01 March 2019

Looking at Monsoon Rainfall Through a New Lens

Researchers discovered different hydrological characteristics between Indian and East/Southeast Asian monsoon systems in a warmer climate.


Summer monsoon rainfall provides the lifeline for agriculture in many tropical and subtropical countries. How monsoon precipitation and hydrological extremes could respond to climate warming is of great social and societal importance. Working with university collaborators, researchers from the U.S. Department of Energy’s Pacific Northwest National Laboratory developed and applied a new analysis method—called local water vapor wave activity, or LWA—to examine how hydrological extremes over Asian monsoon regions will change in the future. They found that a metric for precipitation extremes derived from LWA analysis increased in both the Indian and East/Southeast Asian monsoon regions. Meanwhile, the rate at which water vapor cycled through the atmosphere strengthened in the latter, but not the former, region. 


This research showcases the application of LWA analysis to tropical monsoon systems. This LWA diagnostic allows for more rigorous study of hydrological extremes at a regional scale, with the resultant LWA sink as a useful metric for precipitation extremes. The distinct characteristics in the hydrological cycling rate imply differing dynamical mechanisms governing precipitation extremes between Indian and East/Southeast Asian monsoons.


Globally, the atmosphere may be treated as a “reservoir” of moisture, with precipitation acting as a moisture sink and evaporation a moisture source, and thus the concept of hydrological cycle is well suited for globally integrated moisture. However, this global perspective cannot be readily carried over to regional atmospheric hydrological cycles. In a changing climate, the amount of precipitation that reaches Earth’s surface could fluctuate greatly by region compared to historical trends. Researchers developed an LWA diagnostic method for water vapor to represent local hydrological cycles. They then applied the method to explore the response of hydrological extremes over Asian monsoon systems to a climate warming scenario. Results showed that future water vapor wave activity over the broad Asian monsoon region increased by about 35 percent, largely due to the increase in background moisture. The analysis also found that precipitation extremes measured by the LWA sink of the budget strongly increased in both the Indian and East/Southeast Asian monsoon regions. The hydrological cycling rate, measured by the inverse of the residence time of the column moisture, showed distinct features between the two regions, suggesting different dynamical effects. 

L. Ruby Leung
Pacific Northwest National Laboratory