Subpolar North Atlantic Mean State Affects the Response of the Atlantic Meridional Overturning Circulation to the North Atlantic Oscillation in CMIP6 Models
The North Atlantic Oscillation (NAO) and the Atlantic Meridional Overturning Circulation (AMOC) are leading modes of variability in the atmosphere and ocean, respectively, of the Atlantic sector. Numerous studies have established that NAO is an important driver of AMOC, but the strength of the NAO-AMOC link varies widely across models with potentially important implications for future AMOC predictions and projections. This study shows that the “spiciness” of the model mean state bias in the subpolar North Atlantic (warm-salty vs. cold-fresh) is particularly effective at explaining intermodel spread in the strength of the NAO-AMOC linkage in pre-industrial control simulations contributed to CMIP6. Warm-salty models (including CESM2) are less stratified in the subpolar Atlantic and have less sea ice coverage than cold-fresh models (including E3SM1). While NAO itself is similar in both classes of model, the warm-salty models simulate larger NAO-related heat loss, deep convection, and deep ocean density change. As a result, the NAO-AMOC linkage is much stronger in warm-salty models than in cold-fresh models.
Improved understanding of Atlantic low-frequency modes of climate variability is critical for reducing the uncertainty of decadal predictions and longer timescale projections. This study adds to our understanding of the origins of model structural uncertainty in the simulation of Atlantic variability by showing that the ocean response to NAO forcing depends on both temperature and salinity mean state biases and that warm-salty models (with stronger NAO-AMOC relationship) tend to be more realistic.
Large intermodel spread in CMIP6 simulations of the NAO-AMOC connection is shown to relate to mean state bias in the spiciness of the Atlantic subpolar gyre region. The NAO has a stronger impact on AMOC in warm-salty models that tend to exhibit more realistic stratification and sea ice coverage.