THE STABILITY OF THE NORTH-ATLANTIC THERMOHALINE CIRCULATION IN A COUPLED OCEAN-ATMOSPHERE GENERAL-CIRCULATION MODEL

The stability of the Atlantic thermohaline circulation against meltwat er input is investigated in a coupled ocean-atmosphere general circula tion model. The meltwater input to the Labrador Sea is increased linea rly for 250 years to a maximum input of 0.625 Sv and then reduced agai n to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North A tlantic deepwater and a subsequent reversal of the thermohaline circul ation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reducti on of the Atlantic northward heat transport, resulting in a strong coo ling with maximum amplitude over the northern North Atlantic and a sou thward shift of the sea-ice margin in the Atlantic. Due to the increas ed meridional temperature gradient, the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the No rthern Hemisphere gain strength. We identify four main feedbacks affec ting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence time s of the surface water in high-northern latitudes, which allows them t o accumulate more precipitation and runoff from the continents. As a c onsequence the stratification in the North Atlantic becomes more stabl e. This effect is further amplified by an enhanced northward atmospher ic water vapour transport, which increases the freshwater input into t he North Atlantic. The reduced northward oceanic heat transport leads to colder sea-surface temperatures and an intensification of the atmos pheric cyclonic circulation over the Norwegian Sea. The associated Ekm an transports cause increased upwelling and increased freshwater expor t with the East Greenland Current. Both the cooling and the wind-drive n circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback destabilizes modes without dee p water formation in the North Atlantic, but has been neglected in alm ost all studies so far. After the meltwater input stops, the North Atl antic deepwater formation resumed in all experiments and the meridiona l overturning returned within 200 years to a conveyor belt pattern. Th is happened although the formation of North Atlantic deep water was su ppressed in one experiment for more than 300 years and the Atlantic ov erturning had settled into a circulation pattern with Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude that the conveyor belt-type thermohaline circul ation seems to be much more stable than hitherto assumed from experime nts with simpler models.