Winter oceanographic conditions and water mass balance in the Northern Adriatic (February 1993)

In February 1993 a hydrographic campaign was undertaken in the northern Adriatic on a station network with 5 nM resolution. An interval of more than two weeks, prior to and during the campaign, was characterized by calm weather and small variability in air temperature and atmospheric pressure. The Po river run-off, which can also strongly affect the northern Adriatic circulation pattern, in January 1993 was lower than the long-term average without any short-term high run-off episodes. In the horizontal distribution of temperature and salinity two parallel N-S fronts were observed: one about 20 km from the Italian coast separating colder and fresher surface water of the Po origin from the sea ambient waters, the other frontal zone separates saltier and warmer water advected from south from waters of the basin interior. In the density field only the western surface front, associated with the freshwater inflow, is evident. From the Brunt-Väisälä frequency profiles internal Rossby radii of deformation were calculated for each CTD station. Horizontal distribution of internal radii of deformation closely follows surface isopycnals suggesting that horizontal scales are determined mainly by the freshwater induced stability of the water column. The first Rossby radius of deformation varies from about 1 km in the basin interior to 17 km near the Po estuary where the vertical stability reaches maximum. The vertical structure of the first mode eigenfunction is closely related to the freshwater spreading and saltier water advection from the south. An attempt was made to estimate the water inflow from the south assuming the geostrophic balance. A return flow driven by compensating mechanisms to the geostrophically balanced outflow was found to be of the same order of magnitude as the Po discharge.