Air-Sea Interaction shapes the spurious ‘double ITCZ' in coupled climate models

As a long standing issue, many coupled climate models suffers the spurious double Inter-tropical Convergence Zone (ITCZ) (DI) problem while the corresponding AMIP (Atmosphere Model Intercomparison Project) runs usually do not. This study aims to investigate how air-sea interaction shapes the DI by examining 20 Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) models and a series of AMIP and coupled sensitivity experiments. From an atmospheric point of view, the DI problem is mainly driven by insufficient meridional SST gradient over tropics while the direct forcing from extratropical bias has nearly no impact. The reasons are twofold. First, the same amplitude of tropical SST bias forcing can induce much larger surface heat flux change than the extratropical SST bias forcing. Second, a northward cross-equatorial energy transport is seen for both forcing but there lacks of a southward cross-equatorial moisture transport for the extratropical bias forcing that is crucial for the formation of the spurious ‘Southern ITCZ'. While in coupled model, the extratropical bias (such as the Southern Ocean cloud bias) may contribute to the DI problem via altering the tropical SST (but can only explain about 5% of DI for MME of CMIP5 models). The coupled model SST bias is discussed by comparing the CMIP5 models and the CORE (Coordinated Ocean-ice Reference Experiments) experiments.