Cristina Naranjo

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.

Ten years of marine current measurements in Espartel Sill, Strait of Gibraltar

More than 10 year of Acoustic Doppler Current Profiler observations collected at the westernmost sill (Espartel sill) of the Strait of Gibraltar by a monitoring station have been carefully processed to provide the most updated estimation of the Mediterranean outflow. A comprehensive quality control of the factors affecting the uncertainty of the measurements has been carried out and great care has been paid to infer the current at the bottom layer, where direct observations are lacking. The mean outflow in the southern channel of the sill section has been estimated as −0.82 Sv (1 Sv = 1 × 106 m3 s−1), with an average contribution of the eddy fluxes of −0.04 Sv. This figure is an overestimation, as the mooring measurements, assumed valid for the whole section, ignore the lateral friction. On the other hand, it only gives the flow through the southern channel and disregards the fraction flowing through shallower northern part. Both drawbacks have been addressed by investigating the cross-strait structure of the outflow from hindcasts produced by the MITgcm numerical model, run in a high-resolution domain covering the Gulf of Cadiz and Alboran Sea basins. An overall rectifying factor of 1.039 was found satisfactory to correct the first estimate, so that the final mean outflow computed from this data set is −0.85 Sv, complemented with an uncertainty of ±0.03 Sv based on the interannual variability of the series. The temporal analysis of the series shows an outflow seasonality of around the 8% of the mean value, with maximum outflow in early spring.

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.