Low-frequency flow in the bottom layer of the Strait of Otranto

Abstract

Sub-tidal outflow of the Adriatic Water through the Strait of Otranto (Adriatic Sea) was studied analyzing long-term measurements obtained by means of three bottom-mounted Acoustic Doppler Current Profilers (ADCPs) that were deployed in the core of the density-driven deep current in the layer ∼100m thick just above the sea floor. Sub-tidal currents showed vertically uniform variability with prevailing southward long-term mean flow. However, inversions on weekly scales were rather frequent. The central mooring showed the strongest southward flow and an increase in current speed with depth, suggesting that it was positioned close to the outflowing dense water core. Cross-correlation of the two current components between different moorings shows vector rotation in the opposite sense between the central and outermost moorings. The rotary spectra confirmed this result and revealed that this phenomenon is mainly associated with a weekly timescale. Mean and total kinetic energy calculations showed that the central mooring was the most energetic, confirming that its position was the closest to the outflow core. At the central mooring the mean kinetic energy contributed to a major extent to the total kinetic energy and its relative importance decreased with the distance from the bottom. Additionally, bottom water transports were calculated. Their large interannual and seasonal variability is discussed. The largest outflow on a seasonal scale occurred in late winter and early spring while on an interannual scale it was mainly a function of winter climatic conditions. The most important result obtained from the empirical orthogonal function (EOF) analysis is that the correlation between the station at the sill and different moorings at the Otranto transect varied in different years and seasons. This suggests that the vein changed its equilibrium depth as a function of the outflowing water density.