18 June 2019

Discovering the Cause of Monsoon Depressions

Researchers determine the fundamental processes creating the storms that bring extreme rainfall to South Asia.

Trajectories colored by altitude (km) in an idealized monsoon depression, originating from the strongly precipitating region southwest of the vortex center.
Science

The Asian monsoon is the most energetic monsoon on the planet, constituting a major part of the global circulation of the atmosphere and serving as a prototype for monsoons on other continents. Monsoon depressions are atmospheric vortices that form within this monsoon; they produce a large fraction of South Asia’s rainfall as well as many of its extreme rainfall events. How monsoon depressions form had remained poorly understood, yet was the subject of almost no fundamental research in the past 30 years. In two companion studies, researchers isolated the mechanisms leading to the formation and growth of monsoon depressions. The first study showed that monsoon depressions draw energy from the eastward low-level jet that exists in nearly all monsoon climates. The second study showed that this energy source can combine with precipitation to produce storms that amplify over time. This leads to the process of “moist barotropic instability,” in which monsoon depressions emerge spontaneously from the unstable configuration of winds that are found over Asia during summer.

Impact

Much of the water supply for low-latitude land regions is delivered by propagating vortices embedded within seasonal-mean, continental-scale monsoon circulations. These propagating disturbances frequently produce extreme precipitation that generates catastrophic floods, challenges water management networks, and damages natural ecosystems. Monsoon depressions produce a substantial fraction of the overall rainfall in India and sometimes evolve into typhoons. Understanding the fundamental mechanisms that lead to the formation of this important class of storms can help improve forecast models and lead to improved projections of their long-term trends.

Summary

In two companion studies, UC Berkeley researchers showed how the eastward low-level atmospheric jet that exists in all monsoon regions can become unstable, leading to the spontaneous formation of precipitating atmospheric vortices called monsoon depressions. Both studies used a numerical model of the atmosphere in which the winds were partitioned into the background state of the continental-scale, seasonal-mean monsoon, and perturbations that correspond to the storms that form within that background state. Without precipitation, the low-level monsoon jet can feed energy into vortices, but only fast enough to counteract the effects of friction; this allows monsoon depressions to maintain their amplitude for several days once they are formed, but not to amplify over time. Once precipitation occurs, the depression can amplify in a process the researchers term “moist barotropic instability." Simulations show that this amplification process is distinct from that which occurs in hurricanes and typhoons: it does not require a feedback between surface wind speed and ocean evaporation. 

Contact
William Boos
University of California - Berkeley
Publications
Diaz, M, and WR Boos.  2019.  "Monsoon Depression Amplification by Moist barotropic Instability in a Vertically Sheared Environment."  Quarterly Journal of the Royal Meteorological Society.  https://doi.org/10.1002/qj.3585.