GEOPHYSICAL
RESEARCH LETTERS, VOL. 22, NO. 3, PAGES 207-210, FEBRUARY
1, 1995
The large-scalesummer circulation of the California current
P. Ted Strub and Corinne James
Collegeof OceanicandAtmospheric
Sciences,
OregonStateUniversity
Abstract.
Satellite data from the Geosat altimeter and the
AdvancedVery High ResolutionRadiometer(AVHRR) are
usedto showthe large-scalestructureof the surfacecirculation of the California CurrentSystemin summer.These
data showthe connectionbetweenan equatorward
jet and
temperature
frontoff Oregonthatlieswithin100km of the
of the U.S. west coast [Huyer et al., 1991; Smith, 1992].
The large-scalesatellitefields presentedin this paperprovide more complete,synopticcoveragethan possiblewith
field surveysand add supportto the interpretation
that during midsummerthe convoluted
jet andtemperature
front off
Californiais connectedto the jet and front off Oregon.
coast,similar to that first observedin the 1960's and 1970's,
anda jet thatmeanders
alongthe convoluted
offshoreedge
of a temperature
frontoff California,asrepeatedly
observed Data
in the 1980's.
Altimeters
are satellite radar sensors that measure the sur-
face heightof the ocean. This heightincludesthe height
Introduction
associated
with currentsand the heightassociated
with the
marinegeoid(gravityfield), which wouldbe the heightof
Studiesof seasonalupwellingin the CaliforniaCurrent a perfectlystill ocean. Since the marinegeoidis poorly
System(CCS) off centralOregonand Washingtonin the known, a meanaltimeterheightfield mustbe subtracted
to
1960's and 1970's first describedan alongshoreequator- eliminatethe unknowngeoid, which otherwisedominates
wardjet that developsalonga narrowtemperature
frontin the signal. This also eliminatesthe mean velocityfield,
springand summer(causedby upwellingof colderwater leavingheight 'anomalies',contoursof which are approxinext to the coast). Velocitiesin the jet are of order 0.25 mate streamlinesof the temporallyvaryingflow field.
ms-• [Huyer,1983]. The stronggradient
in seasurface The Geosataltimeterflew in an exact repeat-orbitmistemperature
(SST) andthejet moveoffshoreduringstrong, sionwith 17-dayrepeatsbeginningin November1986. The
upwelling-favorable
(southward)
winds,butremainoriented data usedhere are from the first two yearsof this mission,
fairly uniformlyparallelto thecoast.Thejet andfrontmove after which data dropoutsbecamemore numerous.These
back towardthe coastand becomemore convolutedduring data were regriddedto positionsevery 7 km along subwind relaxations[Holladay and O'Brien, 1975].
satellitetracks (track separationapproximately130 km at
Field studies off central and northern California in the mid-latitudes)and correctedfor environmentalerrorsat the
1980's found strong, narrow jets (velocitiesof 0.6-1.2 Jet PropulsionLaboratory[Zlotnicki et al., 1990]. Flags
ms-•, withwidthsof •20-50 km),alongconvoluted
tem- setduringthis processing
were usedto eliminatedatafrom
peraturefronts revealedby satelliteimages[Breaker and over land or suspected
to be affectedby errorsin the tidal
Gilliland, 1981; Mooers and Robinson, 1984; Kosro and model, pointingerror, etc. Orbit errorswere removedby
Huyer, 1986; Rienecker and Mooers, 1989; Brink and first fitting multiple orbitsto a 1/revolutionharmonicand
Cowles,1991, andotherpapersin the samevolume].While then filtering the data to remove scaleslonger than 3000
much is now known about the structureof thesejets on km. The two year mean height was removedto eliminate
scalesof severalhundredkilometersand less,mostsurveys the marine geoid. A climatologicalmean dynamicheight
have coveredtoo small a region to show their relation to field, relative to 500 m, was calculated from the mean denthe overallstructureof the CCS. Exceptionsto this are sur- sityfield of Levitus[ 1982] andaddedto thedatato partially
veysreportedby Freitag and Halpern, [1981] and Smith, compensate
for the removalof the altimetermean. This cli[1992], which appearto show a connectionbetweenthe matological
meanflowconsists
of a slow(< 0.1ms-•) and
flow alongan SST front near Cape Blanco (43øN) and the broad southward flow.
eddy field farther south. There is still debate,however,as
The altimeter data are flagged as suspectand not used
to the seasonaldevelopment
of the system,the originof the within 30-50 km of the coast,althoughcontoursof height
jets and eddiesand their relationto the larger scaleCali- shownbelow are extrapolatedto the coast.Field data from
fornia Current [Strub et al., 1991]. They have been char- summeroff northernCaliforniagenerallyshowcolderwater
acterizedas offshoresquirtscausedby local convergenceslying inshoreof a strongjet, which flows alongthe sharp
[Davis, 1985], as manifestations
of an offshoreeddy field gradientin SST (seebelow). In this paperwe combinethe
that impingesthe coast[Mooersand Robinson,1984] or contouredaltimeterheight fields with satelliteSST fields
as part of a largeralongshore
jet that extendsalongmuch (from channel4 of the AdvancedVery High ResolutionRadiometer[AVHRR], with 1 km resolutionextendingto the
coast)to show the locationof the temperaturefronts. In
Copyfight1995 by the AmericanGeophysical
Union.
the 30-50 km closestto the coast,the edgeof the coldest
water is a better estimate of the location of the southward
jet than the extrapolatedcontoursof altimeterheights. In
addition,both the altimeterfields, which take 17 days for
Papernumber94GL03011
0094-8534/95/94GL-03011 $03.00
207
208
STRUB AND JAMES: SUMMER
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Fibre 1. Summer 1987. (a) Dynamic height relative to 500 db from a cruise June 9-18, 1987,
contouredat 2.5 cm intervalsand overlainon an AVHRR imagefrom June 16, 1987. Geosatheights
from individual17-daycyclesin (b) Juneand (c) July,1987,contouredat 5 cm inte•als andoverlaidon
SST fieldscoincidentin time with the altimetercycles(June16-18 andJuly 14-15). The climatological
mean dynamicheightrelative to 500 m has beenaddedto the data.
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Figure 2. Summer 1988. Two independentestimatesof the flow fields in mid-July, 1988: (a) Geosat
height from a single cycle in July 1988, contourexland overlaid on a coincidentSST field (July 17,
1988), as in Figure 1; and (b) estimatesof surfacevelocityfrom automatedfeaturetracking(Kelly and
Strub, 1992) using five pairs of SST imagesover a 36 hour period aroundJuly 17, 1988, overlaidon
the sameJuly 17 SST field as in (a).
STRUB AND JAMES: SUMMER CALIFORNIA
completecoverage,andfield surveys,whichtakea week or
more for completion,have the potentialfor distortingcurrentsthat changeoverperiodsof severaldays. The AVHRR
fields, however,are nearlyinstantaneous,
allowingcomparisons between the AVHRR
SST fields and the altimeter and
surveyheight fields to test the degreeof distortionin the
heightfields.
CURRENT
209
intersectthe coastand imply a break in the alongshorejet
near Cape Blanco in June and betweenCape Blanco and
Cape Mendocinoin July,we infer the continuityof thisjet
from the continuousnature of the strongSST gradientin
thesesameregions.
With respectto seasonal
development,
bandsof coldSST
becamepermanentfeaturesof the AVHRR imagesin late
March 1987, appearingnext to the coastalcapes [Kosro
et al., 1991], while the altimeter fields from late March
Results
In Figure l a we showthe dynamicheightfield from a
field surveyin Juneof 1987 between37.5ø-41.5øN, overlaid on an AVHRR imagefrom mid-June.In Figureslb and
l c, we presentthe altimeterheightfieldsfrom single17-day
cyclesduringJuneandJuly 1987,overlaidon mid-Juneand
mid-JulyAVHRR SST fields. Contoursof dynamicor altimetric heightsare approximatestreamlinesfor the surface
flow, with the directionof the flow indicatedby the arrows
(lower heightsto the left when facingin the directionof the
flow). The speedof the currentis inverselyproportional
to
the spacingbetweenthe contours.Besidesprovidingcomparisonsbetweenfield and satellitedata,thesefiguresshow
the differencein the coverageprovidedby the satellitedata
in comparisonto conventionalship surveys.
Field surveysin Februaryand March 1987 did not find
a strongjet in the region50 km and fartheroffshore,while
a strong,equatorwardjet was clearly found in April, May
and June 1987 [Kosro et al., 1991]. In the April-June
surveys,the equatorwardmeanderingjet separatedcolder
(inshore)water from warmer (offshore)water (Figure
In a separatestudy,surfacedriftersdeployedcloserto the
coastbetween37ø- 41øN in March 1987 sampleda strong
jet located20-40 km from the coast[Magnell et al., 1990].
Thesefield data suggesta seasonaldevelopmentof thisjet
in 1987 between37ø- 41øN, with the jet originatingcloser
to the coast. On the offshoreside of the jet in June, 1987
(Figure la), Kosro et al., [1991] found a shallow (20 m
deep)and narrow(10-20 km wide) bandof veryfreshwater
at the northernboundaryof the surveyregion (41.5øN). It
was hypothesizedthat this fresherwater originatedmuch
farther north in the Columbia River plume [Strub et al.,
1991].
The larger fields of satellitealtimeterdata in Figures lb
and l c indicategenerallyequatorwardflow from southern
Oregon(42.0ø- 44.0øN) to southernCalifornia, following
the scallopededgeof the colderwater. In the regionof the
field survey during June, 1987 (37.5ø- 41.5øN), there is
good agreementbetweenthe dynamicheight field derived
from in situ measurements
of density(Figure l a) and the
altimeterheight and AVHRR SST fields (Figure lb). The
locationof the jet in both the dynamicheightfield (Figure
la) and altimeterheightfields (Figureslb and lc) alongthe
stronggradientin the instantaneous
SST field indicatesthat
these height fields do not badly distort the flow field, althoughspecificinstantaneous
featuresmay be misplacedby
50-100 km in the surveyand altimeterfields. The general
agreementof satelliteand in situ data givesus confidence
in the ability of the combinationof altimeterand AVHRR
fields to representthe large-scalestructureof the circulation. Althoughthe extrapolated
contoursof altimeterheight
and April, 1987, (not shown)were the first to reveal lower
heightsin the region 50-100 km from the coast, between
35ø- 39øN and 42ø- 44øN. These regionsof low heights
expandedandconnectedin May andJuneto form the fields
from June and July shownin Figure 1. This structurewas
gone by late December. Thus, thesesatelliteobservations,
in combinationwith the previouslydescribedfield observations lead to the hypothesisthat an SST front and jet developedwithin 20-40 km of the coastby mid-March 1987,
movedoffshorein April and becamethe convolutedjet observedin subsequentsurveysand satellitedata. Previous
observations
off California
in 1981 and 1982 also docu-
mentedthe fairly rapid developmentof a front andjet after
the onsetof southwardwindsand its offshoremigrationbeyond the shelfbreakin April [Strub et al., 1987]. This is
in contrastto observations
off Oregon,wherethefrontand
jet remainoverthe shelffor mostof the springand summer
[Huyer, 1983]. Thus, we believethe resultsfrom 1987 are
representativeof other years.
After a similar seasonaldevelopmentof the SST fields
in 1988, field surveysin June and July 1988 concentrated
on the region between37ø-39øN, where a jet was found
flowing offshore from Point Arena to the southwest,with
peakvelocities
of approximately
1.2 ms-•, as determined
by surface drifters [Swensonet al., 1992]. A ribbon of
fresherwater was again observedon the offshoreedgeof
thejet[Huyer
etal.,1991].InFigure
2a,•healtimeter
field
from July, 1988, showsthat this offshorejet connectsto a
strongcycloniceddy at 37øN, 127øW and then continuesto
the southinto a moreconvolutededdyfield. Thereis good
agreementbetweenthis representation
of the flow field and
the hydrographicand drifter data [Huyer et al., 1991; Brink
and Cowles, 1991].
The satellite data shows even more
stronglythan in 1987 that thejet observedin the surveysat
125øW,37.5øN is part of a continuousflow, this time from
well north of the Columbia River to southern California,
providingsupportingevidencethatthe ribbonof freshwater
came from much farther north.
An independentestimateof the flow field in July 1988
was calculatedusingan automatedfeaturetrackingprocedure [Emeryet al., 1986] on a sequence
of 6 AVHRR imageswithin a 36-hourperiod. Kelly and Strub [1992] describethe methodin detail and show overall agreementbetween the satellite estimates and the velocities derived from
field surveysand surfacedrifter tracks during July, 1988,
althoughthe methodunderestimates
the maximummagnitudesof velocitiesin thejet. Here, the methodis appliedto
a larger,relativelycloud-freeregionbetween35ø- 46øN.
The resultingfield of surfacevelocities(Figure 2b) includesmany of the featuresfound in the altimeterheight
field. Specifically: 1) a narrowjet is found on the offshore
side of an SST front, 20-50 km from the coast off cen-
tral Oregon(44.0ø- 45.0øN); 2) the front and jet separate
210
STRUB AND JAMES: SUMMER
from the shelfaroundCapeBlanco(42ø- 43øN); 3) thejet
formsa largemeanderaroundCapeMendocino,thenflows
onshoreand to the south,into the jet that flows southwest
from Point Arena; and 4) the jet continuesaroundthe cycloniceddyand througha field of two anticyclonic
eddies
that were alsosampledby the drifters[Brinket al., 1991].
The cycloniceddyat 37øN, 127øWwasparticularlywell
documented
by the driftersandshownto persistfor at least
threemonths,entrainingandreleasingdriftersduringJuneAugust[Swensonet al., 1992]. Otheraltimeterfields(not
shown)suggestthat it existedfrom May throughOctober.
The anticyclonic
eddiesat approximately
35øN, 124øWand
35øN, 126øW were shorterlived. The anticycloniceddy
farther offshore at 39ø-40øN, 129ø-130øW in July 1988,
more clearlyseenin Figure2a, can be tracedback in the
altimeterfields to an eddy found in February1988, closer
to the coast. Thus, eddiesassociatedwith the jet may last
up to six months. Closerto shore,eddiessuchas these
have alsobeenshownto causestrongoffshoretransports
of
sedimentand organicmatter[Washburnet al., 1993].
Summary
Geosataltimeter data from two years,along with coin-
CALIFORNIA
CURRENT
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cident AVHRR and field data, show a similar circulation
Kosro, P.M., et al., The structure of the transition zone between
patternin mid-summerfor the CaliforniaCurrentsystema large-scale
jet liesontheoffshoresideof colderwaterover
muchof the regionoff the westernUSA. The observedSST
gradientand inferredjet lie fairly closeto the coast(20-50
km) off Oregonand both are observedin satelliteand field
coastalwatersandthe openoceanoff northernCalifornia,winter
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Levitus,S., ClimatologicalAtlas of theWord Ocean,NOAA Prof.
Pap. No. 13, (U.S. Gov. Printing Office, Washington,D.C.),
data to extend farther from the coast off California, in a
1982.
Magnell, B.A., et al., Convergentshelf flow at Cape Mendocino,
Oceanography,3, 4-11, 1990.
Mooers, C.N.K., and A.R. Robinson,Turbulentjets and eddies
in the California Current and inferred cross-shore
transports,
convolutedmeanderingpattern.Eddiesassociated
with the
jet may persistfor 3-6 monthsand play an activerole in
Science, 223, 51-53, 1984.
offshoretransport.The exactnatureand dynamicsof the
M.M., and C.N.K. Mooers,Mesoscaleeddies,jets and
development
of the large-scale
mid-summer
jet are still un- Rienecker,
frontsoff Point Arena, California,July 1986, J. Geophys.Res.,
known, leavingunansweredquestionsaboutthe difference
94, 12,555-12,569, 1989.
in behavioroff Oregonand California,the locationand dy- Smith, R.L., Modem ocean upwelling, in Upwelling Systems:
namicsof the separationof the jet from the shelf,and the
EvolutionSincethe Early Miocene, SpecialPublicationNo. 64,
C.P. Summerhayes,
W.L. Prell andK.C. Emeis,Eds. Geological
relation of the jet to the surroundingeddies.
Society,London, pp. 9-28, 1992.
Strub, P.T., J.S. Allen, A. Huyer, and R.L. Smith, Large-scale
Acknowledgments. This work wassupported
by fundingfrom
structureof the springtransitionin the coastaloceanoff western
grant958128 from theJetPropulsionLaboratoryandgrantsNAGWNorth America, J. Geophys.Res., 92, 1527-1544, 1987.
2475 and NAF-5-30553
from NASA. Altimeter data were obtained
Strub, P.T, P.M. Kosro, A. Huyer, and CTZ collaborators,The
from the Jet PropulsionLaboratory.AVHRR data were obtained
nature of the cold filamentsin the California Current System,
from Mark Abbott and the dynamicheightfield was providedby
J. Geophys.Res., 96, 14,743-14,768, 1991.
Mike Kosro. The field data were collectedduring the Coastal
Swenson,M.S., P.P. Niiler, K.H. Brink, and M.R. Abbott, Drifter
TransitionZone program, sponsoredby the Office of Naval Reobservationsof a cold filament off Point Arena, California in
search,throughthe CoastalSciencessection,whichalsosupported
July 1988, J. Geophys.Res., 97, 3593-3610, 1992.
someof the work leadingto this paper. Dudley Cheltonmadevalu- Washburn,L., M.S. Swenson,J.L. Largier,P.M. Kosro,and S.R.
able commentson a previousdraft of the paperand four anonyRamp, Cross-shelfsedimenttransportby an anticycloniceddy
mous reviewersmade suggestionsthat resultedin a much clearer
off Northern California, Science, 261, 1560-1564, 1993
and more focusedpaper.
Zlotnicki, V., A. Hayashi, and L.-L. Fu, The JPL-Oceans8902
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