Extratropical sensitivity to tropical sea surface temperatures

Despite numerous investigations of the extratropical response to prescribed observed SSTs in atmospheric general circulation models, a general understanding of the sensitivity of that response to SST anomalies in different parts of the tropical oceans is lacking. In this study, such a general sensitivity analysis has been conducted using the NCAR atmospheric general circulation model (CCM3.10). Ensemble-mean model responses were determined for an array of 43 regularly spaced localized SST anomaly patches over the tropical Indian, Pacific, and Atlantic oceans. A singular vector analysis of these 43 responses shows that the extratropical response is much more sensitive to SST forcing in the western half of the Indo-Pacific basin than in the eastern half, the region of largest observed SST variability. This larger sensitivity is only partly due to the larger thermodynamic sensitivity of the precipitation response to SST anomalies over the warm pool. The dynamic sensitivity of the extratropical response to forcing south of the Asian-Pacific jet is even more important, especially in winter. Remarkably, this sensitivity is of opposite sign for SST anomalies in the Indian and western Pacific oceans. Thus a warmer western Pacific tends to force the positive phase of the PNA pattern, but a warmer Indian ocean the negative phase. Our results strongly indicate the need to improve simulations and predictions of SST in this sensitive western half of the Indo-Pacific basin to improve simulations and predictions of global climate variability from seasonal to centennial scales.