A case study of bora-driven flow and density changes on the Adriatic Shelf(January 1987)

A numerical modelling study of the response of the Adriatic shelf waters an d the Po River plume to a bora wind event has been undertaken using the Pri nceton Ocean Model (POM). After a bora episode from 16 to 20 January 1987, Advanced Very High Resolution Radiometer (AVHRR) satellite image revealed a cold filament protruding from the Italian coastal area toward the open sea . To understand and numerically simulate the offshore spreading of cold wat er from the western coastal area, the effects of three external forcings ha ve been examined: wind stress, heat flux and river discharge. Wind stress a nd surface heat flux, calculated by bulk method from the winds, air tempera tures and humidities obtained by Mesoscale Model 5 (MM5) with space and tim e resolution of 9 km. and I h, respectively, were used to force the oceanog raphic model. River discharge was introduced in the oceanographic model as a source term in the continuity equation. Moreover, it was assumed that riv er water had zero salinity and a 5 degreesC lower temperature than the surr ounding sea water, following some climatological estimations. The results of numerous numerical experiments revealed that the filament ob served in the AVHRR satellite image was formed by the joint action of three external forcings mentioned, and that none of them could be neglected whil e considering the observed pattern. Although the bora is, due to its NE dir ection, downwelling favourable wind for the western Adriatic coast, its alo ngshore variability induces offshore spreading of cold and less saline Po-i nfluenced water. The effects of air stability and wind wave field are impor tant for the wind stress estimation. The role of the Po River is to reduce sea surface salinity and to prevent cold water from sinking-, it is introdu ced in the oceanographic model mainly through the initial salinity field. T he combined effects of the basin topography and surface heat flux create an extra amount of cold water in the western coastal strip, which is then adv ected by bora wind action toward the open sea. High horizontal resolution o f the numerical experiments is important in resolving the alongshore variab ility of the bora ind field and to correctly simulate the narrow filament o bserved in the AVHRR image. (C) 2001 Elsevier Science Ltd. All rights, rese rved.