José C. Sánchez-Garrido

Meteorologically-driven circulation and flushing times of the Bay of Algeciras, Strait of Gibraltar

A primitive-equation model has been used to investigate the meteorologically-driven circulation of the Bay of Algeciras. It is shown that the mean circulation of Atlantic Water (AW) is characterized by an anticyclonic cell, while Mediterranean Water (MW) follows a preferred cyclonic pathway. Meteorological forcing distorts substantially the AW mean circulation pattern, and only modulates that of the MW. Winds drive a vertical circulation cell in the Atlantic layer consistent with Ekman dynamics, whereas the horizontal circulation pattern is markedly dependent on the swift Atlantic jet entering the Mediterranean and changes from clearly anticyclonic to cyclonic as the jet separates or approaches the straits northern shoreline. This occurs through atmospheric pressure-driven acceleration/deceleration of the jet, in agreement with internal hydraulics theory predictions. It is also found that the renewal of AW is largely modulated by tides, with meteorological forcing playing a secondary role. The opposite applies to the renewal of MW.

Modeling the impact of tidal flows on the biological productivity of the Alboran Sea

The control of phytoplankton production by tidal forcing in the Alboran Sea is investigated with a high-resolution ocean circulation model coupled to an ecosystem model. The aim of the modeling efforts was to elucidate the role of tides in sustaining the high biological productivity of the Alboran Sea, as compared with the rest of the Mediterranean subbasins. It is shown that tidal forcing accounts for an increase of phytoplankton biomass and primary productivity in the basin of about 40% with respect to a nontidal circulation, and about 60% in the western Alboran Sea alone. The tidal dynamics of the Strait of Gibraltar is shown to be the primary factor in determining the enhancement of productivity, pumping nutrients from depth to the photic zone in the Alboran Sea. Model results indicate that the biological implications of the propagating internal tides are small. These results imply that nutrient transports through the Strait of Gibraltar have to be parametrized in ocean models that do not resolve tides in order to properly represent the biochemical budgets of the Alboran Sea.

Surface circulation at the Strait of Gibraltar: A combined HF radar and high‐resolution model study

Observations from a high frequency radar system and outputs from a high resolution operational ocean model working at the Strait of Gibraltar have been analyzed and compared during the period February 2013 to September 2014 in order to evaluate their capability to resolve the surface circulation of the region. The description of the mean circulation patterns has been statistically assessed, showing good agreement, particularly in the central region of the strait corresponding with the Atlantic Jet (AJ) stream, although some short scale features are not reproduced by the model. In the frequency domain very high concordance is observed. Tidal maps of diurnal and semidiurnal constituents are in good agreement with previous observations. The analysis of the model and radar response to the wind forcing reveals that the low resolution of the model wind-forcing field and its deeper superficial level smoothes the wind effect on the simulated currents. The first three EOF modes account for the 86% of model and radar variances. The coincidence between the observed and simulated patterns is very significant for the first two modes, which account for the mean velocity field and the latitudinal shifting of the AJ consequence of the flow-topography interaction. The third mode captures the wind-induced circulation, and greater discrepancies are found in this case. Results underline the complementary character of both systems: radar observations improve the model description, resolving short scale processes, while the model completes the radar information when the time or spatial coverage is poorer.

Recent changes (2004–2016) of temperature and salinity in the Mediterranean outflow

Temperature and salinity series near the seafloor at Espartel sill (Strait of Gibraltar) have been used to analyze the thermohaline variability of the Mediterranean outflow. The series shows temperature drops by the end of most winters/early-springs, which are the remote response to Western Mediterranean Deep Water (WMDW) formation events in the Gulf of Lion that uplift old WMDW nearby the Strait. This process distorts the seasonal cycle of colder/warmer water flowing out in summer/winter likely linked to the seasonality of the Western Alboran Gyre. The series show positive trends in agreement with previous values, which are largely increased after 2013. It is tentatively interpreted as the Western Mediterranean Transition (WMT) signature that started with the very cold winters of 2005 and 2006. It was only after the large new WMDW production of 2012 and 2013 harsh winters that WMT waters were made available to flow out of the Mediterranean Sea.

Asymmetric Baroclinic Response to Tidal Forcing Along the Main Sill of the Strait of Gibraltar Inferred from Mooring Observations

Data collected in the north and south channels of the main sill of the Strait of Gibraltar (Camarinal sill) have been used to investigate processes connected to the internal hydraulics of the exchange through the Strait at tidal frequencies. They strongly suggest the setting up of hydraulic jumps at both the western and eastern flank of the sill, the latter associated with the reversal of the Mediterranean undercurrent during spring tides. The northern site is more sensitive to processes triggered by the formation and release of the jump formed east of the sill during intense enough ebb tide cycles, which is thus better traced at this location, whereas the southern site detects the fluctuations and footprints associated with the hydraulic jump regularly formed to the west of the sill during flood tides more neatly. A detailed inspection of the high resolution bathymetry of the area reveals the existence of two enclosed depressions at either side of Camarinal sill, almost certainly carved by the bottom flow over the millennia, whose shape and morphology are suggestive of this spatial differentiation. In addition to the expected fortnightly periodicity of the spring-neap tidal cycle, the observed hydrodynamic features show a pronounced diurnal inequality caused by the tidal currents of the diurnal constituents.

The Mediterranean Overflow in the Gulf of Cadiz: A rugged journey

Bathymetric features determine the internal structure of the Mediterranean outflow plume west of Gibraltar. The pathways and transformations of dense water overflows, which depend on small-scale interactions between flow dynamics and erosional-depositional processes, are a central piece in the ocean’s large-scale circulation. A novel, high-resolution current and hydrographic data set highlights the intricate pathway travelled by the saline Mediterranean Overflow as it enters the Atlantic. Interaction with the topography constraints its spreading. Over the initial 200 km west of the Gibraltar gateway, distinct channels separate the initial gravity current into several plunging branches depth-sorted by density. Shallow branches follow the upper slope and eventually detach as buoyant plumes. Deeper branches occupy mid slope channels and coalesce upon reaching a diapiric ridge. A still deeper branch, guided by a lower channel wall marked by transverse furrows, experiences small-scale overflows which travel downslope to settle at mid-depths. The Mediterranean salt flux into the Atlantic has implications for the buoyancy balance in the North Atlantic. Observations on how this flux enters at different depth levels are key to accurately measuring and understanding the role of Mediterranean Outflow in future climate scenarios.