Boreal autumn to winter North Atlantic atmosphere-ocean coupling in large ensemble climate simulations

Abstract. Recent studies have suggested an observed causal relationship between autumn North Atlantic sea surface temperature (SST) anomalies and the phase of the winter North Atlantic Oscillation (NAO). This autumn SST-winter NAO link, which is mediated by turbulent heat fluxes (THF) and baroclinicity, appears to be underestimated in seasonal prediction models, suggesting possible model limitations in representing air-sea coupling. However, strong atmospheric driving of North Atlantic THF and SST variability at seasonal timescales presents a challenge in establishing a causal ocean feedback onto the large-scale atmosphere. This study examines the representation of autumn-winter North Atlantic atmosphere-ocean variability in ERA5 reanalysis data and historical large ensembles from the sixth phase of the Coupled Model Intercomparison Project (CMIP6). Models adequately capture concurrent North Atlantic atmosphere-SST variability within the autumn and winter seasons, when THF and SST tendencies are mainly atmosphere-driven. However, the leading mode of covariance between autumn SST–winter mean sea level pressure (MSLP) in models differs from ERA5. In ERA5, warm SST anomalies in the central North Atlantic in autumn precede a positive winter NAO anomaly. Conversely, the CMIP6 models show, on average, weak cool autumn SST anomalies precede a positive winter NAO with large model spread. Using an atmospheric analogue method, we separate atmosphere- and ocean-driven components of autumn THF variability and assess their respective relationships with winter MSLP variability. In ERA5, the winter MSLP signal associated with ocean-driven autumn THF anomalies is near zero; in contrast, the winter NAO is linked to atmosphere-driven autumn THF variability. This suggests that atmospheric processes, such as tropical-extratropical teleconnections, could explain the autumn THF–winter MSLP relationship in ERA5 described in a previous study, or that SST anomalies arising from autumn atmospheric forcing persist into winter and contribute to a lagged feedback on the atmosphere. CMIP6 models generally underestimate the winter NAO signal associated with atmosphere-driven autumn THF anomalies, which could reflect biases in feedbacks, atmospheric teleconnections or underpersistence of inter-seasonal North Atlantic SST anomalies.