Wednesday 2 December 2015

Heavy rains bring further flooding to Chennai, Tamil Nadu.

Heavy rains have brought further flooding to the city of Chennai in Tamil Nadu, India, with about 8 cm of rain falling on the morning of Tuesday 1 December 2015, as the city suffers its worst flooding fora century.. Many areas of the city are without electricity, schools are closed, rail services and fights from Chennai Airport have been suspended and many roads are blocked and people are being advised to relocate away from the River Adyar after the Public Works Department was forced to begin releasing water from the Chembarambakkam Reservoir.

 Flooding in Chennai on 1 December 2015. Arun Sankar/AP.

In the neighbouring city of Puducherry 21.8 cm of rain was recored in 24 hours between Tuesday 1 and Wednesday 2 December, with many residents of low lying areas being forced to relocate to relief camps. Widespread flooding has also been recorded in the Tiruvallur and Kancheepuram districts of Tamil Nadu.

Flooding at the Perumal Temple at Mahabalipuram in Kancheepuram District, Tamil Nadu. DT Next.

Heavy rains and flooding have been occurring in Tamil Nadu for about a month, causing widespread disruption and being linked to 188 deaths in the state to date. This has been linked to an area of sustained low pressure over the southern Bay of Bengal, which is in turn probably linked to the El Niño weather system currently affecting the Indian and  Pacific Oceans. Low pressure systems over oceans are caused by warming of the air over the ocean, which causes the air over the affected area to rise, while fresh air is drawn in from elsewhere and in turn be warmed and rise. At the same time the heat causes high levels of evaporation from the ocean, so that the rising air is also waterlogged. Eventually this air rises high enough in the atmosphere that it matches the pressure of the air around it, at which point it stops rising and drifts with the prevalent wind. Eventually it passes into cooler areas, where it starts to lose its water as precipitation (rain), as cooler air cannot hold as much evaporated water as warmer air. As land has different thermal properties to water, this often means that the cooler areas encountered by the warm, waterlogged air are over land, leading to high rainfall in coastal areas.

Satellite image of the low pressure system over the southern Bay of Bengal, taken by the INSAT 3D weather satellite on 1 December 2015. The Hindu.

The El Niño is the warm phase of a long-term climatic oscillation affecting the southern Pacific, which can influence the climate around the world. The onset of El Niño conditions is marked by a sharp rise in temperature and pressure over the southern Indian Ocean, which then moves eastward over the southern Pacific. This pulls rainfall with it, leading to higher rainfall over the Pacific and lower rainfall over South Asia. This reduced rainfall during the already hot and dry summer leads to soaring temperatures in southern Asia, followed by a rise in rainfall that often causes flooding in the Americas and sometimes Africa. Worryingly climatic predictions for the next century suggest that global warming could lead to more frequent and severe El Niño conditions, extreme weather conditions a common occurrence.

 Movements of air masses and changes in precipitation in an El Niño weather system. Fiona Martin/NOAA.

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