An innovative approach to discern variation in long-term regional monsoonal rainfall trend in North Eastern India.

The global hydrological cycle is changing in response to climate change and anthropogenic influence. The rainfall, on an annual or sub-decadal timescale, has exhibited erratic and substantial deviation from the long-term average in different parts of the world. Consequently, there are epochs of higher or lower than average rainfall but these are missed in long-term monotonic trend.

As commonly experienced, the rainfall within different geographical regions also varies significantly in terms of magnitude and timing, and on a smaller spatial scale. Integrating large geographical areas and long timescales for monotonic rainfall trend analyses for the meteorologically homogenous regions provides a general picture which is useful for the purpose of administration, water management and distribution. However, it cannot discern the decadal to multi-decadal rainfall variation in different parts of a meteorologically homogenous region and hence a more advanced and comprehensive approach is required for advancing scientific understanding about the hydrometeorological processes and factors governing multi-decadal rainfall variation.

In this study, an innovative approach is presented which involves: (1) 31 years moving average of percentage departure of seasonal rainfall for 120 years at district level; (2) 15 year sliding slope analyses to identify the year of inflection point based on change in direction of slope; (3) K-Means cluster analyses; (4) normality test of clusters based on Z score; and determination of timeframe during which rainfall trend changed significantly.

This approach was tested in the North East India where the rainfall is derived by one of the most dynamic and complicated meteorological systems. Improving understanding about rainfall variability in Northeast India is also very important from ecological, environmental and strategic point of views. Using the above approach, long-term rainfall data (1901-2020) has been analyzed at a district level in Northeast India. Using K means clustering method time windows of prominent change in rainfall trend have been identified. It is inferred from this study that Northeast India has experienced three major climatological events, during 1929-1941, 1961-1971, and 1984-1992. The first and the third events involving a trend reversal from increasing to decreasing nature around 1929-1941 and 1984-1992 affected respectively 49% and 43% area of Northeast India. The second event involving a trend reversal from decreasing to an increasing trend around 1961-1971 impacted 38% of northeastern India.

The meteorological processes corresponding to these timeframes, which could have caused these major rainfall trend reversals are being examined.