Submesoscale dynamics in the western Mediterranean sea

[eng] The transition from mesoscale to submesoscale dynamics is investigated in the western Mediterranean Sea (WMed) using a set of ROMS model simulations. The research is structured in a series of sequential stages covering the mesoscale-tosubmesoscale range, starting from a regional overview of the WMed ocean circulation and zooming in towards local processes. The mesoscale exploration is assessed in terms of the Lorenz energy cycle (LEC), which provides a quanti cation of the kinetic-potential energy exchanges through eddymean ow interactions. The sources of eddy kinetic energy are analyzed by applying a regional formulation of the LEC to 18 years of the ROMSWMED32 numerical simulation at eddy-resolving resolution (3.5 km), which allows identi cation of whether the energy exchange between the mean and eddy ow is local or nonlocal. The patterns of energy conversion between the mean and eddy kinetic and potential energy are estimated in three subregions of the domain: the Alboran Sea, the Algerian Basin, and the Northern Basin. Results from the LEC analysis reveal that the Alboran Sea is the most energetic region in the WMed. The spatial characterization of the energy conversion routes, together with the physical and dynamical characteristics of the area, hints at two principal submesoscale mechanisms involved in maintaining balance: topographic vorticity generation (TVG) and frontogenesis (FG). The transition toward the submesoscale is explored in the Alboran Sea by means of two nested, realistic simulations covering this region with increasing horizontal resolutions ranging from 1:5 km (WMed1500) to 0:5 km (Alb500). Unbalanced submesoscale dynamics emerge in the ner solution as the model resolution is increased. The occurrence of TVG and FG in Alb500 does not display a clear spatial nor temporal variability which facilitates an overall statistical approach. Instead, our analysis is focused on particular events of FG and TVG which are considered to be representative of the Alboran Sea dynamics. TVG is explored and quanti ed using the barotropic vorticity balance equation, in which the generation of vorticity through ow-topography interaction relies on contributions from bottom stress and form drag, the latter being the principal source. FG is analyzed in a recurrent, intense density front located at the eastern edge of the permanent western anticyclonic gyre (WAG) which has a similar structure to that of the climatological Almeria-Oran front. Alb500 accu