Assessing the Impact of Climate Change on Frequency and Intensity of Compound Coastal Extremes in India

The Indian coastline, flanked by the Bay of Bengal and the Arabian Sea, is prone to the impact of intense low-pressure systems, specifically tropical depressions and storms, which are accompanied by extreme rainfall and storm surges. The vulnerability of the Indian East Coast to compound flooding, characterized by the concurrent occurrence of extreme rainfall and wind-driven extreme storm surges, poses a significant challenge in the face of changing climatic conditions. This study examines how anthropogenic climate change may influence the frequency and intensity of such compound extremes on the Indian East Coast by the end of the 21st century. For this purpose, observed sea level data at three tide gauge (TG) stations (Paradip, Haldia, and Chennai) were used to extract storm surge time series for the period 1980-2010. Daily rainfall was obtained from the 0.25° gridded dataset of the India Meteorological Department (IMD), while mean sea level pressure anomalies and surface wind speeds were extracted from ERA5 reanalysis data within a 500 km radius of the coastal stations along the East Coast of India. A logistic regression model was utilized to identify the suitable atmospheric predictor for storm surge extremes, and the corresponding threshold of the variable leading to storm surge extremes (exceeding the 95th percentile) at the tide gauge station was identified. Subsequently, the bias-corrected GCM simulated precipitation and wind stress (identified from a logistic regression model) variables were obtained at the grid points near the TG stations from 10 models corresponding to CMIP6 simulations for the historical period as well for the end of the century (2070-2100), corresponding to the extreme ssp585 scenario. The compound extremes were identified in the GCM data for both the historical and future periods by using thresholds of simulated rainfall and wind stress (identified from logistic regression) data consistent with those derived from observations. The change in seasonal, annual and decadal variability of the frequency of the compound extremes was investigated for data from each of the 10 models as well the ensemble mean from the models for the future period with respect to the historical period. Initial results show a greater change in the frequency of these extremes in the post-monsoon season than the monsoon season for the majority of the models. Additionally, a higher mean annual intensity of the compound extremes with respect to the historical counterpart was expected to occur under the SSP585 scenario at the end of the century. The synoptic patterns corresponding to the compound extremes were also investigated to understand the changing dynamics of these extremes on the Indian Coast.