On the identification of seasonal trends, dependency and driving forces of precipitation and vertically integrated vapour transport over Northeast India

The vertically integrated horizontal vapour transport (IVT) accompanied by westerlies and southwesterlies mainly contributes as a distant source of moisture to the seasonal precipitation (PR). The moisture transport and PR patterns across the eastern Himalayas (Sikkim, northern Sub-Himalayan West Bengal and Arunachal Pradesh) are strongly modulated by the orographic setup, local land-atmospheric factors and teleconnection with climate indices of the Indian and Pacific oceans. This study elaborates on the changing patterns of PR and IVT, changes in IVT contribution to the seasonal PR under the nonstationary framework and evaluates the nonlinear dependency on local and distant drivers with piecewise regression model. Asymmetries in significant seasonal trends are conspicuous due to multi-altitude interactions. Strong declining trend over Assam and Arunachal Pradesh is evident for PR with concurrent rising trend over the Meghalaya plateau. The IVT signals show a significant decline in post-monsoon (October and November) and winter (December to February) across Assam Plain. The increase in mean IVT contributes significantly to seasonal PR. Significant decline in the contribution of heavy IVT to winter PR is observed over Manipur, Mizoram, Tripura and part of Assam. Surface 2 m temperature and net thermal radiation exhibit contrasting influence on seasonal PR. IVT shows mainly positive association with surface 2 m dew point temperature and latent heat flux across the eastern Himalayas. NINO3.4 index exhibits strong negative influence on post-monsoon PR. The Pacific Decadal Oscillation is strongly associated with summer (March to May) PR. Monsoon (June to September) IVT is negatively influenced by the Pacific Decadal Oscillation over the Sub-Himalayan West Bengal, Gangetic West Bengal, Bihar and Jharkhand. The Southern Oscillation Index is negatively linked with summer IVT. The combination of local driving forces mainly controls the seasonal PR and IVT across all seasons. Besides, distant drivers are prominent in monsoon and post-monsoon. This study reveals insights into recent changes of PR and IVT, their contribution to seasonal water resources in the eastern Himalayas and the controlling strength of local land-atmosphere feedback mechanism and distant land-ocean coupling drivers under nonstationary climate.