The Relationship between Tropical Warm Pool Precipitation, Sea Surface Temperature, and Large-Scale Vertical Motion in IPCC AR4 Models

Abstract

regime sorting analysis is used to identify Caribbean and western Pacific precipitation, sea surface temperature, and large-scale vertical circulation relationships and biases within coupled and uncoupled Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) general circulation models. This analysis shows that an oversensitivity of precipitation to both SST and vertical circulation (as represented by ?500) is inherent in the atmospheric models in both regions, with models using a spectral-type convective parameterization performing best in the Caribbean, but less separation between convective parameterization groups is seen in the western Pacific. The error in magnitude of precipitation for a given SST and vertical circulation causes uncoupled models to overestimate Caribbean and western Pacific mean precipitation. In coupled models, however, errors in the frequency of occurrence of SSTs (the distribution is cold biased in both regions) and deep convective vertical circulations (reduced frequency) lead to an underestimation of Caribbean and western Pacific mean precipitation. In the western Pacific, increased frequency of subsidence regimes in coupled models leads to an overestimation of precipitation at ?500 values above 0 hPa day?1. The varied ability of convective parameterization groups in the two warm pool regions suggests that deficiencies in parameterization groups differ between the two regions, with improvements needed particularly in the deep convective regime in the Caribbean and subsidence regimes in the western Pacific.