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300 R. Sorgente et al.: Seasonal variability in the Central Mediterranean Sea circulation

Fig. 1. Morphology of the Central Mediterranean Sea. Isolines indicate the 200 m and the 2000 m countours. a), b) and c) indicate the
vertical sections at 9◦ E, at the Sicily Strait and along longitude 13◦ E.

fields and dominant variability in the Sicilian Channel dur- phisticated nesting techniques that have been developed in
ing September 1996 (Lermusiaux and Robinson, 2001) at a the atmospheric sciences. Within the modelling effort of
horizontal resolution of 9 km and 20 levels in the vertical, the Mediterranean Forecasting System Pilot Project-MFSPP
using a topography-following (double sigma) transformation (Pinardi et al., 1999), a number of regional scale models have
(Lozano et al., 1994; Sloam, 1996). Sorgente and Zavatarelli been coupled by one-way off-line nesting to the Ocean Gen-
(2002) have applied a numerical simulation of the seasonal eral Circulation Model (OGCM). The OGCM is based on
circulation in the Central Mediterranean area, based on the the rigid-lid Modular Ocean Model (MOM), implemented in
free surface Princeton Ocean Model (Blumberg and Mellor, the Mediterranean basin at 1/8◦ horizontal resolution, with
1987) and adopting the same approach used by Ezer and Mel- 31 levels in the vertical (Demirov and Pinardi, 2003). It has
lor (1994) in diagnosing the north Atlantic circulation. been shown to be capable of reproducing the basin scale sea-
sonal variability of the Mediterranean.
In this work we resort to a nested implementation of a
three-dimensional primitive equation sigma-coordinate nu- This work refers to the MFSPP regional model covering
merical model, permitting an improved grid resolution both the Sicilian Channel, comprising the area of sea prior to the
horizontally and vertically, and producing an additional con- Strait of Sicily and extending towards the east, up to the lon-
tribution to the present knowledge of the spatial and temporal gitude of 15◦ E. The model is based on the free surface POM
variability of the circulation in the region. and is run on a seasonal cycle (perpetual year) simulation for
five years. A one-way off-line nesting method (Zavatarelli et
Nesting is a finite difference modelling technique used to al., 2002) based on bilinear interpolation in the horizontal
simulate a high resolution domain embedded in a lower res- plane is adopted to downscale the large-scale OGCM flow
olution domain. It is a useful method to simulate the small- field, with about 12.5 km of resolution, into the regional sub-
scale dynamics in a limited area, while allowing the large basin scale model with a resolution of about 5 km. The cou-
scales generated on the coarse grid to influence the nested pling to the OGCM is necessary in order to transfer values
grid. A substantial body of literature documenting nest- of variables from the OGCM coarse grid to the finely spaced
ing methods exists, particularly in numerical weather pre- regional model grid at the location of the lateral boundaries.
diction (Koch and McQueen, 1987). For ocean flows, nest- The utilization of the high resolution model has permitted a
ing has, however, only recently been implemented (Oey and more detailed description of the circulation in the region in-
Chen, 1992; Fox and Maskell, 1996; Oey, 1998) and has cluding some mesoscale components that cannot be resolved
not yet been used to take full advantage of the more so-