The Heavy Precipitation Event of December 2015 in Chennai, India
Introduction. At the beginning of December 2015, the Indian state of Tamil Nadu experienced extensive f looding. November had been the second wettest month in Chennai (1049 mm) in more than 100 years, but the main floods were caused by one day of extreme precipitation on 1 December. The commercial center, Chennai (formerly known as Madras), reported 24-hr precipitation from 0830 LT ranging from 77 to 494 mm at 18 stations, with a citywide-average of 286 mm (Fig. 17.1a). The city was declared a disaster area on 2 December after many areas, including the airport, were flooded. Although in the satellite-based CMORPH analysis the largest precipitation amounts were recorded south of Chennai (Fig. 17.1b), we concentrate our analysis on this city because the impact was largest here. Damages were estimated to be as high as $3 billion (U.S. dollars; Wall Street Journal, 11 December 2015). This part of India has its main rainy season during the northeast monsoon (Srinivasan and Ramamurthy 1973; Yadav 2013) in October–December. Sea surface temperature (SST) in the Bay of Bengal typically exceeds the threshold for deep convection throughout the year. Weak vertical shear during May and October–December (months prior to and immediately following the southwest monsoon) makes conditions ideal for tropical storms and cyclones that make landfall on the southeastern coast of India. The extreme rainfall events have sizes of O(100) km. SST in the northern and western Bay of Bengal has hardly increased over the last 35 years, in contrast to most of the rest of the world (Fig. 17.1c). This is likely because of increased air pollution in the region, the “brown cloud,” that blocks more sunlight counteracting the warming due to greenhouse gases, especially in premonsoon maximum temperatures (Padma Kumari et al. 2007; Wild 2012). The CMIP5 historical greenhouse gas (GHG) experiments without aerosols indeed indicate a larger trend in the Bay of Bengal (sea points in 10°–25°N, 80°–90°E), 1.6°C (100 yr)−1 over October–December from 1970 to 2005, than the historical experiments, 1.3°C (100 yr)−1. This is still higher than the observed trend of 0.8°C (100 yr)−1. In line with this, SST anomalies were lower than elsewhere in the Indian Ocean in November–December 2015, although there was a warmer patch just off the coast of Chennai with 0.6°–0.8°C anomalies (Fig. 17.1d). El Niño was also very strong these months.