Saturday, April 20, 2024

20th April 

Understanding the Pacific - the process of formation of the upcoming La Nina has begun!

We had put up an article in Weather Knowledge last year (Weather Knowledge - 48), which had described the El Nino and La Nina phases of the Pacific. 

Now, let us look at the conditions in the equatorial waters of the Pacific to see the situation this year.

El Nino or La Nina usually starts forming in the middle of the year and peaks during the period December-February. 

El Nino - A condition wherein the water in the eastern equatorial Pacific is warmer than normal i.e., the sea surface temperature anomalies are positive (anomalously warm). 

La Nina - A condition wherein the water in the eastern equatorial Pacific is cooler than normal i.e., the sea surface temperature anomalies are negative (anomalously cool). 

Below images show the snapshots from October 2022 to March 2024, which show

  • The ending of La Nina of 2022-2023 (which was actually a 3-year event, from 2020 onwards!)
  • Formation and peaking of El Nino of 2023-2024 
  • Now the decline of the 2023-2024 El Nino conditions over the equatorial Pacific.

The ending of the recent El Nino is also seen in the 90-day animation of sea surface temperature (SST) anomalies below:

     (credits: Climate Prediction Center, NOAA).

In the Pacific Ocean, the surface water is pushed from east to west along the equatorial region due to easterly winds at the surface across the ocean.

Below the surface, water slowly travels west to east, and rises to the surface near the coast of Peru (a process called upwelling).

The sub-surface water in the equatorial Pacific at present is abnormally cool, with a peak cool anomaly of greater than 4°C (i.e., more than 4°C below normal). 

This water is now seen rising up to the surface at the eastern boundary of the Pacific, near the equator (see the animation below). 

And although the sun heats the surface waters, this cold pool of water is cold enough to withstand the warming upon exposure to sunshine, and so is likely to remain cooler than normal, thereby resulting into a La Nina event.

The x-axis covers the extent of the Pacific Ocean and the y-axis is the depth below the surface (credits: Climate Prediction Center, NOAA).

We can also see this cold anomaly off the coast of Peru in the below image of sea surface temperatures:

     (credits: Climate Prediction Center, NOAA).

Therefore, the process of formation of La Nina has begun. As this cold water keeps rising up to the surface, the surface winds will help it spread westwards along the equator. 

Presently, the conditions are neutral over the Pacific. They will gradually shift from neutral to La Nina in the coming months, with La Nina establishing probably from/after August.
Below is the outlook from NOAA's Climate Prediction Center for the 3 monthly phase of ENSO (El Nino Southern Oscillation):

La Nina is known to be beneficial for the Southwest Monsoon over India. So, this development is one favourable factor for this year's Monsoon!

By Vagarian Shreyas 

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