Assessing extratropical impact on the tropical bias in coupled climate model with regional coupled data assimilation

The tropical bias of double-Intertropical Convergence Zone (ITCZ) has been a persistent feature in global climate models. It remains unclear how much of it is attributed to local and remote processes, respectively. Here we assess the extratropical influence on the tropical bias in a coupled general circulation model dynamically, systematically, and quantitatively using the Regional Coupled Data Assimilation (RCDA) method. RCDA experiments show that the model's double-ITCZ bias is improved systematically when sea surface temperature, air temperature, and wind are corrected toward real-world data from the extratropics into the tropics progressively. Quantitatively, the tropical asymmetry bias in precipitation and surface temperature is reduced by 40% due to extratropical impact from outside of ~25°. Coupled dynamics, as well as atmospheric and oceanic processes, play important roles in this extratropical-to-tropical teleconnection. Energetic analysis of cross-equatorial atmospheric energy transport and equatorial net energy input are used to explain the changes in the precipitation bias.