Multicentury Instability of the Atlantic Meridional Circulation in Rapid Warming Simulations With GISS ModelE2

Abstract

AbstractIn multimillennial global warming simulations with the GISS‐E2‐R climate model, we observe multicentennial shutdowns with restoration and fast overshooting in North Atlantic Deep Water production despite the absence of exogenous freshwater input. AMOC (Atlantic Meridional Overturning Circulation) cessation is associated with a sea surface salinity reduction, initiated by increases in precipitation over evaporation as the climate warms. These multicentury shutdowns are the direct result of cooling in the North Atlantic associated with an aerosol indirect effect on cloud cover. The local cooling reduces evaporation within the North Atlantic, while warming elsewhere provides moisture to maintain nearly unperturbed precipitation in this region. As global warming continues, warm temperature (low density) anomalies spread northward at depth in the North Atlantic eventually destabilizing the water column, even though precipitation input at the surface is initially unchanged. Internal ocean freshwater transports do not play an important role in initiating this behavior, as assumed by some standard metrics of AMOC stability. The importance of the aerosol indirect effect in these runs is due to its role in strengthening the sea surface temperature‐evaporation feedback; this suggests a renewed focus on surface flux observations to help assess overturning stability. The length of the AMOC reduction, and its rapid recovery, may be relevant to the onset and end of the Younger Dryas, which occurred within a warming climate during the last deglaciation.