A "nodal line" in tropical SST forcing of the North Atlantic Oscillation and its implications.

The sensitivity of Northern Hemisphere circulation patterns, including the North Atlantic Oscillation (NAO), to tropical SST changes is investigated using the NCAR CCM3.10. An array of localized SST anomalies is specified throughout the Tropics, and the responses to these experiments are summarized in "sensitivity maps" (e.g. Barsugli and Sardeshmukh, J. Climate, Dec. 2002). For the JFM season we find that warm SST anomalies in the Indian Ocean lead to a circulation pattern which projects onto the NAO and AO patterns. In contrast, warm SST anomalies in the western Pacific warm pool produce roughly the same pattern with opposite polarity. This circulation pattern is associated with large surface air temperature anomalies that have a non-zero global mean. SST anomalies in the tropical Atlantic produce an atmospheric response over the North Atlantic which, even though it projects onto the NAO, is structurally quite different. The relevance of these sensitivities is confirmed when limited regions of the observed 1950-1999 SST trend patterns are used to force the GCM. The global trend patterns that are tropically forced result mainly from cancellation of the responses to SSTs in the Indian and western Pacific Oceans. The implication is that regional patterns of climate change, and even the global mean signal, will depend sensitively on the spatial pattern of historical and projected SST change across a "nodal line" of sensitivity separating the tropical Indian and Pacific Oceans.