Shutdown of Atlantic thermohaline circulation and its impact on North Pacific climate

Recent studies of paleoclimate records reveal that large, abrupt climate changes occurred synchronously over many parts of the globe during the last glacial period. The Atlantic thermohaline circulation (THC) is considered to play an important role in reorganizing global climate during these rapid changes. We analyze output from four different ocean-atmosphere coupled general circulation models (CGCMs), and aim to identify the processes in which changes in the Atlantic THC influence climate beyond the Atlantic basin, particularly in the North Pacific.

When the CGCMs are forced with large anomalous fresh water fluxes in the northern North Atlantic, the THC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone (ITCZ) southward. In addition to this Atlantic ocean-atmosphere response, all the models exhibit deepening of the wintertime Aleutian low and cooling of the oceanic frontal region in the North Pacific. This THC-North Pacific connection agrees with analyses of paleorecords and reconstructed sea surface temperature (SST) data.

The barotropic structure of the Aleutian low response suggests that it is caused by remote forcing rather than local forcing. To test this idea, we force an atmospheric GCM with Atlantic-only SST anomalies simulated by one of the CGCMs. The tropical component of the Atlantic SST anomalies is found to be crucial for deepening sea level pressure over the North Pacific during the winter season. The strength of this inter-basin teleconnection depends on the mean convection in the tropical North Atlantic, which many CGCMs underestimate. The strengthened Aleutian low, in turn, cools the North Pacific by increasing the surface heat flux and shifting the sub-polar gyre to the south. Oceanic teleconnections also contribute to the North Pacific cooling: the fresh water input to the North Atlantic raises sea level in the Arctic Sea and reverses the Bering Strait throughflow, which normally transports warmer, fresher water from the North Pacific into the Arctic Sea. When the Bering Strait is closed in a CGCM, the cooling is greatly reduced while the Aleutian low response is enhanced. The closure of Bering Strait during the last glacial period suggests atmospheric teleconnections and local air-sea interactions played dominant roles in the THC-North Pacific climate linkage.