The oceanic atmospheric phenomenon wherein waters in the equatorial Pacific warm up abnormally due to weakening of trade winds is termed as El Nino.
Definition of ‘El Nino’
El Nino, means ‘little boy’ in Spanish. It is a weather system which re-emerges after a gap of about two to five years in the Pacific Ocean and its effects last for about 12 months on an average. El Nino leads to warming of sea surface temperatures, which in turn affects wind patterns and triggers both floods and droughts in different parts of the world. Such an event is usually characterized by extreme weather events in many parts of the world. Few places across the globe are also benefited by El Nino.
What is ONI?
El Nino leads to warming of sea surface temperature (SST) above 0.5oC and Oceanic Nino Index (ONI) is three months running mean of SST anomaly in the Nino 3.4 region, which extends from 120oW to 170oW and 5oN to 5oS.
When does El Nino occur?
El Nino occurs when the threshold value of 0.5oC of ONI is met or crossed for a minimum of 5 consecutive overlapping seasons of three months each.
What is the impact of El Nino on Indian Monsoon?
This phenomenon affects rainfall in India during the Monsoon months. Trade winds normally blow westward from South America towards Asia during Indian monsoon months. Warming of the Pacific results in weakening of these winds. Moisture and the heat content thereby, gets limited and results in reduction and uneven distribution of rainfall across the Indian sub-continent.
The most prominent droughts in India since 1871 have been El Nino triggered droughts, including the recent ones in 2002 and 2009. During an El Nino, monsoon never witnesses excess rainfall baring few exceptions. For instance, 1997-98 was a strong El Nino year but that did not cause drought in India.