Atlantic warm-pool variability in the IPCC AR4 CGCM simulations

Abstract

This study investigates Atlantic warm pool (AWP) variability in the twentieth century and preindustrial simulations of coupled GCMs submitted to the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In the twentieth-century simulations, most coupled models show very weak AWP variability, represented by an AWP area index, because of the cold SST bias in the AWP. Among the IPCC models, a higher AWP SST index corresponds to increased net downward short wave radiation and decreased low-level cloud fraction during the AWP peak season. This suggests that the cold SST bias in the AWP region is at least partly caused by an excessive amount of simulated low-level cloud, which blocks short wave radiation from reaching the sea surface. AWP natural variability is examined in pre industrial simulations. Spectral analysis reveals that only multi decadal band variability of the AWP is significant in observations. All models successfully capture the multi decadal band, but they show that interannual and/or decadal variability is also significant. On the multi decadal time scale, the global SST difference pattern between large AW Pyears and small AWP years resembles the geographic pattern of the AM Ofor most coupled models. Observational analysis indicates that both positive ENSO phase and negative NAO phase in winter correspond to reduced trade winds in theAW Pregion. The westerly anomalies induced by positive ENSO and negative NAOlead to local heating and warm SST from March to May and February to April, respectively. This behavior as a known feature of anomalous AWP growth is well captured by only five models. © 2012 American Meteorological Society.