Contrasting Fast and Slow Ocean Thermal, Carbon and Nutrient Responses to the North Atlantic Oscillation

Abstract Thermal and biogeochemical states of the North Atlantic Ocean are affected on seasonal to decadal timescales by atmospheric forcing associated with the North Atlantic Oscillation (NAO). An NAO–based composite approach is applied to an Earth system model to reveal the fast and slow responses of the ocean to atmospheric impulse forcing. Over the seasonal boundary layer, the atmosphere induces a “fast”, seasonal ocean response driven by anomalies in the air–sea flux, vertical entrainment and Ekman transport. This fast response to an NAO anomaly results in negative temperature and positive carbon and nutrient anomalies over the subpolar gyre, and positive temperature and negative carbon and nutrient anomalies over the subtropical gyre. The “slow” response on inter‐annual timescales involves changes in meridional overturning, tracer transport and entrainment. The slow response leads to a redistribution of anomalies between subpolar and subtropical regions, generating opposing–signed tracer anomalies in the subpolar and subtropical North Atlantic Oceans. These ocean responses also involve changes in the mixed‐layer depth, as well as the associated changes in entrainment and turbulent mixing rates, which are particularly important for the carbon and nutrient responses given their large vertical gradients. Modifications in nutrient concentrations subsequently influence biological activity and biomass production. These thermal, carbon and nutrient responses to atmospheric events linked to the NAO can persist for up to a decade, often characterized by opposing–signed temperature and carbon anomalies, along with contrasting changes in the subtropical and subpolar gyres.