This talk is motivated by two new results from our systematic investigation of the response of the NCAR CCM to an array of Tropical SST anomaly patches. First, over as much as a third of the tropical oceans, a warm SST anomaly actually reduces global mean wintertime temperature and precipitation. Second, both the NAO/AO and PNA circulation patterns--important determinants of regional climate change--respond oppositely to SST's in the two ocean basins. This leads to a partial cancellation in the response of the global and Northern Hemisphere climate to a general warming of the Tropical oceans. These opposing sensitivities are consistent with a large body of theoretical and modeling work.

Yet there are subtle differences in the way the NAO and PNA repsond to SSTs, so that differing patterns of SSTs may emphasize either a Pacific or Atlantic response. We'll use the PNA and NAO/AO patterns, along with the SST patterns that most efficiently force them, as conceptual "building blocks" to understand the response to arbitrary patterns of tropical SST anomalies. We argue that the RELATIVE warming of the Indian vs. Pacific basins is a critical factor in determining the regional climate responses in the Northern Hemisphere. We will discuss the implications for modeling the observed climate trends in the past 50 years.