Can a freshwater forcing in high latitudes induce a permanent El Niño in the tropics - an overlooked mechanism for a rapid climate change.

A rapid climate change in response to the current rise in the atmospheric concentration of greenhouse gases is a matter of considerable concern. Thus far attention has focused almost entirely on changes associated with the deep thermohaline circulation. Little attention has been paid to possible changes in the wind-driven circulation of the ventilated thermocline. This shallow wind-driven circulation controls a significant transport of heat from low to high latitudes and is therefore subject to the constraint of a balanced heat budget: on a certain time-scale, on the order of decades, the heat gain in the upwelling zones of low latitudes must equal the heat loss in high latitudes. Can a freshening of the surface waters in high latitudes significantly reduce this heat transport? How will that affect the tropics? To explore this matter we use a number of idealized general circulation models of varying complexity that capture the main aspects of the thermocline ventilation. Our calculations show that a freshening of the surface waters in high latitudes does reduce the poleward heat transport and deepens the tropical thermocline. For a sufficiently strong freshwater forcing, the poleward heat transport all but vanishes, and permanent El Niño-like conditions prevail in the tropics. Paleo data indicate that this have happened before - there was no equatorial SST gradient in the Pacific Ocean during the early Pliocene.