Variability of currents in front of the Venice Lagoon, Northern Adriatic Sea

Variability time scales and modes of the flow in the water column in the northern Adriatic Sea for late summer 2002 are described based on current recorded from a single bottom-mounted ADCP in the shallow-water area in front of the Venice Lagoon. The time averaged flow was directed 277° E (CCW), roughly aligned with the coastline, with typical magnitudes in the range 4–6 cm/s and a limited, insignificant clockwise veering with depth. Tidal forcing was weak and mainly concentrated in the semidiurnal frequency band, with a barotropic (depth-independent) structure. On a diurnal time scale, tidal signal was biased by the sea-breeze regime and was characterized by a clockwise veering with depth according to the Ekman spiral. A complex EOF analysis on the velocity profile time series extracted two dominant modes of spatial variability that explained more than 90% of the total variance in the current field. More than 78% of the total variance was accounted for by the first EOF mode, with a barotropic structure that contained the low-frequency components and the barotropic tidal signal at semidiurnal and diurnal frequencies. The second mode had a baroclinic structure with zero-crossing at mid-depth that was related with the response of the water column to high-frequency wind-driven diurnal sea breeze variability. Response of low-passed non-tidal currents to local wind stress was fast and immediate, with negligible temporal lag up to mid-depth. Current vectors were pointing to the right of wind stress, as expected from the surface Ekman veering, but with angles smaller than the expected ones. A time lag in the range 10 to 11 h was found below 8 m depth, with current vectors pointing to the left of wind stress and a counter-clockwise veering towards the bottom. The delay was consistent with the frictional adjustment time scale describing the dynamics of a frictionally dominated flow in shallow water, suggesting the importance of bottom friction on motion in the water column.