The sensitivity of global mean temperature and precipitation are investigated using the NCAR CCM3.10. An array of localized SST anomalies is specified, and the responses to these experiments are summarized in "sensitivity maps" (e.g. Barsugli and Sardeshmukh, J. Climate, Dec. 2002). For the DJF season we find that warm SST anomalies in the Indian Ocean lead to a reduction in global mean temperature and a reduction in global mean precipitation. Warm SST anomalies in the western Pacific warm pool have the opposite effect. The contrasting temperature signals forced by these two regions are due mainly to a single circulation pattern that is forced with opposite signs, depending on which side of the "nodal line" the forcing lies.

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 from cancellation of the responses to SSTs in these two areas of contrasting sensitivity. The implication is that regional patterns of climate change, and even the magnitude of 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.