Projected changes in winter-season wet days over the Himalayan region during 2020–2099

The Northern Hemisphere winter-season wet day climatology is extremely important to hydrological and agricultural processes of the Himalayan region (HR). However, knowledge of expected changes in the winter-season wet day climatology under global warming is significantly limited. Hence, this study attempts to quantify the expected changes in winter-season wet day climatological patterns for HR during 2020–2099 in comparison to a baseline period of 1980–2000 under two different warming scenarios, these being representative concentration pathways 4.5 and 8.5 (RCP 4.5 and RCP 8.5). Five climate model products covering the southern Asian region were obtained from the Commonwealth Scientific and Industrial Research Organization initiated Coordinated Regional Climate Downscaling Experiment (CORDEX) of the World Climate Research Programme and used for this purpose. Model biases are estimated with respect to observations for a base line period of 1980–2000. Model ensemble non-linear trends of the winter-season wet days for the periods 2020–2040, 2041–2070, and 2071–2099 are estimated using Sen’s slope estimator, while ensemble average future changes in the number of winter-season wet days are estimated, and attempts made to identify the topographical ranges that are expected to be mostly affected by the changing winter-season wet day climatology. The results show that the CORDEX-regional climate models have a positive bias, ranging between 1 and 30 days, across the high altitudes of the entire Himalayas, and model performance improves with an increasing number of wet days per season. Although the impact of stronger warming (i.e. under RCP 8.5) is noted to enhance the area averaged non-linear trend of wet days over northwestern (0.014) and eastern (0.005) Himalaya during 2071–2099, the model ensemble predicted area-averaged reduction in the frequency of wet days of 0.3 to 1.0 day is highly likely by the end of this century. It is also observed that the Himalayan region within the range of 1000–2500 m above sea level may experience a decline in winter-season wet days by up to 0.8 to 3.2 days under the warming scenarios of both RCP 4.5 and 8.5.