2015-05-21 - By Martin Hedberg

El Niño is Here

El Niño - Southern Oscillation (ENSO) is a global weather system greatly affecting local weather globally, which also affects the prevalence of cyclones in the Atlantic and the Pacific. With reasonably large probability we are approaching a sustained El Niño situation.

There are some global weather systems that have a great and lasting effect on local weather across the world. One of these systems is ENSO, which stands for “El Niño-Southern Oscillation”.  This creates weather connections between continents and generates drought, cloudbursts, good and bad fishing, storms, fires, landslides and crop failure. The changes can be split into categories and the three main scenarios are La Niña, El Niño or a more neutral situation.

The name La Niña comes from the Spanish and means “the girl”. During La Niña the sea water near the equator in the eastern part of the Pacific Ocean is about 3-5 degrees colder than normal. This means that water rich in nutrients flows up the South American coast, which creates good conditions for fishing in the eastern Pacific. During La Niña there are fewer tropical cyclones over the Pacific but those cyclones that do form are pressed westwards increasing the chances of a landfall in China, the Philippines and Indonesia. Over the Atlantic the tropical cyclones become more powerful.

Effects from winds and ocean currents on El Niño

El Niño arises when winds and ocean currents combine so that it becomes warmer in the eastern Pacific near the equator. Graphics from NOAA

El Niño means “the boy” and is the warm phase of ENSO. Around Christmas time each year there is usually a warm ocean current off the coast of Peru and Ecuador. This is called El Niño and it usually occurs for a couple of weeks or up to a month. But every 3-7 years the conditions for positive feedback of El Niño occur allowing it to strengthen itself. The air pressure distribution over the Pacific Ocean changes, which weakens the Trade winds, which in the end change direction. This in turn affects the ocean currents and even more warm water gathers off the South American coast. This strengthens El Niño. These powerful El Niño periods can last from 12-18 months. They affect not just the weather but also the eco-system and the propensity of extreme weather conditions, which in turn affects societies and economies around the Pacific. During El Niño the global temperature rises. In the West persistent high pressures are created that cause droughts in South-East Asia and Australia and can lead to extensive forest fires. Over the central and eastern Pacific there is instead a prevalence of low pressures and humid air. This means that many areas in Central and Southern America gets lots of rainfall, which leads to good harvests but can also cause cloudbursts and landslides. Even fishing is affected. The warm water off the South American coast is poor in nutrients and leads to poorer catches for fishermen.

Even if ENSO is mainly a phenomenon in the Pacific Ocean it affects the weather around the world. In the Atlantic the cyclones become fewer whereas over the Eastern Pacific there are more and more powerful tropical cyclones generated.

Changes between La Niña and El Niño

Changes between La Niña (blue) and El Niño (red). Graphics from NOAA

The conditions for El Niño exist right now.  The Australian Bureau of Meteorology writes:

El-Niño-Southern Oscillation (ENSO) indicators have shown a steady trend towards El Niño levels since the start of the year. Sea surface temperatures in the tropical Pacific Ocean have exceeded El Niño thresholds for the past month, supported by warmer-than-average waters below the surface. Trade winds have remained consistently weaker than average since the start of the year, cloudiness at the Date Line has increased and the Southern Oscillation Index (SOI) has remained negative for several months. These indicators suggest the tropical Pacific Ocean and atmosphere have started to couple and reinforce each other, indicating El Niño is likely to persist in the coming months.

International climate models surveyed by the Bureau indicate that the tropical Pacific Ocean temperatures are likely to remain above El Niño thresholds through the coming southern winter and at least into spring.

The latest three occurrences of a clear El Niño were in 1997-1998 (strong El Niño), 2002-05 (weak, but persistent) and 2009-2010 (average).

Models that NOAA has run continuously during the spring have gradually shown an increased probability for a persistent El Niño. But at the same time they warn that the model’s reliability is reduced somewhat during the period when the northern hemisphere is changing from spring to summer. The conclusion that we are probably about to enter an El Niño period is strengthened by analyses of pressure and temperature of surface and deep water in the Pacific Ocean. NOAA predicts a 70% chance that El Niño continues over the summer for the Northern hemisphere and a 60% chance that it continues for the rest of the year.

 

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Martin Hedberg

Martin Hedberg

Chief Meteorologist

Martin has 20 years’ experience from meteorology and previous officer in Swedish Armed Forces, civil career as meteorologist at Swedish Television and as CEO for a commercial weather company. He also works as a climate expert and advisor about extreme weather and how climate changes affects society, finance and energy markets. Martin holds a Bachelor of Science with a Major in Meteorology from Stockholm University.

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