The Adriatic Sea modelling system: a nested approach

A modelling system for the Adriatic Sea has been built within the framework of the Mediterranean Forecast- ing System Pilot Project. The modelling system consists of a hierarchy of three numerical models (whole Mediter- ranean Sea, whole Adriatic Sea, Northern Adriatic Basin) coupled among each other by simple one-way, off-line nest- ing techniques, to downscale the larger scale flow field to highly resolved coastal scale fields. Numerical simulations have been carried out under climatological surface forc- ing. Simulations were aimed to assess the effectiveness of the nesting techniques and the skill of the system to repro- duce known features of the Adriatic Sea circulation phe- nomenology (main circulation features, dense water forma- tion, flow at the Otranto Strait and coastal circulation char- acteristics over the northern Adriatic shelf), in view of the pre-operational use of the modelling system. This paper de- scribes the modelling system setup, and discusses the simula- tion results for the whole Adriatic Sea and its northern basin, comparing the simulations with the observed climatological circulation characteristics. Results obtained with the north- ern Adriatic model are also compared with the correspond- ing simulations obtained with the coarser resolution Adriatic model. Its morphology and the characteristics of the forcing func- tions acting on the basin determine several notable differ- ences with respect to the whole Mediterranean basin. While the Mediterranean Sea is almost everywhere characterized by a reduced extension of the continental shelf, the northern part of the Adriatic basin lies entirely on the shelf and is charac- terized by very shallow depths (35 m on the average). In the central part depths are gently increasing to 100 m, and the distinctive morphological features are two small bottom de- pressions (the so-called "Pomo" or "Jabuka" Pits) having a maximum depth of 250 m. The southern part of the basin contrasts markedly with the northern one, as depths rapidly increase to maximum values of about 1200 m. The connec- tion with the Ionian Sea in the Otranto Strait is characterized by a sill having a depth of 875 m. The surface heat exchange between the sea and the atmo- sphere determines a net heat loss, estimated by Artegiani et al. (1997a) and Maggiore et al. (1998) on an annual basis at -22 W/m 2 . Monthly values ranges between -250 (winter) and 200 (summer) W/m 2 . The Adriatic Sea has, therefore, a negative heat budget, a characteristic consistent with the whole Mediterranean Sea. On the contrary, the fresh water budget differs strongly from the overall Mediterranean Sea budget. In fact, the Adri- atic Sea shows a significant net fresh water gain, while the whole Mediterranean basin is characterized by a net fresh water loss. Raicich (1996) has estimated the net annual fresh water gain of the Adriatic Sea to be greater than 1 m, mostly determined by the strong river runoff contribution, since evaporation and precipitation almost cancel each other on an annual basis. The climatology of the river runoff into the Adriatic Sea, compiled by Raicich (1994), is shown in Table 1. It indicates that most of the runoff is concentrated in the northern Adriatic Sea, but there are significant contri- butions also in the southern basin due to the rivers located along the Albanian coast, north of the Otranto Strait. The major river discharging in the basin is the Po, with an annu-