How the Freshening of Surface Waters in High Latitudes Affects the Tropics

The impacts of a freshening of the surface waters in high latitudes on the deep, slow, thermohaline circulation (THC) have received enormous attention, especially the possibility of a "shut-down" in the meridional overturning that involves sinking of surface waters in the northern Atlantic. Recent studies (Fedorov et al 2004, 2006) have drawn attention to the effects of a freshening on the other main component of the oceanic circulation - the swift, shallow, wind-driven circulation with which is associated the ventilated thermocline and which adjusts on decadal timescales. That circulation too involves meridional overturning with surface waters sinking in certain subduction zones, but its critical transition affects mainly the tropics. A freshening in the extra-tropics can deepen the equatorial thermocline to such a degree that temperatures along the equator become as warm in the eastern part of the basin as they are in the west, so that the equatorial zonal SST gradient virtually disappears and warm conditions prevail in the tropics. The deepening of the tropical thermocline is caused by the reduction of the meridional density gradient that controls the wind-driven circulation, which is investigated in a hierarchy of idealized and realistic general circulation models. Previous studies of rapid climate changes have shown that the equatorial and south tropical Atlantic warm up significantly when a freshwater perturbation is applied in the northern Atlantic ocean. The results of this work suggest that such equatorial warming is related, to a large extent, to changes in the wind-driven circulation. The oceanic adjustment associated with the shut-down of the THC causes a southward advection of fresh water from high latitudes, which in due course decreases the density gradient between the equator and the extra-tropics. As a result, the equatorial thermocline deepens and the equatorial sea surface temperatures increase (leading to a southward shift of the ITCZ).