Tropical Atlantic SST Forcing of Coupled North Atlantic Seasonal Responses

Abstract Recent observational studies reveal that a fall Pan-Atlantic sea surface temperature (SST) anomaly, composed of a horseshoe-like dipole in the North Atlantic and a southern center in the equatorial Atlantic, tends to precede the winter North Atlantic Oscillation (NAO) and its related SST tripole by several months. This study seeks to understand this relationship using large ensembles of atmospheric general circulation model (AGCM) experiments and experiments with the AGCM coupled to a mixed layer ocean (AGCM_ML). The models are forced either by the North Atlantic horseshoe (NAH) or by the tropical SST anomalies over the boreal winter months. The AGCM results show that the NAH anomaly induces a baroclinic response in geopotential heights throughout the winter, with little projection on the NAO. Since the NAH anomaly is ineffective in forcing the wintertime NAO, it cannot account for observations that the NAH SST leads the NAO. In contrast, in the AGCM_ML, the coupled North Atlantic response forced by the tropical anomaly exhibits a strong seasonal dependence. In early winter, the positive anomaly induces a trough east of Newfoundland with a wave train to the northeast, and in late winter the response projects strongly on a negative NAO. Correspondingly, the extratropical SST response features an NAH-like pattern in early winter and a tripole in late winter. These results suggest that tropical Atlantic SST anomalies can significantly influence the coupled extratropical variability. The observed relationship between the fall NAH SST and the winter NAO (or the SST tripole) may be a consequence of persistent forcing of the seasonally varying atmosphere by tropical SST anomalies. Comparisons with the parallel AGCM results indicate that the largely sign-symmetric NAO responses developed in the AGCM_ML are in part due to active extratropical SST feedbacks. Diagnostic experiments using a linear model further illustrate that, in the absence of transient-eddy feedbacks, an idealized tropical heating induces anomalous flows that are qualitatively similar in early and late winter, with a trough southeast of Newfoundland and a ridge to the northeast. The enhanced seasonality in the SST-induced coupled response likely arises from the seasonal modulation of transient-eddy feedbacks on the heating-forced anomalous flow.

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