A New Variable in the Ocean's Overturning Circulation: Application to the Closure of the Central American Seaway (Invited)

The wind-driven upwelling along the southern edge of the ACC is thought to provide "closure" for the overturning cell that includes NADW. But a rather obvious argument can be made that the upwelling along the ACC is only the first stage of closure. The water upwelled south of the ACC is carried back across the ACC in the surface Ekman layer and combines with thermocline waters from the subtropical gyres to form a series of subantarctic water masses that are emplaced below the thermocline along the northern edge of the ACC. But these water masses are cool and fresh while the precursors of NADW are known to be warm and salty. According to the results from a new paper, the transformation is made in the coastal upwelling zones off Namibia and northwest Africa in the Atlantic and off Costa Rica and Peru in the Pacific. Basically, the upwelling in these areas draws the cool and fresh subantarctic water up to the surface in the tropics where it can then become warm and salty on its way back to the North Atlantic. Unfortunately, this second stage of closure tends not to occur in ocean circulation models because the coastal upwelling regions are barely resolved. In this presentation I will examine how the coastal upwelling alters the ocean's overturning and will speculate about how it changes the sensitivity of the overturning to ocean gateways.