Biological and Environmental Research - Earth and Environmental System Sciences
Earth and Environmental System Modeling
27 April 2020

A New Perspective on the Asian Summer Monsoon

Link between the flow of energy and moisture in the atmosphere can be used to identify the sources of rainfall changes in monsoons.

Monsoon tropics.
Science

One of the challenges of climate science is understanding how warming will change monsoon rainfall. There has been uncertainty across prior model results, suggesting a need for new techniques to pinpoint the physical pathways of the monsoon changes. A research team led by scientists at the Department of Energy’s (DOE’s) Pacific Northwest National Laboratory (PNNL) used a new method for fingerprinting the sources of rainfall changes in tropical circulations. This new method was applied to the Asian Summer Monsoon (ASM) in DOE’s Energy Exascale Earth System Model. Researchers used the link between the flow of energy and moisture—termed the normalized gross moist stability (NGMS)—to quantify the physical processes that give rise to the increased rainfall with warming predicted by the model. This technique confirmed established mechanisms and highlighted the importance of changes to NGMS itself for monsoonal rainfall.

Impact

The ASM brings water to more than a billion people, and understanding its response to warming is a critical need for future water management. The NGMS framework provided a new lens for examining the ASM. Researchers found that changes to the NGMS of the monsoon are an important component of the rainfall response to warming. This work opens up the potential for better understanding tropical circulations via energetic frameworks such as the NGMS framework used in this study.

Summary

Researchers applied a new framework for understanding monsoonal changes by following how energy flows through the atmosphere in DOE’s Energy Exascale Earth System Model, where the divergence of moist static energy (MSE) and moisture have been calculated online. The link between the flow of energy and moisture (NGMS) was used to fingerprint the physical processes that give rise to the increased rainfall with warming predicted by the model. The fingerprint confirmed the importance of known mechanisms while also highlighting the importance of the previously overlooked NGMS term for rainfall response. As the climate warms, the moisture loading of the atmosphere increases such that the same atmospheric circulations import much more water than they export total energy, relative to the current climate. This change in efficacy is as important as other sources to the total rainfall change and is also sensitive to whether the ocean temperature is prescribed or modeled explicitly.

Contact
Ruby Leung
Pacific Northwest National Laboratory
Publications
Harrop, B, P Ma, P Rasch, Y Qian, G Lin, and C Hannay.  2019.  "Understanding Monsoonal Water Cycle Changes in a Warmer Climate in E3SMv1 Using a Normalized Gross Moist Stability Framework."  Journal of Geophysical Research: Atmospheres 124(20): 10826-10843.  https://doi.org/10.1029/2019jd031443.