Mass exchange at the Strait of Gibraltar in response to tidal and lower frequency forcing as simulated by a Mediterranean Sea model

Abstract

The exchange between the Atlantic and the Mediterranean at the Strait of Gibraltar is studied based on numerical simulations of the Mediterranean Sea compared to two sets of observations. The model used has a varying horizontal resolution, highest at the Strait of Gibraltar. Numerical simulations forced by tide, by the subinertial variability, by both and by increasing the diffusion at the Strait are performed and compared to each other. The model successfully reproduces the main observed features of the variability at the tidal and at the lower frequency time scales including the phasing between the barotropic and baroclinic flow components and density variations. The model also simulates the strong mixing at the strait by tide and the resulting fortnightly modulation of the flow, with exchange reduction during spring tides and outflowing waters and acceleration during neap tides and inflowing waters. It is shown that tidal oscillations reduce the two-way exchange by interaction with the subinertial variability. The effects of tide on the Mediterranean Sea thermohaline circulation are also examined using multi-decadal simulations. It is shown that the model reproduces the cooling and saltening of waters crossing the strait in the upper layer and the warming and freshening of waters crossing the strait in the deeper layer, as previously shown by high resolution models of the Strait of Gibraltar. These changes are shown to cool and increase the salinity of the Mediterranean waters especially in the upper and intermediate layers. The water-cooling is shown to lead to a reduction of the heat loss at the sea surface. Based on model results, it is concluded that tide may have an effect on the Mediterranean Sea heat budget and hence on the atmosphere above. A validation of this conclusion is however needed, in particular using higher resolution models.

Keywords

• Strait of Gibraltar
• Mediterranean Sean
• tide
• thermohaline circulation
• water flow
• hydrographic changes