Modeling the Oceanic Circulation in the Area of the Strait of Sicily: The Remotely Forced Dynamics

Abstract

In order to describe aspects of the baroclinic dynamics in the region of the Strait of Sicily a high-resolution multilayer numerical model has been implemented in a central Mediterranean region including the Tyrrhenian and the Ionian Seas. Three layers have been considered representing water of Atlantic origin (MAW), the Levantine Intermediate Water (LIW), and deep water of the Mediterranean. Quasi-stationary circulations representing the local manifestation of the large-scale Mediterranean conveyor belt are obtained [after an adjustment time of O(2 months)] by imposing steady fluxes along the remote open boundaries, in the absence of meteorological forcings. These circulations can be interpreted as possible dynamic scenarios of the seasonal variability in the Strait of Sicily. In the numerical simulations an inflow of MAW and an outflow of LIW through the Strait of Sardinia, an outflow of MAW and an inflow of LIW through the Ionian boundary, and an outflow of MAW through the Corsica channel are imposed, resulting in a vanishing total net transport in each layer. For realistic values of these transports the model captures the main features of the observed circulation, such as (i) the separation of the Algerian Current into two branches, one directed toward the Tyrrhenian Sea and the other entering the strait; (ii) a secondary bifurcation of MAW within the strait giving rise to a southward-moving current that follows the Tunisian continental slope and to a current that flows southeastward along the southern Sicilian coast and then northward along the southern Italian coasts (the so-called Atlantic?Ionian Stream); (iii) a bifurcation of LIW at the strait level leading to a main current directed toward the Strait of Sardinia and to a weaker current that, after having crossed the strait, bends eastward and enters the Tyrrhenian Sea. Sensitivity experiments carried out by imposing different boundary fluxes have shed light on the functioning of the MAW and LIW bifurcations. First of all, for a given net transport of MAW and LIW through the strait (imposed indirectly by the boundary fluxes), the ratio Rmaw between the transport of MAW entering the Tyrrhenian Sea and that entering the strait is found to be virtually independent of the boundary-imposed Algerian Current transport. It is, on the contrary, determined by a local dynamic control, which selects the value Rmaw ≈ 0.43 for a net MAW/LIW strait transport of ±1 Sv, in excellent agreement with observations. Second, for decreasing baroclinic transports the ratio Rmaw is found to decrease up to the limiting value Rmaw ≈ 0.2 (corresponding to the linear regime) for transports <O(0.1 Sv). Finally, Rmaw is found to be very sensitive to the barotropic transport T through the strait, whereas the corresponding ratio for the LIW, Rliw, is virtually independent of T. For T = ?0.5 Sv, Rmaw ≈ 1.1 while for T = +0.5 Sv, Rmaw decreases by one order of magnitude: Rmaw ≈ 0.1. In other words, a weakening of the LIW (or a strengthening of the MAW) net transport through the strait reduces the relative intensity of the Tyrrhenian branch of MAW, and vice versa. On the other hand, for values of T within the same range one always finds Rliw ≈ ?0.3. It thus appears that the local control exerted by the topography through the LIW potential vorticity budget forces the transport of the Tyrrhenian branch of LIW to be always ?1/3 of that directed toward the Strait of Sardinia.