By Dr Julia Maxted, Senior Lecturer in Geography, School of Human and Life Sciences

How does an apparently local event in the Pacific affect the short-term climate and weather in other parts of the world? Julia Maxted examines the links between the 2015/16 El Niño and a variety of recent environmental hazard events across large areas of the globe.

El Niño is the term used for the period when sea surface temperatures are above normal off the South American coast along the equatorial Pacific. Every two to seven years, an unusually warm pool of water – sometimes two to three degrees Celsius higher than normal – develops across the eastern tropical Pacific Ocean to create a natural short-term climate change event. This warming of the ocean, known as El Niño not only affects the local aquatic environment but also causes global disruption in the general circulation of the Pacific Ocean and atmosphere. In turn, this spurs extreme weather patterns around the world, from flooding in the Americas to droughts in Australia and Southern Africa. Particularly strong El Niño periods are also now being investigated in connection with recent events further away from the tropics, such as the flooding in Northern England and parts of Scotland in the winter of 2015.

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March 2016 sea surface temperatures compared to average between 1981-2010. Source: http://www.climate.gov/enso

The meteorologist Tom K. Priddy has commented that Peruvian fishermen have long witnessed periodic changes in the location of fish species usually towards December (Scientific American, 20.10.97). The weather pattern emerges in the mid-Pacific but its effects are felt across large areas of the globe.  Normally it is warmer in the western Pacific and cooler in the east next to South America and this pulls in air from the east, the so-called ‘trade winds’ which return in the upper atmosphere.  The trade winds drive ocean surface currents toward the west along the equatorial Pacific. “Cold water, upwelling from deep ocean currents, provides nutrient-rich food for anchovy, the fishermen’s preferred catch” (op.cit).

El Niño occurs when the trade winds along the equatorial Pacific become reduced or calm for many weeks. Then the upwelling of cold, deep ocean waters slow or stops, allowing sea surface temperatures to increase much above normal in the east and central Pacific. The warm water drives the fish to deeper waters or farther away from usual fishing locations. This happens every 2 to 7 years, and as Prof. Adam Scaife of the UK Meteorological Office reports, can be identified through climate records going back to the late 19th century (BBC Inside Science, 7.1.16).

El Niño is Spanish for ‘the boy’ referring to the Christ child because El Niño peaks around Christmas. The event peaks in mid-winter and then turns over and declines slightly over the next six months back to zero – it takes several months for the ocean to release all the heat and so the impacts of an El Niño continue both in the tropics and elsewhere. While we have made quite good predictions of when it is going to emerge in the mid-Pacific, it is harder to work out how strong the event (measured by the rate of increase in ocean temperatures) will be.  In May 2015, Prof. Adam Scaife of the Meteorological Office predicted that we would be heading for a strong El Nino event and data reveals that the 2015/16 event is likely to have equalled the strongest event since records began in the 1880s, that of 1997-8. Sea surface temperature anomalies can be of the order of 2 to 3 degree C above normal in many parts of the equatorial Pacific in a really big event, and the huge movement of warm water from the Philippines end of the Pacific Ocean across the middle and into the South American coast can raise the coastal sea level here by 40 centimetres.

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Sea surface temperatures. Source: http://www.bbc.co.uk/news/world-us-canada-35235273

Very warm waters in the equatorial Pacific pump more moisture into the air, causing an increase in showers, thunderstorms and tropical storms over a much larger area. The area affected can be so large and deep in the atmosphere that major upper air wind currents are affected. Since major wind currents steer the weather systems in the middle latitudes as well as the tropics, typical storm paths are shifted. Because of the shift of the rainfall pattern in the tropics, there was a very weak Indian monsoon and poor rainfall over the Ethiopian Highlands in 2015, reducing the flow of the Nile and increasing the possibility of drought. Places such as Australia, Indonesia, Brazil, India and Southern Africa can experience drought conditions because moisture-bearing storms are shifted away from these areas. “The risk of coral bleaching increases and populations of marine plants in the eastern tropical Pacific (and the animals that depend on them) sometimes crash”(NOAA: www.climate.gov/enso, accessed 14.4.16).

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Boys play along with the banks of the Zambezi river in Mozambique. The country has been hit by flooding in the north and drought in the south. Source: John Wessels/AFP/Getty Images

We often see an intensification of forest fires during El Niño – and there have been examples of these during the current event in the Philippines and Indonesia. Likewise, Argentina, South China, Brazil and Japan can receive an increase in moisture-bearing storms that cause long periods of heavy rains and flooding. The 2015/16 El Niño system generated intense flooding across South America, with Paraguay, Argentina, Uruguay and Brazil experiencing the worst flooding in 50 years, resulting in the evacuation of more than 150,000 people. Additionally, there is a decrease of tropical storms (hurricanes) in the Gulf of Mexico and Western Atlantic and an increase of tropical storms in the Pacific.

 

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Forest fires in Indonesia in 2015. Source: http://www.bbc.co.uk

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El Niño global climatic patterns. Source: http://www.climate.gov.uk/enso

California is currently in the midst of a serious multi-year drought and the state’s reservoirs are dramatically below the historical average. El Niño could offer some respite because it brings rain but a flip from drought to floods and mudslides is a concern. Parched earth and areas that have experienced wildfires and burns are likely to be unstable or unable to absorb heavy rainfall. Bridges over the heavily channelized Los Angeles River provide shelter for many homeless people and this can suddenly fill with very fast moving water in a rainstorm (BBC Radio 4, Inside Science ,7.1.16)

 

Scientists  are now increasingly turning to the question of whether it is it possible to correlate events happening in El Niño with what happens in Europe? The traditional view is that the Europe is just too remote for El Niño to have an effect. However in the last 5 to 10 years by taking a more careful look and using more sophisticated computer models this view has been overturned so that scientists know now that there is a significant impact on Europe in winter in particular. As Tim Stockdale of the European Centre for Medium Range Weather Forecasting reported in the BBC Radio 4 Inside Science special on El Niño (7.1.16) the very strong heating over the pool of warm water in a strong El Nino event pushes  atmospheric flow upwards into the stratosphere. This is carried around the globe by normal circulation over the Atlantic in the Northern hemisphere and then propagates down again and this disrupts weather patterns. Though the dynamics are yet poorly understood, the net effect is to disturb the jet stream/ In the winter of 2015 the jet stream was pushed so that instead of coming down from the Arctic bringing cold weather with it, it was bringing up warm air from subtropical Atlantic over an extended time period, which is why December 2015 was both a record month for warmth and for rain.

It is not just where this moisture is going but also how much moisture there is. Atmospheric river combines strong winds and high amounts of moisture  in ribbons that are typically 100-200 kilometres wide and thousands of kilometres long, and their impacts are often localised and prolonged. These are weather features and not particularly associated with El Nino but  are becoming more frequent particularly in winter because of global warming. The extreme amounts of precipitation that fell in Cumbria in December 2015 are related to this unusual weather pattern.  Because of the global climate now warmer there is more moisture in the air that fuels a greater intensity of rainfall. Atmospheric moisture is rising at the rate of 1% a decade, essentially making heavy rainfall events (which are part of weather) more severe.