The Gulf of Cadiz Gap wind anticyclones
Alvaro Peliz a,1, Dmitri Boutov a,b, Ana Barbosa Aguiar a, Xavier Carton cQ1a Instituto Dom Luiz, Faculdade de Cincias, Universidade de Lisboa, Lisboa, Portugalb Centro de Oceanografia, Faculdade de Cincias, Universidade de Lisboa, Lisboa, Portugalc Laboratoire de Physique des Oceans, UMR6523 CNRS/IFREMER/UBO, UFR Sciences, Brest, France
a r t i c l e i n f o
Article history:Received 21 January 2014Received in revised form28 June 2014Accepted 2 September 2014
Keywords:Mesoscale eddiesWind curl-induced eddiesGap windsStrait of GibraltarGulf of Cadiz
a b s t r a c t
We describe surface anticyclones developing in summer time after persistent Levanter (i.e., easterly) gapwinds in the Gulf of Cadiz. The process of generation of these eddies is similar to those formed in thetropical Pacific eastern margin in many aspects, but their evolution and fate are different. Theanticyclones are surface intensified structures with a radius of about 30 km, reach velocity maximaexceeding 30 cm/s, and have a strong baroclinic signature in the upper 150 m. They form when thethermocline is thin and shallow, after persistent easterly gap wind jets blowing through the Strait ofGibraltar. A conspicuous cold filament (nearly 8 1C negative anomaly), protruding seaward approxi-mately aligned with the wind jet, is the first observable evidence of the phenomenon. An anticycloniceddy is generated in the northern limb of the filament, and gradually acquires a rounded shape. No clearsign of a cyclonic counterpart is detected. The eddy remains trapped at the slope in close interaction withthe Gulf of Cadiz slope Current (that feeds the Atlantic Inflow into the Mediterranean), and within atime-scale of 12 months the eddy dissipates by interaction with the Inflow and in some cases isswallowed into the Strait of Gibraltar. In 23 years (19912013) of satellite SST images we discovered 16events (0.7/yr). In a 20-yr simulation (19892008), 13 eddies were observed (0.65/yr). The vastmajority of eddies were observed in the AugustOctober period. An event in 1997 is followed in SSTimagery and with in situ hydrology data. This event is reproduced in the model in great detail and ananalysis of its dynamics is presented. Q2
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The process of generation of eddies induced by cross-shorewind jets was first studied in detail by Clarke (1988) and McCrearyet al. (1989). This type of eddies is very frequent near the easternmargin of the tropical Pacific (see a review in Willett et al., 2006),but reports of similar structures in other ocean's margins are stillmissing. These eddies are formed due to the strong curl of a cross-shore wind jet. The winds are narrow inertial flows blowingseaward with cross scales of several tens of kilometers andoffshore scales up to a few hundreds, and time scales of severaldays to weeks. If the thermocline is sufficiently thin and shallow,the pumping produced by the Ekman transport divergenceinduces an uplift of the pycnocline and a dipolar eddy may occur(McCreary et al., 1989). The shallow side tends to erode quickly byvertical mixing with development of a cold filament there. Eitherno clear cyclonic structure is formed or it quickly dissipates(McCreary et al., 1989, and also confirmed in observations byTrasvina et al., 1995; Trasvina and Barton, 2008).
Here, we describe for the first time the formation of anti-cyclones generated downstream of the Strait of Gibraltar by gapwinds associated with persistent Levanter events (sometimescalled Levantine winds). For abbreviation we shall refer to themas Levanter Anticyclones (LAs). LAs have several common aspectswith the eastern tropical Pacific eddies in what concerns theirgeneration, but they are peculiar in many other aspects, namelythe fact that they remain trapped at the slope and also in the waythey dissipate. The oceanographic context where both types ofeddies appear is also very different.
The hydrology and circulation of the shelf and upper slope in theGulf of Cadiz has been the subject of several investigations andreview works in recent years (Snchez and Relvas, 2003; GarciaLafuente and Ruiz, 2007; Relvas et al., 2007; Peliz et al., 2009a;Prieto et al., 2009). The upper layer is dominated by a strongseasonal cycle with a sustained warming during spring and earlysummer reaching SST values in the order of 25 1C by July. Later inthe summer, the shelf and slope zones in the Gulf are dominated byfrequent upwelling favorable winds that prevent further warmingof the upper layer. The slope flow in summer time, is characterizedby persistent equatorward currents, a pattern very clear in thestatistics of the PE buoy (see PE buoy location in Fig. 1) as it isdescribed in Garcia Lafuente and Ruiz (2007), Peliz et al. (2009a),
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Continental Shelf Research
http://dx.doi.org/10.1016/j.csr.2014.09.0040278-4343/& 2014 Elsevier Ltd. All rights reserved.
E-mail address: email@example.com (A. Peliz).1 Tel.: 351 21 7500810; fax: 351 21 7500119.
Please cite this article as: Peliz, A., et al., The Gulf of Cadiz Gap wind anticyclones. Continental Shelf Research (2014), http://dx.doi.org/10.1016/j.csr.2014.09.004i
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and Prieto et al. (2009). In the other seasons, the upper slope flowin the PE buoy site decreases significantly and presents sporadicreversals. This circulation pattern was linked to the seasonal cycleof the atmospheric forcing in earlier papers. Recent works (Pelizet al., 2009a, 2013a) defend that the upper slope flow (the Gulf ofCadiz upper slope Current GCC as coined in Peliz et al., 2009a)is associated with the inflow into the Mediterranean and conse-quently it should be a persistent current with weak seasonality.The apparent contradiction with the clear seasonality at the PEbuoy site is resolved in Peliz et al. (2013a). In the latter work, it isshown that the seasonal currents at the PE buoy site are associatedwith seasonal changes in the intensity, width and core position(in the across-slope direction) of the GCC. In summary, the upperslope zone where the LAs are formed is dominated by equator-ward currents, that feed the inflow into the Mediterranean.In what concerns the wind forcing, the Gulf of Cadiz in summertime is characterized by North-Northwest (N-NW) events alter-nating with East (E) episodes (Sanchez et al., 2007; Boutov et al.,2014). Close to the strait, the orography (Fig. 1) favors zonal flowthrough the Strait of Gibraltar and easterlies become recurrent.Yearly averaged winds are N-NW in the western part of the Gulfand gradually become E near the Strait (see Fig. 1 in Sanchez et al.,2007). The alternation between N-NW and E episodes makes thezonal flow pattern as the dominant mode of variability in surfacewind EOF analysis (see Fig. 5 in Boutov et al., 2014). Duringparticularly strong easterlies (Levanter), the flow-topographyinteraction promotes the development of gap winds (PalomaresLosada, 1999; Capon, 2006; Peliz et al., 2009b) which induce a verystrong laterally sheared jet about 200 km long and a few tenskilometres across. These winds induce significant Ekman pumpingvia surface stress curl leading to a rapidly established cold filament(Peliz et al., 2009b).
Peliz et al. (2009b) simulated the upper ocean response to gapwinds in the Strait of Gibraltar (Fig. 1) and explained the formation
of the filaments, but their experiments were too short to allow forobservations of the development of anticyclones.
In the next section, we describe the data and the simulationsthat were used. Next we describe three events (1997, 2003 and2013) using observations. Afterwards, we analyze first two modelLAs in the same time periods as the reported observations. Thelatest event is not covered by simulations yet. Finally, we summar-ize and discuss the results.
2. Data and methods
A database of SST satellite images of the Gulf of Cadiz coveringthe period from 1991 to 2013 was inspected to search for LAevents. From 1991 to 2005 the images consisted of high-resolutionLevel 2 NOAA-AVHRR (1.11.1 km/pixel) data (Centro de Oceano-grafia, FCUL archive). After 2002 the MODIS data (Aqua and Terra,http://oceancolor.gsfc.nasa.gov/) were also used.
6-hour gridded CCMP (Cross-Calibrated Multi-Platform) winds(ftp://podaac-ftp.jpl.nasa.gov/allData/ccmp/L3.0/flk/) were usedfor the 1997 and 2003 events. For the event of 2013, a time seriesof ASCAT winds at the western mouth of the Strait, is used to showthe Levanter circulation. Daily gridded ASCAT data were down-loaded from the ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/MWF/L3/ASCAT/ archive.
Both databases (ASCAT and CCMP) are on the same grid withresolution of 0.251.
To compare model results with the observations, in-situmoored buoy data (ocean currents at 3 m depth and wind at 2 mheight) from Spanish Puertos del Estado (PE buoy; www.puertos.es) in the Gulf of Cadiz (36.481N, 006.961W) was used. Thesampling period of these data is 1 h.