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Abstract

The North Atlantic Oscillation (NAO) index is one of the most common metrics to characterize atmospheric regimes in the northern hemisphere, revealing how different patterns of atmospheric winds and air-sea fluxes over Europe and North America affect the ocean state. We analyse climate model hindcasts to reveal the impacts of the NAO on the North Atlantic subpolar ocean, which exhibits variability on seasonal to decadal timescales. The ocean response to a single winter NAO event is separated into fast and slow responses. The fast response persists over winter-spring seasons, during which wind stress and heat flux anomalies associated with the NAO rapidly modify ocean temperatures via changes in Ekman transport and ocean-atmosphere heat exchanges. The slow response persists for 3-4 years, during which overturning and gyre circulations redistribute opposing-signed surface temperature anomalies created by the NAO. This redistribution modifies east-west temperature contrasts altering the meridional heat transport associated with gyres and changing the strength of the overturning circulation. Hence, the fast and slow responses lead to opposing-signed subpolar temperature anomalies in time from the competing effects of local forcing and horizontal heat convergence.