The importance of sub-mesoscale processes for the exchange of properties through the Strait of Gibraltar

Abstract

This article presents a detailed analysis of the sub-mesoscale transport processes in the Strait of Gibraltar. The interest is focussed on the Camarinal Sill region, and special attention is paid to the across-strait transport processes, the divergences and convergences in the central zone, and the small-scale circulation patterns along the northern coastal margin. The analysis is based on high-resolution (7 m) SST images acquired by an airborne hyper-spectral scanner, and has been complemented with a rhodamine-release experiment, continuous thermo-salinograph records, acoustic Doppler current (ADCP) profiles from both moorings and vessel-mounted experiments, and numerical modelling. It is deduced from the analysis that the coupling between the upwelling processes, induced by the internal tide and the generation of large-amplitude internal waves, and the cyclonic eddies formed on the coastal margin, seems to be the mechanism that explains the chlorophyll maxima frequently found on the coastal margin of the studied area. Further, as a consequence of the small-scale patterns of circulation induced by the internal waves, the suspended substances are displaced from the coastal margins toward the central zones and later are carried by the westward current toward the convergence zones created by the internal waves, where they may be retained and accumulate. Then, in the eastward phase of the tidal current over the Camarinal Sill, these nuclei of concentrated substances (nutrients, chlorophyll, and plankton) are transported toward the Alboran Sea, where they must contribute, in part, to the primary productivity there.

A high-resolution hydrodynamic-biogeochemical coupled model of the Gulf of Cadiz – Alboran Sea region.

“…Bruno et al, 2002;García-Lafuente et al, 2002) and could be easily identified by remote sensing as a strong cooling of surface waters (e.g. Bruno et al, 2013). Our model, even if correctly reproducing the mean fluxes through the Strait (Peliz et al, 2013b), is lacking these cyclic, recurrent mixing processes and, hence, simulates warmer than observed SST.…”

“…Sarhan et al, 2000;Macias et al, 2008aMacias et al, , 2009 or the Strait of Gibraltar (e.g. Macias et al, 2008b;Bruno et al, 2013;Ramirez-Romero et al, 2014) are much more common.…”

High-Chlorophyll-Area Assessment Based on Remote Sensing Observations: The Case Study of Cape Trafalgar

“…Cape Trafalgar is assumed to act as a source of biomass exporting chlorophyll and nutrient rich waters towards the Alborán Sea. Sub-mesoscale processes have been identified as important mechanisms in the chlorophyll distribution between coastal zones and the main channel of the SoG [19,52], along with the transport of nutrients to the Mediterranean Sea [14], highlighting mainly the role of the fortnightly tidal cycle [12,47]. A lateral advection of coastal waters into the centre of the channel takes place when internal waves are generated over the Camarinal Sill (Figure 1a) during the spring tides [12,52], and also, although it is less evident, during the release of arrested internal waves during neap tides [47].…”

“…As evaporation exceeds the sum of precipitation and river discharge [8], there is a two-layer inverse estuarine circulation that results in an upper Atlantic layer flowing towards the Alborán Sea and an outflow of deep high-density Mediterranean waters towards the Atlantic Ocean [9]. However, this general scheme increases its complexity due to a variety of phenomena such as intense and frequent wind (e.g., [10,11]), transport fluctuations due to the tidal cycles (e.g., [4,12]), or the generation of high-amplitude internal waves associated with periods of spring and neap tides (e.g., [13,14]). Furthermore, the topography strongly modifies this intricate scheme, and is particularly noticeable at the tidal scale (e.g., [15]).…”

Surface mesoscale pico–nanoplankton patterns at the main fronts of the Alboran Sea

“…The generation of internal waves in the Camarinal Sill causes the suction of coastal waters rich in chlorophyll towards the centre of the channel Navarro et al, 2011;Vázquez et al, 2009) and enhances the upwelling of deep nutrient-rich waters in the same region (Echevarría et al, 2002). The coupling of both processes favours the phytoplankton growth during its advection to the Alboran Sea (Bruno et al, 2013) and would support the spread by the AJ as the origin of population cell density observed in mesoscale patches in the NWAF.…”