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Seas of change

We hear the terms El Nino and La Nina and how they affect weather patterns around the world. But what exactly do they mean?

March 10, 2016 03:17 pm | Updated November 17, 2021 02:35 am IST

Hit by drought: Parched earth. Photo: AFP

Hit by drought: Parched earth. Photo: AFP

The year 2015 has been classified as ‘very strong’ El Nino year. Earlier, in 1982-83 and 1997-98 we had experienced similar warming in the equatorial East Pacific. El Nino/ La Nina normally peaks between October and March and is associated with warm or cool temperatures. The El Nino of 2015 has continued into 2016, and is only now showing signs of weakening.

For two years now, the Indian monsoon has failed. El Nino is blamed for this. But, El Nino need not necessarily lead to a poor monsoon. The years 1997 and ’98 had a lot of rain despite being identified as El Nino years.

So, there is but a fragile link between El Nino and the Indian monsoon. Scientists are still studying the complex interaction between the ocean and atmosphere that causes the cyclical event in the East Pacific. They are also trying to find out how this affects not only the monsoon, but also the weather and climate around the world.

El Nino is an elaborate spectacle unfolding in the ocean. It is set to a spirited background chorus of high sea-surface temperature (warming of the ocean due to sun’s heat), atmospheric pressure variations, and winds — their speed, strength and direction.

Warm/cool phases

Given the sheer size of the Pacific and its waters, its influence on weather and climate extends from the Americas to Asia, and by extension, West Asia and Europe.

This is best show-cased by alternating El Nino (abnormal warming) and La Nina (abnormal cooling) of the Equatorial East of the Pacific that lies closer to the Americas. Corresponding to this, the west of the Pacific, lying closer to Asia turns cooler during an El Nino and warmer during a La Nina.

Now, warming of the ocean from the sun’s heat sets up lower atmospheric pressure and triggers convection (the process of cloud-building) and precipitation (rainfall). Cooling is associated with higher atmospheric pressure that suppresses cloud-building and rainfall, and produces dry conditions.

This is what brings about rain-storm-flood or heat-drought conditions in different parts of the world during an El Nino/La Nina. They are part of the El Nino-Southern Oscillation (ENSO), a climate pattern caused by variations of sea surface temperatures and air pressure tendencies in and around the Equatorial Pacific Ocean as discussed earlier. The warm phase of ENSO is known as El Nino and the cool phase, La Nina.

The equatorial Pacific is split into four regions over which sea surface temperatures are monitored. One region, Nino 3.4, is where scientists have found the best linkage between sea-surface temperatures and climate patterns.

For a weather event or pattern to be classified as an El Nino or a La Nina, the sea surface temperatures in the 3.4 region must be at least 0.5 degrees Celsius above or below the long-term average for a minimum of roughly six or seven consecutive months.

El Nino and La Nina events occur every three to five years and reach peak strength between October and March when sea surface temperatures across the equatorial Pacific are the warmest.

Water on earth

Water makes up about 71 per cent of the Earth’s surface, while the other 29 per cent consists of continents and islands. If one were to break down the numbers further, 96.5 per cent of all the Earth’s water is contained within the oceans as salt water. The remaining 3.5 per cent is freshwater lakes and frozen water locked up in glaciers and the polar ice caps.

It is because of the water on the surface that Earth looks like a majestic blue marble from space. While water itself is not blue, it gives off blue light upon reflection.

At 165.25 million square km in area, the Pacific forms the largest division of the World Ocean. It covers about 46 per cent of the planet’s water surface and about one-third of its total surface area, making it larger than all of the Earth’s land area combined.

The Pacific extends from the Arctic Ocean in the North to the Southern Ocean (or Antarctica) in the South and is bounded by Asia and Australia in the West and the Americas in the East.

Oceans play an important part in the Earth’s system influencing the climate system.

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