An Investigation of the Characteristics of Monsoon Low Pressure Systems in the Present Climate and their Sensitivity to Topography and Climate Change

Abstract

Monsoon Low-Pressure Systems (LPS) are synoptic-scale tropical disturbances that periodically form over the Indian subcontinent during the summer monsoon season (June-September). Apart from being a lifeline to agriculture, the LPS-triggered precipitation could cause catastrophic floods. This thesis investigates the large-scale factors that influence LPS characteristics under the current and future climate change scenarios. In the early part of the thesis, a new approach is developed to track the formation and propagation of LPS over the Indian subcontinent. A detailed statistical and visual comparison is made between LPS tracks generated using our approach applied to ERA-Interim reanalysis data and tracks obtained in previous studies. Furthermore, extreme rainfall at locations in the vicinity of LPS is analyzed which could be valuable for flood risk assessment during the monsoon season in central India.

In the latter part of the thesis, a fully coupled version of the Community Earth System Model (CESM 1.2.2) is run at 0.9°×1.25° spatial resolution, and 6-hourly output is generated for track analysis. The model’s ability to simulate the characteristics of LPS is first assessed by performing a present-day control simulation. Simulations to study the sensitivity of LPS statistics to topographical features in the south Asian region (presence or absence of southeast Asian mountains and the height of Tibetan and Himalayan Orography (THO)) and the change in LPS characteristics under climate change are also performed. Simulations without the southeast Asian mountains enable determining the influence of these mountains on the downstream amplified systems (remnants of Pacific tropical cyclones) over the Bay of Bengal. The sensitivity analysis on the influence of the height of THO shows an interesting result: while a decrease in monsoon precipitation with a reduction in the height of THO is simulated, the number of LPS increases. A detailed analysis of the dynamic factors leading to this counter-intuitive result is performed. Finally, the change in LPS characteristics and the associated large-scale SST and circulation anomalies in the Indian Ocean and south Asian region are assessed for the RCP8.5 emissions scenario. It is found that the monsoon circulation is weakened, summer monsoon precipitation over India is enhanced, and the number of LPS remains nearly unchanged in a warmer world.