Present and future long-term variability of the north Ionian surface circulation

In recent years, much work has been devoted to the characterization of the long-term variability of the circulation in the North Ionian Sea, which in the last three decades has been dominated by the so-called Bimodal Oscillation (BIOS). Here we further investigate this topic, both in the context of present climate and in a future scenario, analyzing the results of simulations made with the MED16 model. MED16 is a new, tide-including implementation of the MITgcm oceanic model, which covers the Mediterranean-Black Sea system with a horizontal resolution of 1/16° that is further increased at the Gibraltar and Turkish Straits. Three simulations have been performed: a hindcast run (1980-2010), used to validate the model; a historical run (1980-2005), which provides initial conditions for the scenario simulation; and a future climate (2006-2100) simulation, using atmospheric forcing under the Rcp8.5 emission scenario.

The hindcast run shows a variability of the surface circulation that is in good agreement with the observations, and indicates that during the period 1980–2010 the surface variability in the north Ionian is poorly correlated to that of the wind stress curl. The inversions of the circulation (switch from cyclonic to anticyclonic) that have been observed, and are well reproduced by the model, are apparently controlled by the Eastern Mediterranean Transient phenomenon: the huge volume of dense water produced by the Aegean Sea is the only forcing that may explain the strong anticyclonic surface circulation during the period 1993-1998. After that a prolonged cyclonic phase sets in, which weakens only during the 2004-2006 period.

On the other hand, the climatic projection over the next century shows a prevalence of the cyclonic circulation, well correlated to the prevalent positive wind stress curl, and  three clear inversions in which both the wind stress curl and the anomalies of dense water of Adriatic and Aegean origin appear to play a role. The simulation shows that the variability in the region is also affected by the strengthening and weakening of the cyclonic cell itself, which can modulate the ingression of the Atlantic Ionian Stream from the Sicily Strait and its path. This indicates that the multidecadal variability of the north Ionian circulation can play an important role in the control of the transport of the surface salinity in the Eastern Mediterranean Sea, even in extreme climatic conditions.