Large (100-member) ensembles of GCM experiments are conducted to determine the atmospheric response to the North Atlantic SST tripole featuring warm anomalies off the US coast flanked by cold anomalies off Newfoundland and in the subtropics. The GCM responses to the SST tripole consist of both symmetric and asymmetric components about the sign of the SST anomaly. The symmetric part of the height responses is dominated by a NAO-like dipole with an equivalent barotropic structure over the Atlantic. The strength of the response is about 20 m/K in 500 mb height, roughly 40% of the observed height regression related to the SST tripole. The nonlinearity is manifested in the smaller scale and more wavetrain like dipole response to the positive tripole in contrast to the stronger and more zonally-elongated dipole response to the negative tripole. Similar nonlinearity is identified in observational composites around positive and negative SST troples. This nonlinearity suggests that feedbacks of such atmospheric responses on the ocean may lead to different evolutions of positive and negative tripole anomalies.

Mechanisms for developing and maintaining these GCM responses are investigated through linear model diagnoses and experiments. The SST-induced symmetric NAO response is primarily maintained by a dipolar anomalous eddy forcing that results from an eddy feedback mechanism. To account for the asymmetry of the response about the sign of the SST anomaly, a nonlinear eddy feedback mechanism is proposed that extends the previous mechanism to include the nonlinear self-interaction of the heating-forced anomalous flow and its effects on transient eddy feedbacks.