Abyssal currents generated by diffusion and geothermal heating over rises

A continuously stratified (in both salinity and temperature) diffusive time-dependent one-dimensional-f-plane model over a sloping bottom is constructed. The model is used to investigate the role of mixing of density near the bottom on large-scale abyssal flow near mid-ocean rises. For realistic abyssal values, both geothermal heating from the bottom and diffusion can be important to the dynamics of flow over mid-ocean rises. When diffusion dominates, buoyancy is transported toward the bottom and the theta-S (potential temperature-salinity) relation remains nearly linear. When geothermal heating dominates, the theta-S relation hooks near the bottom and a convectively driven mixed layer forms. Both effects reduce the density and stratification near the bottom. In contrast, bottom-intensified diffusion has the same effect near the bottom but results in an increase of density and stratification some distance above the bottom. If the bottom slopes, a horizontal density gradient results, setting up a geostrophic, bottom-intensified, along-slope flow that can effect mass transport. Evidence of the importance of these processes is found in the abyssal Pacific. Just over the western flank of the East Pacific Rise, a 700-900 m thick layer of low N-2 (buoyancy frequency) is warmer, saltier, and lighter than interior water at the same depth. This layer is described with CTD data from recent hydrographic sections at nominal latitudes 15 degrees S and 10 degrees N. If the interior is motionless, this low N-2 layer transports 4 and 8 x 10(6) m(3) s(-1) equatorward above the western flank of the rise at 15 degrees S and 10 degrees N, respectively. This equatorward current, a direct result of diffusion and heating over a sloping sea-floor, has a volume transport comparable to those of the deep western boundary current at these latitudes. Published by Elsevier Science Ltd.