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

Abstract

A numerical modelling study of the response of the Adriatic shelf waters and the Po River plume to a bora wind event has been undertaken using the Princeton 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 water from the western coastal area, the effects of three external forcings have been examined: wind stress, heat flux and river discharge. Wind stress and surface heat flux, calculated by bulk method from the winds, air temperatures and humidities obtained by Mesoscale Model 5 (MM5) with space and time resolution of 9 km and 1 h, respectively, were used to force the oceanographic model. River discharge was introduced in the oceanographic model as a source term in the continuity equation. Moreover, it was assumed that river water had zero salinity and a 5°C lower temperature than the surrounding sea water, following some climatological estimations. The results of numerous numerical experiments revealed that the filament observed in the AVHRR satellite image was formed by the joint action of three external forcings mentioned, and that none of them could be neglected while considering the observed pattern. Although the bora is, due to its NE direction, downwelling favourable wind for the western Adriatic coast, its alongshore variability induces offshore spreading of cold and less saline Po-influenced water. The effects of air stability and wind wave field are important 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 introduced in the oceanographic model mainly through the initial salinity field. The 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 advected by bora wind action toward the open sea. High horizontal resolution of the numerical experiments is important in resolving the alongshore variability of the bora wind field and to correctly simulate the narrow filament observed in the AVHRR image.