LA NINA & IOD

LA NINA & IOD

Subject: Geography

Context : Skymet Weather, the country’s largest private-sector weather forecaster, has said that initial readings made on the prospects of impending 2021 South-West monsoon indicate that it would be a ‘normal monsoon year’ .

Concept:

• The South-West monsoon 2021 would coincide with a devolving La Niña to start with.
• This trend of the Pacific Ocean temperatures may not lead to an above-normal or excess rainfall, Skymet Weather said, while ruling out the chances of a ‘disfigured’ monsoon from available indications.
• Indian Ocean Dipole (IOD) event, which too has a major bearing on the monsoon, cannot evolve until April.
• The IOD mimics El Nino-La Nina in the Indian Ocean, and early indications suggest it to be ‘neutral’ (neither positive nor negative) around the time of monsoon onset.

About La Nina:

• It means the large-scale cooling of ocean surface temperatures in the central and eastern equatorial Pacific Ocean, together with changes in the tropical atmospheric circulation, namely winds, pressure and rainfall.
• It has the opposite impacts on weather and climate as El Niño, which is the warm phase of the El Niño Southern Oscillation (ENSO).

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.

Indian Ocean Dipole

• Sustained changes in the difference between sea surface temperatures of the tropical western and eastern Indian Ocean are known as the Indian Ocean Dipole or IOD.
• The IOD is one of the drivers of Indian Monsoon. The IOD has three phases: neutral, positive and negative
• Neutral IOD phase: Water from the Pacific flows over to East Indian Ocean (between the islands of Indonesia). Air rises above this area and falls over the western half of the Indian Ocean basin, blowing westerly winds along the equator. Temperatures are close to normal across the tropical Indian Ocean, and hence the neutral IOD does not affect the Indian Southwest Monsoon.
• Positive IOD phase: Westerly winds weaken along the equator allowing warm water to shift towards Africa. Changes in the winds also allow cool water to rise up from the deep ocean in the east.
• This sets up a temperature difference across the tropical Indian Ocean with cooler than normal water in the east and warmer than normal water in the west. Generally this means there is more moisture than normal in the atmosphere over West Indian Ocean & Arabian Sea.
• This changes the path of weather systems coming towards India, often resulting in more rainfall during Southwest Monsoon.
• Negative IOD phase: Westerly winds intensify along the equator, allowing warmer waters to concentrate near Equatorial East Indian Ocean. This sets up a temperature difference across the tropical Indian Ocean, with warmer than normal water in the east and cooler than normal water in the west.
• A negative IOD typically adversely affects the Indian Southwest Monsoon rainfall resulting in below-average rainfall over India.