The weakly stratified bottom boundary layer of the global ocean

The weakly stratified bottom boundary layer (wsBBL) of the global ocean is currently unmapped; even the definition of the wsBBL layer is yet lacking. However, recent studies point to the wsBBL as a region where most of the abyssal water transformation takes place. In this study, historical high‐resolution density profiles are used to map the properties of the wsBBL in the global ocean. We use a density gradient criteria ( urn:x-wiley:21699275:media:jgrc22951:jgrc22951-math-0001 kg m– 4) to define the top of the layer. The thickness of the wsBBL varies from several meters to over a thousand meters and can be used as a rule of thumb to differentiate basin walls from the basin bottom, respectively. Although the thickness varies greatly, the pressure at the top of the wsBBL varies relatively smoothly allowing us to map its distribution across the ocean along with the density of the wsBBL. The neutral density, γwsBBL, and pressure, PwsBBL, of the upper boundary of the wsBBL are highly correlated within each ocean basin. Diagrams of γwsBBL versus PwsBBL clearly differentiate the different basins, connected by the narrow channels, along the pathways of abyssal water circulation. The diagrams give insight into the different mechanisms of abyssal water transformation and highlight locations where transformation happens: inter‐basin channels and over some parts of mid‐oceanic ridges such as found in the Brazil Basin, in the Guiana Basin, and in the Southwest Pacific Basin.