Structure of the Atlantic Meridional Overturning Circulation in Three Generations of Climate Models

Through a large meridional transport of heat and freshwater, the Atlantic meridional overturning circulation (AMOC) plays a fundamental role in the earth climate system. Yet, there exist significant biases in representing AMOC structures in climate models and it is unclear whether or to what extent the biases have reduced through different generations of climate models. To address this question, this study compared the AMOC represented in three phases of the Coupled Model Intercomparison Project (CMIP) models with a similar horizontal resolution: CMIP3, CMIP5, and CMIP6. We found that (a) although the model spread is large, the multi-model ensemble of each CMIP reproduced a similar AMOC magnitude and the overturning depth that are in a reasonable agreement with observations; (b) in each CMIP, there is a smaller temperature and salinity difference between the northward flowing top limb and the southward flowing second limb of the AMOC, which lead to a systematic bias of a weaker meridional transport of heat and freshwater; (c) these biases are generally similar in CMIP5 and CMIP6 and significantly improved from CMIP3.