Characterization of the Double ITCZ Bias in Climate Models

Robust features associated with the double ITCZ bias in climate models are sought by performing a statistical analysis of the differences between the present-day mean climates of 20 models from the IPCC AR4 database. The results indicate that both the double ITCZ and the warm biases along the eastern boundaries of the basins are common to most models but not related to each other. An analysis of atmosphere-only runs of the some of the climate models confirms that the origin of the double ITCZ bias lies in the atmospheric models, which produce excessive convective rainfall on both sides of the equator and an associated positive bias in surface evaporation in the trade wind regions and the equatorial cold tongues. Coupling with the ocean leads to the amplification of the bias in precipitation, but the associated signal in sea surface temperature is weak except along the Pacific equatorial cold tongue.