The Indian state of Tamil Nadu experienced a series of floods this winter. The city of Chennai was flooded between November 13 and 17 and on December 1, 2015 which persisted well into the second week of December. District after district was inundated—Kanchipuram on December 5; Tirunelveli on December 7; and Thanjavur, Thiruvarur and Nagapattinam on December 8. Cuddalore city and the district remained flooded till December 10. More than 500 people died and over 18 lakh people were displaced. Damages and losses have been estimated of the order of 50000 crore (US$7 billion) to 100000 crore (US$15 billion). These floods are by far the costliest natural disaster that the world has seen in 2015.
The period October to December is the Northeast (NE) monsoon season over peninsular India—the major period of rainfall activity over the region, particularly in the eastern half comprising of the meteorological subdivisions of coastal Andhra Pradesh, Rayalaseema and Tamil Nadu-Puducherry. For Tamil Nadu this is the primary rainy season accounting for about 48 per cent of the annual rainfall. Coastal districts of the State get nearly 60 per cent of the annual rainfall and the interior districts get about 40-50 per cent of the annual rainfall. As per the IMD the climatological onset date of NE monsoon over south India is October 20. During this period (later part of October and November) monsoon trough prevails around 13oN latitude (Chennai 13.08°N, 80.27°E) over peninsular India and Bay of Bengal (BoB). During the southwest monsoon sea surface temperatures (SSTs) are high over Indian seas especially over BoB. Climatologically SSTs are above 27oC, which is a threshold for genesis of cyclones in this region. BoB shows secondary peak in SSTs during northeast monsoon period. Fig. 1 shows SST distribution over the globe during November 2015. SSTs above 27oC are seen over BoB and Arabian Sea (AS). High SSTs associated with pressure trough at surface provides favourable conditions for large scale upward motion for formation of clouds. It becomes a breeding ground for formation of low pressure systems over these seas.
2015 El Niño effect
The southwest summer monsoon 2015 was weaker than normal due to the El Niño effect. Weak winds associated with the weaker monsoon were favourable for generating positive SST anomalies (deviation from climatological mean values) over BoB and AS during post monsoon season. These provided extra fuel for formation of low pressure systems over BoB and AS in this season.
Active NE monsoon 2015
Thus the NE monsoon 2015 was active with successive formations of low pressure systems over AS and BoB. A low pressure system formed over the AS which intensified into a depression on November 7 and turned into the extremely severe cyclonic storm ‘Megh’ on November 8. It moved towards the west and hit the Oman coast on November 10. During the same period a low pressure system was developed over BoB near Tamil Nadu coast which intensified into a depression on November 8 and had landfall north of Chennai on November 11.
On November 15, another low pressure system developed over BoB near Tamil Nadu coast. Under the influence of these systems, rainfall over south peninsula in the month of November was 251.5 mm, which was 227 per cent of the long term average rainfall of this area. Chennai received continuous rainfall throughout November. Time series of Chennai rainfall from November 5 to December 7 is shown in Fig. 2.
Mean NE monsoon seasonal rainfalls at Nungambakkam and Minambakkam (Chennai airport) are 750 and 770 mm respectively. Chennai received 1049 mm of rainfall in November, the highest recorded since November 1918 when 1088 mm of rainfall was recorded. The above figure shows heavy rainfalls on November 13 (120 mm), November 17 (265 mm) November 24 (150 mm) and December 1 (350 mm). Chennai consequently witnessed floods during and after these days.
Role of Easterly waves
Moderate to heavy rainfall persisted throughout November under the influence of prevailing surface pressure trough and anomalous SSTs. These provided low level moisture convergence and upward motion. The upward motion was further intensified with the passage of easterly waves. The winds in tropics termed as ‘trade winds’ blow from the Northeast to the Southwest. Near to the equator their direction becomes nearly easterly. Occasionally these easterly winds change their direction. They become south-easterly, southerly, north-easterly and again restoring to easterly direction with height. This type of variation is representation of presence of easterly wave. The wavelength of the wave is of order of 1000-2000 km in east west, north-south direction. The mass divergence occurs in the forward section of the wave. Such divergence in the middle atmospheric levels (2-6 km) provides additional lifting of moisture for formation of deep convection.