Observations of the Mediterranean Undercurrent and eddies in the Gulf of Cadiz during 2001

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  • , F. Neves, T. Ferreira

    The RAFOS floats evidenced the formation of MWeddies at three sites off the Portuguese coast: in Portimo Canyon, off Cape

    The North Atlantic is strongly influenced by the pre-sence of a high salinity and temperature water mass the

    Available online at www.sciencedirect.com

    Journal of Marine Systems 71 (

    Corresponding author. Tel.: +351 2175000 80; fax: +351

    217500009.St. Vincent and in the Estremadura Promontory. The period of eddy generation could be quantified within the range 4560 days.The comparison between the winter and summer cruises has confirmed some aspects of the Mediterranean outflow which had

    been identified in previous data sets, namely the higher temperature and salinity values, the broader lateral extension and the highervelocities of the outflow in winter relative to summer values. 2007 Elsevier B.V. All rights reserved.

    Keywords: Mediterranean outflow; Undercurrents; Eddies; Seasonal variations; Oceanographic surveys; Lagrangian current measurements; NEAtlantic; Gulf of Cadiz

    1. IntroductionInstituto de Oceanografia, Faculdade de Cincias da Universidade de Lisboa, 1749-016 Lisboa, Portugal

    Received 3 May 2007; received in revised form 5 July 2007; accepted 12 July 2007Available online 2 August 2007

    Abstract

    The North Atlantic is strongly influenced by the presence of a high salinity and temperature water mass the MediterraneanWater (MW) lying at intermediate levels. A detailed field survey in the region around the Portimo Canyon, which constitutes thefirst large canyon that the Mediterranean Undercurrent encounters downstream of the Strait of Gibraltar, has been repeated in threedifferent seasonal conditions (winter, spring and summer) of the year 2001, in the frame of Project MEDTOP. This paper is basedon the observations undertaken during MEDTOP and focus on relevant aspects of the thermohaline and velocity fields associatedwith the Mediterranean Undercurrent and the associated eddies, bringing further evidence of hypotheses presented in previouswork on the dynamical behaviour of the Mediterranean Outflow and its intra-annual variability, and describing new observations ofmeddy formation and subsequent evolution in the centre of the Gulf of Cadiz.

    The analysis of the CTD data at the MW levels has shown the presence of four MW cores: the Upper and Lower Cores (centredat about 800 and 1200 m), the Shallow Core (between 400 and 600 m) and, in the region close to the Portimo Canyon, a core withhigher density than the classical Lower Core at depths between 1300 and 1600 m. The analysis of the RAFOS floats trajectories hasshown the Upper and Lower Cores flowing independently along the continental slope, the Upper Core reaching systematicallyhigher velocities than the Lower Core. Within the meddies, there was evidence of a weak vertical coherence of the Cores in theearly stages of the eddy life, and a tendency for acquiring a solid-body rotation as they evolve away from their source.I. Ambar , N. SerraObservations of the Mediterranean Undercurrent and eddiesin the Gulf of Cadiz during 2001

    1E-mail address: iambar@fc.ul.pt (I. Ambar).1 Present address: Institut fr Meereskunde, Universitt Hamburg,

    Bundesstr. 53, D-20146 Hamburg, Germany.

    0924-7963/$ - see front matter 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2007.07.0032008) 195220www.elsevier.com/locate/jmarsysMediterranean Water (MW) lying at intermediate lev-els. This influence is both hydrographical and dynamical

  • arineand has been well documented in many works (e.g.,Reid, 1978; Baringer and Price, 1997; Iorga and Lozier,1999a,b). The presence of the MW not only plays animportant role in the pattern of the thermohaline field ofthe North Atlantic intermediate layers, but also in thetransport of suspended material (Thorpe and White,1988; Freitas and Abrantes, 2002; McCave and Hall,2002; Ambar et al., 2002) and live organisms of Medi-terranean origin (Ambar et al., 2002). The outlet of thiswater mass from the Mediterranean basin into theAtlantic is the Strait of Gibraltar. At the first stagesafter leaving the Strait, the dense MW constitutes abottom current that flows along the continental slope inthe Gulf of Cadiz subject to the deflecting effect of theearth rotation, which maintains it in the northern side ofthe Gulf. As the MWadvances through the layers of theAtlantic Water, its thermohaline characteristics are gra-dually modified by entrainment and its density progres-sively decreases until it reaches the equilibrium level.Downstream from about 8W and away from the upperslope, in the offshore side of the Undercurrent, the MWcorresponds already to an intermediate layer. One of theinteresting aspects of the MW intrusion into the Atlanticis the fact that it presents two vertically separated maincores (e.g. Siedler, 1968; Madelain, 1970; Zenk, 1970;Ambar and Howe, 1979a,b; Zenk and Armi, 1990;Borens et al., 2002), which, not very far away from theStrait of Gibraltar, are easily detectable in the temper-ature and salinity profiles as two maxima, respectivelycentred at about 800 m the Upper Core and 1200 m the Lower Core. As these two main cores proceedwestwards along the continental slope and leave the Gulfof Cadiz, they follow slightly different routes, the lowerone spreading mainly west and southwestwards fromCape St. Vincent (the SW corner of the Iberian Penin-sula) and the upper one bending northward and stayingcloser to the continental slope (Zenk and Armi, 1990;Daniault et al., 1994; Bower et al., 2002).

    In addition to these two classical cores, a shallowercore of MW which will be referred as Shallow Core characterized by comparatively higher temperatures hasbeen identified close to the upper continental slope offthe southern coast of Iberia (Zenk, 1975; Ambar, 1983)and also off the southwestern coast of Portugal (Ambar,1983; Hinrichsen et al., 1993), centred at depths around600 m.

    Observational results seem to indicate an intra-annualvariability of the Mediterranean Outflow with highervalues of temperature and salinity in winter as comparedwith those in summer (Ambar et al., 1999). The compa-rison of several thermohaline data sets collected during

    196 I. Ambar et al. / Journal of Mdifferent surveys in the Gulf of Cadiz for a period ofabout 20 years (December 1982September 2001) hasconfirmed this result (Ferreira, 2004).

    The spreading of MW in the Atlantic is reinforced bythe activity of submesoscale vortices (i.e., with dia-meters of the order of or less than 100 km) which detachfrom the Mediterranean Undercurrent and, being inrotation, tend to conserve for long periods (order ofyears) their original thermohaline characteristics (Armiet al., 1989; Pingree and Le Cann, 1993; Pingree, 1995).Although the existence of the MW in the Atlantic hasbeen known for a long time, the discovery of the exis-tence of these meddies (MW eddies) is relatively recent(late seventies). The first meddy identified as such(McDowell and Rossby, 1978) was found off Bahamas,at more than 6000-km distance from its origin (Gulfof Cadiz), conserving its thermohaline characteristics.Although nowadays there are some doubts about thatstructure being a meddy (Prater and Rossby, 1999), thisstarted a new age in what concerns the understandingof the MW spreading in the Atlantic. Since then, thistype of vortices was identified in many observationalworks in the eastern basin of the North Atlantic, espe-cially off the southwest coast of the Iberian Peninsula(e.g., Kse et al., 1989; Bower et al., 1995) and in theregion of the Canary Basin (Richardson et al., 1989).

    In general, meddies are lens-like structures, in solid-body anticyclonic rotation, with typical diameters ofabout 50100 km and vertical extents of about 700 m(i.e., between depths of 600 and 1300 m), periods ofrotation of about 6 to 8 days and azimuthal velocities ofabout 30 cm s1 (e.g., Armi and Zenk, 1984; Armi et al.,1988, 1989; Richardson et al., 2000). Cape St. Vincentwas identified as a site for meddy formation in the frameof the AMUSE project (Bower et al., 1995, 1997). Thereis also some evidence that their generation can beassociated with other bathymetric features such as thesubmarine canyons off the Iberian coast (Pingree and LeCann, 1993; Prater and Sanford, 1994; Serra and Ambar,2002) or with the Gorringe Bank (Serra and Ambar,2002).

    The dynamical field associated with the meddiesseems to extend vertically much farther than the depthrange where the thermohaline anomaly is detected, andmay reach levels down to 3000 m (Pingree, 1995) or upto the sea surface where meddies signatures have beendetected using satellite data (Stammer et al., 1991;Pingree, 1995; Oliveira et al., 2000).

    In the last decade, field observations off the southerncoast of Iberia (Carton et al., 2002; Serra et al., 2002)were able to detect the presence of cyclones associatedwith meddies to form vortex pairs at the level of the

    Systems 71 (2008) 195220MW. The existence of these cyclones was predicted by

  • potential vorticity considerations (Kse and Zenk, 1996)and is fundamental for the understanding of the trans-lation of meddies away from the generation zone. Alsosome evidence was found that cyclones play a role onthe detachment of the meddies from the MW Under-current (Serra et al., 2002).

    Although much progress has been accomplished inthe last decade, there is still need for a better under-standing of the dynamical aspects of eddies associatedwith the Mediterranean Undercurrent off south Portugal,namely of the role played by submarine canyons andcapes in the flow instabilities and eddy generation.These were the main objectives of the MEDTOP (Medi-terranean Undercurrent eddies and topographic ef-fects) Project which took place between 2000 and 2003.

    This paper is based on the observations undertakeni