El Niño-Southern Oscillation (ENSO) effect[edit]
Main article: El Niño-Southern Oscillation
Effects of El Niño on weather of subcontinent
El Niño is a 'warm' ocean current originating along the coast of Peru that replaces the usual 'cold' Peru or Humboldt Current. This warm surface water reaching towards the coast of Peru with El Niño is pushed westwards by the trade winds thereby raising the temperature of the southern Pacific Ocean. A 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.[27] He noticed that when it was high pressure in Tahiti, it was low pressure in Darwin and vice versa.[27] 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.[28] Walker noticed that the quantity of rainfall in the Indian subcontinent was often negligible in the years of high pressure at Darwin (and low pressure at Tahiti). Conversely, low pressure at Darwin bode well for the precipitation quantity in India. Thus he established the relationship of Southern Oscillation with quantities of Monsoon rains in India.[27]
Ultimately, it was realized that the Southern Oscillation is just the corresponding atmospheric component of the El Niño/La Niña effect (which happens in the Ocean).[27] Therefore, in the context of the Monsoon, the two cumulatively came to be known as the ENSO. The ENSO is known to have a pronounced effect on the strength of SW Monsoon over India with the Monsoon being weak (causing droughts in India) during the El Niño years whereas La Niña years had particularly good Monsoon strength over India.[27]