National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 1996

Abyssal currents generated by diffusion and geothermal heating over rises

Thompson, L., and G.C. Johnson

Deep-Sea Res. Pt. I, 43(2), 193–211, doi: 10.1016/0967-0637(96)00095-7 (1996)

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 θ−S (potential temperature-salinity) relation remains nearly linear. When geothermal heating dominates, the θ−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 N2 (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°S and 10°N. If the interior is motionless, this low N2 layer transports 4 and 8 × 106 m3 s−1 equatorward above the western flank of the rise at 15°S and 10°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.

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