On the bottom density plume on coastal zone off Gargano (Italy) in the southern Adriatic Sea and its interannual variability

Abstract

The Adriatic Sea general circulation model (AIM) was implemented to study a bottom density plume found in coastal zone off Gargano (hereafter Gargano bottom density plume) in the southern Adriatic Sea (SAS), Italy, and its interannual variability. The model has been run with realistic wind stress, surface heat flux, and river runoff forcings continuously for a period from 1 January 1999 to 31 December 2002. The study found that the Gargano bottom density plume, typically as found in spring 2000, is a bottom density current driven by a perturbation in density as a result of successive bora cooling events in the northern Adriatic Sea (NAS) from previous autumn and winter. The density current advects cold and less saline Northern Adriatic Deep Water (NADDW) from the NAS to the SAS, and the propagation speed of the plume can reach 0.1 m s-1 with a downslope component on the order of 0.05 m s-1. The southward mass and heat transport by the Gargano plume can be calculated to be 0.08 Sv and -30 W m-2 at a transect near Gargano Peninsula. These results suggest that about 15% of the NADDW and NAS heat content change produced over previous winter was transported to the SAS by the Gargano plume. One of the most important findings of this paper is that there is a connection between the intensity of Gargano bottom density plume in the SAS and the NADDW production rates in the NAS. The numerical study demonstrated that a continuous heat loss of 175 W m-2 from November to January is required to produce adequate volume of NADDW, so that a density perturbation is formed which is large enough to drive a density current that reaches the Gargano Peninsula in the spring. This minimum requirement of heat loss equates to eight consecutive strong bora events in this period. In addition to the autumn and winter surface cooling, summer preconditioning of the NAS density by the lower Po River runoff is another important factor in the Gargano plume generation. A wet summer results in large Po River runoff and decreases the NAS density. Thus additional heat loss is required for adequate NADDW production in the following winter. Finally, the study also demonstrated that total heat budget of the NAS during the winter season was negative, as the amount of heat lost at the air-sea interface during that period was much larger than the amount of heat gained by horizontal advection. As a result, the NAS during the winter season experienced a strong heat content decrease. Copyright 2006 by the American Geophysical Union.