El Nino and La Nina

Subject : Geography

Section: Physical Geography (Climatology)

Concept :

India is currently witnessing a colder than normal winter mainly due to the north-south winter flow because of the climate phenomenon known as La Niña.
The equatorial Pacific Ocean is experiencing the longest-ever La Niña episode in recorded history.
Having started in September 2020, it has prevailed for three consecutive years and thus has been classified as a “triple dip La Nina”.
However, the forecasts for the 2023 fall and winter are predicting that there is a 50% possibility for the occurrence of El Niño.
In the Indian context, La Niña is associated with good monsoon rainfall, while El Niño is expected to suppress monsoon rainfall.

El Nino

El Nino refers to the large-scale ocean-atmosphere climate interaction linked to periodic warming in sea surface temperatures across the central and east-central Equatorial Pacific.
It is associated with high pressure in the western Pacific. El Nino adversely impacts the Indian monsoons and hence, agriculture in India.

El Nino impacts on India:

In a normal monsoon year (without El Nino), the pressure distribution is as follows:
The coast of Peru in South America has a higher pressure than the region near northern Australia and South East Asia.
The Indian Ocean is warmer than the adjoining oceans and so, has relatively lower pressure. Hence, moisture-laden winds move from near the western Pacific to the Indian Ocean.
The pressure on the landmass of India is lower than on the Indian Ocean, and so, the moisture-laden winds move further from the ocean to the lands.
If this normal pressure distribution is affected for some reason, the monsoons are affected.
El Nino means lesser than average rains for India. Indian agriculture is heavily dependent on the monsoons and because of this, lesser rainfall during the monsoons generally translates to below-average crop yields.

What happens because of El Nino?

The cool surface water off the Peruvian coast goes warm because of El Nino. When the water is warm, the normal trade winds get lost or reverse their direction.
Hence, the flow of moisture-laden winds is directed towards the coast of Peru from the western Pacific (the region near northern Australia and South East Asia).
This causes heavy rains in Peru during the El Nino years robbing the Indian subcontinent of its normal monsoon rains. The larger the temperature and pressure difference, the larger the rainfall shortage in India.

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.

ENSO Cycle

El Nino–Southern Oscillation (ENSO) is an irregularly periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean.
Every three to seven years, the surface waters across tropical Pacific Ocean warm or cool by 1°C to 3°C, compared to normal.
The warming phase of the sea temperature is known as El Nino and the cooling phase as La Nina.
Thus, El Nino and La Nina are opposite phases of what is known as the El Nino-Southern Oscillation (ENSO) cycle.
These deviations from normal surface temperatures can have large-scale impacts not only on ocean processes, but also on global weather and climate.