The Export of Salty Surface Water to Depth in the North Atlantic

Abstract:

Excess evaporation in the subtropics forms a local surface salinity maximum in all subtropical ocean basins. Descent of these waters to depth creates a core layer marked by a subsurface salinity maximum (S-max) spreading towards the equator, an integral part of the lower limb of the meridional subtropical overturning cell. Here we will investigate what governs the evolution of the S-max core layer in the North Atlantic. It is suggested that the export is not just determined by subduction along isopycnals. The S-max core layer is modified along its spreading path by a non uniform diapycnal mixing profile, leading to a migration of the S-max across density surfaces (induced core layer). We propose a combination of diabatic surface fluxes and double diffusion beneath the salty surface waters as crucial for the properties and spreading path of the S-max towards the equator. The regional differences in stratification and potential for double diffusive processes in the central waters of the world ocean basins pose an interesting aspect for future research. It might be an important mechanism responsible for differences in the behavior of the shallow overturning in each basin. For further details on the surface salinity maxima linked to this topic see: Gordon, Giulivi, Busecke, Bingham, submitted to the SPURS Oceanography special issue.