The shallow thermohaline circulation of the Red Sea

The circulation of the Red Sea is driven by strong surface evaporation, but the buoyancy budget alone cannot determine the circulation and thermohaline structure. The circulation could be rapid with weak stratification or slow with strong stratification. These possible states are examined in the dynamical framework of Phillipsʹ similarity model applied to the upper 140 m. A large eddy viscosity in the return flow between 70 and 140 m is required for the solution to match the observed stratification. The circulation for this solution is slow, in accord with the weak geostrophic currents observed in the Red Sea, and partially justifies the neglect of rotation. The internal viscous force required to match the model to observations seems too big to be a proxy for bottom friction on the sloping sides, though the mismatch is not large at the southern end of the Red Sea if the model is modified to allow for decreasing width with depth. The model may also be used to confirm that forcing by the wind stress is of secondary importance. Overall, the Red Sea appears not to be overmixed, though the model displays the counterintuitive result that increased vertical mixing increases the buoyancy difference between surface and bottom.