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

Time scales and modes of variability of the flow inthe water column in the Northern Adriatic Sea for late summer2002 are described based on current record from a singlebottom-mounted ADCP in the shallow-water area in front ofthe Venice Lagoon.The time averaged flow was directed 277 E (CCW),roughly aligned with the coastline, with typical magnitudesin the range 4–6 cm/s and a limited, not significant clockwiseveering with depth. Tidal forcing was weak andmainly concentrated in the semidiurnal frequency band, witha barotropic (depth-independent) structure. On a diurnal timescale, tidal signal was biased by the sea-breeze regime andwas characterized by a clockwise veering with depth accordingto the Ekman spiral.A complex EOF analysis on the velocity profile time seriesextracted two dominant spatial modes of variability, whichexplained more than 90% of the total variance in the currentfield. More than 78% of the total variance was accountedfor by the first EOF mode, with a barotropic structure thatcontained the low-frequency components and the barotropictidal signal at semidiurnal and diurnal frequencies. The secondmode had a baroclinic structure with a zero-crossing atmid-depth, which was related with the response of the watercolumn to the high-frequency wind-driven diurnal sea breezevariability.The response of low-passed non-tidal currents to localwind stress was fast and immediate, with negligible temporallag up to mid-depth. Currents vectors were pointing tothe right of wind stress, as expected from the surface Ekmanveering, but with angles smaller than the expected ones. Atime lag in the range 10 to 11 h was found below 8m depth,with current vectors pointing to the left of wind stress and acounterclockwise veering towards the bottom. The delay wasconsistent with the frictional adjustment time scale describing the dynamics of a frictionally dominated flow in shallowwater, thus suggesting the importance of bottom friction onthe motion over the entire water column.