Tropical monsoon depressions are synoptic-scale (1000-3000 km across) regions of low pressure that are accompanied by cloudiness and rainfall. They often develop over the Bay of Bengal and propagate northwestward towards India. These disturbances can cause up to half of all of India's total monsoonal rainfall.
In spite of their importance to Indian rainfall, very little is known about these depression systems. For a long time, it was thought that these depressions formed due to instabilities that are similar to those of extratropical storms. However, recent work has shown that such a mechanism is unlikely to be operating in monsoon depressions.
My work seeks to understand the structure and propagation of these monsoonal disturbances. Ongoing work uses a combination of observations, modeling and theory to test the hypothesis that moisture plays a key role in the dynamics of these systems.
Using state of the art models combined with statistical techniques, we are able to elucidate the structure of these depressions.
These systems are described as cyclonic anomalies with regions of enhanced moisture and convection. The winds play a key role in the movement of these systems by creating an environment that favors convection. This environment is created when northerly winds induce upward motion, which reduces the environmental inhibition for deep convection.
Previous theories have failed to describe one crucial aspects of monsoon depressions. This suggests that new theories may be needed to fully understand these systems. By including moisture-convection feedbacks into a linear model, we have been able to describe a wave that resembles monsoon depressions. These waves grow to feedbacks describes in the schematic to the right. Future work will test this framework by analyzing model simulations of varying levels of complexities.