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ICES C.M. 1992/C:17Hydrography Committee

DEPENDANCE OF UPWELLING RELATED CIRCULATION ON \VIND FORCINGAND STRATIFICATION OVER TlIE PORTUGUESE NORTlIERN SHELF

Ant6nio Jorge da SilvaInstituto HidrognHico

Rua das Trinas, 49, 1296 LISBOA CODEXPORTUGAL

Abstract

Current measurements over the northern Portuguese shelf during May-September1987 are used together with coastal wind data to infer the shelf circulation associated withupwelling events. The time response of the ocean to the atmospheric forcing was found tobe dose to 1 day. A surt:'lce equatorward jet appeared at a coastal position and, as thewind event persisted, extended vertically to the bottom and consistently moved offshore,its outermost position being dependant on the event duration. Strength and persistence ofupwelling favorable winds seem to have been much more important than stratification indetermining the depth penetration of the coastal jet and the cross-shelf circulation pattern.Three-dimensional effects appeared, on the other hand, to be non negligible.

INTRODUCTION

The circulation over the continental margin off the west lberian coast is dominated, fromJune to Oetober, by theoeeurrenee of eoastal upwelling (WOOSTER, BAKUN and McLAIN,1976). During that period, upwelling favorable northerly winds dominate due to the migration'of the Azores High to the central Atlantic and the simultaneous weakening of the lcelandic Low(FIUZA, MACEDO and GUERREIRO, 1982). RAMALHO and DENTINHO (1928) hadalready notieed an up-sloping pattern of the isolines tOWards the coast during the summer hutthey considered it just as a consequence of the surface current. In a comprehensive analysis ofthe nutrient distribution along the coast and ,the wind regime incoastal stations, BOro (1945)was the first to suggest the occurrenee of upwelling off Portugal.

Using direct current measurements, FIUZA (1984) discussed the circulation over the SWPortuguese continental margin' during the summer upwelling season. The general patterncomprises an equatorward flow in the upper 50-100 m and a poleward undercurrent. No currentmeasurements were, however, available over the northern coastal margin, which is characterizedbya mueh wider shelf and a steeper slope (Figure 1)., Bottom friction should tend to be moreimportant in the northern area, and differences are to be expected in the cross-shelf distributionof the longshore flow, the vertical distribution of the subsurface compensation flow and theposition of the poleward undereurrent (HUYER, 1976; SMITH, 1981).

In this paper results of current measurements off the northem Portuguese coast duringthe summer upwelling season of 1987 are used together with coastal wind data in an attempt toinfer the shelf circulation associated with upwelling events, and to disc10se the differencesbetween circulation patterns in different stratification conditions, as weil as in quarititativelydifferent forcing situations. The data show that strength and persistence of upwelling favorablewinds tend to determine the depth penetration of the eoastal jet arid the position of the eompensa­tion current, with liHle dependance on stratifieation, in a system where three-dimensionalcharacteristics may be important.

THE DATA

As part of a project aimed at studying the conditions underlying the recruitment sueeessof the sardine off the Portuguesecoast, jointly condueted by the National Institute for FisheriesResearch, the Hydrographie Institute and the Faculty of Scienees of the University of Lisbon, : •aHne of moorings was mairitained from May to Getober 1987 off the town of Espinho,comprising 9 Aanderaa RCM4 current meters and 2 thermistor chains (Figures 1 and 2).

timing the whole study, coastal wind data were eolleeted at an Aanderaa automatieweather station a liHle to the north of Peniche, some 200 km south of the mooring line (Figure1), at an altitude of 17 m, in a weIl exposed area' with no distinet topographie features.

CfD dab were colleeted along the mooririg line froiri on board RlV Almeida CarmlllOin rriid May and during the second half of August. During the August cruisethe CfD hit thebottom, the temperature sensor v,'as damaged and the observations were conchided using NallSenbottles equipped with reversing thermometers. All in all, the mooring section was observedtwice in May (13 and 17) and ten times in August (15, and once a day from 19 to 27).

The CfD, data were processed as descnbed in JORGE DA SILVA, MARREIROS andALMEIDA (1992). In some selected files, Brunt-Väisälä frequeney values (initially corresporid­ing to \\'ater eolumns 3 dbar thiek) wei-e smoothed with 5-point rUnning' mearis in order toremove features of small vertical dimension, and tised to assess the stratifieation conditions .edtiring upwelling events (Figure 3). The NallSen cast rlata were interpol(lted to i rri intervalsusing a spline under tension, and Brunt-Väisälä frequencies were ca1culatoo in the same way aswith the CfD data, with no further smoothing.

All current meters over the mid shelf and upper slope gave back good results. TheinstrumentS at the inner mooring were replaced during Oie August cruise. The upper cui-rentmeter has failed to give any eurrent \'alues duririg the' first observational period, 'due tomalfunction. The same mooring has suffered an accident during the second period, and onlythe upper current meter has been recovered after being adrift for severnl months. Therefore,at the inner mooring only the lower (or oottom) layer \\'as observed from May to mid August,while that same layer was not obsei-ved at all from then o,n.

The time series of current and wind, collected at 30-min iritervals, were filtered toproduce hourly values, and subsequently 10w-pass filtered (C == 0.028 cph) with a third orderButtenvonlz filter that preserves 95 % of the signals with 50-hour periods and 5 % of signals \vith

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20-hour periods. Prior to low-pass filtering, the eastWard and riorthward wirid stress.. comj)Qnents ('t'c' 't'N) were calculated using the quadratic rlrag law

('t'E''t') = PaCDIVl(u,v)

where 11 is the wind vector, (u,v) are, respectively, the eastward arid northWard windcomponents, Pa is the air density (1.22 kg mo3

) arid CD is the drag coefficierit (0.0012).

12~hour means of the residual current, centered at 1 pm, were then calculated for theperiods 11 May - 6 lune and 19 August - 16 September, and used to produce "quasi synop/lc"vertical distributions of Cc:1.stward ("cross-shelf", II ) and north\vard ("longshore", v ) currentcomponents (Figures 4 and 5). The initial idCc:1. was to build distributions that could be comparedwith those of the geostrophic currerit obtained from the CfD data, out the method was extendedbeyond the periods of CfD cruises in order to encompass a larger number of upwelling events.Although the extreme current values are Ums minimized arid the time response to the windforcing becomes blurred, the mean values will reveal only the most clear features, on the eventtime scale, a procedure which may be of some advantage when seekirig general patterns.

RESuefS

One major upwelling event was observed in May, during which the wind stress waslarger than 0.06 Pa for about 7 days, reaching more than 0.10 Pa on t\\'O occasions (Figure 3).3 days of calm separated this event from periods of much weaker forcing with slight inversionsof about 1 day. The wirid was generally upweIIing favorable in lune - July, hut only two eventshave occurred with magnitude and duration comparable to the one in early May. IntermittencYwas the main characteristic of the rest of the surrimer season which, as far as the wind wasconcerned, lasted until rriid September.

Although the peaks of up\velling favorable winds in August - September had magriitudesnot much different from those in May, the areas under the wind stress curve were clearlysmaller while the relaxation periods were a IiHle loriger arid more frequent (Figure 3). Theamount of change in the static stability conditions associated with upweIIing events in May andAugust. (Figure 3) provide a good evidence of the different levels of energy transfer between theatmosphere and the ocean at the early and laie stages of the summer up\yeIIing season.

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In what concerns stratification, the main differences between May and August lie on theamourit of charige associated with the development of the upweIIing events. Iri May, the upper30-40 m became almost neutrally stable after upweIIing, while in August the Water columnremained stably stratified (Figure 3). It is worth noting that some upweIIing was alreadyoccurring on 13 May, with shoreward flow along the bottom (Figure 4) being the likely causeof the second maximum at 35 m over the inner shelf.

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Summing up the data on static stability, one might consider the water column before anupweIIing event as spIit in t\\'O layers at about 25-30 m, the stable layer being on top. After anupwelling event the whole water column at the inid and inner shelf was virttially single-Iayeredin May but still two-Iayered in August, rind this was most likely a consequence of weaker windsrather than a result of different initial stratification conditions.

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Atay - J1l11e (Figure 4), . ,

After 3-4 days of \veak S-SW winds, the current field on 11 May was characterized bya longshore riorth\\'ard flow over the \VllOle shelf arid upper slope. Maximum values were foundat mid shelf below 50 m and near the bottom at the slope mooring. The cross-shelf circuIationWas only significani at the outer shelf and especially the upper slope, where it Was shoreward.Very small seaward values were found at the inner shelf, as a possible iridication of coastalconvergence in the surface layer.

With the set up of northerly winds a south\\'ard coastal jet developed and pushed thenorthward flow offshore, while thecross-flow became shoreward at virtually all the observatiorisites. On 13 May the longshore flow was southward over the inner and mid shelf (maXimuminshore) and northward over the slope. Although this flow over the slope could be anundercurrent, there was no sigri of such feature over the shelf. Some recirculation of receritlyupwelled \\'ater might have occurred associated with the lateral shear over the outer shelf.Shoreward velocities were maxiri1lim at the iimer shelf, suggesting that the coastal jet might haveplayed a role in providing the source \vater for the upwelling through bottom friction. Over theslope the near bottom flow was sea\vard, whieh is consistent with a bottom frictioriallayer belowthe north\\'afd current.

These flo\v features basically characterize the ocean response to the upwellirig favorablewind event that lasted until 19 May. However, as soon as the northerly winds reachedsignificant values, aiendency deveioped, for the offshore migration of the coastal jet. Ori 17May it was'located at mid shelf, where the current was above 12 cm S·l, possibly larger eloserto the surface. The northWard counter-current Was then elearly an undercurrent. A seawardflow occupied the upper observecl levels of the mid arid outer shelf between the two cores ofshoreward flow. Itprobably corresponded io the bottom of the wind driven layer that mighthilVe deepened due to ci joint effect of advection and wirid mixing.

On 19 May the core of the coastal jet was displaced further offshore while an inshorecounter-current wasbarely rioticeable at the coastal mooring. The cross-shelf flow was seawardat the inshore mooring, where the whole, water column consisted of a single layer, as shown bythe profile of Brunt-Väisälä fre<iuency (Figure 3). It Was also seaward at the tipper offshorelevels, just as it Was two days before. It is, however, difficult to reconcile the remaining pietureof the crriss-shelf flow on that day with the theoretical idea of a two-dimensional upwe1ling.Orie should perhaps not exclude the possibility of a continuity of the shoreward flow betweenthe shelf edge and the upper mid shelf levels, whieh is concealed by the interpolation algorithm.In such case one would be assuming that the shoreWard flowwas continuous from the riuter tothe mid shelf, with the source \\-'Uter for upwellirig being supplied immCdiately below the (weak)pycnocIine.. AIterriatively, one inay regaf(~ the seaWard flow at mid shelf to be an internal flowreversal as the one analyzed by JOHNSON and MOOERS (1981).

The outermost position of U{e coastal jet Was reached elose to the shelf edge on 20 May.Durlng the subsequent days the wind \\'aS weak and variable, and the southward jet becameprogressively weaker. while both the irishore counter-current and the upper slope undercurrentwere intensified. This is illustrated by the 23 May situation that also shows a generallyshoreWard flow everywhere except in the bottom layer over the slope. The wind peak of 23May caused a temporary decrease of the counter-current (not shown), but it lasted just one dayafter which the previous tendency was resumcd.

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The wind· shifted to southwestcrly on 25 May. Thc southward jet was eroded andrcstrictcd to the upper layers while the undercurrent progressively occupied shallower levels.The current field situation on 26 May v.'as Iikely to approach that of 11 May, inclllding thesuggestion of coastal convergence.

The characteristics of the wind field on 26 May - 3 lune were almost a rcducedduplicaiion of those on 10-20 May. Somewhat expectedly, the current field on 28 May issimilar to that on 13 May. Probably due to the weaker forcirig, the longshore cllrrent valueswere smaller and the cross-shelf flow over the inner and mid shelfat the observation depths wasnegligible. It is possible that the cross-shelf circulation was takirig place at shallo\ver levels,with the onshore flow just beneath a thin surface offshore moving layer.

The wind remairied iHJrtherly imtil 2 lune and the soutl1\\'ard jet \\'as displaced seaoord;just as it happened during 15-19 May. On 1 lune there \\'aS no evidence of deepening of thesurface layer at the offshore side of the jet, contrary to what oos observed on 17 May. In fact,the largest onshore current values were found at the shallowest obserired levels. The mostimmediate explanation for this is that weaker winds directly forced a shallower layer, and theseav.md surface motion did riot reach as far out as in earlier May.

August - September (Figure 5)

The t1rst wind peak in August followed· three days of very weak northerly winds. Thecurrent field on 19 August is a good example of a situation of weak upwelling: (i) a surfacelayer (25-50 m) moving offshore, (ii) a southward coastal jet restricted to the surface layer, withits core at the innermost 20 km, (iii) a pole\\'ard undercurrent restrictOO to the slope region (atmost, affecting the outer shelf) with the corc at the shelf break level, and (iv) a minor shoreoordflow beneath the upper layer, with largest values off the shelf break.

Following the wind decline on 19-20 August, the surface motion completely reversed andit is tempting to suggest that the small northerly wind peak on 18 August acted as an impulsethat set up an oscillation in the ocean. On the other hand, since the northerly wirid event Wasrather weak arid short-lasting, one might have observed what usually occurs at a very early stageof an upwelling event. Actually, the CfD data collected duririg the same period only revealvertical displacemerit of isopycnals at the innermost 6-7 km. Therefore, if there \\'aS any areawhere the coastal jet affected the whole \\'ater column, it \\'as certainly inshore of the moorings.

The wind event that peaked ori 22 August was much stronger and longer-lasting, which\\'aS reflected in the current field until 28 August. As iri May, it was possible to observe theintensificaiion of the coastal jet, followOO by its seaward displacement. However, unlike inMay, the core of the jet did not reach the outer shelf, probably as a consequence of the shorterduration and/or weaker intensity of the August wind event. An inshorc counter-current startedto develop on 25 August and intensified the day after, but was no longer observed on 27 Augustprobably duc to the northerly wind peak on that day. If this interpretation is correct, then theocean response to the wind forcing occurred in less than 1 day.

As in May, the pole\\'ard undercurrent was restricted to the siope area, .Dr at most to theouter shelf, the largest velocities being fouod at the deeper instrument (12.2 cm S·l on 23August), but it appeared to be displacOO dowri\\'ards as the coastal jet moved offshore.Contrasting with the May situation, .the wind driven layer appearOO to have been mostly

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restricted to the upper 30 m, or less. 'fhe innermost (and shaIiowest) current meter 'was thusthe main instrument to detect the cross-shelf seaWard motion~

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Similarly to May, the shoreward subsurface flow was only significant at the outer shelf,but the depth distribution of the observations does not aIlow any conjecture concerning the roleof recirculation in the feeding process ofthe source area of the upwelling. Aetually, the pieturesof the current field from 23 to 27 August seem to favor a three-dimensional proeess.

On 28 August the northerly wind declined, the erosion of the sOllthward jet began, thepoleward undereurrent advaneed onto the mid-shelf and a generaIly shoreward flow wasestablished over the shelf. Associated with the .weak southwesterly wind that persisted until 1September, the motion beeame northward at aIl observational points. The shoreward motion hadmaximum values both off the shelf edge and at the inner shelf eurrent meter, suggesiirig that,over the shelf, most of this motion took plaee immediately below the pyenoeline. H is diffieultto aseribe a elear meaning to the very small westward motion observed at mid-shelf belDw thepyenoeline on 1 September, but it inight have been related to the vertical shear in the longshorecurrent component, in a similar way to what seemed to happeri on 11 May.

,\ •Though the wind beeame northerly 1>y the end of 1 September, the only notieeablechange at the observation points until 3 September was the erosiOil of the northward flow overthe shelf. The presenee of the sDuthward coastal jet was only deteeted by the moorediristruments on 4 September (13 em S·l at the inner shelf). On that day the north\vard flow inthe upper slope region became again an tiridercurrent. Hs maximum velocity \vas found at theshelf break level, which contrasts with the observation on 23-27 August. Apparently, thevertieal momeritum transfer was riot enough to greatly affect the motion at those levels.Horizontal transfer associated with the offshDre migration of the coastal jet after 7 Septemberappeared to have been mueh more effeetive.

The pieture of the current field on 10 September suggests the presenee of the eoastal jetat the outer shelf and shows the poleward undereurrent at the upper slope, its velocity inereasingdownwards. Maximum onshore flow was found elose to the shelf edge and a bottom frictionallayer was refleeted in the sea\\'ard velocities elose to the bottom. Until 16 September the eoastaljet remained offshore, mainly confined to the upper 50 m, the eurrent field being similar to whatwas observed on 26 May. The longshore wind component then ehanged sign again, giving rise eto a new flow reversal over the shelf.

DISCUSSION

The Portuguese northern coast is situated in a latitude zone not mueh different from thatof Oregon. The shelf width is rather similar to that off Cabo Corveiro (with a slightly deepershelf edge), but the stratifieation is about 5-fold the values given by BARTON; HUYER andSMITH (1977) for NW Africa. The summer upwelling season of 1987 was rather untypical,being almost restricted to May - June (VITORINO, 1989), with maximum wind stress valuesbetween the averages given in SMITH (1981) for Oregori and NW Afriea.

As in most upwelling system's (HUYER, 1976; SMITH; i981), the eireulation patternindueed by wind favorable events was charaeterized by longshore shelf eurrents stronger. than

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the cross-shelf flow. An equatorWard jet developed at the coast within one day of the onset ofthe wind, in agreement with the accepted idea that the appropriate time scale üf the pheriomenonis the local inertial period (18.2 h). The cross-shelf flow Was seaWard in the surface layer, witha shoreward compensation flow at some intermediate depth. ,As off NW Africa, the equatorwardrriotion exterided vertically down to the bottom (SMITH, 1981), and a poleward undercurreritwas preserit as a feature of the slope regime rather than of the shelf (HUYER, 1976).

, Relevant parameters for the northern Portuguese shelf in the surrimer of 1987 may beobtairied, following SMITH (1981), from:

Internal, Rossby radiusEkman Iayer thickness (surface ör bottorri, depending on u.)friction velocity - surface layer ('t y longshore wind stress)friction velocity - bottom layer (v

w

velocity outside the layer)I

Thldng the charncteristic depth H to be the thickness of the upper stable layer, using the• average Brunt-Väisälä frequency value iri that layer (Figure 3), and aConolis parameter of.. 9xlO"s s"t,we obtain 7 km for the Rossby radius., It is interesting that this figure supports the

idcil that the situation on 19 August corresponded to a very early stage of upwelling.

. ,If we use an average wind stress of 0.07 Pa, fitting the early May conditions (Figure 3),arid the stratification values for the situation before upwelIing, we obtain 10-11 rri for thethickness of the surface Ekman hiyer. For the other events an average wind stress of 0.04 Pa,with corresponding stratificationvalues, leads to 6-8 in. Iri the boUom layer the stratificationis one order of magnitude smaller. For an interior flow of 10 cm S"l and the straiificationconditions given in Figure 3, the thickness of the bottom Ekman layer will be 6 m, doublirigwith double velocity. In these circumstaIlces the two frictional iayers should be separate, evenat the inner mooring site, as suggested by the cun-ent field on 19 August (Figure 5)~

. As an upwellirig event develops, however, the stratification changes (Figure 3).. Calculating the surface layer thickness with the conditions on 17 May (after upwelIing), weobtain 18 m, opening the possibility for a frictionally dominated flow at the inner rriooring, assuggested by the distributions in Figure 4. According to WINANT (1980), for quasi-steadywind forcing, which isa reasonable approximation in the period 12-18 May, the momentumbalarice should indeed be frictiorial; while for shorter period forcirig, as in 17-19 August, theacceleratiori terms should remain important.

In an analysis of the data series from the t\\'O Oliter mooririgs, ViTDRINO (1989) foundrriaximum mean monthly values of the shoreward flow both at the shelf edge level and in theupper current meter at mid shelf, arid suggested that the shoreward compensation flow waslocated immediately below nie surface Ekman layer. On the event time scale, ho\vever, thepresent analysis p.oin~s to one main core of shoreward flow at shelf edge level, basically as inN\V Africa (BARfON et aI., 1977), although indications of shoreward flow just below thesurface layer were occasiorially obtained. A coritinuOlis cross-shelf flow was only c1ear duringrelaXation penods, suggesting that upwelling developed as a three-dimensional process, assupported by. satellite images thai ofteri reveal the presence of an i'upwelling center" betweenOporto arid the norther border of Portugal.

At the beginning of the upwelling season, when the ,\rind stress values wei-e mostsignificant, a secondary core of shoreWard flow was preserit at the inner shelf near the bottom,

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probably due to friction caused by the equatorward jet. Since no data were obtained in thebottom layer at the inner shelf during the second mooring period, it remains unclear whethersuch feature is more or less permanent, or simply related with periods of high wind stress.

According to SMITH (1981), wide sheives with low stratification and high longshorewinds should have circula;tion patterns like those off NW Africa, the ratio ll. IR! being thecriterion that determines the flow characteristics. Using wind stress values for early May, weobtain a ratio of 0.9 at 100 m and 2.3 at 40 m. For the other periods ~he corresponding valuesare 0.7 and 1.8. According to SMITH and LONG (1976) (cited in SMITH, 1981), when thisratio becomes > 2 the rotational effects diminish. At much higher values, the bottom stresscompletely dominates the Coriolis term in the longshore momentum balance and all the flow isthe direction of the wind. The upwelling circulation pattern breaks down and separates fromthe coast. This has been observed off NW Africa (BARfON et al., 1977), and the present datapoint in the same direction.

Ackllowledgemellts. This work was partly supported by Junta Naeional de Investiga~äoCientffica e Tecnol6gica,under research eontract. •

REFERENCES

BARTON, E.D., A. IIUYER and R.L. SMITH (1977) Temporal vanation observed in the hydrographie regimenear Cabo Corveiro in the northwest Afriean upwelling region, February to April 1974.. Deep-SeaResearch, 24, 7-23.

BÜTO, R.G. (1945) Contribui~äo para os estudos de oceanografia ao longo da costa de Portugal - fosfatos enitratos. Tramux de la Station de Biologie Maritime de Lisbolllle, 49, 102 pp + 3 fig (in Portuguese).

BRINK, K.II. (1985) Some aspects of physical processes iri coastal upwelling. Proeeedings of the InternationalSymposium on Upwelling in West Africa, Vol. 1, Instituto de Investigaciones Pesqueras, Barcelona, 5-14.

FIOZA, A.EG. (1984) Hidrologia e dinämiea das aguas eosteiras de Portugal. Doctoral Thesis, University ofLisbon, 294 pp (in Portuguese). .

FIOZA, A.EG. M.E. MACEDO and M.R. GUERREIRO (1982) Climatological space and time variation of thePortuguese coastal upwelling. Oceallologica acta, 5,31-40.

HUYER, A. (1976) A eomparison of upwelling events iri two locations: Oregon and Northwest Africa. Journal. 0/Marille Research, 34,531-546.· . . e

JOIINSON, D.R.and C.N.K. MOOERS (1981) Internat eross-shelf flow reversals during coastal upwelling. inCoastal Upwelling, EA. Richanls (ed.), Coastal alld Estuarille Sciellces, ''<lI. 1, American GeophysicalUnion, 72-78.

JORGE DA SILVA, A., M. MARREIROS and S.L. ALMEIDA (1992) Processamento de dados de CfD. Allaisdo Instituto /lidrograjico (in Portuguese) (in press).

RAMALHO, A. and L. DENTINHO (1928) Notas sobre as eondi\löes oceanograficas ao largo da eosta dePortugal. lllli'aia de la Stati01; de Biologie Maritime de Lisbolllle, 15, 15 pp ·(in Portuguese).

SMITII, 1.0., arid C.E. LONG (1976) The effect of ttirning iri the bOttom bOundary layer on continental shelfsediment transport. Memoires de la Societe Rayale des Sciellces de Liege, 6hlle serie, 10, 369-396.

SMITH, R.L. (1981) Circulation Patterns in upwelling regimes. Pg 13-35 in E. SUESS and J. THIEDE (ed)Coastal upwelling, its sediment reconl, Part A: ResPonses of the sedimentary regime to present coasttll

. upwelling, Plenum Press, New York, 603 pp.VITORINO, J.P.N. (1989) Circula~äo residual ao largo da eosta NW de Portugal durante a esta\läo de atloramento

de 1987. Anais do Illstituto llidrograjico, 10,25-37.WINANT, C.D. (1980) Coastal eirculation and wind-induced eurrents. Annual Reli~v 0/ Fluid Me~'lallics, 12,

271-301.WOOSTER, \V,S., A. BAKUN and D.R. McLAIN (1976) The seasonal up'Nelling cycle along the eastern

boundary of the North Atlantic. Journal 0/Marille Research, 34, 131-141.

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.. -' ~

"'!I

fI

',V

....>-

40 ::!....a .. ...., . .~

36

40

12 10LONGlTUDE (11)

8

Figure 1. Left: Bathymetrie chart of the Iberian west coast (isobaths in meters). The star indicates theautomatie weather station. Right: detail of the study region with the mooring positions.

Distance to coastline (km)60 40 20

71

100

E-..c:-'Cl.~

CI200

ESPINHO

9IJ

May - Nov 1987

38

119

171

282

'------"------------------'-300

Figure 2. Scheme of moorings. Numbers indicate current meter depths.

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10 15 20 2S ~O

May - June 1987

-. -.-. ~-4

,0

-e ~-e

~

-e _a

-10 -10

-1' -1Z20 2~ ~ 5 10

August - Seplember 1987

-.-f

-e

-e

-'0

-I'U

• ~ (~In--•

.t---~--~----~---''.

INNER SHELF

-- 13 May---_. 17 M"y

INNER SHELF

-- I:> Aueust'.-- •• 23 Augu~t

..t-~-~--~-~~-~~

MIDDLE SHELF

-- 13 Mey._--_. 17 May

MIDDLE SHELF

-- 15 August----- 23 August

Figure 3. Coastal wind stress (upper panel) and squared Brunt-Väisälä frequency (lower panel) in spring(left) and summer (right). Dotted lines correspond to conditions after upwelling. Inner and middle shelf

refer to ca. 15 km and 30 km from the coast, dose to the inner and mid shelf moorings, respectively.

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'00

!....1-...1 JUN 1987

U(cm ._t)

..~~~.:::-.~--=.."'-~-!. :>000Wt.e.a.C'C .. ~h_ (bIl)

'00

~tl

roo

.. ..D1....._ ... _eUt._ (an)

,00

~~

1:...~~~.o;;-~- ..=-~-!.o :>00

DWtaaC'e ta __uu.. (~)

Figure 4. Coastal wind stress and current field in May - June 1987. Values towards Wand S are shaded.

11

'00

:>00.0 ..

Ihat.ac. .. _.u••c (Iunl

...U(cm .-',

~~: '00 ....'" ! ]"'\ 271.UG 1887 ~ ~

! ;U(cm .... ) ... V(cm .... ) ...

- :>00.. zu .. ..~ .. -..tU- (haI ~._ .. _eUi•• (tna)

.~~:

~) 24. AUe I1l87

!:'---7.••:-----::::--~ .........1K'"4' .. e-t ("_I

-.

(kz::v~~2'0 Z6 30 S •• 15

Aucusl - ScpU:tnber J90-;

'00

!~

iV (cm .... ) ...

!:'----::••:-----:..:::-~--!. :>00

o..tA.~ to C'O&eU••e (km.

'00

~---.7..:-~- ..:;;----!.. _.o..ta_ ... -..ll_ (hal

'00

V (ein .-')

~",-"""

*''''j '8 I.UG 1887

~--~••;;-~--::..;;-~--i ,..lh.taaC'le .. coa_lh...."In)

'00

.. ..Oata_ .. __Uiac (klO)

.00 '00

! !~ ;.! ;;

U (ern .-t) ... V (cm .-t)

- -<Cl zo • • 0 .. •DWta_ .. c-'.It_ C-'l Diftaa_ .. e-.u... (kwli

~ ....... 'J'00 ~G~ '00

] !~ '\" 21 .Aue 1987

~

21 Aue 1887 ! ..:. ;,.U (ern .-1) V (ein .-') ...

:>00.. .. zo .. ..o.t.___ e.-utDe (hol Oi.taacoe .. e-etb_ (1r.rn.

'00 l'''! '00 '00

! ! !~ ~ i23 AUe 1987 ! 23 .Aue 1987 I !

U (ern .--) V (CIQ .... ) U(c:m ....) V (cm .-1)

.- :>00 .- .-•• zo • 0 .. ... .. .. ..~ .. --u.... (~) ~ ... _.w._ (laD)

~ .. --w-(ba) JN.ta_ .. _.ua_ (~J

Figure 5. Coastal wind stress and current field in August - September 1987. Values towards W and S are shaded.

.,....~ .

12