Physical and dynamical characteristics of extreme Climate events and their future projections using coupled models over South Asian regions.

Abstract

This thesis explores the characteristics of the SASM rainfall and its extremes in prominent regions selected based on spatiotemporal features. The study analyzes the spatiotemporal variability and related characteristics of SASM rainfall from 1951 to 2015. Five prominent areas in the South Asian region were identified based on the decadal trend of extreme rainfall in these zones. The study then delved into the physical characteristics of extreme rainfall events (EREs) on different time scales within the identified regions. Alongside spatial trends, the study illustrates the evolution and dissipation features of EREs in the chosen areas. It also discusses the interannual variability and probability distribution of mean rainfall and EREs. Furthermore, it explores the association of southwest monsoon rainfall and its extremes with various global climate indices. The regional analysis of rainfall composites for extreme events, along with lead-lag analysis, indicates that, in addition to regional variations, most of the EREs develop within a short span of three days despite their signature of above-normal rainfall appearing almost one week before the event. The probability density function analysis of rainfall in the pre-1980 and post-1980 periods shows more significant differences in the case of extremes compared to mean rainfall. Individual analysis of the correlation of climatological forcing mechanisms with mean and extreme rainfalls reveals spatial variations. Due to the interdependence of various forcing mechanisms influencing the SASM, their coupled action makes the impacts and vulnerability of extreme events unpredictable. The study analyzes favourable conditions for regional extreme rainfall events over the Indian subcontinent by considering circulation and other meteorological parameters. Additionally, it 1investigates the large-scale dynamical factors contributing to various disastrous extreme rainfall events (EREs) in the identified regions before, during, and after the events. The moisture transport analysis indicates that the Arabian Sea is the primary source for extreme events over the west coast, central India, and north-central India. In contrast, the Bay of Bengal serves as the principal moisture source for the northeastern region. The analysis of relative vorticity reveals positive regions during extreme rainfall events, indicating low-level convergence, further supported by low-level moisture convergence. Examining vertically integrated moist static energy shows elevated values in regions experiencing extreme rainfall, aligning with moisture convergence at 1000 hPa. The evolution and dissipation of moisture and vertical velocity, as observed in lead-lag vertical cross-sections, highlight the uniqueness of individual regions. The study validates global climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) for projecting SASM rainfall and its extremes in the future. The most suitable models were selected, capable of accurately replicating historical rainfall (1950–2014) and its extremes in the study region. The intensity of extreme rainfall events and their contributions to seasonal rainfall during the historical period in the study region were assessed. In the final phase, the study presents significant spatiotemporal variations in extreme rainfall across the study region under different future warming scenarios. The selected coupled dynamical models project both significant and insignificant increases in seasonal and extreme rainfall for both monsoonal and non-monsoonal regions over the study area in the future, depending on the scenarios considered. The EC-Earth models forecast a remarkable increase in seasonal rainfall for the west coast, southwestern regions of central India, and the western Himalayas. The NorESM model predicts significant changes in central India and the rain shadow regions of southeast India. In all model projections of extreme rainfall, the increased change in the northern regions of the west coast and southwestern regions of central India could lead to heightened vulnerabilities in these areas, especially under higher forcing scenarios. Moreover, the zone of extreme rainfall extends to the rain shadow regions behind the Western Ghats, resulting in a noticeable reduction in the area of rain shadow regions in the future, particularly in the SSP5-8.5 scenario. These projections underscore the urgent need for climate change adaptation strategies in the region.