Jan 15, 1992 - scanner (CZCS) images, the Florida State University (FSU) monthly ..... break, and the mean pigment concentration in the Oman shelf ..... and erosion of the spring thermocline that occurred in the .... sensing, NASA Tech.
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 97, NO. C1, PAGES 733-750, JANUARY 15, 1992
Interannual Variability in Phytoplankton Blooms Observed in the Northwestern Arabian Sea During the Southwest Monsoon JOHN C. BROCK 1
CooperativeInstitute for Research in EnvironmentalSciences,University of Colorado, Boulder CHARLES R. MCCLAIN
Oceansand Ice Branch, NASA Goddard Space Flight Center, Greenbelt, Maryland Interannual changesin the strengthand seasonalevolution of the 1979through 1982 surfacelevel southwestmonsoonwinds have beenrelatedto variationsin the summerphytoplanktonbloom of the northwesternArabian Sea by synthesisof satelliteoceancolor remote sensingwith analysisof in situ hydrographicandmeteorologicaldata sets.The 1979-1981southwestmonsoonphytoplanktonblooms in the northwestArabian Sea peaked during August-September,extendedfrom the Omani coast to about 65øE,and appearedto lag the developmentof open-seaupwellingby at least 1 month. In all 3 yearsthe bloomwasdrivenby spatiallydistinctupwardnutrientfluxesto the euphoticzoneforcedby the physicalprocessesof coastalupwellingand offshoreEkman pumping.Coastal upwellingwas evidentfrom May throughSeptember,yielded the most extreme concentrationsof phytoplankton biomass,and alongthe Omanicoastwaslimitedin its impacton upperoceanbiologicalvariabilityto the continentalshelf.Ekman pumpingstimulatedthe developmentof a broadopen-oceancomponent of the southwestmonsoonphytoplanktonbloom oceanwardof the Omani shelf. Phytoplankton biomasson the Omanicontinentalshelfwasincreasedduringboththe early andlate phasesof the 1980 southwestmonsoondue to strongercoastalupwellingunderthe most intensesouthwesterlywinds of the four summersinvestigated.Diminishedcoastalupwellingduringthe early phaseof the weak 1982 southwestmonsoon resulted in a coastal bloom that reached a mean phytoplankton pigment concentrationthat was 28% of that seenin 1980. The lack of a strongregionalnorthwesternArabian Sea bloom in late summer1982is attributedto the developmentof persistent,shallowtemperature stratificationthat renderedEkman pumpingless effective in driving upward nutrient fluxes.
INTRODUCTION
the northeast monsoon circulation begins [Bruce, 1968; Wyrtki, 1973; Pickard and Emery, 1982].
The northern Indian Ocean region undergoes a biannual reversal of wind direction. During the southwestmonsoonin June-September [Webster, 1987], surface low level southeasterliesfrom the southern hemisphereextend across the equator to become the southerlies or southwesterlies of the
Monsoon Meteorology of the Arabian Sea
The climatological low level meteorology of the Indian Ocean during the southwest monsoon circulation includes three low-pressure troughs along the southern edge of the
northern hemisphere [Hamilton, 1979]. The near-surface circulationof the northern Indian Ocean is primarily winddriven, and within the monsoon gyre the onset of the
huge Asian continental monsoon low. These low-pressure troughs are positioned over the eastern edges of Africa, India, and the Indochina peninsula [Hastenrath and Lamb, southwest monsoon causes a reversal in the circulation. The 1979; Fu et al., 1983]. Accordingly, the southwest monsoon northeast monsoon, generally November-March [Webster, is split into three branches of intensified wind flow, each to 1987], is marked by westward flow everywhere to the north the east of a low-pressure trough, and located in the western of the equator.The northeastmonsooncirculationcollapses Arabian Sea, the Bay of Bengal, and the South China Sea in April, and usually by May the eastward flow of the [Fu et al. , 1983]. Embedded in the western Arabian Sea branch is a low Southwest Monsoon Current is established[Wyrtki, 1973]. The vigorous and deep southwest monsoon circulation is level jet stream, the Somali or Findlater Jet, which results in typically at peak strength in July and August and takes the higher wind speeds of shorter seasonal duration than are form of a gyre in the Arabian Sea and Bay of Bengal composed of the swift Somali Current directed to the northeast along the east African coast, the broad eastward Southwest Monsoon Current, and the westward South Equatorial
Current (Figure 1). This circulation persistsuntil October, when the southwestmonsoondissipatesand the transitionto
•Nowat Biological Oceanography Division,Bedford Instituteof Oceanography, Dartmouth, Nova Scotia, Canada.
Copyright 1992 by the American Geophysical Union. Paper number 91JC02225. 0148-0227/92/91J C-02225 $05.00 733
observed
elsewhere
in the surface wind field of the summer
Asian monsoon [Fu et al., 1983] (Figure 2). A pronounced intensification of the southwest monsoon airstream, this low level cross-equatorial jet stream is located in the western periphery of the monsoon regime and is centered at an elevation of about 1.5 km. During the northern summer, this 3-km-deep flow originates in the southeasttrade winds of the South Indian Ocean, passesjust to the north of Madagascar, heads north across the flat arid lands of eastern Kenya, Ethiopia, and Somalia, and finally turns to the northeast to cross the Arabian Sea. The monthly mean airflows in the Somali Jet for the months of June, July, and August are similar in their essential features, depicting wind maxima
734
BROCK AND McCLAIN: INTERANNUAL VARIABILITY OF ARABIAN SEA PHYTOPLANKTON
30 ø N
20 øN
NN•
15 ø N ISLAND
OF SOCOTRA
..
10 ø N
,
ARABIAN SEA 50 ø E
I
I
I
55 ø E
60 ø E
70 ø E
65 ø E
Fig. 1. Map of the ArabianSeadepictingcurrentsactiveduringthe southwest monsoon[Wyrtki,1973].
near the northern tip of Madagascarand immediatelydown- jee et al. [1978], related the position of the atmospheric pressureridgeat 500mbarin April to the vigorof the ensuing stream from the Somali coast, a wind minimum along the jet axis near the equator, and a bifurcation of thejet as it crosses monsoon. Verma [1980] identified a link between positive the Arabian Sea. Along the jet axis, maximum wind speeds intermonsoonupper tropospheric temperature anomalies of 25-50 m/s are observed on some days in July, and over the and increased summer rainfall over India. Interannual monArabian Sea at 1 km in the jet maxima off the Somali coast, soon fluctuations were associated by Tanaka [1982] with the July monthly mean is on the order of 18 m/s [Findlater, changesin the strengthof the upper tropospheretropical 1974, 1977, 1981]. Mechanisms of the interannual variability of the low level
easterly jet at 10øN.
monsoon airflow have been divided into those associated
monsoonis essentiallya dynamically stable circulation system whose interannual variability is mostly a result of slowly varying boundaryconditions.Surface boundaryconditions
with internal atmospheric dynamics and those linked to global surface boundary conditions [Shukla, 1987]. Banner-
Charneyand Shukla[ 1981]have suggested that the Asiatic
that are relevant
s
20ON
I
s
10 ø 2,
include the
studies to weak southwest monsoon seasons [Angell, 1981; Rasmusson and Carpenter, 1983; Yasunari, 1990].
0o
10 ø
to the monsoon circulation
extent of northernhemispherewinter snowcover [Hahn and Shukla, 1976;Dey and Bhanukumar, 1982; Dickson, 1984], sea surfacetemperature(SST) of the tropical Indian Ocean [Ellis, 1952;Pisharoty, 1976;Washingtonet al., 1977;Ghosh et al., 1978; Sikka, 1980; Cadet and Diehl, 1984; Shukla, 1984], and the soil moisture content [Shukla and Mintz, 1982]. E1 Nifio events, aperiodic warm equatorial Pacific SST anomalies,have been related by several observational
2.5
20os
Arabian Sea Oceanography Within the Arabian Sea during the southwest monsoon, the Somali Current forms a strong western boundary current
with a transportof about 65 Sv, mostly in the upper 200 m (Figure 1) [Schott, 1983;Swallow et al., 1983;Luther et al., 40OE 50 ø 60 ø 70 ø 80 ø 1985]. Strong upwelling occurs off the coasts of Somalia Fig. 2. July meanwind field at 1 km over the Arabian Sea showing [Bruce, 1974; Schott, 1983] and Oman [Bruce, 1974; Smith the Somali Jet (modified after Findlater [1981]).
and Bottero, 1977; Swallow, 1984; Elliot and Savidge, 1990;
BROCK AND MCCLAIN.'
INTERANNUAL
VARIABILITY
TABLE
1.
Parameter Coastal
zone color scanner
NASA Goddard Space Flight
Mesoscale
Air-Sea
distribution
Current
Surface pigment concentration resolution* Wind stress*
at 4-km
Ekman Transport* Ekman upwelling* SST at 2 ø resolution
Monthly sections
field.
Bauer et al., 1991], and weaker upwelling occurs off southwest India [Wyrtki, 1973]. Mesoscale eddies have been reported from the equator around the rim of the basin to Pakistan [Bruce, 1979;Brown et al., 1980; Evans and Brown, 1981; Bruce and Beatty, 1985; Simmons et al., .1988]. The Monsoon
Interaction
Group, Florida State University Climate Analysis Center (GAG) National Oceanographic Data Center (NODC)
Sea surface temperature (SST) Vertical temperature
Southwest
Comments
Center
Wind pseudostress
735
Sets
Source
(CZCS) data
*Derived
Data
OF ARABIAN SEA PHYTOPLANKTON
off the Arabian
coast is broader
and weaker and is directed to the northeast [Wyrtki, 1973; Swallow, 1984]. Smith and Bottero [1977] recognized two upwelling mechanisms off Oman, a nearshore upwelling driven by the Ekman divergence of surface water offshore under the influence of coast-parallel winds, and an open ocean type upwelling further offshore driven by positive wind stress curl to the north of the axis of the atmospheric Somali Jet (Figure 2). As a result, this upwelling region is broad, extending roughly 400 km seaward and 1000 km along the Omani coast [Luther and O'Brien, 1985; Luther et al., 1985]. Bauer et al. [1991] investigated the influence of Ekman dynamics on the distribution of phytoplankton biomass in the Arabian Sea during the southwest monsoon of
(1) to assess the interannual variability of the southwest monsoon
surface wind field over the western
Arabian
Sea for
the years 1979-1982, (2) to examine the effect of varying
monsoonstrengthduring these four summer monsoonson the intensity of coastal upwelling, Ekman pumping, and mixed-layer formation in the northwestern Arabian Sea, and (3) to relate interannual alterations in these wind-driven
physicalprocessesto satelliteocean color depictionsof phytoplankton biomass distributions during the summer monsoons of 1979 through 1982. METHODS
The data sets used in this study are coastal zone color scanner (CZCS) images, the Florida State University (FSU) monthly mean wind pseudostress fields, NOAA Climate Analysis Center (CAC) sea surface temperature data, and NOAA National Oceanographic Data Center (NODC) expendable bathythermograph (XBT) profiles. The characteristics of these data sets are provided in Table 1. 1987. A total of 160 CZCS images acquired during MayStrong seasonal contrasts in primary productivity have September of 1979, 1980, 1981, and 1982 were processed to been observed in the Arabian Sea [Kabanova, 1968]. During depict surface pigment concentration at 4-km spatial resoluthe northeast monsoon,productivity in the northwestern
ArabianSeaislessthan0.1g C/m2/d,in dramatic contrast to tion for the northwestern Arabian Sea within 10ø-27øN, thehighsouthwest monsoon valuesabove1.! g C/m2/dover 50ø-67øE (Figure 3). Table 2 describes the available CZCS all of the northwestern Arabian Sea observed in this region [Kabanova, 1968]. The high summerprimary productivity of the northwestern
Arabian
Sea has been
attributed
to the
presence of unusually high concentrations of inorganic nutrients (nitrate, phosphate, and silicate) at shallow depths within the euphotic zone [Ryther and Menzel, 1965; Ryther et al., 1966; Kuz'menko, 1977].
data set for the southwest monsoons of 1979, 1980, 1981, and 1982. The SEAPAK software package developed and implemented at NASA Goddard Space Flight Center (GFSC) was used for all processing operations [Darzi et al., 1991; McClain et al., 1991]. The CZCS archive maintained by the National Space Science
and Data
Center
at NASA
GSFC
was browsed
on
video disk [Feldman et al., 1989] to enable the selection of CZCS scenes acquired over the northwestern Arabian Sea OBJECTIVES during May-September of 1979, 1980, 1981, and 1982. The Brock et al. [1991] studied the 1979 summer monsoon entire data set of selected level 1 scenes was ingested at a bloom, and attributed it to vertical nutrient fluxes caused by factor of 4 subsamplingin order to prepare a regional time coastal upwelling and regional upward Ekman pumping. series of overview images covering the northwestern AraGiven that theseprocessesare a resultof vigoroussummer bian Sea at one-sixteenth the original spatial resolution. monsoon southwesterly winds, interannual variability of the The branching, two channel bio-optical algorithm of Gorstrength of the low level southwest monsoon circulation may don et al. [1983] was used to retrieve phytoplankton pigment be expected to yield variations in summer phytoplankton concentration. This algorithm employs two empirical bioblooms in the northwestern Arabian Sea. optical relationships based on data collected in waters adjaThe purpose of this paper is to relate interannual changes centto the UnitedStatesin the Atlantic,Pacific,andGulf of in the strength and seasonal evolution of the 1979 through Mexico [Clark, 1981]. Although these algorithms are based 1982 surface level southwest
monsoon
winds to variations
in
the upwelling-induced summer phytoplankton bloom of the northwestern Arabian Sea. The objectives of this study are
on a limited number of stations in U.S.
coastal waters,
various studies have demonstrated their validity for much of the world ocean [Feldman et al., 1984; McClain et al., 1984,
736
BROCK AND MCCLAIN;
INTERANNUAL
VARIABILITY
OF ARABIAN
SEA PHYTOPLANKTON
30 ø N
25 ø N
20 ø N
15 ø N
10 ø N
50 øE
55 øE
60 øE
65 øE
70 øE
Fig. 3. Location map depictingthe CZCS image region, the regionfor the FSU wind data products and sea surface temperature fields, and the mean pigment subregions.Contours depict the bathymetry at 3000-m depth. The Omani shelf subregion is bounded on the shoreward side by the 30-m isobath.
1986; Abbott and Zion, 1985; Barale eta!., 1986; M•illerKarger et al., 1989, 1990]. Gordon et al. [1983] report an error on the order of 30-40% in pigment concentration in the
range0.08to 1.5mg/m3 for thisalgorithm undera varietyof atmosphericturbidities. The CZCS calibration derived by R. Evans (personal communication, 1991) and used in the global CZCS processing [Feldman et al., 1989] was applied in this study. Atmospheric dust, sun glint, coccolithophore blooms [Holligan et al., 1983], and strong sensor electronic overshoot [Mueller, 1988] or "ringing" present imagingproblems in the study region and necessitated the development of specialized correction procedures. Brock et al. [1991] outline the procedures used for handling dust, sun glint, and ringing.
TABLE 2.
Time Periods and Image Availability for CZCS Composites Early Phase
Late Phase
1979
Time Period
May 19 to Aug. 1
Aug. 17 to Sept. 29
No.
10
25
of Scenes
1980
Time Period
May 22 to July 22
July 23 to Oct. 7
No. of Scenes
4
17 1981
Time Period
May 23 to July 14
Aug. 4 to Sept. 30
No. of Scenes
23
32
Time Period
May 3 to June 26
Aug. 21 to Oct. 4
No. of Scenes
18
31
1982
The•ngstr6mexponents derivedbyBrocket al. [1991]were used in processing the data from 1979-1981. Values of +0.2 were used for the 443 nm, 520 nm, and 550 nm bands. However, a difference in aerosol type and the occurrence of coccolithophoreblooms during the weak southwestmonsoon of 1982 necessitated the application of revised CZCS processing procedures. Following Brock et al. [1991], the "clear water" radiance method [Gordon and Clark, 1981] was applied to several scenesacquired in May and June 1982
in orderto derivea uniquesetof •ngstrOmexponents for May-September 1982.•ngstrOmexponent valuesof -0.15 were obtained for all three bands (Figure 4). Coccolitho-
phore blooms indicated by anomalously high normalized water radiance (nlw) values were masked in the level 2
imagesby applicationof a nlw(550) thresholdof 0.55 mW/ (cm2 /am sr). This thresholdwas derivedby review of nlw(550) scenesdepictingobviouscoccolithopore blooms. Contiguous blooms of breadth greater than 100 km were observed off the north Omani coast on two scenesacquired during the late phase of the 1982 summer monsoon. No
scenesrevealed anomalouslylow nlw(443),nlw(520),or nlw(550 ) radiancesduring1982whichwouldhaveindicated possible E1 Chich6n aerosol contamination [Bandeen and Fraser, 1982].
Once maskedfor the effects of wind-blown dust, sun glint, sensor ringing, and coccolithophores, the level 2 surface pigment concentration images were transformed to a uniform cylindrical equidistant projection. The registration of all scenes to a coastline contour derived from a global coastline data set followed. This resulted in a spatially coregistered1979-1982 southwest monsoon CZCS pigment
BROCKAND MCCLAIN: INTERANNUALVARIABILITYOF ARABIANSEA PHYTOPLANKTON
737
1.2
0
I
1
(-
I:1:
o.8
•
0.6
- :
ß ß•
...,.
'•
0.4
:'
-
o
•
o.•
ß I
oJ
I
ß ;
':
'i
'
I
I ._...-K-: !
" L•
:,
I
I Clear Water Radiance (0.498)
ß
•'
,t
;
I
.ß
ß
I
o
ß
a
I -0.4
-0.2
-0.6
i -0.2
0
I 0.2
i 0.4
0.6
520 nm AngstromExponentValue 0.8
(D
0.7
fit]
0.6
rr'
0.5
ß
.• 0.4. (])
0.3
•
0.2
..,.
o.•
....-•: I
(-
o
•f)
-0.1
! i , -
ß • I =••. ß I& ,,,•,,,•1
_ i •,
o
Z
. ; I
:
:• ß , .•.
I
ß
ClearWater •_ I_I -'Radiance (0.30)
co
ß ß ß
I -
-0.2
-0.6
ß
I
I
-0.4
-0.2
0
I
I
0.2
0.4
b
0.6
550 nm AngstromExponentValue Fig.4. Plotof(a)CZCSband2 •ngstrOm exponent value versus band 2 normalized waterradiance and(b)CZCS
band 3 fkngstrOm exponent value versus band 3 normalized water radiance (mW/(cm 2 /xmst)),showing selection of •ngstrOm value yielding clearwaterradiance value. Eachsymbol ontheplotindicates a normalized waterradiance determination at a northwestern ArabianSeaclearwatersiteononeof several CZCSimages acquired duringtheearly
phase of the 1982 southwest monsoon.
concentrationimage time series. Separate CZCS level 3 surfacepigment concentrationimages were averagedto produce mean pigment fields [McClain et al., 1988]for the early and late phasesof the monsoon.Table 2 gives the compositingtime periods and the number of scenesused to generateeach of these eight biseasonalmean pigmentcon-
Monthly mean wind stress,Ekman horizontaltransport, andEkmanupwellingvelocitywere calculatedfrom monthly pseudostresswind fields for May-September of 1979-1982. These wind pseudostress fields were obtained from the MesoscaleAir-Sea Interaction Group (MASIG) at the Flor-
ida State University, where they were generatedfrom the centrationimages.Estimatedmeanconcentrations of phyto- National Climatic Data Center TD-1129 data set of marine plankton pigment were obtainedfor selectedsubregions surfaceobservations[Leglet et al., 1989]. withinthe earlyandlatephasesouthwest monsooncompos- The initial step in the wind data processingwas the ites. In additionto subregions coveringthe Omanicontinen- conversion of windpseudostress (•-x s, •-•) to surfacewind tal shelf seaward of the 30-m isobath and the central Gulf of stress(•'x, •'y) by useof the expression Oman, two offshore subregionswere defined based on the July monthly mean surface wind stress curl chart of Hastenrath and Lamb [1979] (Figure 3).
('rx,'ry)= PaCd(•'x s, •'•) where
738
BROCK AND MCCLAIN:
INTERANNUAL VARIABILITY
OF ARABIAN SEA PHYTOPLANKTON
30 ø •
PAKISTAN
25 ø N
ARABIAN
PENINSULA 20øN
JUNE 1979 SECTION A
15 ø N
15 ø N
10 øN':;•:•:-::•
1o o N
.............. •:•:•••..../ 50 ø E
55 ø E
60 ø E
65 ø E
70 ø E
50 ø E
55 ø E
ARAB IAN SEA 60 ø E
65 ø E
70 ø E
SEPTEMBER
30ø N ':'"i:i:i:i:!:!:i•::'":':" ":"::i:i:•:•:•-':.:•:i:i:i: ................... ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
IRAN
PAKISTAN ..
N '•*--:.:"':•i '"'"'"'":"':""' "•:'"'•' - '•••ii::•":..
'.................... :::::::::: ............................. •i•'"'" '"'::•-•...'••• ii':':'" ":" '":•:•'•::'::•'-:-'-':-•i :" '""
...... .....'
AUGUST 1979
INDIA
1982
ARABIAN
..... •::.-'.:. -.'.:.......'•:-::•ii
"-•••...'•..:'=.•.=.....::.:'!!!':.'::-':: .................................... •--'
SECI]ON
A
PENINSULA
SEPTEMBER SF.CI]ON
15 ø N
1981 A
SEPTEMBER
SEC•ON
B
.
SEPTEMBER 1982 SF.CTION B
10 o N
ARABIAN
50 ø E
Fig. 5.
55ø E
60ø E
65ø E
70ø E
Wl0
55ø E
60ø E
65 ø E
70' E
Monthly location maps for 1979-1982 temperature sectionsbased on expendablebathythermographdata.
Pa air densityequalto 1.29g/m3 Ca
50 ø E
SEA
drag coefficient,equal to 0.0012 for 0 < W10 < 11 m/s or 0.00049 + 0.000065W10for Wl0 > 11; wind speedat 10 m in meters per second.
(Ue, Ve) =
(Ty, Tx)
(1)
and the Ekman upwellingvelocity (We) was given by
In order to determineCa, the wind speed-dependentdrag coefficient [Large and Pond, 1981], it was necessarywithin
curlz (•/f)
this procedureto calculateWl0, the wind speedat 10 m in
Pw
meters per second, by use of the equation [Legler et al., 1989]
In theseexpressions, Pwis thewaterdensity,1000kg/m3,
and f is the Coriolis parameter. The 1ø resolution monthly W10--[(TxS) 24-(T;)2]1/4 wind stress, Ekman transport, and Ekman upwelling grid Once the surface wind stress had been obtained, the zonal files were filled with a bicubic spline technique to create and meridionalEkman transports(Ue, Ve) were calculated images [McClain et al., 1990]. Transformation of these wind by parameter images to a cylindrical equidistant projection
BROCKAND MCCLAIN' INTERANNUAL VARIABILITY OF ARABIAN SEA PHYTOPLANKTON
June 1979 XBT Section A &,
•
-lOO
•
-200
!
.......
L
739
s z
_
=
-
0 -lOO
'-"'=-
t- -300 .Q.
(D -400
ß
;
ß
'
I
ß . '
ß
ß
ß ' •!4
ß
.
ß
ß
' :
-200
'
-300
'
-400
-500
-500
200
400
600
800
1000
! 200
Distance (km) July 1980 XBT Section N •
-100
-100
•-/-200
-200
..• -300
-500
(L)-400
•
'
-500
'
'("
0
'
200
"; 400
'
;
-400
:
, ',
-5O0
800
600
1000
Distance (kin) August 1979
XBT Section A
•
-100
:
•
-200
-2o0
.•
-300
-300
(D-400
22
I•
-lOO
,
-500
-500
0
200
400
Distance (km) August 1979 XBT Section B
s
-lOO
- lOO
-200
-200
-;;i l'
-300 -400
i,
-300 -400
-500
-500
0
200
400
600
800
1000
Distance (km) Fig. 6. Monthlyverticaltemperature sections basedon expendable bathythermograph data.
resulted in a set of FSU wind-derived environmental fields
that overlie the 4-km-resolutionCZCS regionalimagetime series for May-September of 1979, 1980, 1981 and 1982 (Figure 3). Images of coregisteredmonthly mean Climate AnalysisCenter sea surfacetemperaturefields for MaySeptemberof 1979,1980,1981,and 1982were generatedin a similar manner.
The NODC expendablebathythermographdata was ingestedand binnedby monthfor May-September of the four summer monsoon seasonsunder investigation. Following
the selectionof stationsfor individualsections,griddingand contouringproducedmonthly vertical temperaturesections extendingfrom the surfaceto 500-mdepth(Figures5 and 6). RESULTS AND DISCUSSION
Significant interannual variation of the surface southwest monsoon winds and phytoplankton biomass evolution occurred during 1979-1982. Overall, the surfacewinds over the study region were stronger in 1980 than in 1979 and 1981
740
BROCK AND MCCLAIN' INTERANNUAL VARIABILITY OF ARABIAN SEA PHYTOPLANKTON
September 1981 XBT Section A
N
S
-100
-lOO
-200
-200
-300
-400
-40o
_oo_LLk-' 0
:
200
'-
400
600
, , , , ,
800
1000
-500
1200
1400
Distance (km)
September 1981XBT
Section
B s o -lOO
•
E
-200
-200
c' -300-
:
-
• -400 •
-400
-500
0
-300
-500
2o0
4o0
6oo
Distance (km) September
1982 XBT Section A
s
o
• •
-lOO
E
-200
C- -300 • -400 -500
0
200
400
600
800
Distance (km) September 1982 XBT Section B •._•I=i=:•-.._• ; ••T-"
"
,
ß
i
ß
-lOO -200
C- -300
:
i
s
-•.
I ø -lOO
,.
• -400 -500
•
•00
:,' 400 i_.;. ø' •00;
800
-'øø
1000' -500
Distence (kin) Fig. 6.
(continued)
(Plate 1), and the 1982 southwest monsoon arrived late tial to understanding the variation in late summer phyto(Figure 7), was of diminished vigor, and yielded lower plankton blooms observedfor this 4-year period. phytoplankton pigment concentrationsthan were observed Coastal Upwelling during the previous three summers(Figure 8). Although an examination of the causes of summer monsoon variation Given a surface ocean current flowing poleward in the from 1979-1982 is outside the scope of the current study, northern hemisphere along the eastern margin of a land alterations in wind forcing are significantto physical oceanic mass, horizontal velocity in the Ekman layer resulting from processesthat regulate phytoplankton biomassin the north- a balance between friction and Coriolis forces implies diverwestern Arabian Sea. An examination of interannual genceaway from the coastalboundary and coastalupwelling changesin these processesbetween 1979 and 1982 is essen- [Ekman, 1905; Smith, 1968]. This scenario describesthe flow
BROCK AND MCCLAIN:
INTERANNUAL
VARIABILITY
OF ARABIAN SEA PHYTOPLANKTON
741
f
/?-/..
,JULY
JULY 198!
JULY 19,"'9- 1982 FSU MONTHLYMEAN
TOTAL SURFACE WIND STRESS (N.'M z)
- •2•
0.• H..H
Plate
1.
July FSU monthly mean wind stress fields for 1979-1982.
regime along the Arabian Peninsula in summer, and accordingly, coastal upwelling in the western Arabian Sea occurs primarily during the southwest monsoon [Bottero, 1969; Bruce, 1974; Smith and Bottero, 1977; Swallow, 1984; Elliot and Savidge, 1990]. The most detailed previous studies of
ponent in the surface layer to determine dynamic topographies and mass transports. Horizontal flow between 0 and 100 dbar was to the northeast and obliquely divergent from the coast, suggestingintense upwelling for a narrow band along the Omani coast [Bottero, 1969; Smith and Bottero,
the summer field of motion off the Arabian
1977].
Peninsula made
use of hydrographicdata collected by the R.R.S. Discovery in the summer of 1963 [Bottero, 1969; Smith and Bottero, 1977] and applied a technique first described by Yoshida [1955] that assumes geostrophic flow with an Ekman com-
The FSU
wind
field over the northwestern
Arabian
Sea
became southwesterly during the onsets of the 1979-1981 southwest monsoons in May and June, and the monthly mean surface wind stress fields reached peak intensity in
742
BROCK AND MCCLAIN:
INTERANNUAL
VARIABILITY
OF ARABIAN
SEA PHYTOPLANKTON
0.6
0.5
0.2
0.1
May
Jun
Jul
Aug
Sep
MonthlyTime Period
= 1979 •
1980 _•_1981
= 1982
Fig. 7. Graphof maximum monthlymeanFSU windstress(N/m2) for July1979through1982.
July for each of these three summer monsoonseasons(Plate 1 and Figure 7). Surface wind stress in July 1979 exceeded
reached6.0 m2/s. The July 1981monthlymeanEkman transport as calculated from the FSU wind data again
0.4 N/m2 withina southwest-northeast elongated regionof exceeded 4.0 m2/s.In July 1982,Ekmantransportat the by maximal wind stressroughly 550 km off coast of Oman and Omanicoastwas between2.0 to 4.0 m2/s,diminished Yemen, the surface manifestation of the low level Somali Jet. The FSU monthly wind stress fields for 1980 generally depict stronger winds than were obtained for 1979 and 1981, and in July 1980 a 350-km-wide wind stresspeak reaching
about 30% in comparison with 1979 and 1981. All July-September vertical temperature sections normal to the Omani coast, including August 1979 section A, July 1980 section A, September 1981 section B, and September 0.523N/m2 occurred just eastof the islandof Socotra.In 1982 section B (Figures 5 and 6), depict coastward uplift of July 1981 the monthly mean wind stress field attained a isothermsin the upper 200 m that indicate coastalupwelling. maximum valueof 0.468N/m2, whichcloselymatched the Interannual variation in the divergent southeastwardEkman July1979maximumvalueof 0.448N/m2. transports calculated for July suggest that upwelling along Reverdin and Fieux [1987] noted the occurrence of a weak Oman was strongest in 1980, of intermediate intensity in southwest monsoon and below normal precipitation over 1979 and 1981, and considerably weaker in summer 1982. India in 1982. Similarly, the FSU monthly mean wind stress High surface pigment concentrations shown in the early for July 1982 (Plate 1) was anomalouslyweak in comparison and late phase CZCS compositesover the Omani continental with all other years under investigationin the current study. shelf for all 4 years is inferred to have resulted from coastal The pre-August peak in wind stressthat was obvious during upwelling that began during May and persisted though the the 1979, 1980, and 1981 monsoonsis not apparent in 1982 duration of each southwest monsoon (Figure 8 and Plate 3). (Figure 7). In July 1982 the monthly wind stress maximum The early phase captures the onset of the summer monsoon was 0.416 N/m2, 87% of the averageJuly wind stress (Table 2), observed by Brock et al. [1991] and Banse [1987] maximum fortheprevious 3 years(0.480N/m2),and79%of to precede the development of an extensive phytoplankton the July wind stress maximum for 1980, the year of the bloom in the northwestern Arabian Sea. July, climatologistrongestsouthwestmonsoonobservedin this study. The late cally the peak month for the monsoon [Hastenrath and arrival of the low level summer monsoon circulation in 1982 is Lamb, 1979], was not well sampled by the CZCS during the Thus the late apparent from the June 1982 wind stressmaximum of 0.352 four summer seasons under consideration. N/m2. Thislowvaluemaybe compared withthemaximum phase composites rely almost entirely on CZCS data colwindstress valuesof 0.449N/m2forJune1979,0.409N/m2for lected during August and September (Table 2). June1981,and0.451N/m2 obtained forJune1980(Figure7). Depicted in each early phase composite(Plate 3) is a sharp Horizontal Ekman transport fields for July 1979-1982 pigment gradient near the shelf break that separates eushow Ekman flow directed seaward away from the Arabian trophic coastal water from low-pigment water farther offcoast and imply that coastal upwelling occurredalong Oman shore.Within the regionalbloomsseenon the CZCS late phase during the summers of 1979, 1980, 1981, and 1982 (Plate 2). compositesfor 1979 and 1981, a strong oceanward pigment At the Arabian coast the FSU monthly mean July 1979 gradient persisted at the shelf break, although background surface Ekman transport was estimated to be above 4.0 pigment concentrationswere greatly increased. Overall, the m2/s,dropping fromthepeakof greaterthan18.0m2/sseen elevated mean pigment concentrations of the Oman shelf roughly 300 km off east Africa. The 1980 surface Ekman subregionsduring May-September are inferred to be due to transport fields also reflect a stronger monsoon, and in June active coastalupwellingthroughoutthis time period in all four and July the value of this parameter at the Omani coast summer monsoonsunder consideration(Figure 8 and Plate 3).
Illl
BROCK AND MCCLAIN:
1979
INTERANNUAL
VARIABILITY
1980
OF ARABIAN SEA PHYTOPLANKTON
1981
743
1982
EarlyPhaseSW MonsoonSeason
0
I 1979
I I 1980 1981 Late Phase SW Monsoon Season
+
I 1982
OmaniShelfSubregion _0_ Gulfof OmanSubregion
l- Positive CurlSubregion _•_ NegativeCurlSubregion
Fig. 8. Early and late monsoonphasemean phytoplanktonpigmentconcentrationsin subregionsfor 1979through 1982.
A more extensive coastalregion of phytoplanktonpigment
tinuous bloom that fringed the Omani coast. Only within
concentrations exceeding 5.0mg/m3 occurred withinthelate coastal embayments did pigment concentrations in the bloomapproach thevaluesofover5.0mg/m3seenwithinthe summer 1980 phytoplankton bloom than in AugustSeptember of 1979, 1981, or 1982 (Plate 3). During the late early phasecoastalbloomsof the preceding3 years, and the phaseof the 1980monsoon,pigmentconcentrationsapproach- Oman shelf subregionhad a mean pigmentconcentrationof
ing10mg/m 3wereobserved upto 300kmseaward oftheshelf only 1.29 mg/m3. In the late phasecompositeof 1982, break, and the meanpigmentconcentrationin the Oman shelf
pigmentconcentrationsover the Omani shelf and the along-
subregion wasveryhighat 6.19mg/m 3 (Figure8). In contrast, shore extent of the shelf bloom were diminished in comparwithin the 1979 and 1981 late phaseblooms, pigment concen-
trationsat similarlocations weretypicallybelow5.0 mg/m 3 (Plate 3). The enlarged shelf bloom and extreme pigment concentrationsin the August-September 1980 compositeare attributedto more intensecoastalupwellingin summer 1980. Comparatively weak coastal upwelling in summer 1982 is consistentwith the pigment distributionsshown on the early and late phase compositesfor this year (Plate 3). The May and June CZCS average image depicts a restricted, discon-
ison with the August-Septemberpigment distributionsfor 1980and 1981. By this stageof the 1982monsoon,the Oman shelf subregion mean pigment concentration had risen to
5.38mg/m3 andmoresignificantly thebloomwasrestricted to the north and central Omani coast.
Ekman Pumping
Inhomogeneitiesin the surfacelayer flow related to horizontal variations in the surface winds (the curl of the wind
744
BROCKAND MCCLAIN.'INTERANNUALVARIABILITYOF ARABIANSEA PHYTOPLANKTON
JULY 19.'9
.JULY
ßJULY I%O2
JULY 1979 - 1 FSU HF•HTHLY HEWN
TOTAL SURFACE EKHAN TRANSPORT (N
25 M:,'S 12.0 140 le.e NO
Plate 2.
July FSU monthly mean Ekman horizontal transport fields for 1979-1982.
stress)drive Ekman pumping,a physicalprocessthat can yield vertical velocitiesat the baseof the Ekman layer and
sea upwelling is broader and less intense than the narrow
occurs to the northwest
rangeof 1to 2 x 10-5 m/s(1 to 2 m/d)at 50-mdepth[Smith
coastal upwelling [Bettere, 1969]. Calculations using the vertical nutrient fluxes [Pond and Pickard, 1986]. Previous Discovery wind stress observations yielded divergent Ekinvestigations of the summer monsoon circulation in the man flow and upward Ekman pumping within a region northwestern Arabian Sea [Bettere, 1969; Smith and Bet- extending 1000 km along the Omani coast and out 400 km tere, 1977;Swallow, 1984;Bauer et al., 1991]have suggested into the Arabian Sea [Bettere, 1969].During the heightof the that open-seaupwellingdriven by positive wind stresscurl 1963 southwest monsoon, vertical velocities were in the of the axis of the low level atmo-
sphericSomaliJet. Wind andhydrographicdatacollectedby and Bettere, 1977]. the R.R.S. Discoveryin Juneand July 1963showthis open In spite of the 1979-1982 interannual variation in the
BROCK AND MCCLAIN.'
INTERANNUAL
VARIABILITY
OF ARABIAN
SEA PHYTOPLANKTON
745
EAFLY PHraSE$OUTHgESTMOH$OOH-
o.
e•
I •7
•
P•G•,EHT
% . ' 424
- 4•4 114
'114
•*
248 HO
-.
4
APq•IA .
- e•
FICYEPT C•F h'/TIOF
157
•
Plate 3.
.
-
.
-
CZCS early and late phasephytoplanktonpigmentconcentrationcompositesfor the southwestmonsoonsof 1979-1982.
magnitude of the southwest monsoonwind stress, the July month in 1979. The July 1981 monthly mean Ekman vertical monthly mean Ekman vertical velocity fields derived from velocity field is a close approximation of the July 1979 field the FSU wind pseudostressdata are remarkably similar for for this parameter. The July 1982 monthly mean Ekman all years (Plate 4). The Ekman pumping fields for July vertical velocity field (Plate 4) closely resembles those 1979-1982 all show open-seaupwelling in the northwestern generated for the more vigorous 1979, 1980, and 1981 monArabian Sea, with maximum upwelling in the vicinity of soons, since the monthly mean wind stresscurl distributions Socotra.Weak downwellingin the central Arabian Sea to the were substantially similar. southeastof the Somali Jet was predicted for the summer The dominant influenceson surface and mixed-layer temseasons of all years under study. The July 1979 Ekman peratures in summer in the northwestern Arabian Sea are the pumping calculation yielded upward velocities greater than wind-driven upwelling of cool water, surface heat fluxes and 0.3 x 10-4 m/snearSocotra(Plate4). The 0.1 x 10-4 m/s the absorption of short-wavelength solar radiation by photoisoplethwithin the upward Ekman pumpingmaximum in the synthetic pigments, and the upward entrainment of thermonorthwestern Arabian Sea extended 200 km further to the cline water by wind-driven mixing [Leetmaa and Storereel, northeast along Oman in July 1980 than during the same 1980; Molinari et al., 1986; Shetye, 1986; McCreary and
746
BROCK AND MCCLAIN'
INTERANNUAL
VARIABILITY
OF ARABIAN SEA PHYTOPLANKTON
JULY 1979
JULY 1988
JULY 1981
JULY 1982
JULY 1979- 1982
FSU MONTHLY MEAN EKMANUPIxlELL IHG VELOCITY LAItll
-$ I E-$4
+O2E-84 +O.3E-O4 - +O4E-04 +O.SE-O4 NO
Plate 4.
July FSU monthly mean Ekman vertical velocity fields for 1979-1982.
Kundu, 1989; Sathyendranath et al., 1991]. Given that coastal upwelling is limited to the coastal fringe along Oman and Yemen by the Rossby radius of deformation (about 50 km), the 2ø spatial resolution of the CAC monthly mean SST fields (Plate 5) is appropriate only for the inference of open sea upwelling. The evolution of sea surface temperature shown by the CAC
SST
data
set for each
summer
monsoon
from
1979
through 1982 resembles that shown by surface temperature climatologies [Wooster et al., 1967; Fiel4x and Storereel, 1976; Hastenrath and Lamb, 1979; Brown and Evans, 1981; Rao et al., 1989] and the distributions for summer 1963
provided by Wyrtki [1971]. As depicted by these previous studies, sea surface temperature across the Arabian Sea at the close of the intermonsoon in May is warm, averaging about 29øC, and shows minimal variability within a zonal pattern of isotherms [Wooster et al., 1967; Hastenrath and Lamb, 1979]. The southwest monsoon results in dramatic changes in this pattern of sea surface temperature, as is apparent in the average August surface temperature chart of Hastenrath and Lamb [ 1979] and in the average August CAC monthly mean SST field for 1979-1982 (Plate 5). By August, cooler
water
was at the
surface
over
all of the western
Arabian Sea, and extreme temperature lows existed off the
BROCK AND MCCLAIN:
INTERANNUAL
VARIABILITY
OF ARABIAN
SEA PHYTOPLANKTON
747
II
eS-
Plate $. Mean NOAA Climate Analysis Center sea surface temperature field for August 1979-1982. The jagged shoreward boundary is due to application of a land mask.
coasts of Oman and Somalia, where the monthly mean temperature dropped to less than 25øC. This broad region of summer surface cooling was roughly coincident with the region of upward Ekman pumping in July predicted by the FSU winds analysis for these four summer monsoons(Plate 4). Although surface heat fluxes, changes in shortwavelength absorption due to the seasonal evolution of phytoplankton biomass, and vertical mixing must be considered [Leetmaa and Stommel, 1980; Molinari et al., 1986; Shetye, 1986; McCreary and Kundu, 1989; Sathyendranath et al., 1991], at a coarse scale the summertime decrease in surface temperature seen on the August CAC monthly mean 1979-1982 SST field (Plate 5) provides evidence for upward Ekman pumping in the northwestern Arabian Sea. Diminished surface cooling during the 1982 southwest monsoon is obviousas a warm August SST anomaly, in comparisonwith the 1979-1982 August mean SST field (Plate 6). This warm SST anomaly covers the region of Ekman pumping-induced cooling and exceeds +0.6øC. Offshore uplifts of isotherms that support the interpreta-
tion of active upward Ekman pumping during each of the southwest
monsoons
from
1979 to
1982 are seen on the
following XBT-based vertical temperature fields: August 1979 section B, July 1980 section A, September 1981 section A, and September 1982 section B (Figures 5 and 6). Commonly, temperature contours to depths around 200 m and near 18øC are affected. On these temperature sections, elevated isothermscoincide spatially with the northwestern Arabian Sea region of predicted upward Ekman pumping. The near-surface pigment distributions seen in the CZCS August-September composites for 1979, 1980, and 1981 (Plate 3) are regionally correlated with the June through August monthly mean Ekman pumping fields for these years (Plate 4). The late summer phytoplankton blooms observed by the CZCS in 1979-1981 developed offshore within a 700-km-wide southwest-northeast swath along Oman and Yemen that was coarsely equivalent to the northwestern
tlOM•L' , C'
Plate 6. NOAA Climate Analysis Center sea surface temperature anomaly field for August 1982 relative to the mean NOAA CAC sea surface temperature field for August 1979-1982. The jagged shoreward boundary is due to application of a land mask.
Arabian Sea region of summer upward Ekman pumping for these years. The offshore phytoplankton blooms in the late summers of 1979, 1980, and 1981 covered the positive curl subregion, where the mean pigment concentrations were
2.46 mg/m3, 3.64 mg/m3, and 2.91 mg/m3, respectively. These regional phytoplankton blooms range in pigment
concentration from 2.0 to 5.0 mg/m3 and are attributedto upward nutrient fluxes to the euphotic zone driven by Ekman pumping. The regional northwestern Arabian Sea offshore bloom noted in the preceding three years was not well developed in late summer 1982 (Plate 3), and the positive curl subregion had a low mean phytoplankton
pigmentconcentration of 1.63mg/m3. Interannual Variability of the Regional Phytoplankton Bloom Banse and McClain [1986] suggested that the winter algal production in the northern Arabian Sea is stimulated by mixing primarily due to surface cooling and wind stirring. Inasmuch as seasonally diminished winds and cloud cover may be expected to increase shallow temperature stratification by reducing vertical mixing and altering surface heat fluxes, the weak 1982 summer monsoon allows evaluation of the role of summer thermocline evolution on phytoplankton growth. Further, the diminished phytoplankton bloom in the summer of 1982 occurred during a southwest monsoon that preceded an E1 Nifio event in the equatorial Pacific [Wyrtki, 1985], raising the possibility that El Nifio-monsoon interactions [Yasunari, 1990] may have consequences for northwestern Indian Ocean primary productivity. In the northwestern
Arabian
Sea the arrival
of the south-
west monsoon typically drives deepening of the mixed layer and results in a vertically diffuse thermocline [Colburn, 1975; Hastenrath and Lamb, 1979; Rao et al., 1989]. An early summer thermocline develops in the upper 200 m during April and May [Colburn, 1975], and as has been observed by
748
BROCK AND MCCLAIN:
INTERANNUAL
VARIABILITY
Sastry and D'Souza [1970] and J. C. Brock et al. (manuscript in preparation, 1991), the approximate climatological mixedlayer depth directly off Oman in April and May is limited to only several meters in thickness. Throughout June and July, a mixed layer forms and deepens to about 50 m, in spite of upward Ekman pumping to depths in excess of 200 m that acts to shallow the mixed layer [Bauer et al., 1991]. This is the mixed layer and thermocline evolution that is associated with the climatological southwestmonsoon, and it applies to the moderate to strong southwest monsoonsof 1979, 1980, and 1981.
The deepening of the mixed layer in the uppermost 50 m and erosion of the spring thermocline that occurred in the summers of 1979-1981
shallowed
the nutricline
and rendered
upward Ekman pumping effective in delivering nutrients to the euphotic zone. This vertical mixing combined with open-sea upwelling operating to depths in excess of 200 m stimulated high regional phytoplankton biomass in the late summers
of 1979-1981.
Partial
confirmation
for this scenario
comes from the vertical temperature distributions shown on June 1979 section A, August 1979 section B, and September 1981 section A (Figures 5 and 6). August 1979 section B shows only minimal temperature decrease from 0 to 100 m over much of its extent, in contrast to June 1979 section A in the samelocation, which depictsroughly an 8øCtemperature drop in the same depth range. Also, both September 1981 section A and August 1979 section B show weak vertical temperature gradients north of the wind stress maximum that are inferred
to have resulted
from breakdown
of the
shallow early summer thermocline. Increased
thermal
stratification
and diminished
vertical
mixing during the relatively weak 1982 southwest monsoon [Reverdin and Fieux, 1987] is consistent with the expected increase in net downward
radiant flux due to reduced cloud
cover, the anticipated lower upward latent heat flux, and decreased
turbulent
to the diminished
entrainment
wind
of thermocline
stress. This inference
waters
due
is consistent
with previous studies that noted the importance of both entrainment through the base of the mixed layer and surface heat budget processes as causes of western Arabian Sea mixed-layer cooling and deepening in summer [Molinari et al., 1986;Rao, 1986; McCreary and Kundu, 1989]. Temperature sections September 1982 A and B suggest that the shallow early summerthermocline intensifiedthroughoutthe duration of the 1982 summer monsoon. Indeed, the intense near-surface vertical temperature gradients and greater than 0.6øC increase in SST seen in August 1982 (Plate 6) relative to the August 1979-1982 mean field (Plate 5) suggestthat in 1982 the formation of a mixed layer in the upper 50 m and concurrent thermocline erosion were supplanted by the development of strong stratificationin the upper 100 m. The absence of a strong regional northwestern Arabian Sea phytoplankton bloom in August-September 1982 is attributed to diminished mixed-layer deepening and the development of a shallow temperature stratification under the weak 1982 southwest
monsoon.
CONCLUSIONS
1. Significant interannual variation of the May-September monthly mean southwest monsoon surface wind field over the northwestern Arabian Sea occurred during 1979 to 1982. Although southwesterly winds arrived in June in all
OF ARABIAN
SEA PHYTOPLANKTON
four summer seasons, the 1980 surface flow was stronger than that observed for 1979 and 1981, and the 1982 southwest monsoonwas considerablyweaker than that of the previous three years. Also, the wind stresspeak that occurred in July 1979, 1980, and 1981 was not observed during the weak southwest
monsoon
of 1982.
2. These interannual differences in wind forcing between the summer monsoons of 1979 through 1982 gave rise to variations in the seasonal strength of physical oceanic processes that yield nutrient fluxes to the euphotic zone. Coastal upwelling along Oman was most intense in 1980, intermediate in 1979 and 1981, and greatly reduced during the southwest monsoon of 1982. The monthly mean Ekman vertical velocity fields are similar for all years and predict open-sea upwelling within the northwestern Arabian Sea. The formation and deepening to about 50 m of the mixed layer that occurred between May and September 1979, 1980, and 1981 in the northwestern Arabian Sea was replaced in 1982 by a summertime increase in stratification in the upper 200 m.
3. Phytoplankton biomass on the Omani continental shelf increased during both the early and late phases of the 1980 southwest monsoon owing to stronger coastal upwelling under the most intense southwesterly winds of the four summers investigated. Similarly, lessened coastal upwelling during the weak 1982 monsoon resulted in a diminished coastal phytoplankton bloom. The lack of a strong regional northwestern Arabian Sea phytoplankton bloom in August-September 1982 is attributed to the absence of mixed-layer deepening and the development of sharp and persistent shallow temperature stratification under conditions of anomalously low wind stress and probable elevated oceanic net heat gain. In contrast, during the southwest monsoonsof 1979-1981, vertical mixing and upward Ekman pumping eroded the shallow spring thermocline. This rendered Ekman pumping north of the Somali Jet axis more effective in the upward transport of nutrients and the subsequent stimulation of regional phytoplankton blooms in 19791981.
Acknowledgments. This research was conducted within the Oceans and Ice Branch at NASA Goddard Space Flight Center. J.B. gratefully acknowledges support from the NASA Graduate Student Researcher Program. Funding for C.M. was provided by NASA RTOPS 579-11-01-20, 579-11-02-20, 579-11-03-20, and 578-22-03-20. The authors
thank
Jim Firestone
and Mike
Darzi
of General
Sci-
ences Corp. for assistancein software development for processing of the satellite, wind, and hydrographic data sets. Lola Olsen, Hank Griffioen, and John Vanderpool at the NASA Climate Data System assisted in obtaining the wind stress and SST data sets. The FSU wind fields were provided by D. Leglet and J. O'Brien of the Mesoscale Air-Sea Interaction Group at the Florida State University. REFERENCES
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(Received February 13, 1991; revised July 13, 1991; accepted August 14, 1991.)
