Study of abrupt climate change by a coupled ocean-atmosphere model

This study examines the responses of the simulated modern climate of a coup led ocean-atmosphere model to the discharge of freshwater into the North At lantic Ocean. Two numerical experiments were conducted. In the first numeri cal experiment in which freshwater is discharged into high North Atlantic l atitudes over the period of 500 years, the thermohaline circulation (THC) i n the Atlantic Ocean weakens. This weakening reduces surface air temperatur e over the northern North Atlantic Ocean and Greenland and, to a lesser deg ree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic Continent of the Southern Hemisphere. Upon termina tion of the water discharge at the 500th year, the THC begins to reintensif y, gaining its original intensity in a few hundred years. As a result, the climate of the northern North Atlantic and surrounding regions resumes its original distribution. However, in the Pacific sector of the Circumpolar Oc ean of the Southern Hemisphere, the initial cooling and recovery of surface air temperature is delayed by a few hundred years. In addition, the sudden onset and the termination of the discharge of freshwater induces a multide cadal variation in the intensities of the THC and convective activities, wh ich generate large multidecadal fluctuations of both sea surface temperatur e and salinity in the northern North Atlantic. Such oscillation yields almo st abrupt changes of climate with rapid rise and fall of surface temperatur e in a few decades. In the second experiment, in which the same amount of f reshwater is discharged into the subtropical North Atlantic over the period of 500 years, the THC and climate evolve in a manner qualitatively similar to the first experiment. However, the magnitude of the THC response is 4-5 times smaller. It appears that freshwater is much less effective in weaken ing the THC if it is discharged outside high North Atlantic latitudes. The results from numerical experiments conducted earlier indicate that the inte nsity of the THC could also weaken in response to a future increase of atmo spheric CO2, thereby moderating the CO2-induced warming over the northern N orth Atlantic and surrounding regions. Published by Elsevier Science Ltd.