Discovering the Cause of Monsoon Depressions
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.
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.
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.