The Gulf of Cadiz Gap wind anticyclones

Highlights

• We study gap wind induced eddies in the Gulf of Cadiz (off the Strait of Gibraltar).

• Eddies occur during the stratified period after Levanter events.

• The first response is a cold filament followed by the generation of an anticyclone.

• Anticyclones form at a rate of ~ 0.7/year and last 1–2 months.

• Eddies remain trapped at the shelf break and some are swallowed into the Strait.

Abstract

We describe surface anticyclones developing in summer time after persistent Levanter (i.e., easterly) gap winds in the Gulf of Cadiz. The process of generation of these eddies is similar to those formed in the tropical Pacific eastern margin in many aspects, but their evolution and fate are different. The anticyclones are surface intensified structures with a radius of about 30 km, reach velocity maxima exceeding 30 cm/s, and have a strong baroclinic signature in the upper 150 m. They form when the thermocline is thin and shallow, after persistent easterly gap wind jets blowing through the Strait of Gibraltar. A conspicuous cold filament (nearly 8 °C negative anomaly), protruding seaward approximately aligned with the wind jet, is the first observable evidence of the phenomenon. An anticyclonic eddy is generated in the northern limb of the filament, and gradually acquires a rounded shape. No clear sign of a cyclonic counterpart is detected. The eddy remains trapped at the slope in close interaction with the Gulf of Cadiz slope Current (that feeds the Atlantic Inflow into the Mediterranean), and within a time-scale of 1–2 months the eddy dissipates by interaction with the Inflow and in some cases is swallowed into the Strait of Gibraltar. In 23 years (1991–2013) of satellite SST images we discovered 16 events (~0.7/yr). In a 20-yr simulation (1989–2008), 13 eddies were observed (~0.65/yr). The vast majority of eddies were observed in the August–October period. An event in 1997 is followed in SST imagery and with in situ hydrology data. This event is reproduced in the model in great detail and an analysis of its dynamics is presented.

Introduction

The process of generation of eddies induced by cross-shore wind jets was first studied in detail by Clarke (1988) and McCreary et al. (1989). This type of eddies is very frequent near the eastern margin of the tropical Pacific (see a review in Willett et al., 2006), but reports of similar structures in other ocean׳s margins are still missing. These eddies are formed due to the strong curl of a cross-shore wind jet. The winds are narrow inertial flows blowing seaward with cross scales of several tens of kilometers and offshore scales up to a few hundreds, and time scales of several days to weeks. If the thermocline is sufficiently thin and shallow, the pumping produced by the Ekman transport divergence induces an uplift of the pycnocline and a dipolar eddy may occur (McCreary et al., 1989). The shallow side tends to erode quickly by vertical mixing with development of a cold filament there. Either no clear cyclonic structure is formed or it quickly dissipates (McCreary et al., 1989, and also confirmed in observations by Trasvina et al., 1995, Trasvina and Barton, 2008).

Here, we describe for the first time the formation of anticyclones generated downstream of the Strait of Gibraltar by gap winds associated with persistent Levanter events (sometimes called Levantine winds). For abbreviation we shall refer to them as Levanter Anticyclones (LAs). LAs have several common aspects with the eastern tropical Pacific eddies in what concerns their generation, but they are peculiar in many other aspects, namely the fact that they remain trapped at the slope and also in the way they dissipate. The oceanographic context where both types of eddies appear is also very different.

The hydrology and circulation of the shelf and upper slope in the Gulf of Cadiz has been the subject of several investigations and review works in recent years (Sánchez and Relvas, 2003, Garcia Lafuente and Ruiz, 2007, Relvas et al., 2007, Peliz et al., 2009a, Prieto et al., 2009). The upper layer is dominated by a strong seasonal cycle with a sustained warming during spring and early summer reaching SST values in the order of 25 °C by July. Later in the summer, the shelf and slope zones in the Gulf are dominated by frequent upwelling favorable winds that prevent further warming of the upper layer. The slope flow in summer time, is characterized by persistent equatorward currents, a pattern very clear in the statistics of the PE buoy (see PE buoy location in Fig. 1) as it is described in Garcia Lafuente and Ruiz (2007), Peliz et al. (2009a), and Prieto et al. (2009). In the other seasons, the upper slope flow in the PE buoy site decreases significantly and presents sporadic reversals. This circulation pattern was linked to the seasonal cycle of the atmospheric forcing in earlier papers. Recent works (Peliz et al., 2009a, Peliz et al., 2013a) defend that the upper slope flow (the Gulf of Cadiz upper slope Current – GCC – as coined in Peliz et al., 2009a) is associated with the inflow into the Mediterranean and consequently it should be a persistent current with weak seasonality. The apparent contradiction with the clear seasonality at the PE buoy site is resolved in Peliz et al. (2013a). In the latter work, it is shown that the seasonal currents at the PE buoy site are associated with seasonal changes in the intensity, width and core position (in the across-slope direction) of the GCC. In summary, the upper slope zone where the LAs are formed is dominated by equatorward currents, that feed the inflow into the Mediterranean. In what concerns the wind forcing, the Gulf of Cadiz in summer time is characterized by North-Northwest (N-NW) events alternating with East (E) episodes (Sanchez et al., 2007, Boutov et al., 2014). Close to the strait, the orography (Fig. 1) favors zonal flow through the Strait of Gibraltar and easterlies become recurrent. Yearly averaged winds are N-NW in the western part of the Gulf and gradually become E near the Strait (see Fig. 1 in Sanchez et al., 2007). The alternation between N-NW and E episodes makes the zonal flow pattern as the dominant mode of variability in surface wind EOF analysis (see Fig. 5 in Boutov et al., 2014). During particularly strong easterlies (Levanter), the flow-topography interaction promotes the development of gap winds (Palomares Losada, 1999, Capon, 2006, Peliz et al., 2009b) which induce a very strong laterally sheared jet about 200 km long and a few tens kilometres across. These winds induce significant Ekman pumping via surface stress curl leading to a rapidly established cold filament (Peliz et al., 2009b).

Peliz et al. (2009b) simulated the upper ocean response to gap winds in the Strait of Gibraltar (Fig. 1) and explained the formation of the filaments, but their experiments were too short to allow for observations of the development of anticyclones.

In the next section, we describe the data and the simulations that were used. Next we describe three events (1997, 2003 and 2013) using observations. Afterwards, we analyze first two model LAs in the same time periods as the reported observations. The latest event is not covered by simulations yet. Finally, we summarize and discuss the results.