Upper ocean circulation in the Cape São Vicente (Iberian Peninsula) region: the coastal countercurrent

Paulo Relvas(1) and E. D. Barton(2)(1)CIMA - Universidade do Algarve (F.C.T.), Campus de Gambelas, 8000-117 Faro, Portugal. [prelvas@ualg.pt]

(2) School of Ocean Sciences, Univ. of Wales, Menai Bridge, Gwynedd, Wales LL59 5EY. [e.d.barton@bangor.ac.uk]

CM 2001/W:22

Summary: The coastal upwelling region near Cape São Vicente, the southwestern tip of the Iberian Peninsula, was studied using satellite images of sea surface temperature and time series of sea level heightand wind velocities, recorded at coastal sites within 200 km of the Cape, and in situ CTD and ADCP data. Summer upwelling is more intense and persistent off the western coast than off the southern coast, where a recurrent warm coastal countercurrent flows westward, and at times turns northward along the western coast after reaching the Cape, contrasting with the upwelling regime. Empirical evidence show the presence of an alongshore pressure gradient, stronger in summer, driving the coastal progression of the warm counterflow. Wind forcing plays an important role in the circulation by augmenting or diminishing the effect of the pre-existing alongshore pressure gradient, dictating the extent of the poleward progression of the inshore countercurrent. The hydrographic structure and kinematics revealed by the in situobservations, taken during an upwelling relaxation event, support the mesoscale dynamics that was suggested by the measurements taken from land and space.

22Figure 1 - Thermal infrared NOAA-AVHRR images for the first half of August 1991 (left) and of 20 of July 1986 (right) along with the wind vectors (low pass filtered at 40 hours) for the corresponding periods at Sines, Sagres and Faro, rotated alongshore for the western and south coasts. Image date is marked in the stick diagrams.

Sines Sines

SinesSines

Sines

FaroFaro

FaroFaro

SagresSagres

SagresSagres

Sagres

Faro

Cape São Vicente

countercurrent

filament

The Coastal Countercurrent: Fig. 1 (left) shows a sequence of sea surface temperature satellite images, along with time-series of low pass filtered wind vectors at Sines, Sagres and Faro, so that the ordinate represents the alongshore wind component, with negative values indicating the upwelling favourable wind component. Upwellingwas established along the western coast during early August 1991, in response to relatively intense southward wind, above 8 m/s. The associated equatorward jet appears to overshoot Cape São Vicente, feeding the southward development of a cold filament feature. Upwelling was present also along the south coast by 2 August (day 214), inresponse to a short westerly wind event (7-8 m/s) recorded at Faro. Upwelling later retreated along the south coast as a consequence of the relaxed winds recorded at Faro, and warm water progressed to the west close to shore, even turning clockwise around the cape by 13 August (day 225), profiting from easterly winds blowing at that time along the south coast. An explicit example of the coastal warm countercurrent turning clockwise around the Cape is shown in Fig. 1 (right), for 20 July 1986 (julian day 202) with low passed wind vectors at Sines, Sagres and Faro. Sagres wind is shown twice, unrotated and rotated to the east-west direction, as was done with the Faro data. The ‘nose’ of the warm feature is narrow but well defined, with a width about 10 km and a surface thermal signature of about 2ºC. The pattern in the satellite scene corresponds to the end of a short period of weak south coast downwelling favourable winds (about 3 m/s at Faro and 2 m/s at Sagres). The warm feature turns poleward around the Cape crossing the bathymetry and leaving some cooler water inshore. This occurred during a relaxation of the northerly winds at Sagres and Sines. Further north along the west coast upwelling was still present, as was the usual pattern of cool water along the south coast continental slope.

33The Alongshore Pressure Gradient: After removing tidal and other near diurnal effects from the sea level data recorded hourly from 1982 to 1991 at Sines, Lagos and V.R.S. António tide gauges (applying a Cosine-Lanczos low-pass filter with half power point at 40 hours spanning 121 hours), and correcting the data for the inverted barometer effect (using contemporaneous atmospheric pressure time series at Sines, Sagres and Faro, reduced to the mean sea level and demeaned), monthly mean sea level values were computed (Fig. 2, top). A common pattern of the annual cycle, with minimum spatial variations, is observed at the three tide gauges in close agreement with the sea level annual cycle of the eastern North Atlantic. All tide gauges of the national net are levelled with a maximum error of 0.5 cm (personal communication from Instituto Hidrográfico, responsible for the national tide-gauge chain), and all departures are referred to the national hydrographic zero, the benchmark established for the country. Accepting the levelling of the tide gauges, realistic comparison of absolute sea level height at the different tide gauges stations is possible, indicating an alongshore coastal elevation slope, with sea level rising throughout most of the year from Sines towards Lagos along the west coast, and from Lagos towards V.R.S. António along the south coast, with slight exceptions in November and December. Alongshore sea level slope is stronger in summer, withmaximum values of 3.93×10-7 between Sines and Lagos (142.5 km distance) and 5.00×10-7 between Lagos and V.R.S. António (120 km distance) in August. The total pressure gradient consists of a barotropicand a baroclinic contribution. For a coastline parallel to the x axis, the total alongshore pressure gradient at a depth D (positive z upward) is given by [Chelton, 1984]:

where p is the pressure, g the gravitational acceleration, η the sea surface elevation, ρ the water density, and ρ0 the surface water density. For shallow waters over the continental shelf, D is small and if it is assumed that the vertically integrated alongshore density gradient is small, the barotropic mode is dominant and the pressure gradient is proportional to the surface sea level slope.An independent check on the tide gauge levelling can be done by comparing the mean annual slope from the tide gauges with that from the dynamic height relative to 500 dbar (Fig. 2, bottom), obtained after gridding historical NODC data through an objective analysis scheme [Relvas, 1999]. The slope of the dynamic height has three sources of errors: sparse sampling may affect the result at any grid node; the choice of 500 dbar as the reference level near the slope, where a shallow Mediterranean water vein occurs [Ambar, 1983] may not be completely correct; and the assumption that the sea level slope is the same offshore and at the coast may not hold. However, the slopes estimated from the dynamic height and from the tide gauges have the same sense and order of magnitude. Thus, it seems evident that an alongshore monthly mean pressure gradient force acts during most of the year, favouring a coastal circulation westward along the south coast and northward along the west coast of southwestern Iberia. The strongest pressure gradient occurs in summer, when it is opposed by intense upwelling favourable winds, and the forcing is small or vanishes in winter.

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec200

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onio

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*

Figure 2 - Mean sea level computed from tide gauge measurements, corrected for the inverted barometric effect, at Sines, Lagos and V.R.S.António (top), and the alongshore mean annual slope from the tide gauges - dashed line - and alongshore dynamic height slope relatively to 500 dbar - solid line – (bottom). An inset with the location of the tide gauges is represented.

zx

gx

gxp

D∂

∂∂+

∂∂=

∂∂

∫0

0ρηρ

44The Wind Stress Balance: For a coastline parallel to the x axis, the vertically averaged alongshore component of the linear momentum equation for a barotropic ocean can be written as [Gill, 1982]:

where u and v denote the vertically averaged alongshore and cross-shore components of velocity, respectively, f denotes the Coriolis parameter, σs and σb denote the surface stress imposed by the wind and the bottom stress due to friction, respectively, and H denotes the water depth. It can be expected that the vertically averaged Coriolis term is small, because the depth average cross shelf flow is close to zero. The bottom stress will act to reduce any flow and when the current due to the alongshore pressure gradient is exactly balanced by an opposing wind stress, the bottom friction is zero. In this case, for a steady state flow, the vertically averaged alongshore flow is determined principally by the relative magnitude of the wind stress and the vertically integrated pressure gradient force for a given water depth, and so the momentum equation is reduced to:

with the wind stress written as: , where ρa represents the air density, represents the wind vector, and Cs the surface wind stress drag coefficient. The local alongshore wind stress and the alongshore pressure gradient force act in opposite directions. Simple calculations with ρa=1.225 kg/m3, ρ=1027 kg/m3, Cs=1.2×10-3, and assuming H=30 m, a typical value for the summer pycnocline in the region [Relvas and Barton, 1995], will imply a northerly alongshore wind velocity of 7.9 m/s in order to balance the mean sea level slope of 3.93×10-7

between Sines and Lagos, and a westerly alongshore wind velocity of 9.7 m/s in order to balance the mean sea level slope of 5.00×10-7 between Lagos and V.R.S. António. Thus, in mean terms, the alongshore pressure gradient forces a vertically averaged alongshore flow, westward along the south coast and northward along the west coast, unless the wind velocity exceeds the above values and forces the alongshore flow to reverse in the upper layers. Although assumptions made here are somewhat subjective, it is demonstrate that the wind forcing, under certain conditions, could drive the coastal flow against the alongshore pressure gradient.

aasa uV

HC

xg

ρρη =

∂∂

aasas uVCρσ = ( )aaa vuV ,=

HHxgfv

tu bs

ρσ

ρση ++

∂∂−−=

∂∂

55In Situ Observations: A physical oceanography experiment was realised in the Cape São Vicente upwelling region during the Cruise nº201 of RV POSEIDON, leg 9 (POS 201/9) between 11 and 22 of June 1994, as a collaboration of the Unidade de Ciências Exactas e Humanas/Universidade do Algarve and School of Ocean Sciences/University of Wales, Bangor with the EUROSQUID project AIR 1 CT 92 0573. 132 CTD stations were executed and the data from a hull-mounted Acoustic Doppler Current Profiler (ADCP) operating at 150 kHz were recorded continuously in conjunction with the navigational data provided by the GPS of the ship. During most of the cruise the winds were weak (<6 m/s), with an episode of stronger westerly wind up to 15 m/s after 20 June. No upwelling was evident in the AVHRR satellite imagery, since the survey took place a few days after the cessation of an upwelling event. The situation corresponds to a post upwellingpattern, as revealed by the sea surface temperature image of 16 June 1994, where the dynamic height field of the sea surface relative to 500 dbar (∆D0/500) and the ADCP near surface current vectors averaged for the 15-26 m depth bin, were overlaid (Fig. 3, left). There, a patch of warm coastal water around the Cape is seen to be coincident with the stronger observed northward surface velocities, sometimes exceeding 0.4 m/s, forming a coastal poleward counterflow. Cooler water is seen to lay offshore over the Algarve continental slope, where the flow, up to 0.3 m/s, is predominantly to the south. This flow, limited on both sides by northward velocities, is coincident with the gradient of dynamic height that turns eastward around the Cape, off the continental shelf, advecting predominantly cool water. The southward flow seems to penetrate into a region with an opposed flow regime, while turning eastward around the Cape. The configuration of the ADCP velocity vectors in the shear region between the southward flow and the coastal poleward counterflow suggests that part of the southward flow, turns cyclonically following the dynamic topography. Also, off the south coast close to the Cape, at least part of the near surface flow that turns the Cape is seen to curl cyclonically and contribute to the shallow coastal counterflow. The near surface current vectors (16-25 m bin) and the dynamic height field of the sea surface relative to 30 dbar (∆D0/30) off the west coast, superimposed to the enlarged image satellite (Fig. 3, right), show that the warmer counterflow that turns around the Cape, curls anticyclonically against the coast with velocities up to 0.4 m/s, consistent with the field of dynamic height, and progresses nearshore to the north, inshore of a patch of cold water, presumably originated during the upwelling event prior to the cruise. The 30 dbar reference is not a level of no motion, but the feature observed in the ADCP data is discernible. Further offshore the velocity vectors are weaker (<0.25 m/s) and are seen to point northward, curling slightly westward, advecting some of the warmer water preceding from the south coast round the cold patch to the offshore side, also following the dynamic topography.

-10.5 -10.0 -9.5 -9.0 -8.5Longitude

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Figure 3 - ADCP current vectors averaged for the 16-25 depth bin superimposed on the AVHRR sea surface temperature image and on the dynamic height ∆∆∆∆D0/500 field (left) and ∆∆∆∆D0/30 field for a detailed region off the west coast (right). The scale of the ADCP vectors is shown in the panels and units of the dynamic topography are dyn m. Dark patches represent clouds.

Acknowledgements: Authors would like to thank Institutode Meteorologia and Instituto Hidrográfico, Lisboa, for providing the meteorological and sea level data used in this work. Thanks are due to Peter Bayliss and staff at the Dundee Satellite Receiving Station, Dundee University, and to Steve Groom and Peter Miller, from the NERC/RSDAS at CCMS-PLM, Plymouth, for providing and processing the brightness temperature satellite image. We would like to thank Uwe Piaktowski, chief scientist of the Poseidon 201/9 cruise, for all the collaboration offered to the physical oceanography experiment.

References:

Ambar, I., A shallow core of Mediterranean water off western Portugal, Deep Sea Res., 30(6A), 677-680, 1983.

Chelton, D. B., Seasonal variability of alongshore geostrophic velocity off Central California, J. Geophys. Res., 89(C3), 3473-3486, 1984.

Fiúza, A. F. G., M. E. Macedo, and M. R. Guerreiro, Climatological space and time variation of the portuguesecoastal upwelling, Oceanol. Acta, 5, 31-40, 1982.

Frouin, R., A. F. G. Fiúza, I. Ambar, and T. J. Boyd, Observations of a poleward surface current off the coasts of Portugal and Spain during winter, J. Geophys. Res., 95(C1), 679-691, 1990.

Haynes, R., and E. D. Barton, A poleward flow along the Atlantic coast of the Iberian Peninsula, J. Geophys. Res., 95(C7), 11,425-11,442, 1990.

Haynes, R., E. D. Barton and I. Pilling, Development, persistence, and variability of upwelling filaments off the Atlantic Coast of the Iberian Peninsula, J. Geophys. Res., 98(C12), 22,681-22,692, 1993.

Sousa, F. M.,and A. Bricaud, Satellite-derived phytoplankton pigment structures in the Portuguese upwelling area, J.Geophys. Res., 97, 11,343-11,356, 1992.

Relvas, P., The physical oceanography of the Cape SãoVicente upwelling region observed from sea, land and space, Ph.D. thesis, 244 pp., Univ. of Wales, Bangor, 1999.

Relvas, P. and E.D. Barton, Poseidon Cruise 201/9 Report: Physical Oceanography Data Report, 284pp., University of Algarve, Faro, 1995.

Wooster, W. S., A. Bakun, and D. R. McLain, The seasonalupwelling cycle along the eastern boundary of the North Atlantic, J. Mar. Res., 34, 131-141, 1976.

66Conclusions: From the observations and analysis presented above, it is reasonable to conclude that the coastal circulation off southwest Iberia depends on both the wind stress and the pressure gradient alongshore, whose relative magnitudes govern the direction and strength of the circulation. During the summer off the west coast, the wind stress dominates the pressure gradient, an upwelling regime prevails, and the flow is equatorward for most of the season. A contrasting situation occurs off the south coast, where the pressure gradient dominates, the upwellingevents are intermittent, and the westward flow of a warm coastal countercurrent over the continental shelf, with observed velocities up to 40cm/s, dominates the coastal circulation. However, strong westerly winds are able to provoke coastalupwelling along the south coast and to reverse the alongshore flow, at least in the upper layers. Also, during northerly wind relaxations, the warm coastalcountercurrent along the south coast turns clockwise around Cape SãoVicente and continues northward along the west coast.

11Oceanographic Settings: The coastal ocean off Southwest Iberia is part of the eastern boundary of the North Atlantic. The region is characterised by a marked seasonality related to the large scale wind climatology. A poleward current is a persistent feature of the winter circulation, when the winds decay or are downwelling favourable [Frouin et al., 1990; Haynes and Barton, 1990]. A well defined upwellingseason occurs between March and September induced by prevailing northerly winds [Wooster et al., 1976; Fiúza et al., 1982]. Upwelling causes surface dynamic height to decrease towards the coast and the resulting equatorward geostrophic current is enough to counter the poleward current, establishing a southward jet flow in the upper layers. Deeper layers remains to flow northward as an undercurrent, advecting waters of predominantly subtropical and Mediterranean origin [Haynes and Barton, 1990].The coastline is dominated by the protrusion of the Cape São Vicente, where the south and west coast of Iberia meet in an almost right angle. Such configuration favours the summer development of a filament structure of cold upwelled water, anchored to the Cape [Sousa and Bricaud, 1992; Haynes et al., 1993]. The narrow (±25 km) wide southern shelf slopes gently down to a sharp edge at 100-130 meters depth, defined by a sudden step down to the 700 meters contour. This pronounced feature extends around the southwest tip of Portugal, reaching about 10 km north of Cape São Vicente. One of the most recurrent features during the upwelling season, is the development of a narrow (15-25 km) band of warm water along the south coast of Algarve progressing from the east, forcing the cooler recently upwelled water offshore over the continental slope (Fig. 1). Often it turns the Cape São Vicente, progressing poleward along the west coast, reaching the Cape of Sines at times. This coastal warm countercurrent is the target of the present research.