Cold, cold winds
La Niña has arrived and is causing heavy rainfall across the world
La Niña—analogous to El Niño—is a complex cooling weather phenomenon that is causing large-scale impacts on ocean processes and changing rainfall patterns
LA NIÑA—meaning “the girl” in Spanish—is the cooling phase of the El Niño Southern Oscillation (ENSO) cycle in the equatorial Pacific Ocean, as opposed to the warming phase of El Niño. La Niña is characterised by unusual cooling of the central and east-central equatorial Pacific Ocean, and both El Niño and La Niña are deviations from normal sea surface temperatures that occur due to the anomalous behaviour of worldwide trade winds. In the case of El Niño, the trade winds weaken, leading to warming. However, in La Niña, the trade winds strengthen, leading to cooling. Both events have large-scale impacts not only on ocean processes, but also on global weather and rainfall patterns.
On September 10 this year, the US National Oceanic and Atmospheric Administration (NOAA) said La
Niña conditions were currently prevailing and there was a 75 per cent chance for them to stay till winter. La Niña reduces the wind shear—change in wind patterns between the surface and the upper levels of the atmosphere—allowing hurricanes to grow. NOAA’s analysis was earlier validated by a bulletin of the World Meteorological Organization
(WMO) where it said there was a 60 per cent chance for the formation of a fullfledged La Niña event from September to November.
“At present, we have La Niña-like conditions in the Pacific, favourable for a strong monsoon circulation,” says Roxy Mathew Koll, a climate scientist at the Indian Institute of Tropical Meteorology (IITM), Pune. In fact, India received 27 per cent excess rainfall in August this year. Scientists have partly attributed this to La Niña.
PAST EVENTS
Unlike El Niño, its opposite weather phenomenon has not been studied in detail. However, there is enough evidence to show La Niña has disruptive impacts on global weather systems. Between 1871 and 2019, there were 25 La Niña years, according to IITM. Of this, India suffered from severe floods in nine years, when rainfall was more than 10 per cent of normal. “Taking La Niña and other influences on regional climate into account, the global seasonal climate for September-November indicates elevated probabilities of abovenormal rainfall across south and Southeast Asia,” says WMO.
But what is La Niña and what happens when it prevails? During El Niño, the central and equatorial Pacific Ocean becomes unusually warm. This disrupts global wind patterns, affecting the climatic conditions in tropical regions like Africa and sub-tropical regions like India, causing erratic rainfall and droughts. But in the case of La Niña, the exact opposite happens and India receives more than normal rainfall, leading to floods. El Niño and La Niña episodes typically last nine to 12 months, but some prolonged events may last for years. While their frequency can be quite irregular, El Niño and La Niña eve-nts occur, on an average every two to seven years. Typically, El Niño occurs more frequently than La Niña.
“In India, El Niño tends to produce a weaker monsoon, but only 50 per cent of the weaker monsoons are attributable to El Niño. In contrast, La Niña tends to produce a wetter monsoon,” says Raghu Murtugudde, a climate scientist at the University of Maryland, USA. La Niña can affect India’s winter as well. The winds of winter are from the northeast near the land surface and this is accompanied by the socalled southwesterly jet in the upper atmosphere. During El Niño, this jet is pushed southward which allows more western disturbances from the Mediterranean to bring rain and snow into northwestern India. But La Niña produces a more north-south low pressure system which brings in Siberian air and a cold wave that can extend further south. “We have witnessed frost in Mahabaleshwar and cold waves in the hilly parts of Tamil Nadu, that can be associated with La Niña,” says Murtugudde.
SHIFT IN NATURE
But La Niña itself might be changing its nature. A research paper published in Geophysical Research Letters in January, 2020, says monsoon rainfall over India during La Niña events after 1980 has reduced by six to eight per cent, which is due to changes in the spatial pattern and intensity of La Niña within the tropical Pacific Ocean. Warmer eastern equatorial Pacific Ocean temperatures are impacting the La Niña phenomenon. The warming of the Indian Ocean has also been affecting rainfall during La Niña years after 1980 over central India. So the big question remains unanswered: is the monsoon itself affecting both El Niño and La Niña?
El Niño and La Niña peak in December, January and February, while the monsoon is between June and September. Since El Niño and La Niña tend to be in their growing phase during monsoon, it is difficult to separate the cause and effect, says Murtugudde. Moreover, the cooling caused by La Niña should have brought down temperatures in many regions across the world, but this might not be the case anymore because of human-induced global warming. “Even if a La Niña event develops, its cooling signal will not be enough to counterbalance the impacts of human-induced climate change,” says Petteri Taalas, secretary-general of WMO. Therefore, there is still much to be investigated about ENSO, and La Niña in particular.
THE IMPACTS OF LA NIÑA ARE ALREADY VISIBLE. INDIA RECEIVED MORE THAN NORMAL RAINFALL IN AUGUST THIS YEAR, WHICH CAUSED SEVERE FLOODING IN MANY STATES