North Atlantic overturning and water mass transformation in CMIP6 models

Climate models are important tools for investigating how the climate might change in the future, however recent observations have suggested that these models are unable to capture the overturning in subpolar North Atlantic correctly, casting doubt on their projections of the Atlantic Meridional Overturning Circulation (AMOC). Here we compare the overturning and surface water mass transformation in a set of CMIP6 models with observational estimates. There is generally a good agreement, particularly in the recent conclusion from observations that the mean overturning in the east (particularly in the Iceland and Irminger seas) is stronger than that in the Labrador Sea. The overturning in the Labrador Sea is mostly found to be small, but has a strong relationship with salinity: fresh models have weak overturning and saline models have stronger mean overturning and stronger relationships of the Labrador Sea overturning variability with the AMOC further south.We also find that the overturning reconstructed from surface flux driven water mass transformation is a good indicator of the actual overturning, though mixing can modify variability and shift signals to different density classes.