Double ITCZ in Coupled Ocean-Atmosphere Models: From CMIP3 to CMIP5

Abstract

Recent progress in reducing the double Intertropical Convergence Zone bias in coupled climate models is examined based on multimodel ensembles of historical climate simulations from Phase 3 and Phase 5 of the Coupled Model Intercomparison Project (CMIP3 and CMIP5). Biases common to CMIP3 and CMIP5 models include spurious precipitation maximum in the southeastern Pacific, warmer sea surface temperature (SST), weaker easterly, and stronger meridional wind divergences away from the equator relative to observations. It is found that there is virtually no improvement in all these measures from the CMIP3 ensemble to the CMIP5 ensemble models. The five best models in the two ensembles as measured by the spatial correlations are also assessed. No progress can be identified in the subensembles of the five best models from CMIP3 to CMIP5 even though more models participated in CMIP5; the biases of excessive precipitation and overestimated SST in southeastern Pacific are even worse in the CMIP5 models. Key Points Double ITCZ bias persists from CMIP3 to CMIP5 The CMIP5 MME with overall warmer SST suffers a more serious double ITCZ problem than CMIP3 MME No improvement can be identified in the subensembles of five best models from CMIP3 to CMIP5