Deep-Sea Research I 49 (2002) 1087–1101

Eddy and deep chlorophyl maximum response to wind-shearin the lee of Gran Canaria

G. Basterretxeaa,*, E.D. Bartonb, P. Tettc, P. Sangr!ad, E. Navarro-Perezb,J. Ar!ısteguid

a IMEDEA (CSIC-UIB), Miquel Marqu"es 21, 07190 Esporles, Balears, SpainbSchool of Ocean Sciences, University of Wales, Bangor, UKcSchool of Life Sciences, Napier University, Edinburgh, UK

dFacultad de Ciencias del Mar, Univ. Las Palmas de Gran Canaria, 35017 Las Palmas de G.C., Spain

Received 21 March 2001; received in revised form 19 October 2001; accepted 22 February 2002

Abstract

The physical and biological properties of the warm wake of Gran Canaria were examined during a survey carried out

in June 1998. The sampling region was dominated by the presence of a warm triangular region downwind the island and

an anticyclonic eddy spun off the island. Convergent and divergent frontal regions were generated by the wind

shear zones extending along either side of the sheltered region of the warm wake. With increasing distance from

shore, evidence of convergent/divergent frontal regions weakened, but the influence of the eddy increased.

Both structures, frontal regions and the eddy, clearly altered the vertical phytoplankton biomass distribution as

indicated by chlorophyll-fluorescence. Downwelling on the convergent boundary moved the 26.2 kgm�3 isopycnal and

its associated deep chlorophyll maximum (DCM) below the 1% light zone. Upwelling at the divergent boundary not

only elevated the DCM with its associated isopycnal but also, because of the increased light levels, allowed a shift in the

DCM to higher (deeper) density surfaces (26.4 kgm�3). However, the highest integrated chlorophyll occurred in the

central wake. r 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Islands; Wakes; Fronts; Eddy; Chlorophyll; Wind; Canary islands

1. Introduction

The presence of a generally wind-shelteredregion to the south of Gran Canaria is a wellknown feature for the inhabitants of the island.Sea conditions visibly change from rough white-capped seas to calm in a few hundred meters along

the SE and SW coast. The windless areas give riseto warm ‘wakes’ observable in most sea surfacetemperature (SST) satellite images of the Canaries(Hern!andez-Guerra, 1990; Van Camp et al., 1991).This phenomenon occurs, to some extent, in all theislands of the archipelago but is particularly cleardownwind of the higher islands. The origin ofwarm wakes lies in the intense day-time warmingof the unstirred surface in the wind-sheltered areaof the island, which can extend leeward for manytens of kilometers.

*Corresponding author. Tel.: +34-971-611-734; fax: +34-

971-611-761.

E-mail address: vieagbo@clust.uib.es (G. Basterretxea).

0967-0637/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 6 7 - 0 6 3 7 ( 0 2 ) 0 0 0 0 9 - 2

Barton et al. (2000) have reported differentaspects of the warm wake behind Gran Canaria.They observed that in the boundary zone betweenthe lee and the exposed region winds can changefrom 0 to 15m s�1 in distances of B2 km. Thesewind shears strongly influence the local hydro-graphy. Ekman forcing is weaker in the wind-sheltered region than in the exposed regions. Thegeneral net westward Ekman transport of thewind-driven surface layer is absent from the lee.Therefore, convergent and divergent fronts, andconsequently, sinking and upwelling are generatedbetween sheltered and exposed regions. Bartonet al. (2000) suggest that absence of a solidboundary downstream of the island favors thegeneration of eddies on the length scale of the lee.Eddy shedding in Gran Canaria has been fre-quently observed (Ar!ıstegui et al., 1994; Bartonet al., 1998) and has associated biological con-sequences (Ar!ıstegui et al., 1997; Basterretxea andAr!ıstegui, 2000; Rodr!ıguez et al., 2001).Although the ‘island mass effect’ was first

postulated for the Hawaian archipelago by Dotyand Oguri (1956), there is little information on theoceanography around oceanic islands, and moststudies have been mainly focused on eddy genera-tion by disturbance of ocean currents (Patzert,1969; Simpson and Tett, 1986). Such disturbancescan in principle take place around any sort ofisland, including low atolls, such as Aldabra(Heywood et al., 1990, 1996). In contrast, warmlee formation requires intense winds blowingaround high orography, e.g., where the abruptrelief of oceanic islands of volcanic origin inter-rupts the trade wind systems. Many of these highislands are located in oligotrophic regions wherelocal mesoscale processes are essential to maintainenhanced biological production in their proximity.Notwithstanding their implications for many

island-associated ecosystems, warm lees have beenlittle studied. There is poor knowledge of theirtemporal persistence, spatial extent and biologicalconsequences. In this paper we present results offieldwork in the wake of Gran Canaria conductedas part of the FRENTES project. Sampling re-vealed a clear signal of localised convergent anddivergent fronts formed by the strong horizontalwind shear on the flanks of the warm wake

leeward of the island. It is shown that the structureof the lee is related to the wind forcing and theshedding of an anticyclonic eddy shed from theisland. The resultant perturbations to chlorophylldistribution are documented and discussed.

2. Methods

From 13 to 21 June 1998, a study of the lee ofGran Canaria was conducted during leg 1 of cruiseFRENTES-9806 of R/V Garc!ıa del Cid (Fig. 1).Sampling consisted of CTD transects across thewake, with some fine scale crossings of theboundaries between the exposed and shelteredareas close to the shelf edge. In the detailedcrossings, the ship moved slowly across the windshear zone while the CTD was continuously raisedand lowered in a ‘tow-yo’ mode. On 14 and 15June a time series of 2 hourly CTD casts weremade in the center of the wake over the 1000misobath. Additionally three biological stations(LEE, CV, and DV) were sampled duringleg-3 (2–8 July 1998) near the center of the wakeand just inside the convergent and divergent

Fig. 1. Map of FRENTES-9806 (leg 1) survey showing the

position of the biological (open square) and times-series

stations (open circle).

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011088

boundaries. The positions of the fronts had shiftedwhen the biological stations were sampled andtherefore do not correspond exactly to thoseobserved in the earlier survey.Vertical profiles of temperature, salinity and

chlorophyll fluorescence were obtained with aSBE-911+ CTD probe with attached Sea-Techfluorometer. This system was mounted on a GOrosette sampler with 24 lever action bottles holding12 litres. CTD sensors were calibrated using digitalreversing thermometers and water samples drawnfor salinity determinations with an Autosal salino-meter. Underwater photosythetically active radia-tion (PAR) was measured with a QSP-200L4Sscalar PAR sensor connected to the CTD system.Direct current measurements were obtained

with a vessel-mounted 150 kHz RDI acousticDoppler current profiler (ADCP) down toB300m. The data were post-processed with theUniversity of Hawaii CODAS software to controlquality, derive statistical information on the dataset, and merge it with the available GPS positioninformation to obtain absolute velocities. Theamplitude and phase errors of the ADCP weredetermined and corrected by the method ofPollard and Read (1989).Continuous underway measurements of surface

(B2m) temperature and salinity were logged everyminute with a Seabird thermosalinometer fed fromthe ship’s continuous seawater supply. The cali-bration was checked by comparison with near-surface values from the CTD. Wind speed anddirection, air temperature and atmospheric pres-sure were recorded at 1min intervals approxi-mately 10m above sea level with an ANDERAAmeteorological station.To analyze chlorophyll, 500ml of seawater were

filtered on to Whatman GF/F filters that weresubsequently extracted in 90% acetone overnight.Extracts were measured in a Turner Designsfluorometer previously calibrated with pure chlo-rophyll a (Sigma Co). Although measurementsbefore and after acidification were carried out toevaluate phaeopigment content, data presentedhere are the sum of chlorophyll+phaeopigments.These data were used to calibrate the CTD-fluorometer. For present purposes the depth ofthe fluorescence maximum at a given station will

be equated with the depth of the photosyntheticpigment maximum at that station, and the twofeatures jointly referred to as the Deep Chloro-phyll Maximum (DCM).

3. Results

3.1. Synoptic context

The Sea Surface Temperature (SST) image forB1400 UT 20 June (Fig. 2a) shows a warm wake,aligned with the prevailing wind, extending south-west of Gran Canaria at least two islanddiameters. The temperature gradients that definethe lee (LEE) emerge from the southern flanks ofthe island, but the center of the lee near-shoreremains cooler. The wake is distorted by thepresence of an anticyclonic gyre (A), the size of theisland (50 km diameter), centered at 271 300N161W, 60 km to the southwest. Its western limit isextended into a circular area by entrainmentaround the eddy. However the warmest water(>22.51C) maintains a typically triangular form,because wind mixing outside the lee reducessurface temperature.Trade winds (Fig. 2b) in the exposed region

averaged 10.572m s�1 with a vector mean direc-tion of 2021 while deviations from the meandirection were low (SD=7141). In contrast, windflows in the lee were spatially variable, withaverage speeds of 4.472m s�1. The wake bound-aries seen in SST clearly correspond closely withthe lines of strongest wind shear. The presence ofatmospheric counter-rotating eddies in the lee issuggested by slightly increased speed and theshoreward direction of the wind in the wakecenter. Nevertheless, because the survey tookseveral days, the apparent wind pattern may notaccurately represent the instantaneous wind fieldADCP current vectors averaged over 16–25m,

obtained between 17 and 21 June, show thefeatures that dominated the sampling period(Fig. 2c). Directly south of the island, currentsup to 0.5m s�1 define the northern half of theanticyclonic eddy seen in the SST image. Thestrong shoreward flow in the lee of the islandsweeps eastward nearer to the coast and then turns

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1089

southward before the eastern limit of sampling.The eddy diameter observed in the SST image andthe azimuthal velocity indicated by the ADCPsuggest a rotation period of roughly 3 days. In thewestern part of the south coast, the shorewardflow extends across the wind shear zone to flowcounter to the wind, and at the most northwesternpositions sampled, a smaller cyclonic circulation isindicated. This possibly signals generation of anew cyclonic eddy to be subsequently shed fromthe island. The same patterns of flow, thoughgradually reducing in strength, persisted down tothe limit of ADCP penetration, around 250m (notshown).

3.2. Sections

Fig. 3 (a, c, e, g, and i) is a composite of twodays of sampling across the lee along Section 1.Tow-yo transects were made on both sides fromthe lee into the exposed zone and then the centerwas filled in by normal CTD casts. Dashed verticallines indicate the component parts of the section.

In the wind exposed regions a nearly verticallyuniform surface mixed layer lies above a thermo-cline and pycnocline of spatially varying strength.The steepest temperature gradient is here foundbetween 191C and 20.51C, and the steepestgradient in density anomaly between 26.0 and26.3 kgm�3.Between the wind shear zones the isolines

deepened from west to east. In the western exposedzone the thermocline was shallow (30–40m),horizontal and sharp, whereas, in the wake, itweakened and deepened eastward, towards theconvergence band. The thermocline and pycno-cline deepened by about 90m in a distance of60 km. Remarkably, beneath the cyclonic windboundary the isolines were locally elevated whilebeneath the anticyclonic boundary they werelocally depressed. These regions of apparentupwelling and downwelling extended horizontallyfor 7–8 km and represented local perturbations ofabout 25m in the thermocline depth. In the easternexposed zone, the pycnocline and thermoclinereturned to shallower depths of around 50m.

Fig. 2. (a) Sea surface temperature image of 20 June 1998. The approximate contour of the warm lee (LEE) and anticyclonic eddy (A)

has been marked with a dotted line. (b and c) indicate the wind field and surface currents (16–25m) in the surveyed area.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011090

Fig. 3. Properties along Sections 1 and 2 of Fig. 1: (a) and (b) wind velocity, (c) and (d) surface temperature from thermosalinograph

(dotted line) and CTD at 3m (grey dots). Vertical sections of: (e) and (f) temperature; (g) and (h) salinity; and (i) and (j) density. Black

arrows indicate the position of the wind shear zone. Section 1 plots (a, c, e, g and i) are the composite of two days of sampling. The

dashed line indicates the limit of each day sampling.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1091

Close to the surface (upper 10m) a secondarydiurnal thermocline, related to day-time warmingand near-surface stratification, was evident in thewind protected area. The shallow surface warmlayer typically showed diurnal variations of 0.751Cat 5m depth. The convergent boundary of the lee,where thermocline depth was maximum and SSTgradients strongest, was associated with denseaccumulations of floating debris, ctenophores andphytoplankton colonies in a 500m wide bandparallel to and just inside the line of wind shear.The wind beyond this convergent front wasincreasingly strong, gusting up to 25m s�1.The salinity stratification in general followed

that of temperature. However in the cyclonic windshear zone salinities less than 36.8 extended from50m depth up to the sea surface, which isconsistent with active upwelling. The lower salinitywater appeared to extend at the surface towardsthe wake center as a result of advection by theanticyclone. The highest upper layer salinities ofthe section coincided with the anticyclonic windboundary.Section 2 (Fig. 3b, d, f, h and j), 10 km further

offshore from the island shelf, showed the samebasic structure as Section 1. Weaker upwelling wasseen on the western divergent boundary as uplift ofthe isotherms and isopycnals above 50m, but theconvergent Ekman front showed no clear down-welling. Although this could be attributed to alower sampling resolution, a finely resolved Nu-shuttle transect in the same position made on June12 (data not shown) did not display such evidenceeither. Cool, low salinity surface waters encoun-tered at the easternmost stations outside the leerepresent upwelling filament waters originatingnear the African coast, and sampled earlier in thecruise.Surface temperature records (Fig. 3c and d)

along both sections show clear gradients coincid-ing with the wind shear zones. However, tempera-tures in the lee were only about 11C warmer thanoutside. Within the lee, highest surface tempera-tures corresponded to the two areas of weakestwind. A slightly stronger return breeze in theeastern central wake reduced surface temperatureby mixing. The temperature contrasts were smallbecause the lee was largely sampled at night, when

radiative cooling reduces surface temperature.Hence, these differences represent the minimumof the daily cycle.

3.3. Time variability

A 34 h timeseries station (Fig. 4) near the centerof the northern line shows that thermocline andpycnocline depths varied by only B15m, remain-ing close to 100m throughout. At deeper levels,25m excursions that penetrated to at least 700mdepth had a period close to the semi-diurnal tide.These fluctuations are probably related to thoseseen in Section 1 across the lee, but the correspon-dence of pycnocline displacements of the expectedsign with the wind boundaries seems an unlikely

Fig. 4. Time series in the center of the Lee (Fig. 1) showing

weak semi-diurnal tidal fluctuations of (a) temperature (1C),

(b) salinity, (c) density (kgm�3) and (d) chlorophyll (mgm�3).

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011092

coincidence if these displacements result fromaliased sampling of a tidal signal. Near-surface,day-time warming of around 0.81C was evident atthe time-series station, with maximum temperatureoccurring from 1400–1800 h local time and a pre-dawn minimum. Fluctuations of 50m in the depthof the 211C isotherm indicated convective over-turning during the night and cumulative warmingduring the day. Near-surface salinity showed nosignificant variation. The DCM remained constantin size near the depth of the 26.2 isopycnal. TheADCP velocities (not shown) showed little semi-diurnal variation about the mean in eithercomponent. Surface wind measured at this stationin the lee center did not exceed 3m s�1 during theobservation period. It can be concluded that shortterm wind and tidal variability did not contributesignificantly to the overall patterns observed in thewake cross sections.

3.4. Water masses

Temperature-salinity (TS) diagrams of theupper 250m along Sections 1 and 2 (Fig. 5) areconsistent for densities above 26.6 kgm�3, exceptthat diagrams from more easterly stations showslightly higher salinities at a given temperature. Atthe center of the wake, 26.6 kgm�3 roughlycorresponds to 200m depth. The main differencesamongst the TS plots occur at densities below26.4 kgm�3 (i.e. above the pycnocline) where theTS characteristics clearly bifurcate. Major featuresin the figure are labeled. In the case of observa-tions made during Section 1, closer to the island,the lower salinity branch (1) corresponded to thedivergent front, where a body of salinity 36.75,indicative of upwelling, extended from around65m to the sea surface. The other branch showsmore near-surface variations (see also Fig. 3)corresponding to (2) a low salinity surface parcelat the center of the wake, evidence of earlierupwelling from slightly greater depth, (3) thesubsurface maximum of salinity (>36.9) seen inthe section plot as a narrow band at theconvergent front, and (4) the effects of surfacewarming, which was most evident in the easternhalf of the wake south of Maspalomas andresulted in water exceeding 21.51C.

Section 2 data showed similar patterns. Theupwelled water (1) was observed only at theeasternmost station and did not coincide withthe wind shear zone, because of a change in windorientation. The near-surface, low-salinity, pre-viously upwelled water (2) was again present as itwas in Section 1. The lowest salinity in this featurewas located east of the wake, where surface watersare often supplied from an upwelling filament(Barton et al., 2000). The distribution of the TSplots of type 2 suggests that filament waters mayhave been advected around the periphery of theanticyclonic eddy. Supporting evidence for thisconclusion was found in a SEAWIFS image twoweeks after the cruise in which (surface) water of

Fig. 5. Temperature-salinity diagrams for (a) Section 1 and

(b) Section 2. Numbers indicate characteristics discussed in

the text. (c) Map showing distribution of near-surface water

masses.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1093

relatively high chlorophyll from the filament wasseen to be wrapped around the eddy.

3.5. Chlorophyll

The characteristic vertical distribution of fluor-escence-chlorophyll around the Canaries presentsan homogeneous mixed layer of low chlorophyllabove a pronounced deep chlorophyll maximum(DCM) generally just below the steepest gradientregion of the main pycnocline. During FRENTES-9806 chlorophyll concentrations in the surfacelayer were low (o0.2mgm–3) but similar to othersurveys around Gran Canaria (e.g. Ar!ıstegui et al.,1997). The distribution of chlorophyll alongSection 1 and 2 (Fig. 6a and b) revealed a DCMfluctuating around 90m depth. Although somepatterns can be readily attributed to changes in thepycnocline depth, uncoupling between the biolo-gical (chlorophyll) and the physical (density) fields

was evident, especially in the wake and in Section2. This uncoupling was seen as a spreading of theDCM across isopycnals (Fig. 6c and d), generallytowards lower densities. Within the wake and theeastern wind-exposed region, the chlorophyllmaximum was situated close to the 26.2 kgm–3

isopycnal. In and west of the divergent front theDCM shifted to the 26.4 kgm–3 isopycnal and, as aresult, was less sharp, with higher chlorophyllbeing recorded near the surface.The fluorescence-chlorophyll profiles fell into

four categories corresponding to their locationswith respect to the lee. The far field profiles(Fig. 7a), representing the unperturbed DCM closeto 26.4 kgm�3, were found only west of thedivergent wind shear zone (Fig. 7e). Within andclose to this divergent boundary a weaker DCMwas located at slightly lower densities (Fig. 7b).This water was subjected to upwelling in thecyclonic wind shear zone. However, very similar

Fig. 6. (a) and (b) Fluorescence-Chlorophyll vs. depth and (c and d) chlorophyll vs. density corresponding to Sections 1 and 2 in

Fig. 3. Black arrows indicate the position of the wind shear zones.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011094

chlorophyll profiles were found at the easternextreme of Section 2 where upwelling filamentwaters occurred (Fig. 7e). The bulk of water withinthe lee region and anticyclonic boundary wastypified by a broader DCM of similar magnitudecentered around 26.25 kgm�3 (Fig. 7c). Within thecenter of the anticyclonic eddy the DCM wasreduced in magnitude, spread over a wider rangeof isopycnals, and centered on a shallower surface26.1 kgm�3 (Fig. 7d).Integrated (0–250m) fluorescence-chlorophyll

values were lowest (o55mgm�2) in the eastern

exposed region, where the DCM approximatelycoincided with the 26.4 kgm�3 isopycnal. Theseminimum values occurred where the DCM wassharpest; conversely, higher integrated chlorophyllcoincided with spreading of the DCM over a widerrange of lower densities. This effect was enhancedin the stations more clearly influenced by the eddy.

3.6. Biological stations

Temperature, chlorophyll and nitrate profilesfrom the biological stations (Fig. 8) revealed thedifferences between the frontal boundaries and thewake. The nutrient profiles are taken from arelationship between nitrate+nitrite and densityanomaly established from ‘unperturbed’ stationssouth of the Canarian archipelago in August 1999(Tett et al., in press). They suggest that, except atstation CV, the DCM occurred at nitrate concen-trations between 0.3 and 1.1mmolm�3, and there-fore under conditions of moderate, but not severe,nutrient limitation (see Tett et al., in press).Just inside the divergent boundary (station DV),

which had shifted east by the time the station wasworked, the thermocline was uplifted to almost20m depth. The mixed layer was colder than in thelee center (station LEE) and near the convergentboundary (station CV). A slight near-surfacewarming, permitted by the reduced winds insidethe boundary, was observed at DV. The fluores-cence-chlorophyll maximum at DV occurred atB75m, well above the limit of the euphotic layer(considered as 1% of surface irradiance). At theLEE station, temperature stratification was moreclearly evident in the top 20m, and the mainthermocline was at 60m depth. The 1% light depthwas similar to that near the divergent front(station DV), but here, the DCM was locatedsome 20m beneath it. At CV, the main thermo-cline was located near 90 meters. Water tempera-tures in the upper layer did not differ much fromthose at LEE as the station was located on the leeside of the convergent wind shear zone. Thefluorescence-chlorophyll distribution presented aclearly different pattern here, with a broad peakand 36% decrease in the chlorophyll concentrationat the DCM peak. The 1% light level occurredsome 20m deeper at CV than at the other two

Fig. 7. Fluorescence-Chlorophyll profiles for Sections 1 and 2,

grouped by characteristic structure (groups a–d are discussed in

the text) and a map of the distribution of these groups.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1095

stations, and the DCM was about 20m deeperstill. In all three biological stations the DCMoccurred at densities between 26.25 and 26.30kgm�3. The chlorophyll profiles therefore lie inthe range between those observed on the divergentboundary and in the lee in Fig. 7.

4. Discussion

4.1. Wind shear fronts

Although originally hypothesised (see review inSimpson and Tett, 1986) that island mixing wasdue to disturbance of ocean currents, it seems clearfrom the observations reported here that atmo-spheric processes are at least as important in thecase of Gran Canaria. Perturbations often mani-fested as eddies may occur in the current down-stream of any island, but it has long been knownthat mountainous islands such as Hawaii canperturb strong wind regimes to provide a secondsource of disturbance to the oceanic flow (Patzert,1969). Wind forcing with strong horizontal shearcan be responsible for spin up of energetic oceaniceddies in the absence of significant backgroundcurrents, as well as for localised upwelling, down-

welling and increased vertical mixing in the nearsurface layer, as in the Gulf of Tehuantepec(Barton et al., 1993; Trasvi *na et al., 1995). In thecase of Gran Canaria, the interaction of the islandwith the atmospheric flow is the direct cause of thewind shear fronts, and these in turn play a rolein the generation of oceanic eddies behind theisland. Between two zones of strong wind shear, asheltered area was present, where we were able toobserve the promotion of stratification and en-hanced warming. Ekman pumping beneath theshear zones gave rise to opposed displacements ofthe thermocline either side of the lee, promoting atendency for eddy production.The calmed region of the wake is a particularly

interesting area where two main processes interact.First, the lee is exposed to intense diurnal warmingas well as shelter from wind stirring. The weakwind-driven turbulence is unable to erode thenear-surface stratification and so the surfacewarms. Daytime solar radiation tends to warmand stabilise the surface layer whereas night-timecooling acts to the contrary. Nocturnal surfaceradiative heat loss causes convective instabilityof the surface layer, which by overturning effect-ively cools the near surface water and warmsthat immediately below (Flament et al., 1994).

Fig. 8. Profiles at stations sampled in detail for biology, including temperature, nitrate+nitrite (NOxN), fluorescence, extracted

chlorophyll (DV triangles, LEE circle, CV square), and depths of 1% surface PAR.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011096

However, the local one-dimensional heating cycleis not the only determinant of thermocline depthbecause the two-dimensional effect of the windshear in producing Ekman pumping displaces thethermocline vertically on either side of the lee. Thispumping, and its consequences, constitutes thesecond main physical forcing in this region. Theresultant production of eddies provides a three-dimensional circulation that further affects the leeby introducing water masses from outside. As theeddies develop, they move slowly away from theisland and so conditions in the lee must vary withtime.In SST images, the lee of Gran Canaria is seen

as a triangular region of higher temperature withtwo maxima nearshore separated by a slightlycooler area. This cooler region results fromshoreward wind recirculation related to the pre-sence, as suggested by the wind observations, ofatmospheric eddies. The shape, location andfrequency of these eddies is difficult to estimate,because it took several days to survey the area, andso our wind data provides a smeared view possiblyunrepresentative of the instantaneous wind field.Atmospheric vortex streets extending south fromGran Canaria have been inferred from cloudpatterns (Ar!ıstegui et al., 1994), and an approx-imate description has been given by Chopra(1973), but no observations of the instantaneouswind field at sea level are yet available.Barton et al. (2000) reported that the incident

wind is diverted around the flanks of the islandproducing local wind enhancements and decreasesin pressure. This effect can be attributed to thepresence of a stable and relatively low inversionlayer since layered stratification causes the low-level air mass to flow around the obstacle in anapproximately two-dimensional manner (Baines,1995). The strong wind stress parallel to the coastsresults in Ekman pumping close to the island, butthe absence of a continuous land boundaryrestricts the longitudinal extension of the upwel-ling or downwelling. However, vertical motion isextended beyond the island along the wind shearlines and could be as important a mechanism forgenesis of eddies as the current past the island.Once generated, eddy motions can persist for longperiods. Indeed the anticyclone reported here was

subsequently seeded with Argos drifters thatcirculated within the eddy for B7 months as itdrifted away from the island.It seems likely that variations in trade wind

direction cause the wind shear fronts to sweep toand from downstream on a time scale of 7–10 daysand, as a consequence, to spread the upwelling/downwelling over wider regions. This effect in-creases with distance from shore and would ex-plain the absence of clear upwelling/downwellingregions in our Section 2 despite the presence ofstrong wind shear zones. Notwithstanding thevariable position of the fronts, their regularoccurrence to the south of the island can be con-sidered as a continuous source of eddy productionand enrichment for the local marine ecosystem. Onthe lee boundaries, observed horizontal wind shear15m s�1 in 2 km produces vertical velocities (upand down) as strong as in the African coastalupwelling. The corresponding Ekman transport,assuming a drag coefficient of 1.3� 10�3 at 281N,is 4.3m2 s�1, which would cause upwelling (ordownwelling) of B50md�1 if the vertical motionwere constrained to the 8 km wide bands seen inSection 1. The vertical motion would be propor-tionately smaller for the smeared fronts suggestedby the variation in trade wind direction. Theassociated Ekman current (transport divided byEkman layer depth) averaged over the observed50m deep mixed layer east of the lee would havemagnitude 0.1m s�1, which is large enough toaccount for the accumulated flotsam in theconvergent front. Of course, this accumulation isdue to the combined effect of the frontal flowpatterns and the capability of some organisms anddebris to eliminate one component of the flow field(Olson and Backus, 1985) by swimming orfloating.Island-induced eddies have been observed in

many surveys around Gran Canaria, but this is thefirst occasion on which a well-defined anticyloniceddy has been observed in the process of beingshed from the island. Anticyclonic eddy develop-ment close to Gran Canaria may often be inhibitedby the extension of Cape Bojador (NW-Africa)upwelling filaments towards the south of GranCanaria (Barton et al., 1998). In the contrastingcase of the neighbouring island of Tenerife,

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1097

conditions seem to favour anticyclonic rather thancyclonic eddies.

4.2. Consequences for chlorophyll distribution

The physical processes discussed above, musthave important consequences for biological pro-cesses and distribution patterns. In many areas,fronts and transition zones between different watermasses are, typically, sites of increased biologicalactivity and changes in the biomass and speciescomposition of planktonic organisms (e.g. Holli-gan, 1981; Le Fevre, 1986). They are sometimesregarded as marking a boundary between differentplankton communities (Legendre et al., 1986). Thefrontal regions bounding an island wake may bedifferent because they are the result of intensifiedvertical processes generally occurring within thesame water mass. Here, the key feature is windstress that generates upwelling-downwelling move-ments and affects water column stratification.Therefore, biological variability can not be attrib-uted to differences in the species composition oneither side of the front or to past history of thephytoplankton communities and their evolution,even though these may be significant on longertimescales, such as those involved in interactionsbetween upwelled water in a filament and theisland wake.The most obvious feature of Canarian waters,

outside the African upwelling zone, is the presenceof a deep chlorophyll maximum. Tett et al. (inpress) have demonstrated that the dynamics of theunperturbed DCM can be understood in terms ofsteady–state balance between, on one hand, lightlimited phytoplankton production, and, on theother hand, losses due to exported production andthe local metabolism of microplankton (phyto-plankton+microbial loop). In their work, thisbalance occurred at densities of approximately26.4 kgm�3, mean illuminations between 1% and3% of surface PAR, and nitrate+nitrite (TOxN)concentrations in the range of 0.3–2 mM. Incontrast, our observations indicate that the ob-served DCM is subjected to intense island gener-ated perturbations and in most cases represents astressed community displaced from its steady-statebalance. The major features of DCM intensity and

depth in waters south of Gran Canaria are thuslikely to be the results of a short-term response tothese perturbations.Perturbations must initially displace phyto-

plankton along with the water in which they weregrowing, with the result that the DCM might beexpected to shift with the isopycnal at which it isfound in the steady state. In the sections from June1998, the fluorescence-DCM was most commonlyfound on the 26.2 kgm�3 isopycnal (pattern (c) inFig. 7), and, as evidenced by Fig. 6 and showndiagrammatically in Fig. 9, stayed with thisisopycnal as it was displaced downwards in thewake’s convergent boundary. On the upwellingboundary of the wake, however, the uplift ofisopycnals brings within the euphotic zone waterthat has an increased concentration of nutrients(station DV in Fig. 8). Given time, the DCMmight be expected to shift to a higher density, inthis case to 26.4 kgm�3. The region of enhancedfluorescence-chlorophyll concentration observedto the west of the wake’s divergent boundary(pattern (a) in Fig. 7) could have been a conse-quence of this diapycnal shift, giving access togreater nutrients that support a higher biomass atthe DCM against grazing.In addition, an increased supply of nutrients

into the surface mixed layer should be able tomaintain enhanced production. This is consistentwith the few primary production values obtainedin our study (only three experiments at the sur-face), where higher assimilation numbers (4.670.1mgC (mg Chl a)�1 h�1) were observed in thevicinity of the divergent boundary of the lee(values of 3.970.2 and 3.570.2mgC (mg Chl a)�1

h�1 were obtained for LEE and CV). Themagnitude of this enhancement is presumablylinked to upwelling intensity. Ar!ıstegui et al.(1989) also measured relatively high nitrate con-centrations and primary production rates in thewind-exposed region, in a station close to the coast(B60m isobath).Enhanced phytoplankton biomass (>0.5mg

m�3 as deduced from fluorescence-chlorophyll)occurred only at the DCM on the western marginof the wake. Such enhancements, have, howeverbeen regularly observed (Ar!ıstegui et al., 1997;Barton et al., 2000). In some cases, they have been

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–11011098

attributed to cyclonic eddy recirculation (Ar!ısteguiet al., 1997). However integrated chlorophyllvalues, in our case, were lower at the divergent(upwelling) boundary than within the wake,suggesting that the increases at the DCM werethe result of a vertical biomass concentration atthis level rather than of an overall enhancement.In the divergent frontal region, the intensified

flux of nutrient and consequent increase ofprimary production rates did not seem to resultin biomass accumulation. We speculate that suchregions of enhanced phytoplankton productionattract mesozooplankton, which keep the biomassgrazed to just above a feeding threshold. Indeed,Hern!andez-Le !on (1988) observed gradients ofmesozooplankton biomass and ETS activity re-lated to the wind shear zones. Phytoplanktonbiomass may not be, therefore, the best index ofthe islands’ impact on the production (Simpsonand Tett, 1986).A final complication to the argument presented

here, and sketched in Fig. 9, is the possibility thata filament of coastally upwelled water may beinjecting water into the wake-generated circula-tions (see Barton et al., 1998; Ar!ıstegui et al.,1997). In August 1999, one of the signals of

upwelled water was a DCM shifted to isopycnalsbetween 26.2 and 26.0 kgm�3. It is thereforepossible that the DCM found on 26.4 kgm�3 inthe west of the wake section is the true ‘un-perturbed’ DCM and that on 26.2 kgm�3 in theeast of the section is the result of filamentinfluence.

5. Conclusion

It is well known that islands may impact theocean environment by inducing flow perturbationsand favouring localised vertical and horizontalnutrient fluxes to produce the biological enhance-ment termed the ‘island mass effect’ (Doty andOguri, 1956). In the case of the Canaries, Hawaiiand possibly many more mountainous islands therole of wind shear fronts caused by disturbance ofthe atmospheric flow is significant in contributingto the oceanic perturbations. From the presentstudy we conclude:Wind shear zones downstream of Gran Canaria

give rise to oceanic eddies (or at least stronglyenhance the effect of flow disturbance eddies), e.g.

Fig. 9. Simplified picture of links between physical and biological structure in the wake section, viewed from the south.

G. Basterretxea et al. / Deep-Sea Research I 49 (2002) 1087–1101 1099

the anticyclonic eddy of long duration documen-ted here.

The lee of Gran Canaria is characterized by alocal one-dimensional diurnal heating cyclestrongly modified by vertical thermocline(therefore pycnocline) displacements and eddyrecirculation related to Ekman pumping on thewind shear boundaries.

Downwelling on the convergent boundarymoves the DCM and its associated 26.2isopycnal below the 1% light zone.

Upwelling at the divergent boundary not onlyelevates the DCM with its associated isopycnalbut also, because of the increased light levelsand vertical flux of nutrients, permits a shift inthe DCM to higher (deeper) density surfaces(26.4).

The occurrence of highest integrated chloro-phyll in the central lee is consistent with ideaseither that zooplankton grazing strongly limitsphytoplankton biomass accumulation in thedivergent front or that the anticyclone intro-duces filament waters of African coastal origininto the wake.

Further dedicated studies of this highly dynamicand biologically active region promise to throwlight on the eddy dynamics and their biologicalconsequences. From a global point of view,beyond providing a possibly important source ofvertical and horizontal exchange in the worldocean, island situations provide natural labora-tories where a variety of processes important ingoverning the DCM in many parts of the oceancan be studied because they occur repeatedly onshort time and space scales.

Acknowledgements

We would like to express our thanks to theofficers and crew of R/V Garc!ıa del Cid and to thetechnicians of the UGBO for their support of ourwork at sea. Dr. A. Hern!andez-Guerra kindlysupplied the AVHRR image herein included. Thispaper was completed while Dr. E.D Barton was on

sabbatical at CICESE en BCS, Mexico, withsupport from CONACyT Catedra PatrimonialEX-0100009. This research was funded byFRENTES and CANIGO (MAST3-CT96-0060)projects.

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