A growing number of paleoceanographic observations suggest that the ocean's deep ventilation is stronger in warm climates than in cold climates. Here we use a general ocean circulation model to test the hypothesis that this relation is due to the reduced sensitivity of seawater density to temperature at low mean temperature; that is, at lower temperatures the surface cooling is not as effective at densifying fresh polar waters and initiating convection. In order to isolate this factor from other climate-related feedbacks we change the model ocean temperature only where it is used to calculate the density (to which we refer below as "dynamic" temperature change). We find that a dynamically cold ocean is globally less ventilated than a dynamically warm ocean. With dynamic cooling, convection decreases markedly in regions that have strong haloclines (i.e., the Southern Ocean and the North Pacific), while overturning increases in the North Atlantic, where the positive salinity buoyancy is smallest among the polar regions. We propose that this opposite behavior of the North Atlantic to the Southern Ocean and North Pacific is the result of an energy-constrained overturning.
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