Long-term variability of the South Adriatic circulation in relation to North Atlantic oscillation

The interannual variability of the South Adriatic Gyre and its relation to the wind vorticity and the large-scale climatic pattern (North Atlantic Oscillation - NAO), was studied using the time-series of satellite altimetric data and ocean surface wind products. The cyclonic circulation observed in the South Adriatic area was partly sustained by the local wind forcing, as suggested by the positive correlation between the rate of change of the current vorticity and the wind-stress vorticity. Nevertheless, the influence of vorticity advection from the adjacent area (North Ionian Sea) cannot be ignored and it is more significant during the anticyclonic phase of Adriatic-Ionian Bimodal Oscillation System. The geostrophic current vorticities of the South Adriatic and North Ionian Seas are correlated with a time lag of 14 months, which corresponds to an advection speed of ~1 cm/s. The different wind patterns observed during the two NAO phases in the winter revealed a stronger positive vorticity during the negative NAO phase. Conversely, during the wintertime positive NAO phase the wind vorticity is characterized by lower positive or slightly negative values. Despite a statistically significant negative correlation between the NAO index and the wind vorticity, no unequivocal relationship between large climatic system and the interannual variability of the South Adriatic Gyre intensity was found due to additional effects of the vorticity advection from the Ionian. This can be explained by the fact that the Ionian circulation mode does not depend on the NAO variations. Therefore, the main result of this study is that the interannual variability of the South Adriatic cyclonic circulation is a result of the combined influence of the vorticity advection from the Ionian and the local wind-curl effect.