both bothupwelling upwellingwind wind forcing forcingand and monthly sequence sequence during during La La Nifia Nina conditions is shown monthly conditionsis shownin in Figure 6d, highlighting significant differences between Figure 6d, highlighting significantdifferencesbetweenthe the two phases two phasesof of the theENSO ENSO cycle. cycle.Assuming Assumingaa similar similarlinkage linkage between deficit and and chlorophyll as that betweendeficit chlorophyllas that shown shownin in Figures Figures33 and and 6 6 for for 1997-1998, 1997-1998, the the deficit deficit time time series seriesfrom from 1996 1996 implies a very different annual chlorophyll pattern, implies a very different annual chlorophyll pattern,with with maximum coastal maximum coastal concentrations concentrationsin in summer, summer,coincident coincidentwith with seasonal seasonal wind wind maximum maximum 10 c' C)'25 25 20 8 2o [,15 6 6• 15- • 4 4• 10 5 0 . J FMAM J J A S ON 25 B)25 • 5 0 0 0 88 minimum minimum 6 6 4 5 2 5 0 0 0 Month Month F 8 -6 - .4 4• -22 SONDJ S O N D J FFMAMJ MAM J 1996 (shifted) 1996 (shifted) 20 m 10 FMAMJJ JJASOND A S ON D J 10 10 N D) D) 25 [, 15 15 FMAM 1997-98 1997-98 1010 2 10 10 - 20 2o j J and and ci) D J F 1997 1997 B) forcing forcing concentrations and cross-shelf cross-shelfstructure structureinin winter. winter. Such Such aa concentrations and 8 208 1996 A)2S solar between solarheating heatingproduce producemaximum maximumdifferences differences betweenthe thecoast coast and [Blanco et 2000]. Deficit andoffshore offshorewaters waters[Blanco et al. al. 2000]. Deficit decreases decreases steadily to aa minimum in winter winter (August) (August) and and then then rises steadilyto minimum(1°C) (1øC)in rises again. In 1997-early 1998 the annual cycle begins similarly again.In 1997-early1998the annualcyclebeginssimilarlyin in summer (JanuaryMarch) 1997 more summer(January-March) 1997but butdecreases decreases morerapidly rapidlyin in May to aa prolonged and May to prolonged andlower lower(< (< II °C) øC)minimum minimumfrom fromearly early July through late September. There is a brief recovery Julythroughlate September.Thereis a brief recoveryperiod period in but in in spring springwhen when the the deficit deficitincreases, increases,but in late latespring springand and summer the deficit again to to aa second summerthe deficitdecreases decreases again secondminimum minimumin in December-January, coincident with with the the maximum December-January, coincident maximumEl El Niño Nifio anomalies (Plate 1). anomalies(Plate 1). The The 1997-1998 1997-1998time timeseries seriesisisresampled resampled over the time period of coincident SeaWiFS over the time periodof coincidentSeaWiFSdata data(Figure (Figure6c) 6c) to to (Figure to facilitate facilitatedirect directcomparison comparison to chlorophyll chlorophyllpatterns patterns (Figure 3). 3). The The data datashow showthat thatmaximum maximumcoastal coastaldeficits deficitsdid did not not develop until late in the study period (AprilMay 1998). developuntil late in the study period(April-May 1998). In In September the deficit chlorophyll September the deficitis isinitially initiallylow, low,as asisiscoastal coastal chlorophyll concentration. concentration. The The short-lived deficit deficit increase in late OctoberNovember during October-November duringthe therelaxation relaxationperiod periodbetween betweenthe the large anomalies is is coincident with the largetemperature temperature anomalies coincidentwith thespring spring increase (Figure increasein in chlorophyll chlorophyllconcentration concentration (Figure3). 3). In In December December the coastal the coastaldeficit deficit drops drops to to near nearzero, zero,coincident coincidentwith with the the arrival of of the arrival the second secondEl El Niño Nifio pulse pulseand andvery very low lowcoastal coastal chlorophyll concentrations. The The low chlorophyllconcentrations. low coastal coastaldeficit deficitthrough through the austral summer and the subsequent increase the australsummerand the subsequentincreasetoward towardthe the May maximum are positively May maximumare positivelycorrelated correlatedwith with the the chlorophyll chlorophyll the coastal patterns. For patterns. For comparison, comparison,the coastaldeficit deficitover overthe thesame same 0 10 8 • 10 4• 2 SONDJ FMAMJJ 0 0 S O N D J F MAM Month Month Figure 6. between coastal coastal SST SST (dashed (dashed line) line) and and SST SST Figure 6. Coastal CoastalSST SSTdeficit deficit(dotted (dottedline) line)calculated calculatedas asthe thedifference differencebetween 200 km km offshore offshore (solid (solid line) (°C) in in the the 7 7 day day composite composite SST SST image image time time series. series. The periods beginning in 200 line)(øC) The14 14month month periods beginning in (a) the annual annual cycle cycle of of coastal coastal upwelling upwelling during during La LaNiña (a) January January1996 1996and and(b) (b) January January1997 1997contrast contrast the Nifia and andEl El Nino Nifio conditions respectively. The sequence when chlorophyll data are for conditions respectively. Themonthly monthly sequence whenSeaWiFS SeaWiFS chlorophyll dataare areavailable available arereplotted replotted for comparison to Figure 1997-1998 comparison to Figure 3 3 as asboth both(c) (c)the thecoincident coincident 1997-1998period periodand and(d) (d) the thesame samemonths monthsfrom from1996 1996with with months to facilitate Values for for each each time time period period represent represent aa latitudinal latitudinal mean mean over over the the monthsrearranged rearranged to facilitatedirect directcomparison. comparison. Values study studyarea. area. THOMAS ET AL.: NORTHERN CHILE CHILE LA LA Nlra N11A AND AND EL NIl•10 NITO AL.' NORTHERN 911 911 Arica S) Arica(18.5 (18.5øS) 8 climatology 6 4 2 0 Iquique (20.5 (20.5 øS) S) Iquique B 6 E 4 •E 4 >22 > 0 Antofagasta (23.5 S) Antofagasta (23.5øS) 8 6 4 2 0 1996 1996 1998 1998 1997 1997 1999 1999 Year Y6ar Figure 7. component of Figure 7. The Thevv(alongshore) (alongshore) component ofwind windvelocity velocity(positive (positivenorth) north)off off northern northernChile Chile for for 1996, 1996,1997, 1997,and and 1998 at at three stations (thin (thin line) 1998 threecoastal coastalmeteorological meteorological stations line) at atArica Arica(18.5°S), (18.5øS),Iquique lquique(20.5°S), (20.5øS),and andAntofagasta Antofagasta (23.5°S). For the [Blanco et 2000] of these stations is (bold (23.5øS). Forcomparison, comparison, the10 l0 year yearclimatology climatology [Blanco etal. al.2000] of these stations ispresented presented (boldline). line). pattern would be be consistent with the the two pattern would consistent with two seasons seasonsof of field field region, was was unabated unabated and and (in (in the the north) region, north)even evenincreased increasedduring during measurements presented by by Morales Morales et et al. al. [1996]. measurements presented [ 1996]. the These data the 1997-1998 1997-1998period. period. These data indicate indicatethat thatsurface surface 4.2. Wind Forcing Forcing 4.2. Wind Niflo period are are not of Nifio period not due due to to cessation cessation of upwelling, upwelling,aa view view consistent with the hydrographic analysis of Blanco Blanco et et al., consistentwith the hydrographicanalysisof al., The The year-round year-roundcoastal coastalupwelling upwellingof of northern northernChile Chileresults results from persistent equatorward (upwelling fromseasonally seasonally persistent equatorward (upwellingfavorable) favorable) alongshore wind stress stress [Blanco [Blanco et et al. al. 2000]. 2000]. Relaxations (or alongshore wind Relaxations (or reversals) in local wind stress stress would would result reversals)in local alongshore alongshorewind result in in warmer coastal temperatures and reduced warmer coastal surface surface temperatures and reduced phytoplankton phytoplanktonbiomass. biomass. Previous Previous analyses analyses of of El El Nifio Nifio conditions in in both upwelling conditions boththe theCalifornia Californiaand andPeru-Chile Peru-Chile upwelling temperature and and chlorophyll chlorophyll patterns temperature patternsduring duringthe the1997-1998 1997-1998El El (submitted manuscript, manuscript, 2000). 2000). (submitted 4.3. Comparisons to to Other Other Satellite Chlorophyll Data Data 4.3. Comparisons Satellite Chlorophyll Available satellite pigment pigment data Available satellite datafrom from northern northernChile Chile from from 1979 [Thomas, 1999], 1999], the the only only year year during 1979 [Thomas, duringthe theCZCS CZCS mission mission with in enough to attempt a with observations observationsin enoughmonths monthsto attemptforming forming a systems have have shown shown that that wind seasonal cycle, systems wind stress stressis is often oftennot notreduced reduced seasonal cycle, provide provideadditional additionalcomparison. comparison. Although Although during El events or develop concentrations are are lower lower (0.5-1.0 (0.5-1.0 mg during El Niflo Nifio events or that thatlocal localwind windanomalies anomalies develop CZCS CZCSpigment pigment concentrations mgm3 m'3inin after the the onset onset of [Huyer after of El El Niflo Nifio conditions conditions [Huyerand andSmith, Smith,1985; 1985; the the coastal coastalregion) region) than than those thoseofofSeaWiFS, SeaWiFS,cross-shelf cross-shelf Reinecker and and Mooers, Mooers, 1986]. 1986]. Under local Reinecker Underthese thesesituations, situations, local distances distancesof of maximum maximum concentrations concentrationsare are similar, similar, restricted restricted offshore Ekman Ekman transport transport driven driven by equatorward 30 km. km. However, offshore by continuing continuing equatorward to to within within 30 However,the theCZCS CZCSannual annualcycle cyclein in 1979 1979 wind stress warm a late late fall-winter fall-winter maximum maximum in in concentrations concentrations(April(April wind stresssimply simplyupwells upwellsanomalously anomalously warmand andnutrientnutrient- suggests suggests a poor subsurface water from both Januaiy have no poor subsurfacewater from above abovethe thestrongly stronglydepressed depressed July), July), although althoughboth Januaryand and December Decemberhave no data. data. pycnocline. Hydrographic Hydrographic patterns patterns during during the the 1997-1998 pycnocline. 1997-1998El E! This This is is out outof of phase phasewith withthe theSeaWiFS SeaWiFSdata datafrom fromthe theEl El Niflo Nifio Niflo are consistent consistent with with these (Blanco This inconsistency may be be due in the Nifio are theseprevious previousobservations observations (Blanco period. period.This inconsistencymay due to to inaccuracies inaccuraciesin the et manuscript, 2000). et al., al.,submitted submitted manuscript, 2000). Wind Windforcing forcingalong alongthe the actual actual CZCS CZCS pigment pigmentconcentrations concentrations[Chavez [Chavez 1995] 1995] but but is is northern Chile to be by the northern Chilecoast coastat atArica Arica(18.5°S), (l 8.5øS),Iquique Iquique(20.5°S), (20.5øS),and and also alsolikely likely to be due due to to inadequate inadequatesampling samplingby the CZCS CZCS in in Antofagasta (23.5øS) (23.5°S) is is shown Figure 7 as time and and space space [Thomas [Thomas et etal., Antofagasta shownin in Figure asalongshore alongshoreboth bothtime al., 1994; 1994;Thomas, Thomas,1999]. 1999]. wind (1970Although measurementsare are not windvelocity velocityover overthe thestudy studyperiod periodand andclimatological climatological (1970Although SeaWiFS SeaWiFS chlorophyll chlorophyll measurements not 1997) monthly means means available available from from [Blanco [Blanco et et al. the 1996 1997) monthly al. 2000]. 2000]. available availablefor for the 1996 cold coldphase phaseof of the theENSO ENSO cycle cyclecaptured captured Discussion of and Discussion of wind windanomalies anomalies andtheir theirrelative relativetiming timingduring during by by the the AVHRR, AVHRR, chlorophyll chlorophylldata datafrom fromthe theOcean OceanColor Colorand and the El Nifio Niflo period period are are presented presented by by Blanco Blanco et et al. al. Temperature Sensor (OCTS) aboard the ADEOS satellite the 1997-1998 1997-1998El TemperatureSensor(OCTS) aboardthe ADEOS satelliteare are (submitted manuscript, 2000) 2000) and and will will not here. as global at 99 km (submittedmanuscript, notbe berepeated repeated here. available available as global monthly monthly composites compositesat kmspatial spatial The throughout for the the period November 1996-June 1996June 1997 The important importantpoint, point,however, however,is is that thatwind windstress stress throughout resolution resolutionfor period November 1997 (the (the the region remains equatorward over the entire study period. lifetime of the mission). Differences in the characteristics of of the regionremainsequatorward overthe entirestudyperiod. lifetime of the mission). Differencesin the characteristics Upwelling of subsurface water and offshore transport, the OCTS and SeaWiFS sensors make their respective Upwelling of subsurfacewater and offshore transport, the OCTS and SeaWiFS sensors make their respective although diminishedat at the the southern portion of of the difficult to althoughdiminished southernportion thestudy study chlorophyll chlorophyllconcentration concentrationretrievals retrievals difficult to compare; compare; THOMAS ET ET AL.: AL.: NORTHERN NORTHERN CHILE CHILE LA LA NIIqA NI14AAND ANDEL ELNIl•O Nl1O THOMAS 912 912 indication of trends indication of trends over over the theperiod. period. Both Both cross-shelf cross-shelf extension and and nearshore values of of elevated extension nearshore values elevatedchlorophyll chlorophyll concentrations are areminimum minimum in in November 80 concentrations November1996 1996 (spring) (spring)and and increase to aa maximum in austral and early early fall fall 0 increaseto maximum in austral summer summer and (FebruaryApril). Thereafter both decrease until June (winter) 60 60 (February-April).Thereat•erbothdecreaseuntil June(winter) 1997 when when patterns patterns are aresimilar similar to to those those of of November 1997 November1996. 1996. The summer maximum evident in these data is clearly The summer maximum evident in these data is clearly 40 401 different from The 1996different frompatterns patternsin in 1997-1998 1997-1998(Figure (Figure3). 3). The 1996E 1997 summer maximum is consistent, however, with the 1997 summer maximum is consistent, however, with the 20 20 0.25 climatological seasonal maximum in in upwelling climatological seasonalmaximum upwellingfavorable favorable -1.O-___, .s_____ wind forcing el al., 0 wind forcing [Blanco [Blanco et al., 2000] 2000] as as well well as asboth boththe the AVHRR-measured frontal strengths (Figures4a, 4a,4b 4b and and 5c) A VHRR-measured frontal strengths (Figures 5c) N D J F M A M J and the the temporal temporal pattern patternof of SST SSTdeficit deficit (Figure (Figure 6d) 6d) for for 1996. and 1996. Month A second second indication indication of the the extent extent to to which which the the 1997-1998 1997-1998 seasonal chlorophyll are disrupted by El patterns Figure 8. seasonal chlorophyll patterns are disrupted by El Niflo Nifto The distribution of OCTS satellite-measured Figure 8. The distribution of OCTS satellite-measured be obtained obtained by by examining data from from chlorophyll (mg Chile November 1996 conditionscan can be examiningSeaWiFS SeaWiFSdata chlorophyll (mgm3) m-3)off offnorthern northern Chilefrom from November 1996 conditions to in time the following following year to June June 1997 1997 plotted plotted as as contours contours in time and andcross-shelf cross-shelf the year(1998-1999). ( 1998-1999).Cross-shelf Cross-shelfprofiles profilesfrom fromthe the distance. OCTS data images 1998toto March March 1999 (Figure 9) 9) show distance.OCTS data are are from frommonthly monthlycomposite composite images period period September September1998 1999 (Figure show with resolution. Cross-shelf values are midsummer with99km kmspatial spatial resolution. Cross-shelf values areaveraged averaged maximum concentrations in maximum chlorophyll chlorophyll concentrations in midsummer over extent overthe the latitudinal latitudinal extentof ofthe thestudy studyarea areain ineach eachmonth. month. in maximum in phase with the maximum (January-February), (January-February), in phase with the in climatological wind forcing climatologicalwind forcing (Blanco (Blanco et et al. al. 2000) 2000) and and consistent with the consistent with the summer summer seasonal seasonal maximum maximum of of nearshore nearshore however, valid comparisons can of patterns in time pigment observed in in the the California California Current however, valid comparisons canbe bemade madeof patterns in time phytoplankton phytoplankton pigmentobserved Currentin in 100 lOO 80. C NDJ Month FMAMJ and space. CZCS data data [Strub [Strubet et aL, al., 1990; 1990;Thomas Thomasand andStrub, Strub,1990]. 1990]. and space. The The 88 months monthsof ofOCTS OCTSdata datawere wereacquired, acquired, CZCS subset to the chlorophyll between the the SeaWiFS SeaWiFS spatial spatial patterns patterns evident evident in in Contrastsbetween subsetto thestudy studyarea areaand andthe thecross-shelf cross-shelf chlorophyll Contrasts distribution, averaged over the latitudinal extent of the study September and December of both years are consistent with distribution, averaged overthe latitudinal extentof the study Septemberand Decemberof both yearsare consistent with area, calculated calculatedinineach eachmonth. month. The The 99 km km resolution OCTS [1996]. Maximum those by Morales al. [1996]. area, OCTS those reported reported by Morales el et al. Maximum data do do not not resolve resolve the the upwelling upwelling region as well concentrations concentrations are arerestricted restrictedto towithin within 30-40 30-40 km of of shore, shore,and and data regionand andgradients gradients aswell as the the 44 km SeaWiFS elevated concentrations concentrations extend extend farther farther offshore, offshore, resulting resulting in SeaWiFS data, but the the time time series seriesof of the thecrosscross- elevated in aa shelf chlorophyll concentrations (Figure 8) provide a clear reduced cross-shelf gradient in late winter (September). The shelf chlorophyllconcentrations (Figure 8) providea clear reducedcross-shelfgradientin late winter (September).The Arica 18.5'S Iquique 20.5°S 19.5'S 200 200 200 150 150 150 .0 .8 100 .8 100 a 100 0 50 . 50 0 S J FMA S OOND N D J F M A 0 50 S OOND N D 3 J FMA F M A 0 S Month Month 21.55 21.5øS S OOND FMA N D .1 J F M A S Month Month Month Month 22.5'S 22.5oS 200 2O0 200 200 200• 150 150 150 2oo i....... .......... . .: 150 50 50 50 0 0 Antofagasta Antofagasta23.5'S 23.5øS . 0• '• S OND J FMA 0 S O N D J Month Month F M A S OND FMA S O N D .1 J F M A Month Month 4.. S OND J FMA S o N D J F M A Month Month Figure 9. distribution ofof SeaWiFS satellite-measured chlorophyll (mg Chile from September Figure 9.The The distribution SeaWiFS satellite-measured chlorophyll (mgm3) m-3)off offnorthern northern Chile from September 1998 to April April 1999 at the the six six latitudes latitudes used used in in Plate Plate 2. 2. Data 1998 to 1999plotted plottedas ascontours contoursin in time timeand andcross-shelf cross-shelfdistance distanceat Dataare are subset from 8 8 day day composite composite images imageswith with 44 km km spatial spatial resolution. resolution. subsetfrom THOMAS AL.: NORTHERN NORTHERN CHILE CHILE LA LA NllqA NI1A AND THOMAS ET AL.' AND EL N11O NII•IO temporal patterns of of chlorophyll evident in temporalpatterns chlorophyllevident in Figure Figure99 (non-El (non-El Niflo) clearly clearly track track the the temporal temporal patterns patterns of of SST Nifio) SST and andfrontal frontal activity in 1996 1996 (Figures (Figures2,2,4,4, 5c 5c and and 6b) 6b) as as well activity in well as as the the 913 913 data bins to data are are averaged averagedlatitudinally latitudinally within within cross-shelf cross-shelfbins to provide a a single provide single mean mean cross-shelf cross-shelfprofile profile of of surface surface chlorophyll representative of of the the northern northern Chile Chile chlorophyllconcentration concentration representative study In situ studyarea area for for each eachcruise cruiseperiod. period. In situ data datafrom from four four cruises are shown in Figure cruisesare shown in Figure 10 10 and andcompared comparedwith with the the latitudinally averaged averaged cross-shelf cross-shelf profile profile extracted extracted from from the the latitudinally most These mostclosely closelycoincident coincidentSeaWiFS SeaWiFS 8 8 day daycomposite. composite. These SeaWiFS data features. First, the data data show show three three features. First, the SeaWiFS data systematically overestimate in situ surface chlorophyll in the systematically overestimate in situ surfacechlorophyllin the study region by by aa factor factor of of 2-4. 2-4. As this paper goes to press, a study region As this paper goes to press, a 4.4. Coincident Coincident in in Situ Situ Data Data 4.4. complete reprocessing of of the the SeaWiFS completereprocessing SeaWiFSdata dataisisunderway, underway, Inadequacies in the the atmospheric correction and/or Second, the the SeaWiFS Inadequacies in atmospheric correcticm and/orin-water in-water hopefully hopefiJllyreducing red,icingthis thi.qbias. bia•. Second, SeaWiFSdata data algorithm used to extract chlorophyll concentrations from the successfully capture the mean relative cross-shelf structure of of capturethe meanrelativecross-shelf structure algorithmusedto extractchlorophyllconcentrations fromthe successfully SeaWiFS data version available at present might bias the the chlorophyll in each cruise, with a similar bias during all SeaWiFSdata versionavailableat presentmight bias the the chlorophyllin eachcruise,with a similarbiasduringall that despite despite temporal and spatial spatial patterns patterns shown shown here. here. Precise This suggests suggests that temporaland Precisevalidation validation four four comparison comparison periods. periods. This of [e.g., the absolute concentration inaccuraciesin in the absolute chlorophyll chlorophyll concentration of the theSeaWiFS SeaWiFSdata datarequires requiresspecific specificprotocols protocols [e.g.,Mueller Mueller inaccuracies and from the the SeaWiFS SeaWiFS data, data, the the data data can can be retrieved from be used used to to and Austin, Austin, 1995] 1995] and and cannot cannotbe bereproduced reproducedfor for the the retrieved time/space period analyzed here. provide an initial view of the time and space patterns However, spatial and time/spaceperiod analyzed here. However, spatial and providean initial view of the time and spacepatternsof of within the temporal averages averages can can be be used used to to estimate, at least temporal estimate,at leastto to aa first first relative relativechlorophyll chlorophyllwithin the study studyregion. region. Third, Third, distinct distinct between available approximation, whether general apparent between approximation,whether general trends trendsseem seemreasonable. reasonable. differences differences are are apparent available years. years. Chavez uses this to of summers, Comparisons of in in situ situdata datafrom fromtwo twohemispherical hemispherical summers, Chavez[1995] [1995] uses thisapproach approach to show showthat thatclimatological climatological Comparisons El Niflo December 1997 1997 (the (the height CZCS pigment time time series crossheight to to El Nifio conditions) conditions)and and CZCS pigment seriescapture capturethe thesame samelarge-scale large-scale cross- December December 1998 1998 (non-El (non-El Nifio Niño conditions), shelf in situ situ off conditions),show showthat thatwithin within shelf chlorophyll chlorophyll seasonal seasonalcycle cycle measured measured in off December California but that are apparent nearshore upwelling region, mean chlorophyll California but that systematic systematicdifferences differencesare apparentoff off the the nearshere upwelling region, mean chlorophyll Peru. At the there is is no are Nub Peru. At the time time of of writing, writing, there no doubt doubt that that the the concentrations concentrations aresignificantly significantlylarger largerduring duringthe thenon-El non-E1Nifio SeaWiFS data will be subjected to further reprocessing, year. In December 1998 these concentrations are> 1.0 mg SeaWiFS data will be subjectedto further reprocessing, year.In December 1998these concentrations are> 1.0mgm3 m'3 taking advantage advantage of 20 km of shore and reach a maximum of taking of improved improved atmospheric atmosphericcorrection correction within within20 kmof shore andreacha maximum of1.5 1.5mg mgm3. m-3. schemes, and that chlorophyll schemes, and thatsatellite-derived satellite-derived chlorophyllconcentrations concentrations This This provides providesfurther further evidence evidencethat thatchlorophyll chlorophyllpatterns patterns will to be during will continue continueto be improved improvedupon. upon. duringthe the El E1Niflo Nifio conditions conditionsof of the the1997-1998 1997-1998summer summerare are In coincident anomalous and and that that under under more more normal conditions, conditions, In situ situsurface surfacechlorophyll chlorophyllmeasurements measurements coincidentwith with anomalous the SeaWiFS data are the patterns would the SeaWiFS data are available available from from a series series of of cruises cruises made made chlorophyll patterns chlorophyll would more moreclosely closely resemble resemble the by IFOP conditions structure evident evident in in the by IFOP to tomonitor monitoroceanographic oceanographic conditionsduring duringthe the time/space time/spacestructure the SST SST satellite satellitedata datafrom from 1997-1998 El Nifio Niflo and recovery. These 1996 (Figures 44 and 1997-1998 El and their theirsubsequent subsequent recovery. These 1996 (Figures and6). 6). chlorophyll patterns measured measured by by the the OCTS OCTS (Figure (Figure 8) chlorophyllpatterns 8) more more closely than those of the 1997-1998 El Niflo period. It closelythanthoseof the 1997-1998El Nifio period. It is isalso also of of note notethat thatthe the seasonal seasonalcycle cycle evident evidentin in the the(much (muchmore more extensive) SeaWiFSdata data from from 1998 1998 to to 1999 extensive)SeaWiFS 1999 (Figure (Figure 9) 9) as as well as the the OCTS OCTS data data from from 1996 to 1997 is out out of of phase phase with with well as 1996to 1997 is the 1979 by the 1979 CZCS CZCS data datapresented presented by Thomas Thomas[1999]. [1999]. March 1998 March 1998 December December 1997 1997 1.5 1.5 1.5 .... i .... i .... i .... 1.0 1.0 I. I. /I 0 I. I. 0.5 0.5 0.5 / 0.0 0o0 i - . . , .... 150 200 200 150 i .... - 100 100 ! .... 50 50 0 E 0 50 50 100 100 0 2.5 iI = a. 150 150 I- December 1998 December 1998 May May 1998 1998 1.5 0.0 0.0 200 2OO // 2.0 I- 1.0 / I. it/I 1.5 ....i....i....i.... 1.0 0.5 0.5 o.o 0.0 200 200 150 0.5 100 100 50 50 0 0.0 200 200 --.- • 150 150 100 100 / ..- 50 50 0 Distance Distance from from Shore Shore (km) (km) Figure 10. averaged cross-shelf profiles Chile from from four four time time periods periods of in Figure 10.Latitudinally Latitudinally averaged cross-shelf profilesof of chlorophyll chlorophylloff off northern northernChile situ surface measurements measurements (solid (solid line) (dashed line). situsurface line)and andSeaWiFS SeaWiFSretrievals retrievals (dashed line). 914 914 AND EL EL NllqO Nl1O THOMAS El ETAL.: AL.:NORTHERN NORTHERNCHILE CHILELA LAN111A NllqA AND 5. Summary 5. Summary Satellite data document document the the evolution evolution of of SST Satellite data SST from from the La La Nina conditions of 1996 the El Nifia conditionsof 1996 through throughthe El Niflo Nifio of of 1997 1997and and early 1998 in the the upwelling upwelling region region off off northern northern Chile. Chile. These early 1998 in These confirm the the surface confirm surfacetemperature temperaturedescriptions descriptionsprovided providedby by Blanco et et al. 2000) based based on on in Blanco al. (submitted (submittedmanuscript, manuscript,2000) in situ situ hydrographic data data but temporal hydrographic butprovide provideincreased increased temporaland andspatial spatial resolution. SeaWiFS chlorophyll resolution. SeaWiFS chlorophyll patterns patternsbeginning beginningin in September 1997 provide provide aa first September1997 first estimate estimateof of the thebiological biological response to to the the El El Nifio Niño signal signal and and subsequent subsequent recovery. recovery. response Comparisons to to the the La Comparisons La Nifia Nifia conditions conditionsof of 1996 1996 show showthat that the the first firstpulse pulseof of El El Niflo Nifio anomalies anomaliesin in May May 1997 1997(late (latefall) fall) results in in delayed delayed seasonal of results seasonaldevelopment development of austral australwinter winterSST SST patterns, warmer surface temperatures throughout patterns, warmer surfacetemperatures throughoutthe the study study region, and winter region, andan anearlier earlierand andmore moreprolonged prolonged winterminimum minimum of coastal coastal SST SST deficit deficit but but little in frontal frontal of little detectable detectable difference difference in structure. The structure. The second secondpulse pulsein inDecember December1997, 1997,however, however, arrives in in late late spring-early spring-early summer, summer, resulting resulting in in increased increased SST SST arrives and reduced frontal structure and coastal deficit throughout and reducedfrontalstructureand coastaldeficitthroughoutin in the the study studyarea. area. These Theseconditions conditionsbegin beginto tosubside subsidein in March March 1998 and 1998 such suchthat thatby by April-May April-May(fall), (fall), SST SSTfrontal frontalstructure structure and coastal to those those of coastaldeficit deficitare are similar similarto of 1996 1996 and andearly early 1997. 1997. Chlorophyll patterns patterns show within Chlorophyll showthat thatboth bothconcentrations concentrations within the extension the upwelling upwellingregion regionand andcross-shelf cross-shelf extensionof ofelevated elevated concentrations increase in October concentrations increase in October and and November November 1997, 1997, during the the relaxation relaxation between El Niflo pulses, pulses, to to > > 3.0 3.0 mg m3 during between ElNifio mgm -3 with values of 1.0 mg m3 extending up to 50 km offshore. In withvalues of 1.0mgm-3extending upto50kmoffshore. In early December, December, at at the the time early time of of the thearrival arrivalof of the thesecond secondpulse pulse both metrics both metricsof of chlorophyll chlorophyllbecome becomestrongly stronglyreduced, reduced,and and concentrations are are <<0.5 immediately adjacent to concentrations 0.5mg mgm3 m-3even even immediately adjacent to the most of of the Low the coast coast throughout throughout most the study study area. area. Low concentrations persist persist throughout throughout austral concentrations australsummer summeruntil untilApril April (fall), when when concentrations nearshore increase above 1.0 (fall), concentrations nearshore increaseabove 1.0 mg mg m3 begins to offshore to m '3and andthe the0.5 0.5mg mgm3 m'3isoline isoline begins toexpand expand offshore to reach aa seasonal seasonal maximum maximum in in winter reach winter (August) (August)1998 1998 > > 200 200 km km and and JO JO was wasprovided providedby by IFOP, IFOP, with withfunding fundingby byFondo Fondode de Investigacion Pesquera (Projects (Projects 95-05, 95-05, 96-07 96-07 and and 97-02). lnvestigacion Pesquera 97-02).Funding Funding for NASA Biogeochemistry for MEC MEC was wasprovided providedby bythe the NASAOcean Ocean Biogeochemistry Program and for for PTS Programand PTSby by JPL JPLgrant grant958128 958128(TOPEX) (TOPEX)and andNASA NASA grants (EOS) and and NAG5-6604. Additional funding funding for for grantsNAG5-4947 NAG5-4947(EOS) NAG5-6604.Additional travel in travelfor for all authors authorsto to collaborate collaborate in the theanalysis analysisof of these thesedata datacame came from and an an NSF to grant grant OC-97l1344 fromNASA and NSF supplement supplement OC-9711344(part (partof of the theU.S. U.S. GLOBEC GLOBEC program). program). References References Abbott, of Abbott,M.R., M.R., and andP.M. P.M. Zion, Zion,Satellite Satelliteobservations observations of phytoplankton phytoplankton variability during during an Cont. Shelf Shelf Res., Res., 4, 4, 661variability an upwelling upwellingevent, event,Cont. 661680, 1985. 680, 1985. Amone, definition of of the the bioArnone, R.A., R.A., and and P.E. P.E. LaViolette, LaViolette, Satellite Satellite definition biooptical and thermal thermal variation with opticaland variationof coastal coastaleddies eddiesassociated associated with the the African Current, Current, J. J. Geophys. African Geophys.Res., Res.,91, 2351-2364, 2351-2364,1986. 1986. Barber, of Barber,R.T., R.T., and andF.P. F.P.Chavez, Chavez,Biological Biologicalconsequences consequences ofEl ElNino, Nifio, Science,222, Science, 222, 1203-1210, 1203-1210, 1983. 1983. Barber, and F.P. Barber,R.T., R.T., and F.P. Chavez, Chavez,Ocean Oceanvariability variabilityin in relation relationto toliving living resources during during the the 1982-83 resources 1982-83 El El Niño, Nifio, Nature, Nature, 319, 319, 279-285, 279-285, 1986. 1986. Barbieri, M.A., M. M. Bravo, Bravo, M. Farias, A. Gonzalez, Barbieri,M.A., M. Farias, Gonzalez,0. O.Pizarro, Pizarro,and andE. E. Yáfiez, Phenomena Phenomena associated associated with with the the sea Yfifiez, seasurface surfacethermal thermal structure observed through through satellite satellite images structureobserved images in in northern northernChile, Chile, Invest. Mar. Mar. Valparaiso, Invest. Valparaiso,23, 23, 99-122, 99-122, 1995. 1995. Blanco, J.L., A.C. A.C. Thomas, Carr, and Seasonal Blanco, J.L., Thomas, M.-E. M.-E. Carr, and P.1. P.T. Strub, Strub, Seasonal climatology of of hydrographic conditions in climatology hydrographic conditions inthe theupwelling upwellingregion region off northem northern Chile. Chile. J. J. Geophys. press, 2000. off Geophys.Res., Res.,in in press, 2000. Campbell, J.W., J.M. Blaisdell, Campbell,J.W., Blaisdell,and andM. Darzi, Darzi, Level-3 Level-3SeaWiFS SeaWiFSdata data products: Spatial NASA products: Spatialand andtemporal temporalbinning binningalgorithms, algorithms, NASA Tech. Tech. Memo., vol. 32, pp., 1995. Memo., 104566, 104566,vol. 32, 73 73 pp., 1995. Chavez, F.P., A A comparison of ship Chavez,F.P., comparisonof shipand andsatellite satellitechlorophyll chlorophyllfrom from California and and Peru, J. Geophys. California Peru,J. Geophys.Res., Res.,100, 100, 24,855-24,862, 24,855-24,862,1995. 1995. Chavez, F.P., P.G. P.G. Strutton, Chavez,F.P., Strutton,and andM.J. M.J. 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Farias, Farias,Estudio Estudiodel delproceso procesode desurgencia surgencia enIa la which SeaWiFS-measured The The extent extent to to which SeaWiFS-measuredchlorophyll chlorophyll costa chilena costa chilenautilizando utilizandopercepcion percepcionremota, remota,Invest. Invest.Pesq. Pesq.Chile, Chile, concentrations and patterns concentrations and patternsduring duringthe the1997-1998 1997-1998El El Niflo Nifio 34, 33-46, 1987. period a longer Hill, A.E., periodare are anomalous anomalousawaits awaitsa longertime time series seriesof of data. data. In In Hill, A.E., B.M. B.M. Hickey, Hickey, F.A. F.A. Shillington, Shillington,P.T. P.T. Strub, Strub,K.H. K.H. Brink, Brink, E.D. Barton, situ surface show E.D. Barton, and and A.C. A.C. Thomas, Thomas,Eastern Easternboundary boundarycurrents: currents:A A situ surfacechlorophyll chlorophyllmeasurements measurements showthat thatSeaWiFS SeaWiFS pan-regional pan-regionalreview, review, in: in: The The Sea, Sea,Vol. Vol. 11, 11, edited editedby by A.R. A.R. retrievals may be be high in retrievalsmay high by by aa factor factorof of2-4, 2-4,but butconsistency consistency in Robinson and and K.H. K.H. Brink, Robinson Brink, pp. pp. 29-68, 29-68, John JohnWiley, Wiley, New New York, York, this this bias bias indicates indicatesthat that the the relative relativepatterns patternsover over time time are are 1998. 1998. valid. valid. The The consistent consistentcovariability covariabilityof of SST SSTand andchlorophyll chlorophyll data aa close data during during the the El E! Niflo Nifio period periodsuggests suggests closelinkage linkage between betweenthe the temporal temporalevolution evolutionof ofbiological biologicalspatial spatialpatterns patterns and of chlorophyll andSST SST structure. structure.The Thetime timeseries series ofcross-shelf cross-shelf chlorophyll closely tracks tracks both both the the strength strength and and the the cross-shelf cross-shelf position position of of closely SST zones associated with SST frontal frontalzones associated with upwelling upwellingfeatures featuresas aswell well the between the SST SST index indexof of upwelling. upwelling.IfIfsimilar similarrelationships relationships between surface biological and physical conditions existed during surfacebiologicaland physicalconditionsexistedduringthe the 1996 cold cold La 1996 La Nina, Nifia, phase phaseof of the theENSO ENSOcycle, cycle,available available AVHRR data data from higher AVHRR from this thisperiod periodsuggest suggest highersummer summercoastal coastal concentrations and an increased width of the concentrations and an increasedwidth of the coastal coastalhighhighbiomass by biomassregion. region.These Theseexpected expecteddifferences differencesare aresupported supported by OCTS OCTS data data from 1996-1997, 1996-1997, noncoincident noncoincidentin in situ situdata datafrom from the the study studyregion, region,and andSeaWiFS SeaWiFSdata datafrom fromthe thefollowing following(non(nonEl Nifio) Niflo) year. year. 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Paluszkiewicz, Paluszkiewicz,and and R.L. R.L. Smith, Smith, The The Peru Peru undercurrent: A A study Deep Sea Sea Res., undercurrent: studyin in variability, variability,Deep Res.,Part Part A, A, 38, 38, S247-S271, S247-S271, 1991. 1991. Morales, C.E., J.L. J.L. Blanco, Blanco, M. M. Braun, H. Reyes, Morales, C.E., Braun, H. Reyes, and and N. N. Silva, Silva, Chlorophyll-a distribution and associated Chlorophyll-a distribution and associated oceanographic oceanographic conditions in in the the upwelling region off conditions upwellingregion off northern northernChile Chile during duringthe the winter and and spring winter spring 1993, 1993, Deep Deep Sea Sea Res., Res.,Part PartA, A, 43, 43,267-289, 267-289, 1996. 1996. Mueller, J.L., and and R.W. Mueller,J.L., R.W. Austin, Austin,Ocean Oceanoptics opticsprotocols protocolsfor for SeaWiFS SeaWiFS validation, NASA NASA Tech. Tech. Memo., Memo., 104566, pp., 1995. validation, 104566, vol. vol. 25, 25, 67 67 pp., 1995. Acknowledgments. SeaWiFS Philander,S.G., S.G., El Niño, Nigo, La Nina, Niga, and the the Southern SouthernOscillation, Oscillation,289 Acknowledgments. SeaWiFSlevel level22 data dataare aremade madeavailable availableby by Philander, pp., Academic, Academic, San San Diego, Calif., 1990. the Flight Center Center DAAC. DAAC. We Abbott for for pp., Diego,Calif., 1990. the Goddard GoddardSpace SpaceFlight We thank thankMark Mark Abbott M.M., and and C.N.K. helpful Funding for for ACT Reinecker,M.M., C.N.K. Mooers, Mooers,The The1982-83 1982-83El ElNiño Nitiosignal signal helpful suggestions. suggestions.Funding ACT came came from from NASA NASA grants grants Reinecker, off northem northern California, California, d. J. Geophys. NAG5-6558 1919 (part (part of of off Geophys.Res., Res.,91, 91, 6597-6608, 6597-6608, 1986. 1986. NAG5-6558 and andNAG5-6604 NAG5-6604 and andNSF NSF grant grantOCE-971 OCE-9711919 the program). Data R., and Atlas Oceanográfico de Rojas,R., andN. N. Silva, Silva,Atlas Oceanogr6fico deChile, Chile,vol. vol.1,1,Serv. Serv. the U.S. U.S. GLOBEC GLOBEC program). Datacollection collectionand andsupport supportfor for JLB JLB Rojas, THOMAS AL.: NORTHERN NORTHERN CHILE CHILE LA LA NII•IA Nl14AAND ANDEL ELNIl•10 Nl1O THOMAS ET AL.: Hidrogr. y Oceanogr. de de Valparaf so, Chile, Hidrogr.y Oceanogr. deIa laArmada Armada deChile, Chile, Valparaiso, Chile, 1996. 1996. 915 915 pigment concentrations concentrations across across aa California pigment California Current Currentfrontal frontal zone, zone, .1. Geophys. Res., Res., 95, 95, 13,023-13,042, 13,023-13,042, 1990. J. Geophys. 1990. Shaffer, 0., 0. and Shaffer,G., O.Pizarro, Pizarro,L. L.Djurfeldt, Djurfeldt,S. S.Salinas, Salinas, andJ.J.Rutllant, Rutllant, Van Van Camp, Camp, L., L., L. L. Nykjaer, Nykjaer,E. 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James, James, A comparison comparison of the seasonal and interannual variabilityof of phytoplankton pigment the seasonal and interannual variability phytoplankton pigment concentrationsinin the the Peru J. concentrations Peruand andCalifornia CaliforniaCurrent CurrentSystem, System, d. Geophys. Res., 99, 7355-7370, 7355-7370, 1994. Geophys. Res., 1994. Thomas, A.C., and and P.T. P.1. Strub, Strub, Seasonal Seasonal and and interannual interannual variability variability of of Thomas, A.C., 147, 1995. 1995. J.L. Blanco and J. de Pesquero, Casilla J.L. Blancoand J. Osses, Osses,Instituto Instituto deFomento Fomento Pesquero, Casilla 8-V, Valparafso,Chile. Chile. (jlblancoifop.cl;josses•ifop.cl) jossesifop.cl) 8-V, Valparaiso, (jlblanco•ifop.cl; M.E. M.E. Carr, Carr, Jet JetPropulsion PropulsionLaboratory, Laboratory,California California Institute institute of of Technology, MS MS 300-323, 300-323, 4800 4800 Oak Oak Grove Technology, Grove Dr., Dr., Pasadena, Pasadena,CA CA 91009-8099 (mec@pacific.jpl.nasa.gov) 91009-8099 (mec•pacific.jpl.nasa.gov) P.1. Strub, Sciences, P.T. Strub, College College of of Oceanic Oceanicand andAtmospheric Atmospheric Sciences, Oregon University, Corvallis, OR 97331-5503 Oregon State State University, Corvallis, 97331-5503 (tstrub@oce.orst.edu) (tstrub•oce.orst.edu) A.C. Thomas, Thomas, School School of of Marine University of Maine, A.C. MarineSciences, Sciences, Universityof Maine, Orono, ME 04469-5741 (thomas@maine.edu) Orono, ME 04469-5741 (thomas•maine.edu) (Received August August 30, 30, 1999; revised July July 10, 2000; (Received 1999;revised 10, 2000; accepted July July 27, 27, 2000) 2000) accepted