The importance of tropical Pacific SST changes between the warm pool and the cold tongue
Prashant Sardeshmukh, NOAA/CIRES/CDC, Boulder, CO; and G. P. Compo
A comprehensive atmospheric GCM experiment with anomalous SST only in the Nino-4 area of central tropical Pacific is shown to capture all the major elements of the global response obtained in several multi-decadal AGCM integrations with prescribed observed global SST variations. This area lies at the western edge of the region of warmest anomalous SST during El Nino events, but accounts for most of the global anomalies associated with El Nino. In particular, this experiment suggests that rainfall anomalies over the west Pacific, Africa, central America and Amazonia observed during El Nino are largely forced by remote central tropical Pacific rather than by local SST anomalies. The global response to SST changes in this important area is also significantly nonlinear, as found from comparing large ensembles of GCM integrations with prescribed positive and negative SST anomalies in the region. The response remains similar in pattern but varies nonlinearly in amplitude as the anomalous SST is varied from a 5 degree cooling to a 5 degree warming. As expected, the precipitation response saturates for strong cooling, but varies remarkably linearly from weak cooling to strong warming. The global circulation response, on the other hand, saturates for both strong cooling and strong warming, but at different amplitudes. The negative saturation is closely but not exclusively linked to the negative saturation of the precipitation response. The positive saturation cannot be so linked; it has a dynamical origin. This asymmetry of the circulation response is clearly evident in the PNA, NAO and AO regions of strong regional climate variability.
These results have important implications for the improved modeling of the mean tropical climate as well as tropical variability, tropical-extratropical interactions from monthly to centennial scales, and for assessments of global climate change resulting from anthropogenic forcing. The asymmetric saturation of the circulation response to warming and cooling of the central tropical Pacific is especially relevant in this regard. Because of it, one may expect the global climate to be affected not just by a mean SST change in this area, but also by a change in its variability associated with a change in ENSO dynamics.
Session 9, Climate Model Analysis and Improvement
Thursday, 2 February 2006, 11:00 AM-4:30 PM, A314
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