Hypsographic Control on the Diapycnal Flow in the Eastern Gotland Basin of the Baltic Sea

Abstract The implications of bottom intensified mixing together with an insulating boundary condition at the ocean floor for the ocean overturning has been investigated recently by many authors in many different settings. Here, we apply similar ideas to a brackish coastal sea, the eastern Gotland Basin in the Baltic Sea. The deep overturning in the basin is estimated using high-resolution temperature, salinity, and bathymetry data for a number of assumed vertical buoyancy flux and diffusivity profiles. The overturning structure in the basin is, in many ways, found to be similar to that inferred for the global ocean in recent studies, having rising water in a bottom boundary layer and sinking waters in the stratified mixing layer above. Moreover, the vertical structure of the overturning is found to be controlled by the hypsographic curve, with strong transformation occurring in certain depth ranges. These transformation hotspots are, however, not visible when water mass transformation is averaged in buoyancy space indicating that the effect of these hotspots is spread naturally over a large range of water masses.