Single to Double-ITCZ transitions in an idealized moist GCM

On Earth, precipitation can have one or two maxima near the equator that each correspond to an intertropical convergence zone (ITCZ). For example, during spring in the Eastern Pacific, precipitation is most intense north and south of the equator; at the equator, there is a precipitation minimum. This has implications for the seasonal cycle of precipitation over the ocean because shifts of the ITCZ across the equator can be abrupt, without an intermediate precipitation maximum on the equator. In addition, many state-of-the-art general circulation models exhibit a double-ITCZ bias over the Pacific, overestimating the strength of precipitation south of the equator and underestimating it north of and on the equator. In this study, we characterize the transition from single to double-ITCZs in an idealized moist aquaplanet general circulation model (GCM) with and without seasonal cycle. We present energetic constraints on the location of the double-ITCZ branches and provide insights into why and when transitions from single to double ITCZs occur. By relating changes in the local energy balance of the atmosphere to circulation changes, we find that double ITCZs require a negative atmospheric net energy balance at the equator and that the locations of the two branches of double-ITCZs are related to the atmospheric net energy balance at the equator and to the curvature of the energy flux across the equator. This has implications for comprehensive climate models and provides a framework in which precipitation biases in models can be analyzed and understood.