Friction and mixing effects on potential vorticity for bottom current crossing a marine strait: an application to the Sicily Channel (central Mediterranean Sea)

We discuss here the evolution of vorticity and po- tential vorticity (PV) for a bottom current crossing a ma- rine channel in shallow-water approximation, focusing on the effect of friction and mixing. The purpose of this re- search is indeed to investigate the role of friction and ver- tical entrainment on vorticity and PV spatial evolution in channels or straits when along-channel morphology varia- tions are significant. To pursue this investigation, we pose the vorticity and PV equations for a homogeneous bottom wa- ter vein and we calculate these two quantities as an integral form. Our theoretical findings are considered in the context of in situ hydrographic data related to the Eastern Mediter- ranean Deep Water (EMDW), i.e., a dense, bottom water vein that flows northwestward, along the Sicily Channel (Mediter- ranean Sea). Indeed, the narrow sill of this channel implies that friction and entrainment need to be considered. Small tidal effects in the Sicily Channel allow for a steady theoret- ical approach. We argue that bottom current vorticity is prone to signifi- cant sign changes and oscillations due to topographic effects when, in particular, the current flows over the sill of a chan- nel. These vorticity variations are, however, modulated by frictional effects due to seafloor roughness and morphology. Such behavior is also reflected in the PV spatial evolution, which shows an abrupt peak around the sill region. Our diag- noses on vorticity and PV allow us to obtain general insights about the effect of mixing and friction on the pathway and internal structure of bottom-trapped currents flowing through channels and straits, and to discuss spatial variability of the frictional coefficient. Our approach significantly differs from other PV-constant approaches previously used in studying the dynamics of bottom currents flowing through rotating channels.