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The double ITCZ syndrome in GCMs: A coupled feedback problem among convection, clouds, atmospheric and ocean circulations

Zhang, Guang J., Song, Xiaoliang, Wang, Yong
Atmospheric research 2019 v.229 pp. 255-268
General Circulation Models, absorption, atmospheric circulation, convection, surface water temperature
The appearance of a spurious double ITCZ south of the equator in coupled general circulation models has been a stubborn problem ever since the beginning of coupled model development. This article reviews the past research in this area, with a focus on three possible major contributors to the double ITCZ biases: 1) the southeastern Pacific marine stratus clouds and associated warm sea surface temperature (SST) biases; 2) the extratropical shortwave absorption biases over Southern Ocean; and 3) convective parameterization. The negative biases in marine boundary layer clouds in the southeastern Pacific lead to warm SST biases. Through coupled atmosphere-ocean interactions, it contributes to the double ITCZ bias. Positive shortwave absorption biases over Southern Ocean is believed to be an extratropical contributor to the double ITCZ biases in models. Since the heat from excess shortwave absorption must be transported to the northern hemisphere by the Hadley circulation, the position of the ITCZ must shift southward. However, later research finds that reducing the extratropical shortwave absorption has little effect on the ITCZ position. One possibility is that some feedback mechanism involving subtropical low-level clouds as intermediary is missing. For convective parameterization, changes in various elements in convection schemes can have large impacts on the ITCZ simulation. It involves a complex chain of interactions among convection, large-scale atmospheric circulation, SST, and upper ocean circulation. In one scheme, changes in the scheme lead to the elimination of double ITCZ in all seasons, giving hope that we can finally take on the double ITCZ problem.