The Gibraltar Strait and its role in the dynamics of the Mediterranean Sea
In: Dynamics of Atmospheres and Oceans. Elsevier: Amsterdam; New York; Oxford; Tokyo. ISSN 0377-0265; e-ISSN 1872-6879, more
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| Abstract |
The Strait of Gibraltar is the only dynamically relevant communication of the Mediterranean Sea with a large ocean basin, the North Atlantic. Although quite constrictive, important water exchanges occur through it over a broad frequency range.With respect to the tides, considering the Mediterranean as a long zonal channel closed at both ends indicates that the principal tidal signal observed in its interior is astronomically forced. However, the tidal wave incoming from the Atlantic, although strongly reflected (~ 94%) at the entrance to the strait, has about a 10% integrated contribution to the observed tide within the sea. In the strait most of the tidal flow (~ 92%) is barotropic, but a clear baroclinic tide is discernible from observations. The correlation between tidal currents and depth variations of the interface (separating Atlantic and Mediterranean waters) at Gibraltar's main sill represents up to 13 of the mean transport in each layer.At subinertial frequencies, periods from days to a few months, the strait restricts the meteorologically forced flows modifying the simple isostatic response of sea level to atmospheric pressure within the sea. A simple analytical model, consisting of two basins (the western and eastern Mediterranean) and two straits (Gibraltar and Sicily), indicates that the atmospheric pressure field over the sea accounts for 65% of the barotropic subinertial flows at Gibraltar, 68% of sea level variability in the western basin but only 41% of that in the eastern basin. These results are obtained by applying a linear friction coefficient to limit the barotropic flows at both straits, Gibraltar and Sicily.The long-term (seasonal to interannual) two-layered baroclinic exchange through the strait has historically been related to the integrated effects of the mass and salt balances in the sea. Recent works, based on the hydraulic behaviour of the two-layer flow in from the Atlantic at the surface and out of the Mediterranean at depth, allow speculation on the possible forcing and control mechanisms for this exchange, as well as their repercussions on the circulation and water characteristics within the sea. However, the true dynamic role played by time-dependent processes on these exchange flows remains a subject of debate and intense research. |
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