El Niño’s Role in Deadly Chennai Rains; Progress in Paris Climate Talks

Conditions in far southeast India are slowly improving after five weeks of frequent torrential rain that has led to more than 250 deaths. A region of low pressure positioned near Sri Lanka during much of the period channeled moisture from the Bay of Bengal and the record-warm eastern Indian Ocean toward the region. Much of the suffering has been in Chennai, an urban area of more than 9 million people that ranks as the largest in South India and among the world’s 40 largest metro areas. Parts of Chennai have spent days inundated by as much as eight feet of polluted water, with widespread power outages exacerbating the crisis. At least 18 patients died in a Chennai intensive-care unit after backup power to ventilators was knocked out. Last month Chennai recorded 1218.6 mm (47.98”) of rain, the highest observed for any November in more than 100 years of recordkeeping. Then, on December 1-2, a total of 345 mm (13.58”) fell in 24 hours, which smashed the city’s all-time 24-hour record rainfall of 261.6 mm on December 10, 1901. Estimated losses in the region have already topped $2 billion US.


Figure 1. People wade through a flooded street in Chennai, in the southern Indian state of Tamil Nadu, on Wednesday, December 2, 2015. Image credit: AP.

Along with the heavy rains, the Indian Express cited infrastructure issues as a key part of the disaster: “While officials at the India Meteorological Department have said the exceptionally strong El Niño, along with a rare ‘coincidence of various factors’, has resulted in the heavy rain, there’s no denying that Chennai has failed in maintaining an effective storm water drainage system.” At Slate, Eric Holthaus reviewed how fast-growing industrialization made matters worse in Chennai: “For a city built on a floodplain, development has essentially gone unchecked: Critical infrastructure--like the airport, automobile manufacturing plants, and IT centers as well as countless houses--has been built over streams and marshes, and plastic bags clog drainage networks.”

Holthaus also noted the well-established relationship between warming global temperatures and the intensification of short-term rainfall events, which has been observed in many parts of the globe. This effect appears especially strong in the tropics, according to a study published earlier this year in the journal Current Climate Change Reports by Paul O’Gorman (Massachusetts Institute of Technology).


Figure 2. Chennai’s airport was closed for four days last week, with some 4000 people and dozens of aircraft stranded. At one point, all runways were under water. This photo is from Thursday, December 2, 2015. Image credit: Atul Yadav/ Press Trust of India via AP.

El Niño’s contrasting effects on the Indian monsoon
Cyclic, large-scale oceanic and atmospheric features--including El Niño--helped set the stage for the South India disaster. El Niño has a dual effect on monsoonal rains across India, hinging on the two phases of the monsoon itself. By far the biggest player in India’s climate is the summer, or southwest, monsoon, in which moist air sweeps across the nation from southwest to northeast during the late spring and summer. The phenomenon’s alter ego is the winter, or northeast, monsoon, in which northeast winds push back across the nation during autumn and winter. For most of the nation, the northeast monsoon has a drying effect, since the winds are bringing cool, dry air from interior Asia. But as the northeasterlies pass over the Bay of Bengal during autumn, they pick up moisture that is often deposited across far southeast India. Chennai typically receives more than half its moisture this way, with rainfall averaging around 11” in October, 16” in November, and 7” in December. (Chennai’s annual average is around 55”).

El Niño tends to reduce rainfall during the southwest monsoon, as was the case this year, but it also raises the odds of heavy rainfall during the northeast monsoon. The latter effect doesn’t get as much attention, in part because the southwest monsoon is the one that affects the bulk of India. On a global scale, the northeast-monsoon effect in India is small enough that it often gets omitted from maps showing how El Niño affects regional climate. However, it still affects a huge number of people: the population of Tamil Nadu state alone (where the northeast monsoon is the dominant one) is 72 million. Moreover, year-to-year variability is higher for India’s northeast monsoon than it is for the southwest monsoon.

Another player: the Indian Ocean Dipole
Another factor lined up this autumn for a strong northeast monsoon is the Indian Ocean Dipole. During the positive phase of the IOD, warmer-than-average waters extend across the central and western tropical Indian Ocean, with cooler-than-average waters toward the eastern tropical Indian Ocean. A 2004 study by R.H. Kripalani and Pankaj Kumar (Indian Institute of Tropical Meteorology) was one of the first to relate the Indian Ocean Dipole to the strength of the northeast monsoon. It found that a positive IOD was associated with heavier northeast monsoonal rains in South India.


Figure 3. State of the Indian Ocean Dipole, 1982 - present. Crosshatches show monthly readings; the red and blue graph is a three-month running average. Image credit: NOAA Ocean Observations Panel for Climate

The IOD is currently in a strongly positive mode (see Figure 3). Based on NOAA data, the three-month running average this autumn has been in the +0.8°C to +1.0°C range, which ranks among the top five positive IOD events in the last 30 years. So between the IOD and El Niño, two major factors have been in place for unusually heavy rains in and near Chennai this autumn. India’s seasonal forecasters were on top of this risk. In its northeast monsoon outlook issued on October 16, the India Meteorological Department gave 88% odds for a wetter-than-normal monsoon over the South India peninsula, with 90% odds for Tamil Nadu, the state where Chennai is located.

In South Florida, a December deluge
South Florida--another place where El Niño tends to boost cool-season rainfall—has slogged through some of its heaviest December rains on record over the last several days. Miami Executive Airport picked up 8.92” in 24 hours, with 10.11” observed at The Hammocks. For the first six days of the month, Miami International Airport received 8.48”. With more than three weeks to go, this total already beats any December in 104 years of official Miami recordkeeping, with just two exceptions: 12.08” in 1905 and 9.03” in 1929. The first six days of December were wetter than any other week in Miami’s weather history during meteorological winter (December through February), as noted by NHC’s Eric Blake. Although the heaviest rains from Thursday through Saturday targeted the Miami area, South Florida as a whole averaged more than 2”, according to the South Florida Water Management District.


Figure 4. Demonstrators participate in a climate march on Sunday, December 6, 2015 in the coastal city of Oostende, Belgium. Image credit: Nicolas Maeterlinck/AFP/Getty Images.

Draft climate agreement hammered out in Paris
The first half of the two-week UN climate summit in Paris ended on a relatively high note, with a draft agreement delivered on time. The agreement is still peppered with hundreds of bracketed words and phrases, indicating fine and not-so-fine points that need to be worked out over the next week.


Figure 5. Mayors from cities around the world convene at the Paris city hall on Friday, December 04, 2015, for an event called Cities for Climate. Roughly a thousand mayors gathered to coordinate their own efforts to reduce greenhouse emissions. The mayors signed an agreement to work toward 100% renewable energy in their cities, or an 80% reduction in greenhouse gases, by 2050. Image credit: Thierry Chesnot/Getty Images.

This week the negotiation process shifts from diplomats to high-level government ministers from each UN member. These ministers will sign off on the final agreement, including not only the stipulated terms but which ones (if any) are considered legally binding. The latter point is heavily influenced by the United States, since the U.S. Congress has already made it clear it will not approve any legally binding agreement. More likely to emerge is some type of requirement for transparency and regular progress reports from each nation, which would employ international peer pressure rather than legal muscle. One climate policy expert interviewed by AP likened this to playing a soccer game without a referee: “Everything happens in the open in the stadium….So if someone fouls another player, even if he doesn't get a red card, he will be booed by the audience.” Another key aspect to be determined is whether the final draft will refer to a goal of keeping global temperature at no more than 1.5°C above preindustrial levels, a far more challenging goal than even the oft-cited 2.0°C goal. As the draft global agreement evolves, a group of 11 legal experts (including two representatives from each UN region, plus one representative from small-island developing states) will be scrutinizing it for legal and linguistic clarity and consistency.

We’ll have more details on the Paris meeting later this week as negotiations proceed. I will be appearing each night this week on the Weather Channel’s Weather Underground program (#WUTV) to discuss the happenings in Paris. These segments will air at 6:40 pm EST on most if not all nights. A UN website includes more background on how this week’s negotiations will unfold. Many of the proceedings can be viewed on TV only within the convention center (and in some cases, not even there). If you’re interested in the step-by-step evolution of the agreement, complete with sometimes snarky context, check out this annotated Google Doc, which was updated frequently last week by a pair of college students from New Zealand who will be on site for the duration.

We’ll have our next post by Wednesday at the latest. WU blogger Steve Gregory has a new post covering the "warm wave" spreading over much of eastern North America over the next week, plus what may follow later in the month.

Bob Henson