MULTIPLE-CENTURY RESPONSE OF A COUPLED OCEAN-ATMOSPHERE MODEL TO AN INCREASE OF ATMOSPHERIC CARBON-DIOXIDE

To speculate on the future change of climate over several centuries, t hree 500-year integrations of a coupled ocean-atmosphere model were pe rformed. In addition to the standard integration in which the atmosphe ric concentration of carbon dioxide remains unchanged, two integration s are conducted. In one integration, the CO2 concentration increases b y 1% yr(-1) (compounded) until it reaches four times the initial value at the 140th year and remains unchanged thereafter. In another integr ation, the CO2 concentration also increases at the rate of 1% yr(-1) u ntil it reaches twice the initial value at the 70th year and remains u nchanged thereafter. One of the most notable features of the CO2-quadr upling integration is the gradual disappearance of thermohaline circul ations in most of the model oceans during the first 250-year period, l eaving behind wind-driven cells. For example, thermohaline circulation nearly vanishes in the North Atlantic during the first 200 years of t he integration. In the Weddell and Ross seas, thermohaline circulation becomes weaker and shallower, thereby reducing the rate of bottom wat er formation and weakening the northward flow of bottom water in the P acific and Atlantic oceans. The weakening or near disappearance of the rmohaline circulation described above is attributable mainly to the ca pping of the model oceans by relatively fresh water in high latitudes where the excess of precipitation over evaporation increases markedly due to the enhanced poleward moisture transport in the warmer model tr oposphere. In the CO2-doubling integration, the thermohaline circulati on weakens by a factor of more than 2 in the North Atlantic during the first 150 years but almost recovers its original intensity by the 500 th year. The increase and downward penetration of positive heat and te mperature anomaly in low and middle latitudes of the North Atlantic he lps to increase the density contrast between the sinking and rising re gions, contributing to this slow recovery. The recovery is aided by th e gradual increase in surface salinity that accompanies the intensific ation of the thermohaline circulation. During the 500-year period of t he doubling and quadrupling experiments, the global mean surface air t emperature increases by about 3.5 degrees C and 7 degrees C, respectiv ely. The rise of sea level due to the thermal expansion of sea water i s about 1 and 1.8 m, respectively, and could be much larger if the con tribution of meltwater from continental ice sheets were included. It i s speculated that the two experiments described above provide a probab le range of future climate change.