Monsoon Low Pressure Systems over Indian subcontinent in CESM1.2.2

Abstract:

Monsoon low pressure systems (LPS) are synoptic scale disturbances which form over the Indian subcontinent along the quasi stationary monsoon trough during the monsoon season (June to September). In a recent study, we found that 60-70% of monsoon rainfall in north, east and central India and about 80% of extreme precipitation events in the country are associated with LPS. For millions of people whose livelihood depends on agriculture, and lakhs of people whose lives and livelihood are endangered due to monsoon floods every year, understanding the evolution and propagation of these LPS and their future change becomes a necessity. Coarse resolution Global circulation models have been used to understand the features of tropical disturbances in the past. As model resolution plays a key role in simulating the climatology of tropical storms, finer resolution (of the order of 20-100km) models are required for analyzing the genesis and propagation of these storms.

In this study, the present-day control simulation of the Community Earth System Model (CESM1.2.2), generated at 0.9°×1.25° horizontal resolution, is used to understand characteristics of monsoon LPS over the Indian subcontinent. We find that CESM is capable of reproducing the overall characteristics of monsoon over the Indian subcontinent in terms of seasonality, propagation of monsoon rainfall and mean monsoon winds. However, precipitation simulated by CESM is generally larger than observed IMD (India Meteorological Department) precipitation. LPS are tracked in the CESM simulation and in the ERA-Interim Reanalysis by a recently developed tracking algorithm called the Automated Tracking Algorithm using Geopotential Criteria (ATAGC). Genesis of LPS is found to be spread over both land and ocean in CESM simulation, as opposed to being concentrated in north Bay of Bengal in ERA-Interim Reanalysis. A southward latitudinal shift is evident for the median track of LPS in CESM simulation relative to that in ERA-Interim Reanalysis. Rainfall contribution of LPS to the total mean monsoon rainfall on the Indian subcontinent is larger in the case of CESM-simulated tracks. The percentage frequency of extreme rainfall events attributable to LPS is also larger in CESM simulation, while the percentage of mean monsoon rainfall attributable to extreme rainfall events caused by LPS is larger in ERA-Interim Reanalysis. The ability of CESM to simulate the major features of monsoon LPS provides confidence in using it to project the possible changes in LPS and related extreme rainfall events under climate change scenarios. The assessments based on our modeling study could be helpful to devise mitigation/response strategies to cope with natural hazards (e.g., floods, landslides) triggered by LPS related extreme precipitation events over India.