Observations of coastal filaments in the Adriatic Sea

At the end of August 1990, 1991 and 1992, temperature readings from satellite AVHRR images provided a description of cold filaments departing from the Croatian coast directed offshore in the Adriatic Sea. The main filament was observed in the middle Adriatic extending from the Croatian to the Italian side along a topographic escarpment, giving rise to a double recirculation cell. Surface velocities associated to the offshore flow, estimated with the maximum cross-correlation technique, exhibited values on the order of 10-20 cm/s. ECMWF wind data indicate the Adriatic was characterized by winds with intensity of 5-7 m/s (breeze superposed to the climatological signal) excluding the eventuality of a wind driven filament. The filament formation process was also analyzed in terms of residual currents associated to tidal rectification processes. Comparisons between velocity values predicted by this theory and observations indicate the dynamics associated to the observed structures cannot be explained in terms of the theory of tidal residual flows. Although much more experimental evidence is required to finally prove the following hypothesis, it is shown that the baroclinic extension of the theory of the Rossby adjustment over a bottom escarpment can ascribe the filament formation in the middle Adriatic to interaction of steady coastal currents with bathymetry.