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0.11. 1990/0:3drography'Committee

MEZESCALE VARIABILUY OF HYDROPIIYSlCAL PIELDS GENEIlATED

BI Z!E CANARY CUR...ttErl'T UFWELLING PROM· Tm: NAVIGATIONAL

AND SU'ELLITE OBSERVATIOn DAU

by.

A.G.Xostyenoy end Yu.V.stepanov

Atlentic Scientiric Research Institute of Mar'ne Fieheries end

Oceenogrnphy (.A.tlantllIRO), 5, Dm.Donskogo ·str.,· Kalin1ng:rad,

236000, tlSSR

. Abstract'

During the hydroPhYsical research' 01' canary ~pwellinE; in

'February 1987 on the R/V nMonocrystall" the places·ofappearence.

of mostly cold upwelling waters on the oeeen surface ware localized.

The evolution 01' the filament cr cold upwel~g water, BO km in

length, was demonstra.ted. An 1ntratermoeline eddy 35 km. in di·tl.lneter

• end 150 m in thickness was 1'0und'in the 50-200 m depth layer. The

lII9.:in ch2.rachristies 01' :Lntratermoeline end bottam' lenees, which .

were l'ound 'in the historieal dtlta, ~e presented. It 113 supposed

that the intratermocline ed:l1es are formed as a result .of geo­

atrophie adjastment oihomogeneous bottom lenses, generated by

the :Lntense m1%:lng :Ln the bottom layer cl the shelr dur1ng the

upwelling svent.

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Complex·hydrophysical end hydrobiologic~l studies o! the

canary Current upwelling, which is of great importance tor the

fisheries, showed a'considerable mezoscale variability o! the

t~mperature, aalinity,'hydrochemical and biological element !ie11s'

(Voitov, Zhurbaa, 1986; Monin et al., 1981; Fedorov, 1'98.3; Fcdo- ,~. :,.

rov; GinzbUrg~ 1988; HalpeI""\ 'et al., '1911. 'Hayer, 1916; Mittel-estacdt, 1982; TOalczakj 19'73.; Tomczak; Hughes, 1980)., As is evident

!~om the observations, the rise of the water~ to the ocean our~. .face during the upwelling event,is o!'s diocreto-continuous pat-

tern, i.e. ,c~nte.s o! rise,o! the coldest waters associated \Vith

peculiar bottom topcgraph,y a.nd shore line type are observed dthin

the narrow band 0: the coastBl zone. Tho'width o! this.band 1s

determined by a baroclinic radius of Rossby'sdeformation, and

usually does not exceed 50 km (Brüllt, 1983). ,The upwelled water

atructure in a wider zone 200-300 km,orr the shore 18 0: n patchy

pattern. In:the Canary Current upwelling, the cross secUort of

these patches varies trom 20 to 50 km.'the~ occur'~O to 200 km

apart, end are' 0.4-1°0 colderthan the temperature cf the ambient

waters (Tomozak, 1973). In seme hypotheses, the appe~~ce of

these patchea 1s partly related to breakixlg., the interntll waves eagainst the ahel! edge (Tomczak. 1973), to cyclonic eddy format­

io~ (BoWrnan et' 81•• 1983; La Violette, 1974) ~d generation of

intrathermocline anÜcycloniC eddics (ITE) in the Cana.ry Current

upwelling (Koatyanoy. Rodionov, 1986). According to Kostyanoy

and Rodionov (1986). the formation of these eddies may rcsult

Ire ~ geostrophic adaptation or homogeneous lellDcs ol. tran.'Jfol':ned.' " .

upwelled waters whlch "stream down" the ahel! into the ocean

depths Bleng 1sopicnal sur!ace. Also the generation of ITEs may

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be caused by instability of the'aubsurface front running between

the Northatlantic ani Soutbatlantic central water massives with

meandcrs und eddies normally 20 km in'e%tent'occurring near the

front (Bartoll"1987). Thermohal1ne intrusions develop along the'

·".ront spreading, aa tor as' 30 km; trom 1t. The year of 1982 was

lloted tor a ,owerful intrusion 150'm thick reaching 100 km in

lcngth and having a +0.7 salinity gradient in the forefront (Bar-. ,

ton, 1987). When broken off, these +.hick intrusive tongues may

also cause the formation of anomalously wr-rm and saline whirling

lenses as a result ol geostrop~ic adaptation.

Such a diveraity of·mesoscale at~ctures in the Canary Current

upwelling must .c,ertainlY call !orth ,a', 'local variability of the,

phyto- and zooplankton and fish aggregations•. Theretore the in-

- depth knowledge of physical processes taking p~ace in thc area

under cons1deration will ensure a h1ghe~ efficiency of the·search

of fish aggregations.

For this purpOSG the research works were carried out on the

RV "MonokristaUn (AtlantNIRO) on 2-10 February 1987 'in the area

between 16-21°N 16-18oW. The resulta of the survey were used to.

e.imate the biomase of tlQ.1n commereial 'Hah species in the area.

The saa surtaoe'temperature field eSST) was constru~ted based on

55 latitudinal tacks made at 9 km interyals with 2 km disereetness.

Besidee, 10 hydrographie lattitudinal·transeots' Ware J:ladn each

'50 km on average (a total or 58 stat10nseaeh 10-20 ~).

,Tne SST cbart dro~ trom·ihe survey data showe~ the ceAters

of the coldest waterrise (T<1?5°~) at 19°10'. 17°20', 16°55'

and 160 20'N. The~e,locations"werealso characterized by amaller

3alinity values (S~35.5) st the ses surface. A single' patch cf..

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of cold upwolled waters (T< 15°0, AT= -2°0) 25 km in diameter

was recorded at 19°40'N and'16°55'N at a'distance of 50 km.ofr, ,

the share. The SST chartsrecelved:from the'satellite NOAA-9

over the' same perlod contirmed the'existence ol thls patch at

least ove: the period trom 7 to 9 February. /

The satellite SST chatts provlded a broader coverage'(1~- ~

24°N 15-200 W) whicn allowed a:detection of a cold stream of the

upwelled waters in the area between 2JoN end 18°W, and observance

of its evolution. In the coastal upweliings,the transversal jet

systems are narrow (10-60 km) breriks of relatively cold ( AT=

= -1.5-2°0) and nutrient~rich B~face waters directed fro~ the

-shore towards the op~n ses. at distancesfrom some 'tens to '500 km

(Ginzburg, Fedotöv. 1985).~Tranaveraa~ jets can,be orten seen in

satellite images of the Oregon',Oalitornia and Benguella Current,

upwellings, arid,single cases were recorded in the Canarj ~~rent

upwelling ares. oft the Marooco .coast (J20 10 (Bulletin ••• , 198).

, A jet 50 km long o.nd 25 km wide appeare~ on th~ SST cho.rt

on J February (fig. 1). Its tronts'were indistinct and the tempera­

ture gradient was -0.8°0. BY·1 Februery, with sizes unchanged.

the fronte became more distinct and the teIIJper3.t~a gra.dient edropped to -t.6°0. 'In a day•. the jet extcnded",to 8Q km, and the

temperature gradient rea.ched the maiio~ value cf -2.400. Approxi­

mate11 on 9'February, tho jet re~axation proceas began, which

man:1.fested in jet widening tq 45 km, tront weakening ani reduot-,I '

ion of the,temperature gradient to -1.6°0. During its existence.

the Jut actually did not chanie the position h3ving just bcnded, ,

under the ~~lence of the aloDg coastal c~rent. liownere in the

upwelling regions the jet direction 'was intl'lf:nc.;ld: by the current

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(Ginzburg, Fedotov, 1985).

Lens-11ke .distribution of the temperature T, aalinity S

und density 69 fields (fig. 2) charaeteristie of intrathermoeline

entieyelonic eddie5 observed before in the Canary Current upwel-.

~ling region was.foundalo~ one of the hydrographie transects.

"""'made along 200 05'N in the 50-200 m layer (Kostyanoy, Rodionov•....,....,. .

1986). Sueh a lens was not seen along neighbouring transects

aiong 20°20' and 19°35'N. At 120-140 m depths, it contained an

isolated nucleus with the encmalously warm end saline water'w1th

T ::>,16°C (Tmax > 16.5°C) end 8::>.35.9 (Smai...35.94). In·the sa11nity.

fleld, the isolated nucleus was contoured by the 35.75 isohaline

and was 80 m thick. Isopicnal enomalities T end S were +1~50C

end +O.4% o ,respeetively.oln the density field, the isopicnals.

ware sagged up and down relative to '6~ =26.40 ·(Z=120 m) with· the

surfaea 1nelined in accordance with the riae of aeasonal ;'icno­

cline during the upwelling event, and lifte~ to the 60 m horizon

above the eheli. The le~s diameter was estiroated at 35 km.

The eddy was evide~tly at the final stage oi ita iorm~tion

aa it waS not yet completely isolated by a clearly pronouneed

~lensitY front on the shelf water aide. A similar situation was

rcported by K03tyanoy and Rodionov (1986) who assumed that such

edaies could be iormed as a result of geostrophic adaptation of

thc upwelled water lenses· which nstr'eam down" into the oeean

ueptho ao isopicnals. Separate profiles of hydrophysieal anQ

~ydrochem1cal parame~era at the ahel! stations confirmed tae fact

of intensive mixingin the near-bottom layer during the upwelling

event. This haS called forth the necessity cf analysis of histo-

~ie hydrographie data tor the aren ur..lC-:•

investigatian in o:aer

,

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to detect mixed isolnted lensea in the shel! near-bottom layer.

These pear-bottom lenses were !ound to occur quite often

both above the ahel! and outside the shel! edge. where'they break

oft the bot tom end C~~~l into anticyclonic eddies of the ITE ' '

,,;. «!g. 3)••iP'cal ,.,. .,••• ver,r~. 20 '0 40 kdhor'ZOn;ally_...l

end !rom 60 to 80 m vertically. Usually the lens 'waters are.mixed 4IIfIIIICk--act"e- "',to complete homogeneity. Ma~ ITE and near-bottom l~ns UU4.~ ~

rist1cs are g1ven in the table.

The fact that all ITE and thermostad lenses are located along

the 100 m 1sobath, which actually'ls the shelf edge. 1s of

interest. Theretore 1t 181mportant to observe the eddy lens

.evolut1on and 'conatruct actual.trajectories of their movement in

the Canary Current upwelling region.

In conclusion'it should be noted"that a large meaoscale dy_.

namie varlabl1ity of hydrophyslcalflelds caused by ~ complex

current system, discrete cont~uous ribd of,the upwelled wnters

to·the sea surface, formation of a peculiar patcby surface water

structure. formation or both sur!ace and subaurface eddies carry-

~ both signa and c! transversal jets 1s observed in the Canary

Current upwelling region. All these, mesoscale eddy formations may ~be of great importance for the !1sheries.'In some other upwelline

areas. the most'biologicallY productive regions very orten 00-

incided with locatlons contoured'by frontal boundaries of cOld

eddles end upwelled transversal eddles which yielded the'largea~

,catch o! the fiah (Bowman' et,al.,' 1983; Laurp et al., 1984; Tra­

ganza et al., 1980)~

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He!erenees

1. BartoIl E.D•.• 1987.' Meanders, eddies and intrusiOIlS iIl

tbe tbermohaline front off Northwest.Atr1ca. J. Oceanolog1ca Acta.

, 10 (~): 267-283. ."

__ 2. Barton E.D., Hue;hes P. 1982. Isolated ho:ooganeous layers

.- over the contiIlental slope off Northwest Africa. J. Deep-Sea Res.

29 (1): 13.5-14,3.

3. Bowma:o. M.J. et al. Coastal upwe1l1ng cyologenes1s and

squid fiehiIlg neer Cape Farewell, New Zealand. 1983. Ld. 'by

Gada H.G. et al. NAFO Conference aeries, Bar.IV: Marine·se1ences.

11 : 279-310.

4. Brj,nk E.H. 1983. The nea.r-:surface d;rnamica of coastal

upwel.liIlg. J. Progr. Oce8n~6l'. 12 (3)1 22;-257."

5. Bulletin mensuel SATMER. Le Centre de Meteorologie .spati­

als. Lannion. Oct. 19B3. ~o.2. p. 21.

6. Fyodorov K.li. 198,3. I'bys1eal nature and strueture of

oceanic .fronts. G1drometizdat,' Leningud. 296 pp. (In !lussian).

7. Fyodorov K.N.,·Ginzburg A.U. 1988. The near-sur!ace

.. elw:Uc' layer~ Gidrometizdat, Leningrad. 303 pp. (In Russian).

8. Ginsburg A.l., Fedotov K.N. 1985. ~ansversal jet system

i.D. insllore up"elliIlg zones: satellite date. end phys10al hypo-

I theEis. J. Earth Res. from Cosmos. No. 1: .3-10. (In Hussien).k~ 9. Halpern D. et al. Cross welt' ciroulation on the conti-

nental deli' off llorthwest Africa during upwell1ne;. 1977. J. llar.

Jes. 35 (4): 757-79C.

10. Bayer A. 1976. A oomparison of upwelliIlg events in two

locations: OreE;On and Northwest·Africa. :J. Mar. R~!J. V. ;41551-;;46.

-8-

11. Kos-c;yenoy A..G•• Rodionov V.B. 1986. On 1ntrathermoel1ne

eddy formation 'in the Cenary upwelUng. J. OeeanolOgia. 25 (6):

'892-895. <In Hussien).

12. Kostyanoy .1. __ ., Rodionov V.B. 1986. The inshore upwelling ...

zone as the souree 01' intratt:-0rmoeline eddy formation. intrather~cline eddies in the oeeen, pp. 50-55. Ed.b;y Fyodorov K.N. IOAN"""- .USSR. l.Ioscow., (In Ruf:pian):

13. La 7iolette T.E. 19740• .1 satell1ta-a1J:eraft therI:lal

S'tUdy 01' the upwelled water off Speniah Sahara. J. I'h;ys. Oeeen0Gr.

,40 (5): 679-684.1976

140. Laurs R.M. e1: al:v11baeore tuaa catch distribution rela-

tive to enviro:amental features observed !rom satellites. J. Deep­

-Sea Res. 31 (9): 1085-1093. _

15. nttelstaedt·E. 1982. The upwelling area.,off Northwest~,

·A.frica• .1 descriPtio~ 01' phenomena rnlated to eoastal upwelling~

.. ' J. Progr. Oeeanoe;r. V.12: 3r:t1-332.

'16. Uonin .1.8. et al~ 1987~: On hydrophysleaJ. mezostrueture.

01' the inshore upwelling. Rep.,o! .lead. 01' Ses. 01' US~. 297 (3):

706-710 (In Hussian) •.

17. Feters' H. 1977.Meendera' and eddles off Northwest .\i'riCc.~Folymode News. No. 25= 1, 4.

18. Tomczak M., Jr. 1973. 'An investigation into occurence and

development of cold water patcbes in the upwelJ.ing region 01';:

N. 'N. Uriea Crtosabrelten:-,;Expedltion 1970); "Me;;eor n Fors:ehun;;s­

ergebo.1sse. No • .113: 1-402•.

19. ,Tomczak M•• Hughas P. 1980. Three dimenzional variabilit;·

'or wate: lllasses end currenta in the Cana.r;y Curren.t upwelling

re.;1on. "Lleteor" ForschungserGebnisse. No. A21: 1-24.

9 -

20. Traganza E.D. e't al. 1980. Batellite observa"Cicn of '­

nutZ'1ent upwelling off the coast cf Calilornia. J. GeophJ's. lies.

85 (C?): 4101-4106.

21. Voitov V.1•• Zhurbas V.M. 1986. Complex studies of thc

Canary Current upwelling. Phy~1cal and oceanographic research in

~&OPiCal Atlant1c. pp. 112-128. Nauka. Mosco\v. (In Russian) •

•

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Figure legends

Figure 1. Satellite SST charts.

NOAA-9 for 0).02.67 (a), 07.02.87 (b). 08.02.87 (c),

09.02.87 (d). Figures indicate temperature contrasts

between isolines estimated at O.8°C.

2. Transects across anticyclonic intrathermocline eddy in

temperature (a),-salinity (b) and potential density

(c) fiel.ds.

Figure 3. Transects across near-bottom lenses in the temperature

!1eld made on 12.02.85 (a) and 29.08.86 (b) •

•

I!

I J

a)

-~~,-- -~~-----

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b)

d)

I

I

.100

~oo -----_--:...

-13-

b)

~g. 2

,I

- 14 -

100

200_~

0)

. Pig. 2

..; 15 -

a)

fig. ')

.-_2~----':~'~b~)-::- . ~. Aua.'I1'ffirro

3aK!l3 II58 203 ~. ~

'18,0

• '0