CLIMATE STABILITY AS DEDUCED FROM AN IDEALIZED COUPLED ATMOSPHERE OCEAN MODEL

The stability of an idealized climate system is investigated using a s imple coupled atmosphere-ocean box model. Motivated by the results fro m general circulation models, the main physical constraint imposed on the system is that the net radiation at the top of the atmosphere is f ixed. The specification of an invariant equatorial atmospheric tempera ture, consistent with paleoclimatic data, allows the hydrological cycl e to be internally determined from the poleward heat transport budget, resulting in a model that has a plausible representation of the hydro logical cycle-thermohaline circulation interaction. The model suggests that the stability and variability of the climate system depends fund amentally on the mean climatic state (total heat content of the system ). When the total heat content of the climate system is low, a stable present-day equilibrum exists with high-latitude sinking. Conversely, when the total heat content is high, a stable equatorial sinking equil ibrium exists. For a range of intermediate values of the total heat co ntent, internal climatic oscillations can occur through a hydrological cycle-thermohaline circulation feedback process. Experiments conducte d with the model reveal that under a 100-year 2xCO(2) warming, the the rmohaline circulation first collapses but then recovers. Under a 100-y ear 4xCO(2) warming, the thermohaline circulation collapses and remain s collapsed. Recent paleoclimatic data suggest that the climate system may behave very differently for a warmer climate. Our results suggest that this may be attributed to the enhanced poleward freshwater trans port, which causes increased instability of the present-day thermohal ine circulation.