Investigating the Changing Heavy Rainfall Climatology of North East India

The increase in the variability of rainfall as the climate of the world warms up is a concern for many regions. Studies in the past have associated this change in variability to rise in heavy rainfall events. Thus a regional analysis of heavy rainfall characteristics to evaluate its response to a changing climate becomes important. In this study, we are putting this focus on the North East Region (NER) of India, which boasts of being one of the wettest regions of the world. This study examines the heavy rainfall characteristics from 1901 till 2020 over NER using the IMD gridded daily rainfall product. The examination showed that although the annual and monsoonal rainfall over the NER has been decreasing, the intensity of heavy rainfall has increased by ~5mm over the decades. Singular Spectrum Analysis is used to identify the long-term trend, which showed a change in heavy rainfall characteristics around 1970 and hence further investigation is carried out over two time blocks (Pre-1970: 1901-1970 and Post-1970: 1971-2020). The investigation of the frequency of heavy rainfall events showed that its increasing trend, prior to 1970, transitions to a decreasing trend, post 1970. It also showed that the area under a negative trend of frequency has increased significantly after 1970. Furthermore, a non-parametric probability distribution approach has also been implemented to interpret the frequency and intensity relationship of heavy rainfall together. This showed that post 1970, the probability of occurrence of a very heavy or extreme rainfall events has increased. The increase in probability did show a spatial variability. The increase in probability is more for the pre-monsoon season compared to the monsoon. This finding corresponds to the fact that the contribution of pre-monsoon rainfall to annual rainfall has increased while that of the monsoon rainfall has decreased over the decades. To investigate the local causes of the observed changes, the 2m temperature (T2), 2m Dew-point temperature (TD2) are investigated using a cross-sample entropy analysis. Interestingly, both T2 and TD2 showed a significant increasing trend over NER. Coincidentally, locations with increasing heavy rainfall intensity and frequency are also the locations with increasing TD2. Also, the pre-monsoon show a stronger increase in TD2, i.e., more moisture is available for convection, compared to T2, which could explain the higher probability of heavy rainfall events. Thus, increasing intensity and decreasing frequency can be explained by the inter-relationship between T2, TD2 and the convective processes.