El Nino and La Nina
The Indian famine of 1899-1900 affected around one third of the country and one quarter of the population, who were still recovering from the 1896-97 famine. The Indian Meteorological Department had been using projections of monsoon rains based on the spring snow cover of the Himalayas, but these projections were increasingly failing. Gilbert Walker, a mathematician from Cambridge was brought in to improve the projections. He utilized a huge amount of weather data from India and other countries and described the “great seesaw oscillation of atmospheric pressure between the Indian and Pacific Ocean, and its correlation to temperature and rainfall patterns across much of the earth’s tropical regions, including India” (https://en.wikipedia.org/wiki/Gilbert_Walker_(physicist)). This was initially based on observed fluctuations between atmospheric pressure in Darwin, northern Australia and Tahiti.
The El Nino Southern Oscillation (ENSO) describes the cycle of warm and cold sea surface temperature (SST) of the central and eastern Pacific Ocean close to the equator (https://en.wikipedia.org/wiki/El_Nino). El Nino (the ‘boy child’) occurs on average every four years (range from two to seven years) warm water builds up in the eastern Pacific near the coast of Peru, usually around Christmas (https://www.noaa.gov/understanding-el-nino). On the western side of the Pacific the SST decreases. This SST gradient coincides with, or possibly is driven by, the equatorial Trade Winds reversing and blowing from west to east (https://oceanservice.noaa.gov/facts/ninonina.html). The opposite side of the ENSO is La Nina (the ‘girl child’) where warm water builds up in the western Pacific and cold water accumulates in the eastern Pacific.
This cold water upwelling off the Peruvian coast brings nutrients to the surface, and with it an abundance of small fish like anchovies and larger marine predators higher up the food chain. The trade winds return to their normal east to westerly direction, allowing marine vessels to cross the Pacific against the westerly winds prevailing at higher latitudes.
The warming associated with El Nino is also associated with reduced rainfall over Indonesia, northern Australia and the Indian monsoon. For La Nina this pattern is reversed with increased rainfall over Indonesia, and an increased South Asian monsoon, but there may be drought in east and north east Africa (Horn of Africa), depending on the Indian Ocean Dipole (IOD).
The definition of an El Nino (or La Nina) varies from country to country. Australia, the USA and Japan all use slightly different indices and reference regions (NINO 3, or 3.4) of the Pacific Ocean, but generally it requires a differential of +0.5oC Oceanic Nino Index. (NOAA) for several months.
It is not clear if El Nino and La Nina are influenced by climate change, although there is recent evidence that the frequency of El Nino is increasing with global warming. Examination of the pattern of El Nino and floods in Bangladesh suggest that there is some link, but it is not strong, with certain flood years being strong El Nino years (1988, 1998, 2007, 2010) but other flood years being neutral or La Nina years (1963, 1974, 1982, 1990, 1994, 2004).
Sources:
- NOAA National Weather Service Climate Prediction Center
https://www.cpc.ncep.noaa.gov/products/precip/CWlink/MJO/enso.shtml#current
- Australian Government Bureau of Meteorology
http://www.bom.gov.au/climate/enso/
- Japan Meteorological Agency
https://ds.data.jma.go.jp/tcc/tcc/products/elnino/outlook.html
