Wind induced variability of hydrographic features and water masses distribution in the Gulf of Cadiz (SW Iberia) from in situ data

s 63 (2006) 130–140www.elsevier.com/locate/jmarsys

Journal of Marine System

Wind induced variability of hydrographic features and water massesdistribution in the Gulf of Cadiz (SW Iberia) from in situ data

F. Criado-Aldeanueva ⁎, J. García-Lafuente, J.M. Vargas, J. Del Río,A. Sánchez, J. Delgado, J.C. Sánchez

Departamento de Física Aplicada II, Universidad de Málaga, Málaga, Spain

Received 6 April 2006; received in revised form 19 June 2006; accepted 21 June 2006Available online 8 August 2006

Abstract

In May–June 2001, the GOLFO 2001 survey was carried out in the Gulf of Cadiz area (South-West Iberia). The surveyconsisted of three legs that were accomplished under different wind regimes. In Mesoscale 1 (under westerlies) North AtlanticCentral Water was located in the northern region and Surface Atlantic Water in the outer part and this distribution maintained forthe entire upper water column, with the only exception of some traces of Shelf Water in the surface layer closest to the Guadalquivirriver mouth. In Mesoscale 2 (under easterlies), Shelf Water was advected westwards by the coastal countercurrent that developedunder this regime, thus flooding the surface layer of the eastern (and partly the western) continental shelf and restricting theupwelled North Atlantic Central Water to a smaller region west of 8°W. This pattern only affects the upper 20–25 m of the watercolumn, then recovering the common distribution with North Atlantic Central Water in the northern area and Surface AtlanticWater in the outer part of the basin. The wind induced variability modified the surface TS diagrams and these variations have beenused to further investigate the water masses distribution in terms of the TS properties of three “standard” points defined on the TSdiagram.© 2006 Elsevier B.V. All rights reserved.

Keywords: Wind-driven circulation; Meteorological forcing; Water masses; TS diagrams; Surface layer; Gulf of Cadiz (South-West Iberia)

1. Introduction

TheGulf of Cadiz is the sub-basin of the NorthAtlanticthat connects the Atlantic Ocean and the MediterraneanSea through the Strait of Gibraltar. The most outstandinggeographical features are Cape St Maria (CSM), Cape StVincent (CSV) and Cape Trafalgar, CT (see Fig. 1 upper).

⁎ Corresponding author. Dpto. Física Aplicada II, E.T.S.I. Informá-tica, Campus de Teatinos, Universidad de Málaga, 29071 Málaga,Spain. Tel.: +34 952 132849; fax: +34 952 131450.

E-mail address: [email protected](F. Criado-Aldeanueva).

0924-7963/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2006.06.005

CSM divides the continental shelf into two halves. Tothe east of the Cape, the continental shelf is wide (some30–50 km) and has a gentle slope, whereas to the west it isnarrow (b15 m) and its bottom is dotted with submarinecanyons — Faro, Lagos, Portimao, St Vincent — (seeFig. 1 upper, again).

The main part of the studies carried out in the Gulf ofCadiz have focused on the Mediterranean Water outflowthrough the Strait of Gibraltar (Zenk, 1970; Bryden andStommel, 1982; Zenk and Armi, 1990; Ochoa and Bray,1991; Baringer and Price, 1999; Ambar et al., 2002) andits subsequent mixing with Atlantic waters. Moreover,the formation of eddies of Mediterranean origin, the so-

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Fig. 1. Upper panel: Map of the Gulf of Cadiz showing the position of locations and other geographical features mentioned in the text. CT, CSM andCSV stand for Cape Trafalgar, Cape St. Maria and Cape St. Vincent, respectively. Grm, Orm, Trm and Gqrm stand for the mouths of Guadiana, Odiel,Tinto and Guadalquivir rivers, respectively. The star marks the position of the Red de Aguas Profundas (RAP) oceanographic buoy mentioned in thetext. Lower panels show the station grid of the Mesoscale 1 and Mesoscale 2 legs of GOLFO 2001 survey.

131F. Criado-Aldeanueva et al. / Journal of Marine Systems 63 (2006) 130–140

called meddies, has also been matter of special interest(Serra and Ambar, 2002). The surface circulation of theGulf of Cadiz is integrated into the general circulation ofthe Northeast Atlantic: the Azores current, whichtransports some 15 Sv between latitudes 35°N–40°Nto feed the Canary Current, frequently forms meandersthat separate themselves from the main flow (Alveset al., 2002). The surface circulation of the Gulf of Cadizcould be understood as the last meander of the Azorescurrent that, as entering through the Strait of Gibraltar,counters the outflow of Mediterranean Water.

Most of the studies on the surface circulation deal withremotely sensed Sea Surface Temperature (SST) imagesand climatological data. Stevenson (1977), combining insitu and SST satellite observations, identified an interest-ing thermal feature formed by a sequence of warm–cold–

warm waters in NW–SE direction between Cadiz andHuelva (the so-called ‘Huelva Front’). Fiúza (1983), usingwind data and SST images corresponding to the summerof 1979, correlated the occurrence of upwelling off thesouthwest coast of Iberia (and the appearance of theHuelva Front) with westerlies and the development of awarm coastal countercurrent stretching east–west witheasterlies (Fiúza et al., 1982; Fiúza, 1983). Folkard et al.(1997) analysed infrared SST satellite images throughoutthe year between July 1989 and March 1990 and reporteda bimodal pattern in SST images related to wind regime inthe summermonths.Westerlies enhance the upwelling offCSVand promote its extension to the east. Additionally, asecond core of upwelling appears off CSM that eventuallymerges with the one of CSV. Easterlies restricts theupwelling off CSV to a smaller area while, at the same

132 F. Criado-Aldeanueva et al. / Journal of Marine Systems 63 (2006) 130–140

time, a warm coastal countercurrent develops from east towest and displaces the upwelled waters offshore (García-Lafuente et al., in press).

Relvas and Barton (2002) used a combined set ofsatellite data (1200 images between 1981 and 1995),coastal meteorological (wind, atmospheric pressure) andsea level observations at strategic points to study differentareas of interest: the upwelling off the western Portuguesecoast, the upwelling off the south coast of the IberianPeninsula (Gulf of Cadiz), the filament off Cape StVincent and the warm coastal countercurrent, all of themin relation tometeorological patterns. Sánchez and Relvas(2003) have analysed databases of hydrographic stationscontaining data for the whole 20th (1900–1998) corre-sponding to spring and summer on the southwest coast ofthe Iberian Peninsula and have depicted the climaticpatterns of circulation in the Gulf of Cadiz during theseseasons. The surface circulation in the Gulf of Cadiz ispredominantly anticyclonic with some mesoscale mean-ders. Off Cape St Maria, the main current turns to thesouth and then to the north and continues flowing parallelto the Spanish coast. Finally, the branch that separatesfrom the Strait of Gibraltar forms an anticyclonicmeanderin the easternmost region to feed the Canary Current(Sánchez and Relvas, 2003; Criado-Aldeanueva et al.,in press). This pattern is compatible with long current-meter observations collected by the Red de AguasProfundas (RAP) network of Puertos del Estado, Spain(see Fig. 1 for location), although these records also reportnorthwestwards velocities in wintertime. Some furtherresearch on wintertime circulation and its correlation withlarge scale and local wind forcing have been also recentlyaccomplished by Sánchez et al. (in press).

Vargas et al. (2003), analysing an 8-year series of SSTimages have showed that the spatial pattern of the firstempirical mode (which explains more than 60% ofvariance) indicates accumulation of warm light water inthe middle of the basin, compatible with anticyclonicgeostrophic circulation. But the time coefficients of the

Table 1Characteristics of the three legs of the GOLFO 2001 survey

Mesoscale 1

Sample dates 14/05/01–16/05/01No of transects 7Longitude range 6.92°W–8.18°WLatitude range 36.58°N–37.08°NInclination _Origin coordinates (8.30°W–36.54°N)Number of points 12×15

Inclination refers to the angle of deviation from the meridians. The last twoSection 2).

mode showed important seasonal variability with mini-mum values in winter which weaken the horizontalthermal gradient and may cause a reversal in the circu-lation during this season. Sub-surface temperaturemeasurements taken in situ during previous surveyscorrelate reasonably well with satellite surface patternsand also show variability. Cruises carried out in 1995 and1997 during the summer period showed differentoceanographic patterns in the basin: in 1995, a pool ofwarm water near to the Guadalquivir River mouth wasobserved, whereas in 1997 a cold water front wasregistered in the same area (Rubín et al., 1997, 1999;García et al., 2002).

The GOLFO 2001 cruise provided a great amount ofin situ data and more research has been recently carriedout both on physical and biological topics. García-Lafuente et al. (in press) and Criado-Aldeanueva et al.(in press) depict the water mass circulation pattern on thecontinental shelf and the outer region of the Gulf of Cadizand describe some mesoscale features like the filament ofCSM. The variability of the surface currents andgeostrophic transports is discussed as well as somemodifications in the continental shelf patterns due to windinduced variability (García-Lafuente et al., in press) but,except for the continental shelf, no attention is paid to thevariability of the 3-D thermohaline patterns and watermasses distribution. Physical–biological coupling studieshave identified different phytoplankton populations inconnection with the variability in the water masses spatialdistribution pattern (Reul et al., in press; Navarro et al.,in press) and claim for further knowledge on this topic, towhich is mainly devoted this paper.

The work is organised as follows: Section 2 presentsthe data and methodology; Section 3 discusses thevariability observed both in the TS diagrams, in the 3-Dfields and in the water masses distribution. To performthe latter, a technique based on the selection of threereference points in the TS diagram, is presented. Finally,Section 4 summarises the conclusions.

Macroscale Mesoscale 2

17/05/01–24/05/01 29/05/01–02/06/0112 116.27°W–9.28°W 6.51°W–8.59°W36.01°N–37.13°N 36.57°N–37.08°N15° _(9.20°W–36.00°N) (8.70°W–36.54°N)22×31 12×23

rows are related to the optimal interpolation technique parameters (see


2. Data acquisition and methodology

The interdisciplinary surveyGOLFO 2001was carriedout in the waters of the Gulf of Cadiz between 14May and3 June 2001 onboard the oceanographic Research VesselHespérides, within the framework of the project MAR99-0643 ‘Distribution and Dynamics of Plankton and Sestonin the Gulf of Cadiz: Variability Scales and Control byPhysical and Biological Processes’. The survey was di-vided into three legs, called Mesoscale 1, Macroscale andMesoscale 2, whose characteristics are summarised inTable 1 and whose geographical scope can be seen inFig. 1 (lower). More details of the station spacing,profiling time or data processing can be found in Criado-Aldeanueva (2004).

The data set includes hydrological data (Conductivity–Temperature–Depth, CTD Idronaut MK 137 with nephe-lometer and fluorometer SeaPoint and thermosalinogra-pher SEABIRD SBE 21 for continuous recording ofsurface data), Acoustic Doppler Current Profiler (ADCP)velocity data (ADCP RDI VM150-NB) and biologicaldata (Rosette General Oceanics 1015 of 24 bottles). Ad-ditionally, air pressure and wind velocity was recorded bythe RAP oceanographic buoy of the Gulf of Cadiz. Due toits offshore location, these data were considered as repre-sentative of the mean wind and pressure fields over theGulf of Cadiz. Remote sensing SST data were obtainedfrom the Deutshes Zentrum fur Luft und Raumfahrt(DLR) through the public access gateway. Daily SSTmaps were composed using up to seven different NOAAsatellite acquisitions through Advanced Very High Reso-lution Radiometers (AVHRR) onboard the NOAA. Thespatial resolution is 1.1 km at the nadir. However, cloudyweather prevented the acquisition of good quality imagesduring most of Mesoscale 1.

Following the directions of UNESCO (1985) thequantity γt, obtained from the temperature and salinity

Fig. 2. Stick diagram of the wind regime from the RAP buoy (see Fig. 1 for lindicated. Westerlies prevailed during Mesoscale 1 and the previous days; Mmore frequent and intense. Mesoscale 2 was influenced by the moderate to

fields through the state equation, is used to express thedensity field. The use of γt instead of the potential densityγθ is appropriate due to the shallow depth (b1000 m) ofthe sampled stations. The interpolation of the hydrologicaldata has been carried out by means of the optimal orstatistical interpolation technique, OI henceforth. Thismethod, widely presented in the literature (Gandin, 1963;Thiébaux and Pedder, 1987; Ruiz Valero, 2000; Gomiset al., 2001), is based on the condition that the differencesbetween real field values and the results of the analysis areminimised statistically. The OI technique requires theadjustment of several parameters: for the tendency's de-gree of the polynomial, as low values are recommended(Thiébaux and Pedder, 1987), n=2 has been used. Thespatial scale correlation has been established as 20 km,and the noise–signal ratio as 0.001 for CTD data (seeCriado-Aldeanueva et al., in press for details). The samespatial grid step has been chosen for the three legs: 0.1° incoordinate x (longitude) and 0.0548° in coordinate y(latitude). The difference lies in the origin of the grid and,consequently, in the number of grid points. The values ofthese parameters for each leg are shown in Table 1.

3. Results and discussion

The wind regime drastically changed from westerliesduring Mesoscale 1 and part of the Macroscale toeasterlies during Mesoscale 2 (see Fig. 2). This changehas made it possible to study the response of the watermasses distribution and 3D fields to this forcing. Due toits rather different wind regime, comparison will bemainly established between the two mesoscale sam-plings. In the Macroscale leg, widely discussed inCriado-Aldeanueva et al. (in press), westerlies weredominant but some of the oceanographic features relatedto this regime weaken and lead to those of easterlies. Inthis sense, it can be considered as a typical intermediate

ocation) during May–June 2001. Time period of each survey has beenacroscale was sampled under both regimes although westerlies were

strong and rather persistent episode of easterlies.


situation between the two mesoscales (Criado-Aldea-nueva et al., in press).

3.1. Variability of the TS diagrams

Strictly speaking, a water mass is defined by its tem-perature and salinity taken as conservative parameters onlyaltered bymixing. Following this requirement, some of thewater classes described below (SAW, SW) are not a watermass in strict sense, although, by extension, in some casesall of them will be referred as water masses in general.Fig. 3 illustrates the TS diagrams obtained in the twomesoscale legs and different water masses were identified.Mainly, North Atlantic Central Water (NACW), charac-terised by a linear relation in the TS diagram (11.0 °C≤T≤17.0 °C; 35.6≤S≤36.5) and bounded by isopycnals26.6 kgm−3≤γt≤27.3 kgm−3, was found in both cruises(see Fig. 3). Below a certain depth, that is, from a certainisopycnal (27.1 kg m−3≤γt≤27.3 kg m−3 depending onthe location), the TS diagram diverges from its formerlinear behaviour due to mixing with the underlying, saltyMediterranean Water (MW), that presents two mainmaxima in the TS diagram corresponding to the classicaltwo cores (upper and lower) with different densities,γt≈27.5 kg/m3 and γt≈27.8 kg/m3 (see Fig. 3), widelyreported in the literature (Madelain, 1970; Zenk, 1970;Ambar and Howe, 1979a,b; Ambar et al., 2002).Comparison of Fig. 3a and b evidences that the previousdescription is rather permanent within the two legs, thusshowing (as expected) that NACW and MW are notsensitive to short time variations in the local wind regime.

Air–sea interactions modify the upper layers of thewater column and NACW becomes Surface AtlanticWater (SAW) characterised by TN16.0 °C, S≈36.4 and

Fig. 3. Temperature–Salinity (TS) diagrams of all the stations of Mesoscale 1NACW, SAW, SW and MW are the acronyms for North Atlantic Central Wrespectively. The labels are roughly located at the theoretical positions of thsuperimposed for clarity.

γt≤26.7 kg m− 3 (see Fig. 3). Recently, Criado-Aldeanueva et al. (in press) have described Warm ShelfWaters (SW) over the eastern continental shelf. Thesewaters are warmer and fresher than SAWand correspondto the points of the TS diagram with TN16.0 °C and35.9≤S≤36.5 that are placed outside the line of NACW.Contrary to NACW and MW, SAW and SW suffernoticeably short-term, wind induced variability due to itsshallower depth. According to García-Lafuente et al.(in press), the presence of localised spots of water warmerthan the average requires the existence of a heat supply tomaintain the surface thermal signature against heat ad-vection and diffusion. Off Guadalquivir river mouth andCadiz embayment, the heat source would be, apparently,the land. In this area, M2 amplitude is higher than 1 m andthe tide progresses inland through the different arms of theGuadalquivir river (similar happens in the neighbourhoodof Cadiz embayment) flooding a few squared kilometresof marshes. Some concomitant mechanisms but, special-ly, the flooding of marshes that have been heated by sunradiation during the previous low tide lead to a greaterabsorption of energy per mass unit that is brought back tothe sea during ebb tide (García-Lafuente et al., in press).The accumulation ofwarmer (and, hence, lighter) water inthis part of the shelf creates an east towest along-shore seasurface slope that, if not baroclinically compensated,produces a pressure gradient in the interior. Westerlies(Mesoscale 1) increase this sea level slope as they pile upwater against the coast but wind drag cancels the excess ofsea level gradient force, thus preventing a surface counter-flow to the west. This wind set up disappeared in Meso-scale 2 (under easterlies) and the warm water pool was nolonger retained near Guadalquivir river mouth but it wasreleased, invading the surface layer of the eastern (and

(panel a) and Mesoscale 2 (panel b) legs during GOLFO 2001 survey.ater, Surface Atlantic Water, Shelf Water and Mediterranean Water,ese water masses on the TS diagram. Some γt isopycnals have been

Fig. 4. Temperature (°C, panels a, d), salinity (panels b, e) and density (γt, kg·m−3, panels c, f) at 10 m depth from Mesoscale 1 (upper panels) and

Mesoscale 2 (lower panels) data. Notice the high variability detected both in the temperature and density fields.


partly western) continental shelf. Its fluvial influencewould also explain the lower salinity of SWin comparisonwith SAW (García-Lafuente et al., in press). For thisreason, in Mesoscale 2, surface TS points have highertemperature than in Mesoscale 1 (compare Tb18 °C inFig. 3a with T above 20 °C in Fig. 3b) and most ofthem fall within the region of SW and SAW. Aboveγt≈27.0 kg m−3, NACW suffers influence of SW. Thisfact along with reciprocal influence between SAW andSW confers the upper TS diagram a typical triangle shapethat will be used in Section 3.3 for further discussion.

3.2. Variability of the 3D fields

The temperature field at 10 m shows significant dif-ferences between Mesoscale 1 (Fig. 4a) and Mesoscale 2(Fig. 4d). The mean temperature at this depth is more than3 °C lower in Mesoscale 1 (Table 2), sampled underwesterlies. Additionally, NACW upwelled in Cape StMaria is clearly detected and it stretches eastwards fromthe Cape, giving rise to the surface thermal signature ofthe Huelva front and pushing to the east the pool of

Table 2Mean values of temperature, salinity and density fields at 10 m, 50 m and 1

Leg Variable 10 m

Mesoscale 1 Temperature (°C) 15,57Salinity 36,00γt (kg/m

3) 26,60Mesoscale 2 Temperature (°C) 19,04

Salinity 36,01γt (kg/m

3) 25,81

Mean temperature at 10 m depth is more than 3 °C lower in Mesoscale 1 and100 m depth the mean values in both legs are rather similar for the three va

warmer water over the continental shelf (Fig. 4a). On thecontrary, in Mesoscale 2, accomplished under easterlies,upwelled waters have been confined to a smaller regionwest of Cape St Maria and pushed offshore to the southdue to the spreading to the west of a SW coastal countercurrent (Fig. 4d) that strengthens under easterlies (Fiúzaet al., 1982; Fiúza, 1983; Folkard et al., 1997; Vargas et al.,2003; García-Lafuente et al., in press). Macroscale leg(not shown) corresponds to an intermediate situationbetween the twomesoscales, although it is more similar toMesoscale 1, as westerlies were dominant (Fig. 2). Someof the features detected in Mesoscale 1 weaken (e.g., theupwelling off Cape St Maria) and those of Mesoscale 2begin to appear (e.g., the development of the warmercoastal counter current).

Contrary to the temperature, salinity field at 10 mdepth is rather uniform in both mesoscales (Fig. 4b, e)and the value of the mean salinity is the same (Table 2).As previously shown, changes in wind regime promoteheat inputs from land but, due to its land origin, nosignificant inputs of salty or fresh water are expected.Additionally, salinity of surface waters is rather similar

00 m depth for the two mesoscale legs

50 m 100 m

±0,38 15,12±0,87 14,76±0,42±0,05 36,10±0,09 36,01±0,08±0,09 26,77±0,08 26,84±0,03±0.71 15,72±0,33 15,08±0,21±0,09 36,14±0,08 36,11±0,04±0,09 26,68±0,04 26,80±0,02

hence, mean density at this depth is higher in this leg. Both at 50 m andriables.


to that of sub-surface ones (compare Fig. 4b, e withFig. 5b, e), that is NACW becomes SAWor SW mainlyby heating processes and salinity remains the same. Inboth legs, the surface signature of the filament of fresherwater off Cape St Maria (see Criado-Aldeanueva et al.,in press) is clearly detected. Although the geographicalscope of the samplings does not allow determining theposition and dimension of the filament accurately, thereare pieces of evidence that it could be slightly displacedto the west in Mesoscale 1. It might be that the windinduced variability of the geostrophic current thatadvects to the south the upwelled waters and givesrise to the filament (see Criado-Aldeanueva et al., inpress) promote some variations in its location.

Due to the relatively slight spatial variation of thesalinity field in relation to temperature, the density field isdominated by temperature. Thus, the variability observedin the temperature field at 10 m depth (Fig. 4a, d) isalso reflected in the density field at the same depth (seeFig. 4c, f). The areas of cold upwelledNACWcorrespondwith denser regions and those ofwarmer SAWcorrespondwith less dense ones. Consequently, the maximum valueof mean γt at 10 m is detected in Mesoscale 1 and theminimum in Mesoscale 2, being the difference about0.8 kg m−3 (see Table 2).

At 50 m depth (Fig. 5), the temperature field adopts acommon pattern for both mesoscales, being the centralwarm core of SAW and the upwelled NACW over thewhole continental shelf the most outstanding character-istics (Fig. 5a, d). The mean temperature is rather similarfor the two legs at this depth, less than 1 °C higher inMesoscale 2 (Table 2). The salinity field (Fig. 5b, e)

Fig. 5. Temperature (°C, panels a, d), salinity (panels b, e) and density (γt kMesoscale 2 (lower panels) data. Stations shallower than 50 m have been blanat this depth.

hardly shows response to the wind induced variability inthe spatial patterns at 10 m depth and neither does it at50 m. Saltier waters are located in the central warm coreand fresher ones over the continental shelf. The densityfield (Fig. 5c, f) is again dominated by temperature as itcoincides denser waters with colder (although fresher)ones and less dense waters with warmer (although saltier)ones. Anyway, the spatial distribution of the density fieldis not sensitive to wind variations at this depth. Finally, at100 m depth (not shown) the spatial distribution of thehydrological variables does not differ from the onedescribed for 50 m and its mean values are almost thesame for both legs (Table 2).

The above description suggests that some hydrologicalsurface patterns, particularly related to temperature and,hence, to density, are influenced by the wind regime.Wind changes produce great temporal variability in somespatial features (the upwelling off Cape Santa María, thesurface signature of the Huelva Front, the region of warmcoastal waters, etc) in a relatively shallow layer (b50 m,see Section 3.3 for details). Below this surface layer, thehydrological characteristics are fairly independent of me-teorological forcing.

3.3. Variability of the surface water masses distribution

As previously shown, the short term, wind inducedvariability only affects some tens of metres of the watercolumn. In this section, research on this variability isrefined by studying the composition of the surface watersreferred to three “standard”water classes defined in termsof the TS diagram.

g m3, panels c, f) at 50 m depth from Mesoscale 1 (upper panels) andked. Surface horizontal variability shown in Fig. 4 is no longer detected

Fig. 6. Detail of the upper layer of the TS diagrams for Mesoscale 1 (panel a) andMesoscale 2 (panel c). The triangle shape characteristic of this upperlayer has been emphasised by drawing the lines between the three apices selected (see Table 3 for the coordinates). The criteria used for this selectionare discussed in Section 3.3. The colour code for each apex is in connection with the averaged upper 20 m spatial distribution showed in the rightpanels for Mesoscale 1 (panel b) and Mesoscale 2 (panel d). Notice that in Mesoscale 1 (under westerlies), the balance is almost established betweenwaters A and B whereas in Mesoscale 2 (under easterlies), high concentration of water C is detected in the northern and eastern continental shelf, thusshowing the wind induced variability that affects the surface layer. Further details are discussed in the text.

Table 3TS coordinates of the three apices selected for the triangle shape of theupper Ts diagrams (see also Fig. 6)

Apex Temperature (°C) Salinity

A 13.20 35.80B 17.50 36.55C 21.30 35.97

The criteria used for this selection are discussed in Section 3.3.


3.3.1. TS triangle in the surface layerFig. 6a, c emphasises the typical triangle shape of the

upper layer of the TS diagram above γt≈27.0 kg m−3.Three apices have been selected for the triangle. Point A(green) corresponds to the TS properties of NACWlocated at γt≈27.0 kg m−3. Criado-Aldeanueva et al.(in press) show that this NACW suffers very littlemixing and has well-defined temperature and salinityvalues (T≈13.2 °C, S≈35.8). Point B (blue) is locatedat the end of the linear TS relation of NACW and wouldcorrespond with the upper layer of NACW (that is,SAW) before being modified by air–sea interactions(mainly heating by radiation) that may raise its tem-perature. Finally, point C (red) is associated with SWand has been selected so that most of the TS points fallinside the triangle. As point C has not conservative TSproperties, not only mixing processed but others likehorizontal advection or air–sea exchanges may be in-volved. However, this selection has been proved to beuseful to analyse the wind induced variability in the

waters spatial distribution pattern. Not only physicalproperties of the water masses have been considered inthe selection of the apices, but also biological ones. Reulet al. (in press) have identified different phytoplanktonpopulations connected with the lines connecting eachpair of apices and claim for further research on thephysical underlying. The TS coordinates of the threeapices are summarised in Table 3.

Once defined the three previous apices, temperatureand salinity of any arbitrary point P of the TS diagram,

Fig. 7. Spatial distribution pattern of the percentages (expressed in %) of the three “standard”water types A (panels a, b), B (panels c, d) and C (panelse, f) for the upper 20 m water column in Mesoscales 1 (left panels) and 2 (right panels). Calculations have been carried out from Eqs. (1) (2) (3).Thewind induced variability of the waters spatial distribution is revealed. In Mesoscale 1 (under westerlies), the balance is almost established betweenwaters A and B whereas in Mesoscale 2 (under easterlies), high percentage of water C is detected in the northern and eastern continental shelf.

1 Water A (B, C) in the following discussion really means waterwith TS properties similar to those of “standard” water A (B, C).


can be expressed as a linear combination of temperatureand salinity of the reference points A, B and C, that is:

TP ¼ adTA þ bdTB þ cdTC ð1Þ

SP ¼ adSA þ bdSB þ cdSC ð2Þwhere TP and SP are temperature and salinity of point P.TA, TB and TC are temperature of the reference points A,B and C and SA, SB and SC refer to salinity of the samethree points. For the inner points (remind that the apiceshave been selected so that most of the TS points fallinside the triangle), the normalisation condition muststand:

aþ bþ c ¼ 1 ð3ÞEqs. (1)–(3) can be solved for all the TS points of the

TS diagram to obtain a, b and c. Due to the existence ofa set of values a, b and c for any point P, the solution isdependent of the three spatial coordinates, thus pro-viding spatial information on the water classes distribu-tion. For practical purposes, the equations have beensolved for vertical 5-metre averaged TS diagrams so thatsome spatial smoothing of the coefficients is achieved.

3.3.2. Water masses distribution in the surface layerCriado-Aldeanueva (2004) has performed a detailed

analysis for 5-metre averaged TS diagrams concluding

that the variability of the 3-D fields mainly affects thefirst 20–25 m of the water column. Below this depth,proportion of water C is almost negligible everywhereand Eq. (3) is satisfied with the only contribution ofcoefficients a and b (corresponding to the “standard”waters A and B, respectively). Below 20–25 m, water Ais mainly located in the northern part of the Gulf, overthe continental shelf, and water B occupies the centralpart of the Gulf, in good agreement with the descriptionof Section 3.2 for 50 m depth. Much more interestingappears to be the discussion on water masses distribu-tion based on the shallower 20-metre averaged TS dia-grams, displayed in Fig. 6b, d. Colours are in coherencewith that assigned to the triangle apices (green for waterA, blue for water B and red for water C).

In Mesoscale 1 (Fig. 6b), carried out under westerlies,high concentration (about 80%, see Fig. 7a) of water A1 isfound in the northern part of the basin, also stretching tothe east of the Cape Sta Maria, whereas the central part isfilled with water B (above 80%, see Fig. 7c), in goodagreement with the description of Section 3.2 for 10 mdepth. Water C is scarcely found (under 20%, Fig. 7e)except in the northeastern part of the sampling region(about 20–30%, see Fig. 7e again), in coherence with thefew TS points that separate from the NACW line in the TS


diagram (see Fig. 3a). This distribution does not differsignificantly from that of 50m depth, except for the tracesof water C in the northeastern region. In Mesoscale 2,accomplished under easterlies, things are completelydifferent.Water B is again located in the central part of theGulf, but is modified by the presence of water C (comparethe bright blue tones of Fig. 6b with the ones of Fig. 6d).Water A is restricted to a smaller region west of Cape StaMaria and also suffers the influence of water C (per-centage of water A in Fig. 7b, about 60%, is lower than inFig. 7a). Precisely, water C is the most abundant (above70% in the northeastern region) in this shallower layer(see also the TS diagram, Fig. 3b), as it floods almost thewhole continental shelf from Cadiz to west of Cape StaMaria advected by the coastal countercurrent thatstrengthens under this wind regime (Fiúza et al., 1982;Fiúza, 1983; Folkard et al., 1997; Vargas et al., 2003;García-Lafuente et al., in press). The above descriptionconfirms the wind induced variability depicted in Section3.2 and restricts its influence to a relatively shallow layer(20–25 m). As previously pointed, Macroscale sharesfeatures of bothmesoscales, mainly that ofMesoscale 1 aswesterlies were dominant, and could be considered as anintermediate (averaged) situation between the twomesoscales.

4. Conclusions

From the data collected during GOLFO 2001 survey,a study has been conducted on the distribution of watermasses in the Gulf of Cadiz in the spring of 2001, and onits meteorologically forced variability. In Mesoscale 1,westerlies were dominant while Mesoscale 2 was aperiod of strong easterlies. This variability has made itpossible to study the response of the oceanographicfeatures to meteorological forcing using in situ data. Thetemperature field at 10 m shows significant differencesbetween Mesoscale 1 and Mesoscale 2, the mean tem-perature at this depth being more than 3 °C lower inMesoscale 1. During this leg, NACW upwelled in CapeSt Maria is detected stretching eastwards from the Cape,leaving a clear surface thermal signature of the Huelvafront and pushing the pool of warmer water over thecontinental shelf to the east. On the contrary, in Meso-scale 2, upwelled waters were confined to a smallerregion west of Cape St Maria and pushed offshore to thesouth due to the spreading of the warm SW to the westadvected by the coastal counter current.

This variability also reflects in the upper layer of theTS diagrams. Reciprocal influence between NACW andSWand between SAWand SW confer this upper layer atypical triangle shape, in which three “standard” TS

apices have been selected. Point A corresponds to theTS properties of NACW at γt≈27.0 kg m−3, point Bcorresponds to SAW before being modified by air–seainteractions and point C is associated with SW and hasbeen selected so that most of the TS points fall inside thetriangle. Considering the shallower 20 m averaged TSdiagrams, in which most of the variability is concen-trated, some remarks are of concern: in Mesoscale 1,high concentration of water A is found in the northernbasin, also stretching to the east of Cape Sta Maria,whereas water B fills the central basin. Water C isscarcely found except in the northeastern part of thesampling region. In Mesoscale 2, water B is again lo-cated in the central part of the Gulf, but noticeablymodified by the presence of water C. Water A is res-tricted to a smaller region west of Cape Sta Maria and isalso influenced by water C, the most abundant in thisshallower layer, as it floods almost the whole continentalshelf from Cadiz to west of Cape Sta Maria advected bythe coastal countercurrent that develops under easterlies.

Acknowledgements

We would like to thank the UIB-IMEDEA (SpecialAction CICYT, REN2000-2599-E) for the dissemina-tion of the software used for the optimal interpolation.Wind data from the RAP buoy has been kindly yield byPuertos del Estado. We are also indebted to the crew ofBIO Hespérides for his work during GOLFO 2001survey. F. Criado Aldeanueva is much obliged to theSpanish Ministry of Education and Science for awardinghim a F.P.U. grant (reference no. AP2000-3951).

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Wind induced variability of hydrographic features and water masses distribution in the Gulf of Cadiz (SW Iberia) from in situ data