Modeling the oceanic circulation in the area of the Strait of Sicily: The remotely forced dynamics

In order to describe aspects of the baroclinic dynamics in the region of th e Strait of Sicily a high-resolution multilayer numerical model has been im plemented in a central Mediterranean region including the Tyrrhenian and th e Ionian Seas. Three layers have been considered representing water of Atla ntic origin (MAW), the Levantine Intermediate Water (LIW), and deep water o f the Mediterranean. Quasi-stationary circulations representing the local m anifestation of the large-scale Mediterranean conveyor belt are obtained [a fter an adjustment time of 0(2 months)] by imposing steady fluxes along the remote open boundaries, in the absence of meteorological forcings. These c irculations can be interpreted as possible dynamic scenarios of the seasona l 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 imp osed, resulting in a vanishing total net transport in each layer. For reali stic values of these transports the model captures the main features of the observed circulation, such as (i) the separation of the Algerian Current i nto two branches, one directed toward the Tyrrhenian Sea and the other ente ring the strait; (ii) a secondary bifurcation of MAW within the strait givi ng rise to a southward-moving current that follows the Tunisian continental slope and to a current that flows southeastward along the southern Sicilia n coast and then northward along the southern Italian coasts (the so-called Atlantic-Ionian Stream); (iii) a bifurcation of LIW at the strait level le ading to a main current directed toward the Strait of Sardinia and to a wea ker current that, after having crossed the strait, bends eastward and enter s the Tyrrhenian Sea. Sensitivity experiments carried out by imposing different boundary fluxes h ave 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 R-maw between the transport o f 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 sele cts the value R-maw approximate to 0.43 for a net MAW/LIW strait transport of +/-1 Sv, in excellent agreement with observations. Second, for decreasin g baroclinic transports the ratio Rmaw is found to decrease up to the limit ing value R-maw approximate to 0.2 (corresponding to the linear regime) for transports <0(0.1 Sv). Finally, R-maw is found to be very sensitive to the barotropic transport T through the strait, whereas the corresponding ratio for the LIW, R-liw, is virtually independent of T. For T = 20.5 Sv, R-maw <approximate to> 1.1 while for T = 10.5 Sv, R-maw decreases by one order of magnitude: R-maw approximate to 0.1. In other words, a weakening of the LI W (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 R-li w approximate to -0.3. It thus appears that the local control exerted by th e topography through the LIW potential vorticity budget forces the transpor t of the Tyrrhenian branch of LIW to be always similar to1/ 3 of that direc ted toward the Strait of Sardinia.