Understanding and Reducing the Double ITCZ Bias in Coupled Ocean-Atmosphere Models

Abstract:

Double Inter-Tropical Convergence Zone (ITCZ) in the central equatorial Pacific is a chronic bias in coupled global ocean-atmosphere climate models. This bias affects the ability of climate models to simulate ocean biogeochemical cycles and teleconnection of the tropical climate with middle-latitude atmospheric circulations. In this presentation, we first assess what progress has been made in reducing the double ITCZ biases from CMIP3 to CMIP5. We then show why double ITCZ occurs in observations in some years during the northern Spring while not in other years. Large-scale atmospheric and oceanic controlling factors are identified that favor double ITCZ. We finally show how and why the Community Atmospheric Model (CAM5) and the CESM simulate the double ITCZ. Sensitivity results of model simulations to the parameterizations of convective mass flux and triggering conditions are discussed.