Context: The UN’s World Meteorological Organization (WMO) has predicted that there is a 75-80% chance of a weak El Niño weather pattern forming by February and a slightly lower likelihood that it will continue through the northern hemisphere winter of 2018-19.

Background:

An El Niño – a warming of ocean surface temperatures in the eastern and central Pacific that typically happens every few years – last occurred in 2015-2016 and caused weather-related crop damage, fires and flash floods.

What is ENSO?

ENSO is nothing but El Nino Southern Oscillation. As the name suggests, it is an irregular periodic variation of wind and sea surface temperature that occurs over the tropical eastern Pacific Ocean. ENSO affects the tropics and the subtropics. The warming phase of ENSO is called El Nino, while the cooling phase is known as La Nina.

What is El Nino?

El Nino is a climatic cycle characterised by high air pressure in the Western Pacific and low air pressure in the eastern. In normal conditions, strong trade winds travel from east to west across the tropical Pacific, pushing the warm surface waters towards the western Pacific. The surface temperature could witness an increase of 8 degrees Celsius in Asian waters. At the same time, cooler waters rise up towards the surface in the eastern Pacific on the coasts of Ecuador, Peru, and Chile. This process called upwelling aids in the development of a rich ecosystem.

What causes El Nino?

El Nino sets in when there is anomaly in the pattern. The westward-blowing trade winds weaken along the Equator and due to changes in air pressure, the surface water moves eastwards to the coast of northern South America. The central and eastern Pacific regions warm up for over six months and result in an El Nino condition. The temperature of the water could rise up to 10 degrees Fahrenheit above normal. Warmer surface waters increase precipitation and bring above-normal rainfall in South America, and droughts to Indonesia and Australia.

What are El Nino’s effects?

• El Nino affects global weather. It favours eastern Pacific hurricanes and tropical storms. Record and unusual rainfall in Peru, Chile and Ecuador are linked to the climate pattern.
• El Nino reduces upwelling of cold water, decreasing the uplift of nutrients from the bottom of the ocean. This affects marine life and sea birds. The fishing industry is also affected.
• Drought caused by El Nino can be widespread, affecting southern Africa, India, Southeast Asia, Australia, and the Pacific Islands. Countries dependent on agriculture are affected.
• Australia and Southeast Asia get hotter.
• A recent WHO report on the health consequences of El Nino forecasts a rise in vector-borne diseases, including those spread by mosquitoes, in Central and South America. Cycles of malaria in India are also linked to El Nino.
• The rise in sea surface temperature may be intensified by global warming. From the current study, we learn that El Nino can exacerbate global warming and hence the process could become a vicious circle.

What is La Nina?

La Nina is a climate pattern that describes the cooling of surface ocean waters along the tropical west coast of South America. It is considered to have the opposite effect of El Nino. It brings greater than normal rainfall in Southeast Asia and Australia, and causes drier-than-normal conditions in South America and the Gulf Coast of the United States. La Nina events sometimes follow El Nino events.

How long does it last?

An El Nino or La Nina episode lasts nine to 12 months. Some may prolong for years. Its average frequency is every 2 to 7 years. El Nino is more frequent than La Nina.