The atmospheric response over the North Atlantic to decadal changes in seasurface temperature

Decadal fluctuations in the climate of the North Atlantic-European region m ay be influenced by interactions between the atmosphere and the Atlantic Oc ean, possibly as parr of a coupled ocean-atmosphere mode of variability. Fo r such a mode to exist, a consistent atmospheric response to fluctuations i n North Atlantic sea surface temperatures (SST) is required. Furthermore, t his response must provide feedbacks to the ocean. Whether a consistent resp onse exists, and whether it yields the required feedbacks, are issues that remain controversial. Here. these issues are addressed using a novel approa ch to analyze an ensemble of six integrations of the Hadley Centre atmosphe ric general circulation model HadAM1, all forced with observed global SSTs and sea-ice extents for the period 1949-93. Characterizing the forced atmospheric response is complicated by the presen ce of internal variability. A generalization of principal component analysi s is used to estimate the common forced response given the knowledge of int ernal variability provided by the ensemble. In the North Atlantic region a remote atmospheric response to El Nino-Southern Oscillation and a further r esponse related to a tripole pattern in North Atlantic SST are identified. The latter, which is most consistent in spring, involves atmospheric circul ation changes over the entire region, including a dipole pattern in sea lev el pressure often associated with the North Atlantic oscillation. Only over the tropical/subtropical Atlantic, however, does it account for a substant ial fraction of the total variance. How the atmospheric response could feed back to affect the ocean, and in particular the SST tripole, is investigat ed. Several potential feedbacks are identified but it has to be concluded t hat, because of their marginal consistency between ensemble members, a coup led mode that relied on these feedbacks would be susceptible to disruption by internal atmospheric variability Notwithstanding this conclusion, the au thors results suggest that predictions of SST evolution could be exploited to predict some aspects of atmospheric variability over the North Atlantic, including fluctuations in spring of the subtropical trade winds and the hi gher latitude westerlies.