Intermodel spread of the double‐ITCZ bias in coupled GCMs tied to land surface temperature in AMIP GCMs

Abstract

AbstractGlobal climate models (GCMs) have long suffered from biases of excessive tropical precipitation in the Southern Hemisphere (SH). The severity of the double‐Intertropical Convergence Zone (ITCZ) bias, defined here as the interhemispheric difference in zonal mean tropical precipitation, varies strongly among models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) ensemble. Models with a more severe double‐ITCZ bias feature warmer tropical sea surface temperature (SST) in the SH, coupled with weaker southeast trades. While previous studies focus on coupled ocean‐atmosphere interactions, here we show that the intermodel spread in the severity of the double‐ITCZ bias is closely related to land surface temperature biases, which can be further traced back to those in the Atmosphere Model Intercomparison Project (AMIP) simulations. By perturbing land temperature in models, we demonstrate that cooler land can indeed lead to a more severe double‐ITCZ bias by inducing the above coupled SST‐trade wind pattern in the tropics. The response to land temperature can be consistently explained from both the dynamic and energetic perspectives. Although this intermodel spread from the land temperature variation does not account for the ensemble model mean double‐ITCZ bias, identifying the land temperature effect provides insights into simulating a realistic ITCZ for the right reasons.