Drivers of the mean biases of the tropical atmospheric circulation in a moist static energy framework

Abstract

Abstract In this study, we apply the moist static energy for first baroclinic mode (MSEB) model to examine the drivers of the mean tropical atmospheric circulation biases over oceanic regions. The model diagnoses the vertical motion in an air column of the tropical regions based on net energy heat flux and advection of moisture or heat into the air column in relation to the stability of the air column due to the gradients in moist static energy. Analysis of CMIP and AMIP simulations helped to identified errors intrinsic to the atmospheric models or errors due to atmosphere-ocean coupling process with a focus on some of the main large-scale biases (double-ITCZ, Pacific cold tongue, southward shift of ITCZ over the Atlantic, and dipole bias over the Indian Ocean). Despite some limitations of the MSEB model we find that the primary drivers of the tropical circulation biases result from biases in the top of the atmosphere long wave radiation and the surface latent heat flux, suggesting a link to biases in the hydrological cycle. Biases in the advection of moist static energy also play an important role, while biases in the gross moist stability profiles play only a minor role. Most biases are intrinsic to the atmospheric model, but some are significantly enhanced or altered in pattern in the coupled model simulations.