The Atlantic Meridional Overturning Circulation (AMOC) is an active component of the global climate system. It transports large amounts of heat from tropical oceans to the northern high latitudes, where heat is released into the atmosphere.   

Earlier studies suggested that changes in the AMOC could impact the Atlantic or even global climates. The multi-model ensemble in IPCC AR3 / AR5 suggests that only under future extreme scenario, the Greenland Ice Sheet (GrIS) melting (equally distributed around the Greenland) can induce a noticeable AMOC weakening. However, it should be noted that there is no coupled ice-sheet module in most of the IPCC AR5 models. Recent observations indicated that the GrIS melting is accelerating and will likely discharge large amount of freshwater into the northern high latitudes. Such a freshening could reduce the sea surface density and likely suppress deep ocean convection in the Atlantic, potentially resulting in a weakened AMOC.

Dr. YU Lei and Prof. GAO Yongqi from the Institute of Atmospheric Physics, cooperating with the Nansen Environmental and Remote Sensing Center and the Bjerknes Centre for Climate Research, have investigated the response of AMOC to the enhanced freshwater discharge into the northern North Atlantic and the impact of a such weakened AMOC on East Asian climate using a series of freshwater-perturbation numerical experiments.

"We found the parameterization scheme of vertical mixing in ocean general circulation model plays an import role in modulating the behavior of AMOC in responses to enhanced freshwater input.” Dr. YU introduced their finding, “Under the stratification-dependent mixing paramerization, for example, the AMOC has an ability of ‘self-recovery’ from an initial weakening to a gradual recovery. This result differs from the ‘slow-down’ or ‘shut-down’ of AMOC in other model-based studies.”

They also found for the East Asian climate, as AMOC is reduced to a critical threshold value, a weaker East Asia Summer Monsoon can be seen in model simulations. The mechanism can be described as follows: the sea surface temperature (SST) anomalies in the North Atlantic corresponding to a weakened AMOC can influence the SST variability in the Pacific Ocean through an atmospheric bridge, resulting in a decadal ENSO-like SST anomalies mode in the eastern tropical Pacific Ocean. Meanwhile there is a westward shift in upward branch of Walker circulation, eventually leading to a weakened East Asia summer monsoon.

"The AMOC is more sensitive to an enhanced freshwater forcing along the eastern coast of the Greenland than an equally enhanced freshwater forcing along the entire coast of Greenland.” Prof. GAO concluded, “Therefore, we need a better estimation of a temporal and spatial distribution of the GrIS melting to assess the response of the AMOC in a warmer climate in our future work.”