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.:
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200
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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,
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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).
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1996 cold
cold La
1996
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Nifia, phase
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theENSO
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available
AVHRR data
data from
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highersummer
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an
increased
width
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by
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Theseexpected
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OCTS
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(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