Ocean response
A snapshot from the two-way coupled Adriatic model (2 km resolution ocean/ 4-km resolution atmosphere) on September 29 2002 0600 UTC illustrates the Adriatic bora structure. Bora events are generated by synoptic pressure systems interacting with orography. During a bora event, cold northeasterly winds flow over the Dinaric Alps and result in large-amplitude gravity waves and wave breaking along the western slopes near the Adriatic shore. The strongest winds are found in the lee of mountain gaps, where wave breaking is minimized and bora “jets” are orographically accelerated. A gap in topography on the mainland, to the northeast of the Croatian Islands (close to this cross-section) accelerates flow across Kvarner Bay. There is also a wind speed maximum over the southern Gulf of Trieste in the far northern Adriatic. The highest terrain forces larger amplitude waves that overturn and break, thereby inducing a downstream “wake” of low-speed winds such as is found off of the Istrian Peninsula. The jets and wakes are organized in bands perpendicular to the axis of the Adriatic and possess a characteristic width less than 25 km (Grubisic, 2004). The alternating jets and wakes generate a wind stress curl pattern that drives a double gyre circulation in the ocean below - with a counter-clockwise cell dominating the northern coastal waters, and a clockwise circulation nestled against the Istrian Peninsula.
The rapidly moving air in the bora flow is typically 10°C colder than the sea, creating unstable atmospheric conditions. The near-surface air warms in the fast moving winds through air-sea interaction processes as it transits the basin in contact with the warmer sea temperatures. The atmospheric boundary layer (ABL) depth over the ocean is approximately 1 km. Above the bora flow, the unstable convective marine ABL is capped by a temperature inversion. In the ocean, large upward heat flux leads to a >20 m deep surface mixed layer as evidenced by the vertical temperature contours.
In addition to the strong atmospheric forcing, the northern Adriatic Sea is influenced by discharge from the Po River, which has an anomalous salinity and temperature signature and strongly affects the surface waters.
