Climate Change

Expect a wet September, late monsoon withdrawal due to La Niña

There is a 50-55% chance for the development of La Niña condition in the equatorial Pacific Ocean during the autumn of 2020

By Akshit Sangomla
Published: Friday 14 August 2020

There is a 50-55 per cent chance for the development of a La Niña condition in the equatorial Pacific Ocean during the autumn of 2020, according to the latest update by the United States National Oceanic and Atmospheric Administration.

There is a 50 per cent chance that it might continue into the winter season as well. Currently, El Niño Southern Oscillation (ENSO) neutral conditions are prevailing in the equatorial Pacific Ocean.

During the last four weeks, equatorial sea surface temperatures were below average from the International Date Line to the eastern Pacific and were above average in the western Pacific. The International Date Line is an imaginary line on the Earth's surface, defining the boundary between one day and the next.

La Niña is the cooling phase of the ENSO cycle in the equatorial Pacific Ocean, as opposed to the warming El Niño phase. It is characterised by the unusual cooling of the central and east-central equatorial Pacific Ocean.

Both, El Niño and La Niña, are deviations from normal surface temperatures that occur due to the anomalous behaviour of trade winds. In the case of El Niño, the trade winds weaken, leading to warming.

In La Niña, the opposite happens and the trade winds strengthen, leading to cooling. Both these events can have large-scale impacts not only on ocean processes, but also on global weather and climate.

During El Niño, the central and equatorial Pacific Ocean becomes unusually warm. This disrupts global wind patterns, affecting climatic conditions in tropical areas like Africa, sub-tropical areas like India and extra-tropical areas like North America.

In India this often, but not always, causes erratic monsoons and droughts. In the case of La Niña, the exact opposite happens and India receives more rainfall than normal, 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 events occur on average every two to seven years. Typically, El Niño occurs more frequently than La Niña.

“El Niño tends to produce a weaker monsoon but only 50 per cent of the weaker monsoons are attributable to El Niño. La Niña tends to do the opposite and produce a wetter monsoon,” Raghu Murtugdde, a climate scientist at the University of Maryland in the United States, said.

Considering that the monsoon is below normal along much of the west coast and central India, La Niña could produce more rain during the rest of August and into September, according to Murtugudde.

The inter-annual variability of the monsoon, in fact, tends to be the largest in September. Hence, La Niña may produce a wetter September and a delayed withdrawal if it grows stronger, he added. La Niña itself has been changing over the past four decades.

“A La Niña can affect India’s winter. As you know, the winds during the winter are from the northeast near the land surface and this is accompanied by a so-called Southwesterly Jet in the upper atmosphere. During an El Niño, this jet is pushed southward and this allows more western disturbance to bring rain and snow into northwestern India. But a La Niña actually produces a more north-south low pressure system which brings in Siberian air and the cold wave can extend much further south. During some years, we have had frost in Mahabaleshwar and cold waves in hilly parts of Tamil Nadu and so on. These can be associated with a La Niña,” Murtugdde added.

A research paper published in Geophysical Research Letters in January this year, said the monsoon rainfall over India during La Niña events after 1980 has reduced by six-eight per cent. The main reasons for this are the 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 the rainfall during La Niña years after 1980 over central India, which could have an impact on the long-term water availability of the region.

One instance of a deficit monsoon season in the region during a La Niña year was in 2010, according to a research paper published in Meteorological Applications in June 2012.

The paper says that “large areas covering east‐central India, the Bay of Bengal and extending eastward into the South China Sea and the Philippines received deficit monsoon rains” during the year. This is when north west India and Pakistan had extensive floods during the same period of time.

Murtugudde pointed out that “the big question we haven’t answered very well is whether the monsoon itself affects El Niño and La Niña.” 

El Niño and La Niña have their peak in December-January-February, while the monsoon is between June and September. Thus, the key question is - how does the monsoon affect El Niño or La Niña?

In the meantime, since El Niño and La Niña tend to be in their growing phase during the monsoon season, it is hard to separate the cause and effect, Murtugudde said.

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