PNNL

Abstract

One of the grand challenges of climate science is understanding the changes of the tropical rain belts and monsoon systems owing to CO2-induced warming. A promising path forward links the fluxes of energy and moisture to tropical circulation features. To this end we make use of the E3SMv1 model, where the divergence of moist static energy (MSE) and moisture have been calculated online, and a normalized gross moist stability (NGMS) diagnostic framework employed to understand the linkages between changes in the flow of energy and moisture within the monsoon systems. We focus on the Asian summer monsoon system and utilize a series of atmosphere-land and atmosphere-land-ocean simulations to understand the connection between fluxes and monsoons. Uncoupled simulations with prescribed sea surface temperatures indicate that decreases in NGMS over land are important in explaining precipitation increases in response to both SST and CO2 increases. In fully coupled experiments, NGMS decreases remain an important contributor to the increase in P-E, but the coupled simulations highlight the importance of consistent ocean and land responses in interpreting the monsoon changes. This study indicates that transient eddy fluxes play an important role in NGMS decreases and that a time-mean view of the monsoon circulations is insufficient to quantify the link between future changes in the fluxes of energy and moisture. Compensation between dynamic and thermodynamic components of vertical MSE advection occurs, with the thermodynamic contribution dominating. The compensation is shown to be sensitive to relative humidity, with higher relative humidity leading to a stronger thermodynamic component.