Unravelling the causes of 2015 winter monsoon extreme rainfall and floods over Chennai: Influence of atmospheric variability and urbanization on the hydrological cycle

The present study investigates the causes of extreme precipitation events over the coastal Chennai city in India during the winter monsoon season of 2015. Our investigation reveals that atmospheric and anthropogenic variability influenced the hydrological cycle in producing the Chennai floods. Atmospheric perspective linked the extreme rainfall to the natural variability over tropics in the form of convectively coupled equatorial waves (CCEWs). This finding is entirely new compared to Chennai floods' earlier attribution to Bay of Bengal warming and El Nino related easterly bursts. Along with the natural atmospheric variability, anthropogenic variability due to unprecedented urbanization multiplied the catastrophe scale. A prominent land cover change over the Chennai region was found in 2015 with respect to 1992 land cover and land use classification satellite dataset. A kilometre-scale regional climate simulation clearly demonstrated the urbanization impact on the extreme rainfall over Chennai. Also, we found unprecedented changes in hydrologic conditions like surface discharge of rivers in 2015 over the Adyar basin of Chennai using the Global Flood Awareness System simulated dataset. We revealed a daily mean surface discharge >1000 m 3 s − 1 of rivers at every 100 km 2 area over the Chennai city after the extreme rainfall events. •Atmospheric natural variability is the cause of Chennai extreme rains.•Anthropogenic variability due to urbanization influenced the hydrological cycle to result in extreme rainfall.•Unprecedented urbanization over Chennai in 2015 modified the hydrological cycle.•Convective coupled equatorial waves were found to contribute to natural variability.•Northward propagating ‘tropical disturbance’ triggered extreme rain events along the southeast coast of India.