Interbasin Exchange of Bottom Water in Ocean General Circulation Models

Abstract

The abyssal part of the thermohaline circulation can pass through narrow channels between ocean basins, which can potentially be resolved by high-resolution ocean models such as the Fine Resolution Antarctic Model. However, in the FRAM, as in other models, topographic smoothing filled in these channels, severely weakening the abyssal circulation and altering the baroclinic structure at shallower depths. The authors describe a model covering the Vema Channel region of the South Atlantic, which was nested in the FRAM, with a straight, single gridpoint channel joining the Argentine and Brazil Basins. Initialization with the FRAM results produced a weak northward flow through the channel, which decayed after 6 months. Spinning up the model toward the FRAM results produced southward flow, whereas spinning up toward the Levitus climatological fields of temperature and salinity produced northward flow of similar strength to observations. An experiment with higher cross-channel resolution gave a similar result. Topographic smoothing therefore results in incorrect abyssal flow and, therefore, thermohaline circulation in the present generation of both eddy-resolving and climate ocean general circulation models.