Abrupt Climate Change Triggered by GIN Sea Convection

In the IPCC report, the warming at the end of 21st century is projected as between two and four degree Celsius. This statement is based on the equilibrium sensitivity, and the climate system is bound to exhibit wild swaying of the abrupt climate change due to transient and nonlinear dynamics, especially in the polar region. In order to provide more accurate projections for the future, it is necessary for us to demonstrate that the IPCC-type models have the capability to simulate abrupt climate change, such as the classic example of Bølling-Allerød (BA) warming, which began 15,000 years ago and eventually ended the last ice age.

We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation of the transient evolution of global climate from the Last Glacial Maximum 21,000 years ago to the abrupt BA warming 14,500 years ago. Our model reproduces the major features of the evolution of deglacial climate change, including the magnitude of the climate response. The model simulates the BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to the termination of imposed freshwater discharge associated with Heinrich event 1 (H1).

In the ramped forcing scenario, the freshwater discharge slows down gradually in the North Atlantic ocean from H1 to BA, however, an abrupt warming event occurs before the freshwater forcing zeros out: in four decades, the surface air temperature in GIN Sea shoots up from -10 degree Celsius to above freezing. This abrupt warming is triggered by the sudden switch–on of the GIN Sea convection when AMOC switches from the glacial cold mode to the interglacial warm mode. The ocean subsurface warming after H1 and the surface evaporation after the sea ice opening appear to be important processes that generate positive salinity anomaly in the GIN Sea and trigger the mode switch of AMOC.