Eulerian current measurements in the Strait of Otranto and in the Southern Adriatic

The main features of the mean flow across the Strait of Otranto are studied using long-term (2-years) direct current measurements. The flow is characterized by a cyclonic horizontal shear between the inflowing currents from the Ionian into the Adriatic to the east and the outflowing currents from the Adriatic into the Ionian to the west, and by a two-layer structure in the central region, where in the bottom layers Adriatic Deep Water (ADW) outflows into the Ionian. The currents were also measured in the Southern Adriatic during summer. These data reveal the cyclonic circulation around the South Adriatic Pit, while the flow over the Otranto Sill has a two-layer structure with inflow near the surface and outflow near the bottom. The subtidal flow variability across the Strait of Otranto during winter, summer, and autumn is also analyzed. The space-time structures of the flow are obtained using an Empirical Orthogonal Function (EOF) analysis. It is shown that in winter and autumn the greatest portion of the current variance is associated with the intense fluctuations in the upper layers along the strait flanks. This subtidal variability is superimposed over the outflow along the western shelf and over the inflow along the eastern strait margin, with a time scale on the order of 10 days. The fluctuations are mostly driven by the local north–south winds, which through the Ekman transport and consequent geostrophic adjustments modulate the mean circulation. The barotropic-like flow fluctuations trapped over the western continental slope, with the intensification in the ADW layer, are prominent in summer, probably because the intensity of the wind-forced fluctuations along the strait flanks is attenuated. This barotropic-like pattern seems to modulate ADW outflow, blocking it completely from time to time. From the presently available data sets the main force driving these fluctuations has not been identified. However, they are partially associated with the barotropic forcing exerted through the sea level slope between the Northern Adriatic and Northern Ionian seas.