The source of Double ITCZ induced by the SST bias over the tropical western Pacific as reflected in CAS-ESM2 Model

Abstract

“Double ITCZ” is a common precipitation bias over the tropical Pacific in current climate models and Earth system models, but the reasons for its formation are still worth exploring and discussing. In this study, we adopted the second version of Chinese Academy of Sciences Earth System Model (CAS-ESM2), by comparing a set of sea surface temperature (SST) bias correction experiments over the tropical Pacific, to explore the possible mechanism of SST bias on inducing the “Double ITCZ” from the perspective of the climatic annual mean bias in the coupled model. We revealed that the simulated climatic annual mean SST bias over the tropical Pacific can affect the bias of latent heat flux through the saturation specific humidity, and the bias of latent heat flux can further affect that of vertical velocity of humid air by the condensation release mechanism, and finally modulate the simulated bias in precipitation. Furthermore, through the inter-comparison between different experiments, it is found that the source of Double ITCZ might mainly come from the annual mean SST bias over the tropical western Pacific through the proposed air-sea coupled process of “SST-saturated specific humidity-latent heat flux-vertical velocity-precipitation”, indicating a possible way on reducing the couple biases in models over the tropical Pacific to improve the accuracy of CAS-ESM2 for climate simulation.