La Nina and Indian Ocean Dipole Influence on Distribution of Daily Rain Intensities in India

Abstract:

The Indian monsoon (IM) and daily extreme rainfall events (EREs) have a major influence on the welfare of 1.2 billion people in South Asia. ERE s have killed tens of thousands of people and severely affected millions of people in the past decade alone in India.

Current evidence suggests that the dynamics of the IM, as well as the corresponding frequency and magnitude of daily EREs are changing (Krishnaswamy et al 45(1-2), 175-184, Climate Dynamics 2014).Variability in the Indian monsoon (IM)and associated rain intensitiesis known to be modulated by two ocean atmosphere phenomena, El Nino-Southern Oscillation (ENSO) and the Indian Ocean Dipole (IOD). This is compounded by changes in the hydrological cycle due to global warming. There is growing evidence that the proportion of moderate rains have declined and those of sparse and more intense rains have increased. We investigated the role of La Nina and IOD on the time-aggregated as well as time-evolving distribution of daily rain intensities India using data from rain gauges and gridded rain data. We applied dynamic linear models, generalized linear and additive to compare the relative influence of La Nina and IOD on frequency of different daily rain exceedance levels (25, 50,100,150 and 200 mm day^-1) as well as trends since 1950.

Our results indicate that La Nina has a very significant influence on annual frequency of moderate rain intensities (<100 mm day^-1), whereas IOD has a greater influence on annual proportion of total rain accounted for by rain intensities above 100 and 150 mm day^-1. The observed declining role of La Nina in India in recent decades, potentially more moisture availability under climate change, and the strengthening of the IOD due to warming of the Western Indian Ocean could strengthen the shift towards more extreme rain events in the coming decades.