The Gulf of Cadiz Gap wind anticyclones

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<ul><li><p>Research papers</p><p>The Gulf of Cadiz Gap wind anticyclones</p><p>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</p><p>a r t i c l e i n f o</p><p>Article history:Received 21 January 2014Received in revised form28 June 2014Accepted 2 September 2014</p><p>Keywords:Mesoscale eddiesWind curl-induced eddiesGap windsStrait of GibraltarGulf of Cadiz</p><p>a b s t r a c t</p><p>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</p><p>&amp; 2014 Elsevier Ltd. All rights reserved.</p><p>1. Introduction</p><p>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).</p><p>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.</p><p>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),</p><p>123456789</p><p>101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566</p><p>676869707172737475767778798081828384858687888990919293</p><p>Contents lists available at ScienceDirect</p><p>journal homepage: www.elsevier.com/locate/csr</p><p>Continental Shelf Research</p><p>http://dx.doi.org/10.1016/j.csr.2014.09.0040278-4343/&amp; 2014 Elsevier Ltd. All rights reserved.</p><p>E-mail address: ajpeliz@fc.ul.pt (A. Peliz).1 Tel.: 351 21 7500810; fax: 351 21 7500119.</p><p>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</p><p>Continental Shelf Research () </p><p>www.sciencedirect.com/science/journal/02784343www.elsevier.com/locate/csrhttp://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004mailto:ajpeliz@fc.ul.pthttp://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004</p></li><li><p>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).</p><p>Peliz et al. (2009b) simulated the upper ocean response to gapwinds in the Strait of Gibraltar (Fig. 1) and explained the formation</p><p>of the filaments, but their experiments were too short to allow forobservations of the development of anticyclones.</p><p>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.</p><p>2. Data and methods</p><p>2.1. Observations</p><p>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.</p><p>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.</p><p>Both databases (ASCAT and CCMP) are on the same grid withresolution of 0.251.</p><p>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.</p><p>123456789</p><p>101112131415161718192021222324252627282930313233343536373839404142434445464748495051525354555657585960616263646566</p><p>676869707172737475767778798081828384858687888990919293949596979899</p><p>100101102103104105106107108109110111112113114115116117118119120121122123124125126127128129130131132</p><p>200 200</p><p>200</p><p>200</p><p>200</p><p>200</p><p>600</p><p>600</p><p>600</p><p>600</p><p>600</p><p>600</p><p>600</p><p>1000</p><p>1000</p><p>1000</p><p>1000</p><p>1000</p><p>2000</p><p>2000</p><p>2000</p><p>3000</p><p>3000</p><p> 9oW 8oW 7oW 6oW 5oW 30 </p><p> 35oN</p><p> 30 </p><p> 36oN</p><p> 30 </p><p> 37oN</p><p> 30 </p><p>St. </p><p>Gib</p><p>CSM</p><p>Gua</p><p>dalq</p><p>uivir</p><p>Med</p><p>. Sea</p><p>Gulf of Cadiz</p><p>PEbuoy</p><p>Fig. 1. Map of the Gulf of Cadiz and Strait of Gibraltar. The colors show the land topography with the green-red shading indicating the mountains north and south ofMediterranena Sea entrance responsible for the gap Winds. The Guadalquivir estuary and the Cape Santa Maria (CSM) are also indicated. The curved lines represent theevolution of the Levanter Anticyclone observed in the summer of 1997. The blue curved lines correspond to the envelop of the eddy's warm patch at several moments asdetected by its signature in the SST infrared satellite imagery. The moment when the eddy was first detected is plotted in magenta and the last detection is marked red. Theblack lines represent the isobaths. The location of the PE buoy is indicated with a red star at about 71W, 36.51N. (For interpretation of the references to color in this figurecaption, the reader is referred to the web version of this article.)Q4</p><p>A. Peliz et al. / Continental Shelf Research () 2</p><p>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</p><p>http://oceancolor.gsfc.nasa.gov/ftp://podaac-ftp.jpl.nasa.gov/allData/ccmp/L3.0/flk/ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/MWF/L3/ASCAT/ftp://ftp.ifremer.fr/ifremer/cersat/products/gridded/MWF/L3/ASCAT/http://www.puertos.eshttp://www.puertos.eshttp://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004http://dx.doi.org/10.1016/j.csr.2014.09.004</p></li><li><p>A CTD section of CANIGO 97 (Section 5, 1213 September)cruise is used to describe the internal structure of one LA. TheseCTD data have been used in several papers (e.g., Borens et al.,2002).</p><p>2.2. Model simulations</p><p>A 20-year long (19892008) 2 km (32 levels) resolutionsimulation is used to describe the dynamics of the model LA.The model is documented in Peliz et al. (2013a,b). These simula-tions are forced with high-resolution model winds described inSoares et al. (2012). We use 3D 2-day averaged data of salinitytemperature and velocity to describe the LAs events in the model.The dates of the outputs correspond to the central time of the2-day averaging period.</p><p>2.3. Detection of events</p><p>After testing different criteria for the identification of eddies,like for example looking for all SST images after 5-day periods ofeasterly winds above 10 m/s (our first guess), we discovered thatLA events could be detected in SST quicklook databases very easilywithout the need for reprocessing all images. To facilitate theprocess the analysis was restricted to the period MayNovember,and all images were checked regardless of wind conditions.It wasconcluded that LA situations are very unequivocal since they havea very sharp filament-like SST signature at the beginning, and aprolonged very warm core eddy-type signature during a substan-tial period usually lasting beyond several passing synoptic situa-tions. What is not so clear sometimes, is whether the spotted LAsin SST scenes are independent or the same event, after longperiods of clouded sky. At least in 2003, two situations seem tocorrespond clearly to two independent events.</p><p>In what concerns the model outputs, the signature of LA in thedynamical fields is so clear that there is no need for disambigua-tion. No other process or type of eddies of such magnitudes wereobserved in those places. The model LAs were easily traced in thesurface flow magnitude and vorticity fields.</p><p>3. Results</p><p>3.1. Wind forcing during Levanter events</p><p>Fig. 2 shows the wind vector time series (the axis of the windvectors was rotated 901 anticlockwise such that easterlies areclearly seen and point to the bottom of the page) near the Straits ofGibraltar, with 6 h interval for 1997 (a) and 2003 (b) and dailyaveraged for 2013 (d). The 6 h averaged observed winds at theCadiz PE buoy are shown for 2003 only (c), since there were nodata available for the study period of 1997.</p><p>The three events are slightly different in what concerns thewind persistence. The easterlies in 1997 seemed to be more pulse-like whereas in 2003 the episodes were shorter but more stable.Both events happened during AugustSeptember and lasted onthe order of 23 weeks. The winds in 2013 are a mixture of theprevious situations. They have a period of persistent Levanterbetween 22 June and 10 July, and a more pulse like period after31 July.</p><p>Fig. 3 shows the spatial structure of the wind jet for the periodsbefore and during the peak of the event (1997, 2003 and 2013cases). As it is typical of summer time, the north winds prevailmost of the time. The establishment of easterlies (Levanter events)produces a significant change of the wind forcing in the Gulf ofCa...</p></li></ul>