Investigating the causes of the response of the thermohaline circulation to past and future climate changes

The Atlantic thermohaline circulation (THC) is an important part of the Earth's climate system. Previous research has shown large uncertainties in simulating future changes in this critical system. As an activity of WCRP CMIP/PMIP committees, the responses of the THC to idealized freshwater perturbations and the associated climate changes have been intercompared. This intercomparison is among models ranging from the Earth system models of intermediate complexity (EMICs) to the fully coupled atmosphere-ocean general circulation models (AOGCMs) in order to better understand the causes of the wide variations in the THC response. The robustness of particular simulation features has been evaluated across the model results. In response to 0.1 Sv freshwater input in the northern North Atlantic, the multi-model ensemble mean THC weakens by 30% after 100 years. All models simulate some weakening of the THC but no model simulates a shutdown. The multi-model ensemble indicates that the surface air temperature could present a complex anomaly pattern with a cooling south of Greenland and a warming over the Barents and Nordic Seas. The Atlantic ITCZ tends to shift southward. In response to 1 Sv freshwater input, the THC switches off rapidly. A large cooling occurs over the North Atlantic. The annual mean Atlantic ITCZ moves into the Southern Hemisphere. Models disagree in terms of the reversibility of the THC after its shutdown. In general, the EMICs and AOGCMs obtain similar THC responses and climate changes with more pronounced and sharper patterns in the AOGCMs.