North Atlantic SST Forcing of the NAO and Relationships with Intrinsic Hemispheric Variability

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Abstract
Large (100‐member) ensembles of GCM experiments are conducted to examine the atmospheric response to a North Atlantic SST tripole with warm anomalies off the US Eastcoast and cold anomalies north of 40°N and south of 25°N. The response varies seasonally with the model's intrinsic variability, producing a strong NAO pattern only in February–April, the same months in which the model's internal variability projects strongly on the NAO. The response is significantly different for positive and negative SST tripoles, reproducing nonlinearity present in observational composites. The anomalous surface heat flux associated with the strong late‐winter/spring response implies a significantly reduced damping of the SST anomaly, and in some areas even a positive thermal feedback between the atmosphere and the ocean.