page_title: Monsoon of South Asia
section_title: El Niño-Southern Oscillation effect
hierarchical_section_title: Monsoon of South Asia / Theories for monsoon variability / El Niño-Southern Oscillation effect
caption_attribution_description: During warm ENSO episodes the normal patterns of tropical precipitation and atmospheric circulation become disrupted. Related image: Image:La Nina regional impacts.gif
context_page_description: The monsoon of South Asia is among several geographically distributed global monsoons. It affects the Indian subcontinent, where it is one of the oldest and most anticipated weather phenomena and an economically important pattern every year from June through September, but it is only partly understood and notoriously difficult to predict. Several theories have been proposed to explain the origin, process, strength, variability, distribution, and general vagaries of the monsoon, but understanding and predictability are still evolving.
  The unique geographical features of the Indian subcontinent, along with associated atmospheric, oceanic, and geophysical factors, influence the behavior of the monsoon. Because of its effect on agriculture, on flora and fauna, and on the climates of nations such as Bangladesh, Bhutan, India, Nepal, Pakistan, and Sri Lanka — among other economic, social, and environmental effects — the monsoon is one of the most anticipated, tracked, and studied weather phenomena in the region. It has a significant effect on the overall well-being of residents and has even been dubbed the "real finance minister of India".
context_section_description: El Niño is a warm ocean current originating along the coast of Peru that replaces the usual cold Humboldt Current. The warm surface water moving toward the coast of Peru with El Niño is pushed west by the trade winds, thereby raising the temperature of the southern Pacific Ocean. The reverse condition is known as La Niña.
  Southern oscillation, a phenomenon first observed by Sir Gilbert Thomas Walker, director general of observatories in India, refers to the seesaw relationship of atmospheric pressures between Tahiti and Darwin, Australia. Walker noticed that when pressure was high in Tahiti, it was low in Darwin, and vice versa.  A Southern Oscillation Index (SOI), based on the pressure difference between Tahiti and Darwin, has been formulated by the Bureau of Meteorology (Australia) to measure the strength of the oscillation. Walker noticed that the quantity of rainfall in the Indian subcontinent was often negligible in years of high pressure over Darwin (and low pressure over Tahiti). Conversely, low pressure over Darwin bodes well for precipitation quantity in India. Thus, Walker established the relationship between southern oscillation and quantities of monsoon rains in India.
  Ultimately, the southern oscillation was found to be simply an atmospheric component of the El Niño/La Niña effect, which happens in the ocean. Therefore, in the context of the monsoon, the two together came to be known as the El Niño-Southern Oscillation (ENSO) effect. The effect is known to have a pronounced influence on the strength of the southwest monsoon over India, with the monsoon being weak (causing droughts) during El Niño years, while La Niña years bring particularly strong monsoons.
caption: Effects of El Niño on the weather of the subcontinent.
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