Multidecadal Variability in Global Surface Temperatures Related to the Atlantic Meridional Overturning Circulation

Multidecadal internal climate variability centered in the North Atlantic is evident in sea surface temperatures and is assumed to be related to variations in the strength of the Atlantic meridional overturning circulation (AMOC). In this study, the extent to which variations in the AMOC may also alter hemispheric and global surface air temperature trends and ocean heat content during the past century is examined. Forty-seven realizations of the twentieth-century climate change from two large ensembles using the Community Earth System Model (CESM) are analyzed. One of the ensembles shows a much wider spread in global mean surface air temperature between its members. This ensemble simulates diverging trends of the AMOC strength during the twentieth century. The presence and strength of deep convection in the Labrador Sea controls these trends. The AMOC strength influences the air-sea heat flux into the high-latitude ocean, where a strengthening of the AMOC leads to decreased storage of heat in the Atlantic, and a larger fraction of the heat taken up by the global ocean accumulates in the top 300 m, compared to the case of a weakening AMOC. The spread in the amount of heat stored in the global ocean below 300 m is similar across the CESM members as in a set of CMIP5 models, confirming the AMOC as a control knob'' on deep-ocean heat storage. By influencing the ocean heat uptake efficiency and by shifting the pattern of heat uptake, global surface air temperatures are significantly altered on a multidecadal time scale by AMOC variability.