Page 1 - angeo-21-299-2003

Annales Geophysicae (2003) 21: 299–322 c European Geosciences Union 2003 Annales
Geophysicae

Seasonal variability in the Central Mediterranean Sea circulation

R. Sorgente1, A. F. Drago2, and A. Ribotti1
1CNR-IAMC, Sect. Oristano, c/o International Marine Centre, Loc. Sa Mardini, 09072 Torregrande-Oristano, Italy
2Physical Oceanography Unit, IOI-MOC, University of Malta, c/o 36, Old Mint Str., Valletta VLT12, Malta

Received: 23 August 2001 – Revised: 4 June 2002 – Accepted: 20 June 2002

Abstract. A high resolution eddy-resolving primitive equa- kins, 1978). This results in a two-layer flow through the
tion numerical model, based on the Princeton Ocean Model Strait of Sicily with a fresh Modified Atlantic Water (MAW)
(POM), is used to study the seasonal variability of the gen- eastward flow near the surface and a deeper salty Levantine
eral circulation in the Central Mediterranean Sea. The model Water outflow. Both flows are characterized by a significant
is run on a seasonal cycle, perpetual year simulation for five seasonal and interannual variability. In the past, investiga-
years, with nesting to the coarser resolution Ocean General tions have relied on hydrographic measurements, sub-surface
Circulation Model (OGCM), covering the whole Mediter- current meter data and lagrangian drifting buoys, with an
ranean Sea. The model results are compared to the current emphasis on the Strait area (Manzella, 1994; Grancini and
knowledge on the hydrography and dynamics of the area, Michelato, 1987; Poulain, 1998). Several authors have at-
with a special focus on the annual cycle of the Modified tempted to establish an estimate of the seasonal and inter-
Atlantic Water (MAW), on the circulation in the Sardinia annual variability of the transports across the Strait mainly
Channel, the water exchange across the Strait of Sicily, and on the basis of two-layered models (Bethoux, 1980) or
on the transition and fate of the Levantine Intermediate Wa- geostrophic flow calculations (Manzella et al., 1988; Moretti
ter (LIW). The results show that the adopted coupling tech- et al., 1993; Manzella, 1994; Astraldi et al., 1996). The re-
niques between the two models give a proficient downscaling sults are, however, often incompatible and no clear situa-
of the large-scale OGCM flow field into the regional scale tion emerges. The circulation is further influenced by strong
model. The numerical solution is also used to highlight the mesoscale signals with periods from 3 to 10 days (Manzella
seasonal characteristics of important dynamical features in et al., 1988). They are triggered mainly by intrinsic dynam-
the area, as well as to shed light on the scarcely known cir- ics, by the interaction with the bottom topography and by the
culation regimes along the north African shelf and slope. synoptic scale atmospheric forcing. With a Rossby radius
of about 10 km in the shelf areas, these mesoscale phenom-
Key words. Oceanography: general (numerical modelling); ena cannot be detected and followed unless observations are
Oceanography: physical (currents; general circulation) available with sufficient detail in both time and space. Nu-
merical model simulations offer the solution to these defi-
1 Introduction ciencies and constitute an important tool to study the phe-
nomenology of the hydrodynamics in the area.
The Central Mediterranean area, comprising the Sicilian
Channel, is a large and dynamically active area connecting Modelling studies of the climatological seasonal struc-
the eastern and the western Mediterranean sub-basins across ture of the circulation and water masses in the area have,
the shallow sill at the Strait of Sicily (Fig. 1). The region however, addressed the problem only within the wider
is known to be characterized by a number of significant dy- basin-scale applications (Malanotte-Rizzoli and Bergam-
namical processes covering the full spectrum of temporal and asco, 1991; Pinardi and Navarra, 1993; Roussenov et al.,
spatial scales. Although it plays an important role in the wa- 1995; Zavatarelli and Mellor, 1995; Wu and Haines, 1996;
ter mass exchanges between the two basins, especially for the Horton et al., 1997a) and consequently, with a coarse spatial
budget of the eastern Mediterranean basin, it has not received resolution. More specific model applications are the now-
the same attention as the Strait of Gibraltar. cast/forecast trials restricted to the southern and southeast-
ern Sicilian coast, and based on dedicated cruises conducted
The general flow is mainly driven by the slow (vertical) in 1994, 1995 and 1996 (Robinson et al., 1999). Combined
Mediterranean thermohaline basin scale circulation (Hop- with an intensive data collection survey, a nonlinear primitive
equation dynamical model in rigid-lid configuration (Ler-
Correspondence to: R. Sorgente (r.sorgente@imc-it.org) musiaux, 1999) has been used to simulate the mesoscale