La Niña

La Niña

Subject: Geography

Context: Global climate models suspect that the ‘neutral’ conditions (neither El Nino nor La Nina) in the Equatorial Pacific might give way to a return of La Nina conditions into the autumn and winter even as the annual monsoon in India, a known La Nina beneficiary, readies to cross the half-way stage

Concept:

The neutral phase

• In the neutral state (neither El Niño nor La Niña) trade winds blow east to west across the surface of the tropical Pacific Ocean, bringing warm moist air and warmer surface waters towards the western Pacific and keeping the central Pacific Ocean relatively cool.
• The thermocline is deeper in the west than the east.
• Warm sea surface temperatures in the western Pacific pump heat and moisture into the atmosphere above. In a process known as atmospheric convection, this warm air rises high into the atmosphere and, if the air is moist enough, causes towering cumulonimbus clouds and rain.
• This now-drier air then travels east before descending over the cooler eastern tropical Pacific. The pattern of air rising in the west and falling in the east with westward moving air at the surface is referred to as the Walker Circulation. 

La Niña

It is a weather pattern that can occur in the Pacific Ocean every few years. In a normal year, winds along the equator push warm water westward. Warm water at the surface of the ocean blows from South America to Indonesia.

• As the warm water moves west, cold water from the deep rises up to the surface. This cold water ends up on the coast of South America.
• In the winter of a La Niña year, these winds are much stronger than usual. This makes the water in the Pacific Ocean near the equator a few degrees colder than it usually is.
• Even this small change in the ocean’s temperature can affect weather all over the world.
• Rain clouds normally form over warm ocean water. La Niña blows all of this warm water to the western Pacific. This means that places like Indonesia and Australia can get much more rain than usual.
• However, the cold water in the eastern Pacific causes less rain clouds to form there. So, places like the southwestern United States can be much drier than usual.
• The 2020-2021 La Niña event has ended and neutral conditions (neither El Niño nor La Niña) are now likely to prevail over the tropical Pacific in the next few months, said the World Meteorological Organization (WMO)

Weather Changes due to La Nina:

• The Horn of Africa and central Asia will see below average rainfall due to La Niña.
• East Africa is forecast to see drier-than-usual conditions, which together with the existing impacts of the desert locust invasion, may add to regional food insecurity.
• It could also lead to increased rainfall in southern Africa.
• It could also affect the South West Indian Ocean Tropical Cyclone season, reducing the intensity.
• Southeast Asia, some Pacific Islands and the northern region of South America are expected to receive above-average rainfall.
• In India, La Niña means the country will receive more rainfall than normal, leading to floods.