Decadal variations of circulation in the Central Mediterranean and its interactions with mesoscale gyres

The most prominent oceanographic features in the Central Mediterranean are decadal reversals of the Northern Ionian Gyre (NIG), interpreted in terms of internal processes. Altimetry data, drifter data and model surface salinity products are used in this paper to define some specific features of the circulation related to the anticyclonic or cyclonic NIG modes. Results not only highlight different shapes and intensities of the Mid-Ionian Jet and northern Ionian meander among the two circulation mode, but emphasize distinct behaviour within the same mode, imputable to the variability of the mesoscale quasi-permanent features. The wind-driven Messina Rise Vortex, on the western side of the northern Ionian, and the Pelops Gyre, on the eastern side of the northern Ionian, show different behaviour during the first (1993–1996) and the second (2006–2010) anticyclonic periods, related to the cyclonic activity along the dense water pathway. When the dense water was of Aegean origin (1993–1996; “Aegean” anticyclone), the 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 strengthening of cyclonic mesoscale activity on the western flank of the Ionian caused the shape change of the Messina Rise Vortex, favouring its longitudinal extension. The zonally-elongated Messina Rise Vortex reduced the inflow of Atlantic Water in the northern Ionian, and induced a strengthening of the eastward flowing Mid-Ionian Jet in the southern Ionian and Cretan Passage. From these results, it emerges that the interannual variability of the wind-driven quasi-permanent mesoscale gyres in the Ionian is influenced by internal forcing (location of the deep water formation site – Aegean or Adriatic Sea), in turn related to the major local climatic shift occurred in the last decades (the Eastern Mediterranean Transient). We show that the interplay between decadal and interannual variability, or rather between basin-wide and mesoscale circulation, affected the intrinsic characteristics of the NIG reversals.