Tropical Atlantic Sea Surface Temperature Forcing of Quasi-Decadal Climate Variability over the North Atlantic–European Region

Abstract

Effects of Atlantic sea surface temperature (SST) anomalies on the North Atlantic low-frequency atmospheric variability are examined by analyzing two ensembles of integrations of the ARPEGE general circulation model (GCM) forced with differently configured observed SSTs and sea ice extents (SIE) over the 1948?98 period. An attempt is made to separate the forced atmospheric response from internal atmospheric variability by using a signal-to-noise maximizing empirical orthogonal function (EOF) analysis. This method yields an estimate of the most detectable common forced response given the knowledge of internal variability provided by the ensemble. Applying the algorithm to North Atlantic atmospheric model data leads to an atmospheric response associated with a tripole pattern in North Atlantic SSTs. The spatial structure of the forced response, which is most consistent in winter, shows a dipole pattern in mean sea level pressure projecting onto the North Atlantic Oscillation. Examination of other atmospheric variables shows a very coherent signal with a quasi-barotropic signature. Additional atmospheric integrations with idealized SST anomaly patterns demonstrate the primary role of the tropical North Atlantic SST anomalies in generating the forced response. The physical mechanism involves related changes in tropical convection, Hadley circulation, and the modulation of the stationary and transient planetary-scale waves by the low-frequency variability in subtropical winds induced by the persistent tropical circulation anomalies.