Impact of La Nina on Northeast monsoon

Context:

With the  Northeast monsoon onset expected soon, the India Meteorological Department (IMD) has issued a ‘yellow’ alert over Kerala, Tamil Nadu, coastal Andhra Pradesh, Yanam, Rayalaseema and south interior Karnataka, warning of thunderstorms and lightning.

Relevance:

GS-I: Geography (Climatology, Important Geophysical Phenomena), GS-III: Environment and Ecology (Environmental Pollution and Degradation, Impact of Climate Change)

Dimensions of the Article:

1. Key points
2. La Niña
3. El Niño
4. ENSO
5. How does La Nina impact India’s monsoon?
6. Why have La Nina conditions continued for three years?
7. La Nina conditions and cyclone formation

Key points:

• With the likely setting-in of northeasterly winds in the lower atmospheric levels over the Bay of Bengal and southern peninsular India, the Northeast monsoon rain is likely to commence over the Southeast Peninsula from around October 29.
• It will remain under the influence of the continuing La Ninã conditions.

Onset of the Northeast monsoon:

• The onset of the Northeast monsoon over south India was delayed. This is often attributed to the delayed withdrawal of the Southwest monsoon in recent years. It has been noted that the subsequent Northeast monsoon onset, too, gets delayed.
• Climatologically, after the complete withdrawal of the Southwest monsoon, the monsoon trough shifts southwards and the wind direction reverses from the southwest to the northeast after mid-October.
• The Northeast monsoon onset is declared when the monsoon trough reaches near central Tamil Nadu, the main beneficiary of this monsoon as it receives about 48 per cent (447 mm) of its annual rainfall between October and December.

Regions receiving rainfall during North-East Monsoon:

• Also known as winter monsoon affecting a smaller geographical area, the rainfall activity during October to December is largely restricted to the southern peninsular-Tamil Nadu, Puducherry, Yanam, Mahe, Kerala, south interior Karnataka and Lakshadweep.
• The winter monsoon also brings rainfall over some South Asian countries, including Maldives, Sri Lanka and Myanmar.

La Niña

• La Niña is a coupled ocean-atmosphere phenomenon that is the colder counterpart of El Niño, as part of the broader El Niño–Southern Oscillation (ENSO) climate pattern.
• During a period of La Niña, the sea surface temperature across the equatorial Eastern Central Pacific Ocean will be lower than normal by 3 to 5 °C (5.4 to 9 °F).
• An appearance of La Niña persists for at least five months.
• It has extensive effects on the weather across the globe, particularly in North America, even affecting the Atlantic and Pacific hurricane seasons, in which more tropical cyclones occur in the Atlantic basin due to low wind shear and warmer sea surface temperatures, while reducing tropical cyclogenesis in the Pacific Ocean.
• La Niña is a complex weather pattern that occurs every few years, as a result of variations in ocean temperatures in the Equatorial Pacific.
• It occurs as strong winds blow warm water at the ocean’s surface from South America across the Pacific Ocean towards Indonesia.
• As this warm water moves west, cold water from the deep sea rises to the surface near South America.
• As a result, it is considered to be the cold phase of the broader El Niño–Southern Oscillation weather pattern, as well as the opposite of El Niño weather pattern.
• La Niña impacts the global climate and disrupts normal weather patterns, which as a result can lead to intense storms in some places and droughts in others.

El Niño

• El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO) and is associated with a band of warm ocean water that develops in the central and east-central equatorial Pacific (between approximately the International Date Line and 120°W), including the area off the Pacific coast of South America.
• The ENSO is the cycle of warm and cold sea surface temperature (SST) of the tropical central and eastern Pacific Ocean.
• El Niño is accompanied by high air pressure in the western Pacific and low air pressure in the eastern Pacific.
• During the development of El Niño, rainfall develops between September–November.
• The cool phase of ENSO is La Niña, with SSTs in the eastern Pacific below average, and air pressure high in the eastern Pacific and low in the western Pacific.
• The ENSO cycle, including both El Niño and La Niña, causes global changes in temperature and rainfall.

ENSO

• El Nino and the Southern Oscillation, also known as ENSO is a periodic fluctuation in sea surface temperature (El Niño) and the air pressure of the overlying atmosphere (Southern Oscillation) across the equatorial Pacific Ocean.
• El Nino and La Nina are complex weather patterns resulting from variations in ocean temperatures in the Equatorial Pacific Region. They are opposite phases of what is known as the ENSO cycle.
• El Nino and La Nina episodes typically last nine to 12 months, but some prolonged events may last for years.

How does La Nina impact India’s monsoon?

• El Niño years in India have witnessed extreme heat and below normal rainfall levels during monsoon, even though El Niño might not be the only factor or even have direct links to them.
• In 2014, a El Niño year, India received 12 per cent deficient rainfall from June to September.
• On the other hand, La Nina years are known to favour the Indian summer monsoon.
• This year, India has received 740.3 mm of rainfall, quantitatively 7 per cent higher than the seasonal average till August 30.
• Among the 36 states and union territories, 30 have received rainfall that is categorised as either ‘normal,’ ‘excess’ or ‘large excess.’
• Uttar Pradesh, Manipur (-44 per cent each), and Bihar (-39 per cent), however, remain the worst affected states this season.

Why have La Nina conditions continued for three years?

• Climate change could be a driving factor behind such uncommon conditions.
• El Niño has been associated with rising heatwaves and extreme temperatures, such as in parts of the US, Europe and China recently.
• India’s Northeast monsoon rainfall remained subdued during past La Niña events, but the 2021 monsoon remains an exception in recent years.
• In 2021, the southern Indian peninsula experienced its wettest recorded winter monsoon since 1901, receiving a whopping 171 per cent surplus of rainfall between October and December, IMD data stated.

La Nina conditions and cyclone formation

• Intense hurricanes and cyclones have frequently occurred in the Atlantic Ocean and the Bay of Bengal during La Nina years.
• Over the North Indian Ocean as well, the chances of an increased number of cyclones are due to multiple contributing factors, including high relative moisture and relatively low wind shear over the Bay of Bengal.
• The post-monsoon months, from October to December, are the most active months for cyclonic developments over the North Indian Ocean, with November as the peak for cyclonic activity.

-Source: Indian Express