Study of abrupt climate change by a coupled ocean atmosphere model

This study examines the responses of the simulated modern climate of a coupled ocean-atmosphere model to the discharge of freshwater into the North Atlantic Ocean. Two numerical experiments were conducted. In the first numerical experiment in which freshwater is discharged into high North Atlantic latitudes over the period of 500 years, the thermohaline circulation (THC) in the Atlantic Ocean weakens. This weakening reduces surface air temperature over the northern North Atlantic Ocean and Greenland and, to a lesser degree, over the Arctic Ocean, the Scandinavian peninsula, and the Circumpolar Ocean and the Antarctic Continent of the Southern Hemisphere. Upon termination of the water discharge at the 500th year, the THC begins to reintensify, 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 Ocean 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 multidecadal variation in the intensities of the THC and convective activities, which generate large multidecadal fluctuations of both sea surface temperature and salinity in the northern North Atlantic. Such oscillation yields almost abrupt changes of climate with rapid rise and fall of surface temperature in a few decades. In the second experiment, in which the same amount of freshwater 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 weakening the THC if it is discharged outside high North Atlantic latitudes. The results from numerical experiments conducted earlier indicate that the intensity of the THC could also weaken in response to a future increase of atmospheric CO ₂, thereby moderating the CO ₂-induced warming over the northern North Atlantic and surrounding regions.