Spiciness of the subpolar North Atlantic affects the response of the Atlantic Meridional Overturning Circulation to the North Atlantic Oscillation in CMIP6 models

Climate models are a valuable tool to study the interaction between ocean and atmosphere. Nevertheless, they are known to suffer from various biases and uncertainties. In the subpolar North Atlantic typical biases among models from the Coupled Model Intercomparison Project phase 6 (CMIP6) are found in the mean surface temperature and salinity, and in the mean sea ice concentration. These biases will affect the air-sea interaction.

In this study, we are investigating the diversity of CMIP6 models with respect to their response of the Atlantic Meridional Overturning Circulation (AMOC) to the North Atlantic Oscillation (NAO) in pre-industrial control experiments. This response is sensitive to the mean spiciness of the North Atlantic. Thus, we focus on two categories of models: Models that are spicy (warm-salty) and models that are minty (cold-fresh) within the subpolar gyre of the North Atlantic. Spicy models tend to have a lower sea ice cover in the Labrador Sea (LS) and larger LS heat loss during a positive NAO, compared to minty models. Also, spicy models have a weaker stratification in the LS. Sub-surface density changes 1 to 3 years after the NAO are larger in the spicy models and establish a zonal density gradient that can cause a stronger delayed AMOC response that is also more coherent across latitudes.

Although some metrics seem to be more realistic in the spicy models, other characteristics seem less realistic compared to the minty models, like the mixed layer depth relative importance between the eastern and the western subpolar North Atlantic. This could be a sign for how some mean states or processes might be right for the wrong reasons and stresses the need for model improvement.