The Relevance of Air-sea Coupling in the North Atlantic for Decadal Predictability of the Large-scale Circulation in the Atlantic/European Sector

In a series of numerical experiments with the EC-Earth global coupled atmosphere-ocean general circulation model the potential decadal predictability of the large-scale circulation in the Atlantic/European sector in relation to the state of the Arctic Ocean is investigated. In this perfect model experimental setup, several 10-year long ensemble simulations with EC-Earth have been undertaken, starting out from particular extreme states of the Arctic Ocean from a multi-century pre-industrial pre-industrial control simulation with EC-Earth. These cases are defined by large either positive or negative decadal trends in the total fresh water content in the Arctic Ocean. The analysis of these predictability experiments indicates that the extreme freshwater exchanges have a limited effect on the state of the North Atlantic and that this effect only is detectable in the first few years of the simulations. In this study, we therefore investigate the relevance, which air-sea coupling has for these limited effects of the freshwater exchanges for the North Atlantic and the large-scale circulation in the Atlantic/European sector. The air-sea coupling is described by specific patterns maximizing the covariance between the state of the atmosphere and the underlying sea surface temperatures in the North Atlantic as well as the sea-ice cover in the Nordic Seas, both for situations where the atmosphere leads the sea surface temperatures or the sea-ice extent and for situations, where the atmosphere lags the state of the underlying oceans. The analysis of the air-sea coupling indicates that in the course of the year alternating phases, where the ocean leads the atmosphere and where the atmosphere leads the ocean. In our study we investigate, whether this impact of the atmosphere can explain the aforementioned limitation of the decadal predictability of the large-scale circulation in the Atlantic/European sector. Moreover, we analyse to which extent the anomalies of the sea surface temperatures in the North Atlantic and the anomalies of the sea-ice extent in the Nordic Seas associated with the initial states of the various sets of simulations correspond to the preferred patterns of air-sea interactions with the ocean leading the atmosphere.