EQUATORIAL UPWELLING IN THE EASTERN ATLANTIC:PROBLEMS AND PARADOXES

B. Voituriez

Antenne ORSTOM, C.O.B. B.P. 337, 29273 Brest Cedex,France

Abstract. Recent oceanographic cruises haveyielded new information that requires revision ofideas a~d hypotheses concerning equatorial upwel­ling· in the Atlantic. Some of them concernedmainly with the eastern equatorial Atlantic areconsidered, including seasonal variations inequatorial upwelling, the physical processesaccounting for seasonal sea-surface cooling andnutrient enrichment, and primary and secondaryproductions.

Seasonal variations in sea-surface temperaturein the eastern equatorial Atlantic are comparablewith those of the eastern equatorial Pacific. Var­iations of the zonal pressure gradient induced byvariations of the wind stress along the equatorexplain the east-west asymetry of the sea-surfacecooling during the upwelling season. In the east­ern part of the Atlantic, where the thermoclineis shallow in July and August, the southeasterlywind induces, south of the equator, an equatorialdivergence causing marked cooling of the sea sur­face. During the same season, the thermocline inthe western Atlantic is too deep to permit signi-

. ficant sea-surface cooling by the equatorial di­vergence. Unlike the eastern equatorial Pacific,nutrient enrichment of the surface layer is re­stricted to the period July to September, butthis does not necessarily result in an increase inprimary and secondary production during the per­iod.

Introduction

The actual role of equatorial upwelling and theimportance of variations in it have been under­estimated for some time. The Equ~lant interna­tional cruises in 1963 and 1964 were the firstaimed at a biological and physical description ofthe tropical Atlantic Ocean. Until recently theEqualant observations were considered as refer­ences of the Atlantic equatorial upwelling. How­ever, it is now clear that 1963 was actually anexceptional year in that upwelling did not occuror was very weak, as suggested by Sedikh and Lou­tockhina (1971). This is clearly shown by compar-

ison of the sea-surface temperatures in summer,1963 (Fig. 1) and those in more recent atlases.Except in the far easte&" part of the ocean, theSST was greater than 24 C everywhere in the equa­torial zone in summer, 1963 whereas the atlasesshow that on the averaRe temperatures are below22.5 (Mazeika, 1965) or 220C (Neumann, Eeatty andEscowitz, 1975) in the Guif of Guinea in summer.The anomaly is also shown by nutrient distribu-.tions in the surface layer. The maps of thedistributions of phosphate in the surface layer(Fig. 2) show no equatorial enrichment either inFebruary and March, 1963 (Equalant 1) or in S~­

mer 1963 (Equalant 2). However, the systematic·observations made from the R.V. Capricorne in theeastern Atlantic along 40W between 1971 and 1979show nutrient enrichment of the surface layer tobe the rule from July to September (Voituriez andHerbland, 1977).

Seasonal Variations of Equatorial Upwellingin the Gulf of Guinea

The observations of the R.V. Capricorne along40 W in the Gulf of Guinea make it possible todefine the cycle and amplitude of seasonal varia­tions of the upwelling (Fig. 3). The sea-surfacetemperature falls from 290C in March and April to2loC in July and August, an SoC amplitude. Theanomaly of the summer of 1963 (Equalant 2) isclearly shown on Fig. 3. The figure shows thestriking similarity of the seasonal. variations inequatorial upwelling in both the eastern Atlanticand Pacific oceans. The seasonal cycle and thesurface temperature signal are roughly the samein both oceans, a point to be emphasized. Itwill be shown that no such similarity exists in"the variations of nutrient enrichment, which ismuch less in'the Atlantic than in the Pacific.The difference between the temperature and thenutrient signals probably tended to mask the im­portance of upwelling in the equatorial Atlanticwhen the scientific community was more concernedwith the consequences of upwelling on biologicalprocesses than on the climatic processes. Hisard

95

80-W 40- 30- 20- lO- o- JO-!N20-

EQUALANT 2 AUG.1963

:~SEA SURFACE TEMPERATURE ..

2T ///T- < 24-

2~ 10·

ZO'~--..L. -.L .r.- ....L... --'- --/. -.L--IS

Fig. 1. Sea-surface temperature of·the equatorial Atlantic Ocean in sUDllller 1963 (Kolesnikov, 1973) •

.:...

(1980)' 'p.ointed out that the silll1larity betveenthe eaàeèrn Atlantic and Pacific oceans probablyiacluded inter-anoual temperature variations; hesuggested tnat anomalies such as. those observedin 1963 (Equalant 2) can be considered sim1lar tothe occurrence of E2 Nina in the Atlantic Ocean.

The Hechanisms of the Equatorial Upwellingin the Atlantic Ocean

The great interest in recent years in the equa­torial upwelling of the eastern Atlantic stemsfrom the idea that equatorial divergence couldnot account for the upwelling observed in thearea, because the annual upwelling in the Gulf ofGuinea was not associated with changes in thelocal winds (Hisard, Citeau, and Voituriez, 1977;Voituriez and Herbland, 1977; Hoore et aL., 1978).The idea was supported by other studies showingthat in the coastal upwelling of the Gulf ofGuinea there is no relationship between variationsof the sea-surface temperature and Ekman transportas deduced fram the observed winds (Berrit, 1977;Ba~un, 1978). Other hypotheses were proposed taexplain the variations of the sea-surface temper­ature and summer cooling in the Gulf of Guineasuch as vertical lIl1xing in the vertical shearzone between the westward surface current and theequatorial undercurrent (Hisard et a~., 1977;,Voituriez and Herbland, 1977) and an upwellingsignal, generated by increased westward windstress in the western Atlantic and travelling to

96 VOITURIEZ

the eastern Atlantic as an equatorially trappedKelvin wave (Moore et aL., 1978; O'Brien, Adamec,and Moore, 1978). However, the hydrographic and~teorological observations along 40~ by R.V.cap~OrnB· show that the new hypotheses are pro­bably premature for two reasons: 1) There isgood agreement betveen seasonal variations in se_surface temperatures and the variations in thewindstress. This 15 clearly illustrated by Fig.4; the sea-surface temperature is minimum whenthe wind stress is maximum. 2) The distributionof the sea-surface temperature agrees well viththe qualitative model proposed by Cromwell (1953)linking the equatorial divergence ta the tradewinds.

This model Ls presented in Fig. 5. It showsthat the equatorial divergence is right a~ theequator with easterly winds and is shifted south­wards with southeasterly winds, exactly as ob­served during the CIPREA cruises in summer, 1978.Fig. 6 shows the divergence marked by the minimumsea-surface temperature ta be south of the equatorwith southeasterly wind. The rotation of the windand the inc~ease of the zonal component of thewind stress between 4 and 90 W lead ta an equator­ward motion 'of the,equatorial divergence in accor­~ance with Cromwell's model. Ta conclude, inopposition to what was thought before, there is noapparent contradiction between equatorial upwel­ling events in the Gulf of Guinea and observedchanges of the local wind and subsequent equa­torial divergence.

W. so· 40'

EQUALANT 2AUG.63

O' E.

N.

10·

10·

S.

EQUALANT 1MARCH.63

~

.," .

W. SO· 40' 30·

cO.2

~0.2

10· O·

0.2

10· E.

N.10'

10·S.

Fig. 2•. Phosphate-phosphorus in surface (Ilg-at/l) during Equalant 1 (Harch 1963) and Equalant 2(August 1963) (Kolesnikov, 1976).

t7 +---,.-,F:-T-,"I:-r-,A-,r-:-:"':-r-7""'"I---:"........:-"...-:S;""'T'""";O:"'T""-::N:"'T""';;'O........-:..::-:-•• 1h

Fig. 3. Seasonal variations of sea-surface temp­erature in the Eastern Equatorial upwelling re­gions of the Atlantic and Pacifie oceans: lowesttemperature measured between 0° and 5°S alang 4

0W

(1971-1979 R.V. Capricozone) and9SoW (1967-1968Eastropac cruises).

A problem.remains: the asymetry between theEastern and the western Equatorial Atlantic dur­ing the upwelling season. Fig. 7 shows that thesea-surface temperature is lower in the Easternthan in the western Atlantic whereas the windstress along 2.50 5 is greater in the west than inthe East. Paradoxically, the cooling of theseasurface is the most important. where the windstress is the weakest. This seems to be in con­tradiction to the previous simple conclusions onthe role of Equatorial divergence in the upwel­ling events in the Gulf of Guinea, but the ano­maly can be explained when the entire EquatorialAtlantic is considered. Merle (1980) pointed outthat the Equatorial homogeneous surface layer is~uch thicker in the western than in the EasternAtlantic (Fig. 8) because of the accumulationa$ainst th$ American coasts of water carriedwestwards by the South Equatorial Current. There­fore the energy or the wind stress intensityneeded to bring the cold, deeper waters to thesurface is less in the Eastern than in the westernAtlantic. Thus the sea-surface temperature is not·always a good indicator of the intensity of theEquatorial divergence. In fact surface coolingin the Equatorial divergence depends on two fac­tors. the wind stress. which controls the inten-

T'C

25

20

1"""o 1

\ '2 2} :,::"~, , ••

\.V

\- 0/'06T.8'SPAC 1FIC 9S' W.

PHYSlCAL PROCESS OF UPWELLING 97

-_. ---- ._----_._-_.•-------_.__ .__...__....

•__ - - - - - - - --0 0

-0.1

25

F M A M

't:t.

·: '.· .... ..• •i ....0"'

A SON 0 ,M ••th

crease of the thermocline depth. lt can be co~­

cluded that the greater the zonal pressure gradi­ent, the greater the cooling of the sea surfacein the eastern equatorial Atlantic. Thus theunusually high sea-surface temperatures of August1963 'appear ta correspond ta an anomalously veakzonal pressure gradient (Fig. 10). Hisard (1980)suggested that such abnormal conditions in theAtlantic âre similar ta El Nina in the Pacific~

That rather simple explanation of the equator­ial ~pvelling in the Atlantic does not excludethe other mechanisms, the trapped Kelvin vave andvertical mixing in the vertical shear zonebetveenthe surface current and the undercurrent; bathcan act to cool the eastern Atlantic. The Kelvinvave has not yet been detected in the equatorialarea. lt vould act on the zonal pressure gradientand th~s vould DOt change the previous conclusionson the equatorial divergence. The role of verti­cal mixing in summer in the vestern Atlantic(30OW) vas pointed out byKaiser and Postel (1979)and by Cornus and Meincke (1979). This iG not in

Fig. S. The equatorial divergence model of Crom­vell (1953). Intenaity and directions of thevind and total transport are assumed constant.The angle of the vind vith the total transportdirection depends on the latitude so that themeridional transport pattern depends on the vinddirection.

( --- lotol t'''''lport. ). - .... ,Ilt/ollol lro"lport.

• ~ t1 ,. ",t, .

\ .-~·N,, 1

·f,

\ •,, ,'\

" t" ,'\'\ ,

'-----, t, ." ~\

" t.0 DIY '\ -O·

,..- '. tt

,~

-.. •~-

- !.ODIY-~

1

J..-,-,

l ,

l , , -~.. S. ,.1

1

1,-1

f ;

Fig. 4. Seasonal variations of the sea-surfacetemperature (lovest temperature betveen 0 and SoS)and the vind stress (mean value betveen 0 and SoS)in the equatorial upvelling regian of the AtlanticOcean along 40 W (Data collected by the R.V. ~ricorne from 1971 ta 1979.). t: total wind stresa,t x : zonal camponent of the wind stress.

sity'~f the divergence, and the depth of the ther­mocline. Consequently, in spi te of a possiblymore intense divergence in the vestern part of theAtlantic. surface cooling can be greater in theeaat than in the west.

The increase fram east to west of the· mixedlayer depth accounts for the zonal pressuregradient. along the equator as shawn by Newœannet al. (1975) and Katz (1977) (Fig. 91. Katz(1977) bas pointed out tvo facts: 1) There isa seasonal variation of the zonal pressure gra­dient. The maximum occurs iD August and Septem­ber and the min1,mum in March (Fig. '10). Thismeans that the seasonal variations of the slopeof the surface along the equator caine ide viththe variations of the sea-surface temperature inthe eastern Atl~\tic as described bafore (Fig. 3).2) The zonal pressure gradient is correlated withthe wind' stress; the stronger the vind stress',the greater the zonal pressure gradient. Thesummer increase of the wind stress has tvo con­sequences: 1) An increase of the intensity of the'equatorial divergence over the ocean, and 2) Anincrease of the zonal pressure gradient~ i.e., anincrease of the depth of the thermocline in thevest and a decrease of it in the east (Fig. 8)\Accordingly the consequences of .the equatorialdivergence on the surface conditions (sea-surfacetemperature. nutrient concentrations) are ampli­fied in the eastern Atlantic by the decrease ofthe thermocline depth and. on the contrary, canbe neutralized in the western Atlantic by the in-

OR UnTT1Tl:1TJ::~

EAST WIHD

DivERGENCEAT

THE EOUATOR

"'\'\

'\

"SOUTH EAST WINO

DiVERGENCESOUTH

OF THE EQUATOR

Fig. 6. Sea-surface temperature in the Gulf of Guinea in August. 1978 (CIPREA cruises of the research vessels Suroit. Capri­corng,and Ni2e~y). The mean wind (direction and speed) betveen 0 and SoS is given for 9 and 4

0Wvith the total vind stress and

the ratios of the zonal to the meridional wind stress T /T •X Y

-~L.-_--O-

CIPREAAUG. 1978SEA SURFACE T·C

3-

ICAPRICORNE 12- 1NIZERY

1

ter ~----h~~~o-r--,-2­

3-'

4­

5­S.

..............

..1

eu

. '00••c~..,

...1

2

50

a WIND STRESS ALONG 2.~· S

June. July. AUQul1

(~e"erman 1967)

tO· E. Lonllitude

N­to-

10· E.

-b

'0-'-- ............ ~ ....... ......_ _'__..... __"_.....__.....I S;

",

Fig. 7. Sea-surface temperature and total WiDd stress iD the equatorial areaof the AtlanticOcean in sommer. a: total wind stress along 2.50 5 (data from Hellerman, 1967); b) sea-surfacetemperat.ure in July from Nelllll&1U1 st aZ., 1975).1

contradiction vith the observations in the Gulfof Guinea. . In fact the relative importance ofthese processes on sea-surface conditions dependson the season and on the longitude. The equator­ial divergence seems to be the most importantfactor in cooling the sea surfaœ in the easternAtlantic in sommer when the thermocline is shal­love During other seasons in the same area andduring aIl seasons in the vestern equatorialAtlantic, the thermocline is deeper and the in­fluence of the equatorial divergence on the seasurface i. probably negligible, sa vertical mix­ing can become the primary cause of the smallvariations of the sea-surface temperature.

Nutrient Enrichment

Nitrate enrichment of surface vaters Along 40woccurs only in July, August, and September (Fig.

11). The rest of the year, the mixed layer isnitrate impoverished. This contrasts with theeastern Pacific Ocean vhere surface nitrate con­centratious higher than 6 ~g-at/l vere observedaIl the year during Eastropac observations at950 W. Rovever, there are definite seasonal var­iations in the eastern Atlantic when the totalnitrate of the upper 100 m is considered. Thevariations are in good agreement vith the varia­tions in upvelling; enrichment is maximum in sum­mér (August) and minimum in March and April. Thedifferences betveen the Atlantic and the Pacificare interesting. While the effects of equatorialprocesses on the sea-surface temperature are sim­il.r, the effects on nutrient enrichment differmarkedly. For the same sea-surface temperaturethe Pacific Ocean i. richer than the Atlantic by6 ~g-at/l of nitrate-nitrogen, and the lovest

100 VOITURIEZ

SOW 4QW 30W 20W IOW 0 IOE

A

8

Fig. 8. a) Mean annual temperature in the 0 to20 S band from the Brazilian Coast (500 W) to theAfrican Coast (120 E). Data averaged by 40 long­itude ~from Merle, 1980); b) Depth variation ofthe 23 C isotherm for the four seasons of theyear along the section shown in Fig. 8a (fromMerle, 1980). The 230 C isotherm depth is takenfor the thermocline depth.

tempera~ure gt vhich 6 ~g-at/l of nitrate occur­red vas'20,5 C in the Atlantic and the highest. 0(26 ) vas in the Pacifie (Table 1).

Such differences may arise from the origin ofthe upvelled vaters in the Atlantic, vhich isthe equatorial undercurrent floving in the ther­mocline layer. The undercurrent carries a salin­ity maximum from vest to east (Fig. 12); itsorigin is subtropical, high-salinity, nutrient­depleted vater; on a Meridional section the equa­tor shows a nitrate minimum. The upper 30 m ofthe thermocline can be nitrate-depleted (Voitur­iez and Herbland, 1977). For example (Fig. 13)at 60 E in 1971 betveen 20 May and 15 June thesea-surface temperature dropped 40 C and isothermsshoaled by 30 m. At the same time the nitrateisopleths shoved a 30-m rise, but the nitratecontent remained undetectable down to 20 m. Thisis important; Herbland and Voituriez (1979) usedsurface nitrate concentrations to define tvotypes of situations in the tropical ocean: thetypical tropical situation characterized by anabsence of nitrate in the surface layer and theupvelling situation vith significant surfacenitrate concentrations. The eastern equatorialPacifie is usually in an upvelling situationvhereas the eastern Atlantic is in a typicaltropical situa.tion nine months (October to June)out of tvelve. With the same annual temperaturesignal nitrate does not limit primary productionin the eastern Pacifie, vhereas it probably doesmost of the year in the eastern Atlantic.

Primary and Secondary Production

Seasonal variations in primary and secondaryproduction in the equatorial Atlantic are muchless than one might expect from the variations ofthe nutrient regime.

Primary production. The CIPREA cruises at 40 Win 1978 and 1979 shov the usual seasonal varia­tions of sea-surface temperature, nitrate at thesurface, and integrated nitrate over 100 m. Hov­ever, neither chlorophyll nor primary productionfolloved the same pattern (Fig. 14). Variationsin primary production vere small, values beingslightly more than 100 mg C m- 2h- l The chloro­phyll content had the same value in April andAugust (Table 2). The conclusion is that in \1978and 1979 primary production vas rather high dur­ing aIl seasons in the equatorial area at 40 W andthat the summer nutrient enrichment did not inducean increase of primary production and chlorophyll.

Secondary production. Similarly, zooplanktonbiomassdoes not alvays differ significantly be­tveen the cold season in July and the varm seasonin April (Fig. 15).

The equatorial area in the Gulf of Guinea isalvays rather productive, but the levels of pro­duction and biomass are not significantly increa­sed by the appearance of nutrients in the surfacelayer. The absence of contrast of production be­tveen the tvo regimes has probably led to the un­derestimation of the importance of the inter­annual variations of the equatorial upvelling inthe region. Indeed phenomenon similar to EZ Ninain the eastern Atlantic, as suggested by Hisard(1980) for 1963 (Equalant 2), has a marked eff~cton the sea-surface temperature but not on thebiological production, vhich remains at a highlevel vith or vithout upvelling.

Several authors such as Sorokin, Sukhanova.Konoyalova, and Pabelyeva (1975) have commentedon the lov level of production compared to nutri­ent availability in the equatorial upvelling re­gions of the Pacifie Ocean. The anomaly vascalled "the paradox of the nutrients" by Walsh(1976); vhy are nutrients not taken up in greaterquantity vhen light and nutrient conditions arenot limiting? Several explanations have beenproposed for the paradoxe The lack of chelatorsvas proposed by Barber and Ryther (1969), andintense vertical mixing in the vertical shearzone .betveen the surface current and the equator­ial undercurre~t vas proposed by Sorokin et aZ.(1975). Walsh (1976) suggested that the dominantfrequencies of variability of the physical envi­ronment May define the importance of grazingstress as a tonstraint· to nutrient utilization inthe sea. For example the relatively high stabil­ity of the lov-frequency regime of the equatorialupvelling could lead to an herbivore populationexerting a quasi-continuous grazing pressure andconsequently a lov utilization of nutrients.

The discussion on this subject is probably not

PHYSlCAL PROCESS OF UPWELUNG 101

100.--------------------------,

8,.....---.-.;,.---------------,l'

::. 5•B 4.,;•.. 3:::~.. 2

lAT! 0111111'41

+ CIIAWI'OI'lQGAn z I,uel(,t1'41 GAT! 3 +

-1-0(1t1'41

__++-1_ !azl l''UI

-t-;c•..

la , Il'.3)

o l--,--2.,..........~3:....--4--'......,-6-T....".7-,.--8......,-9-T~'~O....-1l.......--12...J......Fig. 9. .Dynam1c height of the 0, 50, .and 100- dbar surfaces relative to500 in the AtlanticduriDg GATE-Phase 2 (27 July"- 17 Auguse 1974) (Katz, 1977).

:8.-------------------....,

l 1 la 1 Il,n 1

o~~'T"""-:2.....-:3~--:4-".,.....'::-"~6:"""'l""""':1':-r'""':'8.....-:9:-"'l"""":':'O~~'~\...,.-:1~2......

T lai---H1-- IIIUI

+ CRAWI'OI'lQ...n z "(tI~el

(111'41 GA n s +--1-(111'41

-+-lAT! 011'''1'4\

l'

•c.: ,

·:. 5••!4

Fig. 10. The Atlantic equatorial zonal pressuregradient (50/500 dbar) vest of 100W as a functionof the observational period and independent ofyear (Katz ct at.~ 1977).

over and the camparison of the two eastern oceanswhere the variations of the nutrient regime arequite different is a fascinating point which canhelp ta solve the problem of the fertilization inthe equatorial upvelling.

Conclusion

The several points discussed in this paper showthat equatorial processes are complexe Many pro­blems are st111 ta be solved. What is the raieof the different equatorial processes in nutrientenrichment and sea-surface cooling? What are thespace and time variabilities of equatorial upvel­ling? Why do nutrient uptake and production seemrelatively low in equatorial upvelling regions?Ta answer these questions we need more observa­tions in the equatorial region, which IllUSt be con­sidered as a whole because upwelling is not onlya local phenomenon but it is the result of pro­cesaes involving the whole equatorial basin.Hovever some progress in our knovledge could beIIIIIde. now by IIUlldng use of aimllarities and con-

102 VOlnnuEZ

a 1 NO, N ln lurfac.20

CONCENTRATION MAXIMUM BETWEEN O·and S·S

.• 10~

z-­o

z

1167 -1'"

/', \ ATLANTIC 4·W

,~ \ CA,.'''COIIN[ 1t71-1t7'

l 'l "O~~~""'~""~L::-::.,=-J!"''''-'--r--r-.,.....,.-''''!.;-'''-t--r--

F l' A l' JAS 0 N 0 MON TH

ISOO b : L;" NO, N

MEAN VALUE BETWEEN O·llnd S·S71 ATLANTIC 1171-1171

..i..;1000

zÔz

71

.' ..SOO+-J......-F.,.....,-..-r-A'"""T-..---.-J---.r---r-A......-S-r--O.......-N-r-O......- ..-O-N tH

Fig. Il. Nitrate distribution in equasoria1 upwe11ing regions. a) Maximum concsntration ofnitrate in the surface between 0 and 5 S in the eastern equatoria1 Atlantic at 4 W (data co11ec­ted by the R.V. Capricorne from 1971 to 1979) and in the Pacifie at 950Wo (Eastropac 1967-1968);b) Integrated value of nitrate over 100 m in the Atlantic Ocean a10ng 4 W: mean value between 0and 50S (data co11ected bythe R.V. Capricorne from 1971 to 1979).

TABLE 1. Nitrate concentrations at the ~urface i8 the equatoria1upwe11ing regions of the eastern Pacifie at 95 W (EASTROPAC67-68) and the eastern Atlantic at 40W(CIPREA 78-79)

Sea Surface(ToC)

N03-N (surgace)Pacifie 95 W(\lg-at/1)

N03-N (surgace)Atlantic 4 W(\lg-at/1)

ôN03-Nl\lg-at!l>

20.5

12

6

6

6

o

6

PHYSlCAL PROCESS OF UPWELLING 103

:: ..

20 WAY liTt 15 .lU NE l'Tt

,ON 0° 1° •o ..J.I__.......I ...I_

RI.

:.:~';.

~~~~

"\ Î100 V·RI.

,OS1

.0.•

0°,'ON,

'ON 0°

SALiNITY APRIL 1·878CIPREA li 4·W./ //A 011011, > JI "10o

-- __ NII,aloo "101'"

SONo

SO

ISO•

100

~IPREA 2 APRIL 1878 4·W.

~....L. NO. N .

cO.,

0.1.1 •~ ----,---- -- &0

Fig. 12. Transect a10ng 4uWin April 1979. CIPREA2. a) sa1inity and nitrate isop1eths; b) int~grated

value of nitrate over 100 m.

NO) N Plia'. 1.1

100RI.

Fig. 13. Nitrate and tempera~ure section a10ng60 E.· Left: ~O Hay 1971. Right: lS June 19710Upper part: T C. Lower part: Nitrate in ~g-at/1

..,.JI:...,e

c.J,;.ec.2u~

c.J...0• À:...

2S .§À:

200

ucou

100

20q. . rOd. /.

&\ ~I~ - ----:-<. ".......- / .30 \ .. '\

/0. ./' c:."\ \.~ 9-'~ 'b

\\

\\\

\ .

....," NOs·N (surfoce)

0~20~T"'""~~-?:";-=-:=-:"";'-..:--:-:..:-r=-~-+-=-:-~-:;,:,--:::-:"':-r-=-:=-":lF-:":--':"ï-_~~1~OA 5 ON 0 J FM AM J J

1978 1979

zS

~o..-

..

...:.

Fig. 14. Variations ~f temperature, primary production, ch10rophy11, and surface nitrateconcentration a10ng 4 W in 1978-1979 (Mean values between 0 and 5°S of the CIPREA cruises).

TABLE 2. ~ean values between 0° and 5°S in 1978-1979 at 40W. CIPREA cruises of R.V. Capricorne~

Cruise 7802 7802 SOP 7906 7910 7912Aug. 78 Sept. 78 Jan. 19 Apr. 79 Jun. 79 Oct. 79

! (Surface) 21.6 22.8 27 28.5 24.6 25.5

- ~03 (surface) 4 1.5 0 0 0.7 0.12l'g-at/1

Integrated nitrate 1275 1170 580 990 802content, 20 to 100 m.(mg-at/m )

Integrated chloro- 33 27.5 33.1 27.5 27.3phy11 content, 0to 10~ m.(:ng/m )

Primary produc- 114 104 120 98thity(mg C m-2h- l )

PHYSlCAL PROCESS OF UPWELLING 105

References

WP Z H., 0- 500 ...

(- J~lr 1177)••• .. "iI 1.7t

cesses of upper waters in the Atlantic Ocean.Report of the International Workshop on the .GATE Equatorial Experiment. Miami. 28 Februaryto 10 March. 1977.

Kaiser. W. and L. Postel. Importance of the ver­tical nutrient flux for biological productionin the Equatorial Undercurrent region at 300 W.Marine BioLogy, 55, 23-27. 1979 •

Katz. E.J. and collaborators, Zonal pressure gra~

dient along the equatorial Atlantic. Journal. ofMarins Research, 35, 293-307. 1977.

Kolesnikov. A.G •• Equalant 1 and II oceanographieatlas,' Vol. l, Physical Oceanography. UNESCO,Paris, 1973.

Kolesnikov, A.G., Equalant 1 and II oceanographieatlas, Vol. 2, Chemical and Biological Oceano­graphy, UNESCO, Paris, 1976.

Mazeika. P.A•• Mean Monthly Sea Surface Tempera­tures and Zonal Anomalies of the Tropical At­lantic. Serial Atlas of the Marine Environment.Folio 16. American Geographie Society. 1968.

Merle, J •• Seasonal heat budget in the equatorialAtlantic Ocean. Journal of PhysicaL Oceancgra.­phy, 10, 464-469, 1980.

Moore. D•• P. Hisard. J. McCreary. J. O'Brien.J. Picaut. J. Merle. J.M. Verstraete. and C.Wunsch. Equatorial adjustment in the easternAtlantic. G60physicaL Researeh Lette~s, 5,637-640. 1978.

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8- S. Lotitudf

.......~..-

'"••z·o·

.1

:-ï 11 1• 1

"'...'"

H. ~

1000

.•;ZooO

trasts observed between the equatorial areas ofthe three oceans on one hand and between theeoastal and the equatorial upwelling on the otherband.

Fig. 15. Zooplankton biomass along 40 W in July1977 (upwelling season) and April 1979 (warm sea­son).

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