A strong bora event in the Gulf of Trieste: a numerical study of wind driven circulation in stratified conditions with a pre- operational model

This study investigates the circulation driven by a severe bora wind event in the Gulf of Trieste during the stratification season. A preliminary numerical analysis of the wind driven transport in the surface boundary layer is carried out for an idealized, laterally unbounded, shallow domain. Then, the simulations are focused on the Gulf of Trieste, using a realistic bathymetry and assuming stably stratified initial conditions. First, the model is driven by constant wind forcing, then qualitative and quantitative estimations of the dynamics of the basin are made reproducing the strong Bora event of the 25 June 2002. Numerical results show good agreement with in situ measurements and remotely sensed images. The relevance of coastal upwelling and its persistence are also assessed. The results prove that mixing and coastal upwelling (both wind-driven) govern the circulation of the basin. When bora starts blowing, buoyant surface water is moved by EKMAN transport offshore inducing a bottom onshore current. Simultaneously, wind driven stirring breaks the initial stable stratification. The surface offshore current generates a sea surface level setup on the northern boundary. When bora decreases, this surface tilt is no longer sustained, and a reversal of the circulation is observed in the upper layer. The sensitivity to the wind forcing is also discussed. All of these issues are crucial to environmental problems such as pollutant dispersion or anoxia phenomena in the bottom layers of the Gulf.