Future changes in monsoon extreme climate indices over the Sikkim Himalayas and West Bengal

Abstract

Utilizing an ensemble of regional climate models (RCMs) from Coordinated Regional Climate Downscaling Experiment (CORDEX) South Asia, projected changes in mean and extreme climate indices are analysed for future period (SCEN; 2021–2060). The parametric transformation bias correction technique is applied to enhance the model agreement in calibration period (1961–2005) and to produce reliable results for evaluation period. The observed pattern of temperature and precipitation indices is illustrated for control period (CTRL; 1961–2005), which exhibits high spatial variability with the altitudinal differences, especially over the Himalayas. Strong meridional moisture flow from the Bay of Bengal contributes higher, heavier and more intense precipitation events over the Sikkim Himalayas and northern Sub-Himalayan West Bengal (SWB). Despite spatial difference in the occurrences of return value for 5-, 10- and 50-year levels, some indices show close similarity between the Sikkim Himalayas and Gangetic West Bengal (GWB). The projected uncertainties under the RCPs remain very high for extreme temperatures over the Himalayas. Change in minimum temperature is largely significant. Conversely, maximum temperature shows insignificant change. Diurnal temperature range shows an expected decline for projected scenarios. Lower atmospheric easterlies towards east-central India enhance future moisture transport, which don’t eventually bring positive change for precipitation amount. Instead, the Sikkim Himalayas and the Eastern Ghats exhibit expected rise in higher, heavier and intense precipitation events. Consecutive dry days exhibit significant future rise over the plain and lower plateaus, which show significant decline over the mountains. The topographic forcings to the convective mechanism as well as model sensitivity evidently control the spatial pattern and occurrences of extreme events. Future rise in moisture level is more conspicuous under RCP8.5 along with the significant changes in extreme climate indices.