| ABSTRACT | The upper layer circulation of the Ionian Sea is strongly influenced by quasi-decadal reversals of the Northern Ionian Gyre (NIG), due to the mechanism called BiOS (Bimodal Oscillating System) powered by the difference in salinity between the salty and warmer waters originating from the eastern Mediterranean, and the less saline water of Atlantic origin entering from the Sicily Channel. Altimetry data, drifter data and model surface salinity products were used to define the temporal phases and peculiar features of the circulation related to the anticyclonic and cyclonic NIG modes. Results not only highlight different shapes and intensities of the Mid-Ionian Jet and northern Ionian meander between the two circulation modes, but they emphasize also distinctive behaviour within the same mode, imputable to the variability of the mesoscale quasi-permanent features. The Messina Rise Vortex and the Pelops Gyre, on the western and eastern sides of the northern Ionian, respectively, showed different behaviours during the first (1993-1996) and the second (2006-2010) anticyclonic periods, related to the strong cyclonic activity along the dense water pathway. When the dense water was of Aegean origin (1993-1996; “Aegean” anticyclone), the strong cyclonic mesoscale activity on the eastern flank of the Ionian overwhelmed the anticyclonic wind forcing, and led to the disappearance of the Pelops Gyre. When the dense water was of Adriatic origin (2006-2010; “Adriatic” anticyclone), the strong cyclonic mesoscale activity on the western flank of the Ionian increased the longitudinal extension of the Messina Rise Vortex. The zonally-elongated Messina Rise Vortex reduced the inflow of Atlantic Water in the northern Ionian, and induced a cyclonic-like event in the southern Ionian and Cretan Passage. From these results, the interannual variability of the wind driven quasi-permanent mesoscale structures appear further influenced by internal forcings (i.e. deep water formation site). The interplay among decadal, interannual and climatic variability affects the characteristics of the basin-wide circulation and the water masses advection in the Ionian Sea. |