Northeast India (NEI), the wettest place on the Earth, has experienced a rapid decrease in summer monsoon rainfall (about 355 mm) in the last 36 years (1979–2014), which has serious implications on the ecosystem and the livelihood of the people of this region. However, it is not clear whether the observed drying is due to anthropogenic activities or it is linked with the global natural variability. A diagnostic model is employed to estimate the amount of recycled rainfall, which suggests that about 7% of the total rainfall is contributed by the local moisture recycling and decrease in recycled rainfall is about 30–50 mm. Using gridded observed rainfall and sea surface temperature data of the last 114 years (1901–2014), here we show that the recent decreasing trend of NEI summer monsoon rainfall is rather associated with the strong interdecadal variability of the subtropical Pacific Ocean. The strong interdecadal variability over NEI suggests a possibility of skillful decadal prediction of the monsoon rainfall, which may have important implications in terms of long‐term planning and mitigation.

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With the water and food security of one-sixth of the world's population strongly tied to it, the future of the seasonal mean Indian Summer Monsoon Rainfall (ISMR) is of serious concern for policymakers and farmers alike. Unmistakable evidence of the increasing trend of global mean temperature (T g) attributed to anthropogenic greenhouse forcing with high confidence (Houghton et al., 2001; IPCC, 2007, 2014) together with a similar increasing trend of annual mean temperature over the Indian monsoon region (Kothawale & Rupa Kumar, 2005; Ross et al., 2018) leading to enhanced moisture content over the region and in landocean temperature contrast (Sutton et al., 2007), a forced increasing trend of ISMR is reasonable to expect at longer time scales. While an increasing trend of ISMR may be expected due primarily to about 7% increase in the precipitable water per K increase in mean temperature (Mears et al., 2007) also referred to as the Clausius-Clapeyron (CC) scaling, climate models simulate only 1%-3% increase in global mean precipitation/K (Held & Soden, 2006; Wentz et al., 2007). Although the moisture content in the atmosphere increases according to the CC scaling with increase of temperature, regional precipitation like the ISMR is also influenced by dynamics in addition to thermodynamic

Numerical simulations of a thunderstorm event that occurred in the pre‐monsoon season over the North‐Eastern region of India (NEI) and Bangladesh are performed using Weather Research and Forecasting (WRF) Advanced Research Weather Research and Forecasting Model (ARW, version 3.8.1). Doppler weather radar indicates severe convective activity lasted for more than 10 hr. These extremely deep convective clouds with minimum cloud‐top temperature −70 °C at 19 km were triggered by the mixing of a moist air mass transported from the Bay of Bengal in the south and dry air transported from the northwest. A cyclonic circulation observed over the Tibetan plateau is likely to be associated with the strong southerly low‐level wind over NEI, as the plateau acts as a source of heat‐lows during the pre‐monsoon season. The coexistence of ice particles and supercooled water in the storms resulted in a large number of lightning flashes during the storm as observed from the Tropical Rainfall Measuring Mission Lightning Imaging Sensor (TRMM‐LIS). Co‐location of supercooled cloud water droplets helps in forming graupel through riming that plays a vital role in these convective systems. Lightning flashes calculated from WRF simulation using the Morrison microphysical and cloud top height based dynamical lightning parametrization scheme was found comparable with the observed flashes from TRMM‐LIS. Since the WRF model could simulate the thunderstorm, we recommend using this state‐of‐the‐art regional model in thunderstorm and lightning predictions for northeast India which would be useful in preparedness for such extreme events.

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