Long-term variability of the North Ionian Gyre (BiOS); characteristics and causes

Long-term variability of the multiannual circulation inversions of the North Ionian Gyre (Bimodal Oscillating System - BiOS) was analyzed using the surface geostrophic vorticity time-series. The vorticity evolution within the period 1992-2021 in the South Adriatic Pit (SAP) area was compared to the North Ionian one to look for their possible relationship. In parallel, variations in the thermohaline properties from Argo float data in the SAP were analyzed to search for the role of baroclinic forcing in generating the vorticity variability. The long-term variations of the North Ionian vorticity show a clear BiOS signal with prevalently constant amplitude of the cyclonic mode. On the other hand, the anticyclonic mode displays a maximum in the early 1990s and then it decreases in amplitude. The maximum anticyclonic amplitude during this period is associated with the Eastern Mediterranean Transient. The vorticity curve of the SAP shows positive values (cyclonic curl) over the entire period with amplitude which changes rather weakly in the first part of the record up to 2006, while after 2006 it displays large multiannual oscillations showing also a higher long-term average value.  A comparison between the vorticity time-series and the salinity in the SAP shows that large amplitude variations in the second part of the vorticity record are in counterphase with respect to the average 0-150m salinity. This suggests that these large amplitude vorticity variations after the end of the EMT are driven by the horizontal density gradient, which is largely associated with the salinity variations, i.e., baroclinic in origin. As for the long-term trend in salinity over the 30-year time-series, the increase is quite significant.