Mesoscale-resolving simulations of summer and winter bora events in the Adriatic Sea

This paper presents simulations of the Adriatic Sea response to two distinct bora wind events, one in summer (11-20 August 2001), when the water is stratified, and the other in winter (11-20 February 2003), when it is vertically homogeneous. The simulations employ the DieCAST model on a 1.2-min grid (about 2-km resolution) and resolve the mesoscale variability because the grid size falls below the first baroclinic deformation radius (about 5 - 10 km) and the model has very low horizontal dissipation. The model is initialized with seasonally averaged temperature and salinity data and spun up with climatological winds. The summer of August 2001 event leads to the generation of a coastal current directed paradoxically to the left of the wind and identified with the summertime Istrian Coastal Countercurrent. Analysis of the physics simulated by the model leads to the conclusion that this current is caused by baroclinic geostrophic adjustment of the Istrian coastal waters following a rapid but strong wind impulse. According to both satellite observations and model simulations, the current persists for more than a week after the bora event. The winter event of February 2003 generates a slightly less complicated situation because the shallow northern Adriatic is then vertically homogeneous, but in situ observations at the time of this particular event permit a comparison of model results with observations and thus an evaluation of the model performance. In particular, the model simulates correctly a series of currents, bifurcations, and confluences of the wind-driven currents across the northern Adriatic basin. A lesson learned is that a bora event, though generally strong, especially in winter, leaves a legacy that does not obliterate completely what existed prior to the event. In other words, the state of the northern Adriatic basin is fashioned by sequential events, and one bora event may not be viewed in isolation but must be considered as one episode in an unfolding succession of events.