Processes controlling the seasonal evolution of Mixed Layers in the Red Sea

The seasonal evolution of oceanic mixed-layers (MLs) in the Red Sea, together with their spatial patterns of variability, have been analyzed using conductivity-temperature-depth (CTD) hydrographic observations and the results of a very high resolution (1/100°) numerical model simulation forced with downscaled (5km) ERA-interim atmospheric fields, for years 2001-2015. The sparse observations reveal the basic features, suggesting that the seasonal evolution of mixed layers in the Red Sea is governed not only by strong surface air–sea buoyancy fluxes, but is also strongly modified by advective fluxes of both potential temperature (θ) and salinity (S). This is further explored analyzing closed and complete θ and S budgets, integrated over the MLD, from this 15-year long high resolution simulation. This provides for the first-time a detailed quantitative estimate of the relative role of individual physical processes in the seasonal ML depth (MLD) evolution in various parts of the Red Sea. We separately consider the importance of the atmospheric forcing, diapycnal mixing and divergence of advective flux of θ and S. Deep wintertime MLs develop in the Northern (up to 150 m deep) and Central (~80 m) Red Sea, where seasonal variability of θ plays a dominant role in MLD variability. Even in winter, MLs are shallow in the Southern Red Sea (~ 40m) and seasonal variability of S due to exchange of water with the adjacent Gulf of Aden predominantly governs their seasonal evolution. Strong and persistent eddies related to strong jets blowing through the mountain gaps strongly influence the mixed layer depth distribution and variability, especially in the Central Red Sea.