Climatic Change and Ocean Bottom Water Formation: Are We Missing Something?

Abstract

The global thermohaline circulation is driven by negative buoyancy sources associated with surface cooling and salinity enhancement, with subsequent upwelling over all the world’s oceans. Relatively shallow, thermally-driven convective overturning occurring at high latitudes which arises from heating of the atmosphere by the oceans is fairly well represented in current numerical ocean models, but deep convection from surface salinity sources including North Atlantic Deep Water (NADW) and, particularly, Antarctic Bottom Water (AABW) is not. A schematic model is developed for AABW which incorporates brine rejection during the seasonal sea ice cycle and sinking in boundary layers along the continental shelf and slope of Antarctica. This model predicts the formation of 30–40 Sv of bottom water from an annual sea ice freezing rate of order 1 Sv. These values and the predicted top-to-bottom potential density increment in the Southern Ocean are in reasonable accord with observations. When the amount of sea ice freezing is expressed as a function of hemispheric mean sea surface temperature (calibrated from the seasonal cycle), the model predicts the possible cutoff of AABW and most of the thermohaline circulation for a four degree Celsius global mean warming, an event which could be triggered in the next century by anthropogenic greenhouse gases.