Seasonal variability in the Central Mediterranean Sea circulation

International audienceA high resolution eddy-resolving primitive equation numerical model, based on the Princeton Ocean Model (POM), is used to study the seasonal variability of the general circulation in the Central Mediterranean Sea. The model is run on a seasonal cycle, perpetual year simulation for five years, with nesting to the coarser resolution Ocean General Circulation Model (OGCM), covering the whole Mediterranean Sea. The model results are compared to the current knowledge on the hydrography and dynamics of the area, with a special focus on the annual cycle of the Modified Atlantic Water (MAW), on the circulation in the Sardinia Channel, the water exchange across the Strait of Sicily, and on the transition and fate of the Levantine Intermediate Water (LIW). The results show that the adopted coupling techniques between the two models give a proficient downscaling of the large-scale OGCM flow field into the regional scale model. The numerical solution is also used to highlight the seasonal characteristics of important dynamical features in the area, as well as to shed light on the scarcely known circulation regimes along the north African shelf and slope. Key words. Oceanography: general (numerical modelling); Oceanography: physical (currents; general circulation

Numerical simulation and decomposition of kinetic energy in the Central Mediterranean: insight on mesoscale circulation and energy conversion

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. <br><br> 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. <br><br> 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

Central Mediterranean Sea forecast: effects of high-resolution atmospheric forcings

International audienceOcean forecasts over the Central Mediterranean, produced by a near real time regional scale system, have been evaluated in order to assess their predictability. The ocean circulation model has been forced at the surface by a medium, high or very high resolution atmospheric forcing. The simulated ocean parameters have been compared with satellite data and they were found to be generally in good agreement. High and very high resolution atmospheric forcings have been able to form noticeable, although short-lived, surface current structures, due to their ability to detect transient atmospheric disturbances. The existence of the current structures has not been directly assessed due to lack of measurements. The ocean model in the slave mode was not able to develop dynamics different from the driving coarse resolution model which provides the boundary conditions

Indication of recent warming process at the intermediate level in the Tyrrhenian Sea from SOOP XBT measurements

The Tyrrhenian Sea is a sub-basin of the western Mediterranean crossed by intermediate and deep waters from the eastern basin. Across this sub-basin, temperature profiles of the water column from expendable bathythermographs (XBT) have been acquired for sixteen years along transects realized thanks to the use of commercial vessels. Since 1999 an increase of temperature has been observed at intermediate depths even if interspersed with periods of decrease. This increase involves deeper and deeper depths along the years then involving the whole sub-basin in the range 200-800 m in September 2014 when largest anomalies over the whole period are found. The paper shows evidences of this rapid heating, giving insights into the origin and the diffusion of the warmer intermediate waters then showing its evolution in years and its relationship with the Eastern Mediterranean Transient

Observations of a phytoplankton spring bloom onset triggered by a density front in NW Mediterranean

Phytoplankton blooms in the northwestern Mediterranean Sea are seasonal events that mainly occur in a specific area comprising the Gulf of Lion and the Provençal basin, where they are promoted by a general cyclonic circulation, strong wind-driven mixing and subsequent re-stratification of the water column. At the southern boundary of this area, a persistent density front known as the north Balearic front can be found. The front is presumed to cause an early phytoplankton bloom in its vicinity because (a) it enhances the transport of nutrients into the euphotic layer and (b) it promotes the speedy re-stratification of the water column (through frontal instabilities). In February and March 2013, a glider, equipped with a CTD (conductivity, temperature, and depth device) and a fluorometer, was deployed on a mission that took it from the Balearic Islands to Sardinia and back. The frontal zone was crossed twice, once during the outbound leg and the once on the return leg. The data provided by the glider clearly showed the onset of a bloom soon after a decrease in wind-driven turbulent convection and mixing. The in situ observations were supported and confirmed by satellite imagery. It is shown that frontal dynamics play a key role in the promotion and acceleration of re-stratification, which is a necessary pre-conditioning factor for the onset of blooms much like other relevant processes such as an enhanced biological pump. Swift re-stratification stimulates new production by inhibiting mixing. Finally, viewing the blooming phenomenon from a regional perspective, it seems that Sverdrup's critical depth model applies in the northern well-mixed area whereas, in the south, front-related re-stratification seems to be the principal cause

Effects of the 2003 European heatwave on the Central Mediterranean Sea: surface fluxes and the dynamical response

International audienceThe 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

density front in NW Mediterranean

www.ocean-sci.net/10/657/2014/ doi:10.5194/os-10-657-201