The effects of the 2003 European heatwave on the sea surface layer of the Central Mediterranean were studied using a regional 3-D ocean model. The model was used to simulate the period 2000 to 2004 and its performance was validated using remotely-sensed and in situ data. Analysis of the results focused on changes in the Sea Surface Temperature (SST) and on changes to the surface and sub-surface current field. This permitted us to identify and quantify the anomalies of atmospheric and sea surface parameters that accompanied the heatwave. The dominant annual cycle in each variable was first removed and a wavelet analysis then used to locate anomalies in the time-frequency domain. We found that the excess heating affecting the sea surface in the summer of 2003 was related to a significant increase in air temperature, a decrease in wind stress and reduction of all components of the upward heat flux. The monthly averages of the model SST were found to be in good agreement with remotely-sensed data during the period studied, although the ocean model tended to underestimate extreme events. The spatial distribution of SST anomalies as well as their time-frequency location was similar for both the remotely-sensed and model temperatures. We also found, on the basis of the period of the observed anomaly, that the event was not limited to the few summer months of 2003 but was part of a longer phenomenon. Both the model results and experimental data suggest the anomalous heating mainly affected the top 15 m of ocean and was associated with strong surface stratification and low mixing. The skill of the model to reproduce the sub-surface hydrographic features during the heatwave was checked by comparison with temperature and salinity measurements. This showed that the model was generally in good agreement with observations. The model and observations showed that the anomalous warming also modified the currents in the region, most noticeably the Atlantic Ionian Stream (AIS) and the Atlantic Tunisian Current (ATC). The AIS was reduced in intensity and showed less meandering, mainly due to the reduced density gradient and low winds, while the ATC was enhanced in strength, the two currents appearing to modulate each other in order to conserve the total transport of Modified Atlantic Water.

Oceanic fronts are dynamically active regions of the global ocean that support upwelling and downwelling with significant implications for phytoplankton production and export. However (on time scales the inertial time scale), the vertical velocity is 10³–10⁴ times weaker than the horizontal velocity and is difficult to observe directly. Using intensive field observations in conjunction with a process study ocean model, we examine vertical motion and its effect on phytoplankton fluxes at multiple spatial horizontal scales in an oligotrophic region in the Western Mediterranean Sea. The mesoscale ageostrophic vertical velocity (∼10 m/day) inferred from our observations shapes the large‐scale phytoplankton distribution but does not explain the narrow (1–10 km wide) features of high chlorophyll content extending 40–60 m downward from the deep chlorophyll maximum. Using modeling, we show that downwelling submesoscale features concentrate 80% of the downward vertical flux of phytoplankton within just 15% of the horizontal area. These submesoscale spatial structures serve as conduits between the surface mixed layer and pycnocline and can contribute to exporting carbon from the sunlit surface layers to the ocean interior.

The spatial and temporal variability of eddy and mean kinetic energy of the Central Mediterranean region has been investigated, from January 2008 to December 2010, by mean of a numerical simulation mainly to quantify the mesoscale dynamics and their relationships with physical forcing. In order to understand the energy redistribution processes, the baroclinic energy conversion has been analysed, suggesting hypotheses about the drivers of the mesoscale activity in this area. The ocean model used is based on the Princeton Ocean Model implemented at 1/32° horizontal resolution. Surface momentum and buoyancy fluxes are interactively computed by mean of standard bulk formulae using predicted model Sea Surface Temperature and atmospheric variables provided by the European Centre for Medium Range Weather Forecast operational analyses. At its lateral boundaries the model is one-way nested within the Mediterranean Forecasting System operational products. The model domain has been subdivided in four sub-regions: Sardinia channel and southern Tyrrhenian Sea, Sicily channel, eastern Tunisian shelf and Libyan Sea. Temporal evolution of eddy and mean kinetic energy has been analysed, on each of the four sub-regions, showing different behaviours. On annual scales and within the first 5 m depth, the eddy kinetic energy represents approximately the 60 % of the total kinetic energy over the whole domain, confirming the strong mesoscale nature of the surface current flows in this area. The analyses show that the model well reproduces the path and the temporal behaviour of the main known sub-basin circulation features. New mesoscale structures have been also identified, from numerical results and direct observations, for the first time as the Pantelleria Vortex and the Medina Gyre. The classical kinetic energy decomposition (eddy and mean) allowed to depict and to quantify the permanent and fluctuating parts of the circulation in the region, and to differentiate the four sub-regions as function of relative and absolute strength of the mesoscale activity. Furthermore the Baroclinic Energy Conversion term shows that in the Sardinia Channel the mesoscale activity, due to baroclinic instabilities, is significantly larger than in the other sub-regions, while a negative sign of the energy conversion, meaning a transfer of energy from the Eddy Kinetic Energy to the Eddy Available Potential Energy, has been recorded only for the surface layers of the Sicily Channel during summer.