Response of Monsoon Low Pressure Systems to a Warming Environment

The Low Pressure Systems (LPS) are cyclonic storms embedded in the large scale monsoon circulation. Like tropical cyclones, these storms are also rotating vortices; however, with a weaker intensity and larger diameter (~1000km) than a tropical cyclone (~500km). Also, the LPS form over both ocean and land, whereas a tropical cyclone forms only over warmer oceanic region. Although LPS are found in all monsoon regions of the world, they are most prominentover the Indian region during summer monsoon season. During a typical summer monsoon season, about 12 LPS form over the Bay of Bengal and the Gangetic plains, which then traverse a north-west trajectory and contribute more than 60% of the total seasonal rainfall received over the fertile central Indian region. Thus any changes to the number and intensity of these storms in a warming climate can make significant societal impacts. However, the variability of these rain-bearing storms in response to Green House Gas (GHG) induced warming is not yet explored by the climate science community. A great hurdle in the understanding of the response of LPS to an enhanced GHG warming is that these storms are poorly resolved by the current generation climate models. Typical grid size of a climate model is in the range of 100 – 250km which is inadequate to resolve a storm. In order to overcome this issue we performed numerical experiments with a High Resolution Atmospheric Model (HiRAM) which has a grid size of 50km. Two sets of simulations are performed – one with current climate conditions and the other with enhanced GHG conditions. Unlike the climate models that are used for Inter-Governmental Panel for Climate Change (IPCC) assessment, HiRAM realistically simulated the characteristics of LPS in the current climate experiment. The future climate simulations with GHG induced warming show that the monsoon LPS activity over India would decline by about 45% towards the end of the 21 st century. It is also important to understand the physical mechanisms that are responsible to the weakening of the LPS in a warming climate. A diagnostic analysis of the future climate simulations reveals that an enhanced stability of the atmosphere over the core LPS genesis region of the Bay of Bengal suppresses the formation of the storms.