The long arm of El Nino that suppresses monsoon rains | Number Theory

How exactly does El Nino suppress temperature gradient between Indian subcontinent & Indian Ocean which weakens monsoon? Part 2 of this series will answer that.

It is now almost certain that 2026 will be an El Nino year when the rains will disappoint. The word, which is Spanish for “little boy”, has a lot of importance for India because of its impact on the monsoon rainfall which accounts for 75% of the annual rainfall in India. What exactly is the El Nino? What is its relationship with rainfall in India? This two-part series will use some meteorological maps and data points which explain it for

Women shield themselves from the scorching heat on a hot summer day in Rishikesh on Monday. (PTI)

The long arm of El Nino that suppresses monsoon rains

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  The monsoon is not the typical sea breeze

  As of now, the Indian Meteorological Department (IMD) expects monsoon to hit Kerala coast by May 30. Next week onwards, India will obsessively track the movement of the monsoon, best captured in a curved line moving through the subcontinent’s landmass. The monsoon, essentially speaking, is moisture being carried by a wind pattern which originates in the Indian ocean. Intuitive geography would tell us that it is normal. Land heats faster than water and the air over the ocean moves in towards land, creating the proverbial sea-breeze. However, the monsoon is not your typical sea-breeze. For example, during the peak monsoon month of July, surface air temperatures (this is what is commonly known as just temperature) over the Indian ocean are higher than those over western peninsular India.
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  What moves the monsoon is the temperature higher up

  Why do monsoon winds travel from a warmer Indian ocean to relatively cooler Indian peninsula in June-September? The answer is in the sky rather than the surface temperatures, which is where the difference in temperatures or gradient over land and sea is favourable for the monsoon to move towards India. In fact, scientists believe that this gradient – averaged in a scientific way over roughly 3-12 kilometers altitude – is a good predictor of monsoon onset and rains. Prince K. Xavier and other scientists have shown this in a May 2007 paper in the Quarterly Journal of the Royal Meteorological Society.
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  El Nino appears to suppress this temperature difference

  This can be seen in the fact that the detrended gradient (the gradient with any linear trends removed) shows a negative correlation with the index that tracks El Nino: the Oceanic Nino Index (ONI). El Nino conditions are said to be in place when the ONI is positive with values of 0.5 or higher. Negative ONI values of similar magnitude signify La Nina conditions, El Nino’s meteorological counterpart, characterised by a cyclical cooling of the central-eastern equatorial Pacific. Almost all gradient values are negative in El Nino conditions and positive in La Nina ones. To be sure, the chart tracks Pacific status in the following winter because El Nino/La Nina is typically still developing in June-September. To be sure, we might now be in a period where monsoon will also face structural rather than cyclical headwinds because of a long-term decline in the gradient, as the Western Indian Ocean experiences warming. This is a finding published in a June 2015 paper in Nature Communications by Roxy Mathew Koll and others.