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Natural Disasters

The perilous Christ Child

Experts believe that the weather phenomenon, El Niño, is very likely to occur this year. What will this mean for global weather?

09.07.2014

Tough times could lie ahead for fishers in Ecuador, Columbia and northern Peru. The fish, and in particular the anchovy stocks from which they earn a living, are always among the first of El Niño's victims. The weather phenomenon generally begins to have a strong impact off the tropical Pacific coast of South America towards the end of the year, cutting off fish from contact with nutrient-rich deep water. But South America would not be the only place to experience El Niño's impact: there are noticeable changes to weather conditions in many parts of the world. Current observations are making experts sit up and take notice. Sea surface temperature, which is averaged over a section of the tropical Pacific for use as an indicator, is currently around 0.5 degrees Celsius above the longtime mean. The US National Weather Service's Climate Prediction Center classifies a phase as an El Niño event when this deviation is achieved or exceeded over a seven-month period, with the measurements taken as five consecutive, overlapping three-month averages. At the present time, experts expect this scenario to develop between the summer of 2014 and the spring of 2015. This would be El Niño's first return since 2009/2010. It has occurred every three to five years since the 1950s.

Depending on how much the temperature deviates from the mean, a distinction is made between a weak (+ 0.5°C or more), moderate (+1.0° or more), and a strong (+1.5° or more) El Niño event. The intensity of the weather extremes triggered by the phenomenon also increases in line with its strength. However, the terms "weak" and "moderate" can be misleading, as there can be quite a severe impact in all three cases. Since the deviation from the mean temperature must persist over a longer period, a substantial El Niño event can only be classified after some time has elapsed. But what exactly lies behind this weather phenomenon, which is also known as ENSO (El Niño-Southern Oscillation)?

Experts divide this climate oscillation into three phases: the neutral phase, which merges at its boundaries into an El Niño or La Niña. During the neutral phase, warm surface water in the eastern equatorial Pacific (i.e. off the tropical coast of South America), is pushed westwards by the trade winds. The cold water of the Humboldt current, which is much richer in nutrients, can therefore mix easily with the warm surface layer that has been thinned out by the wind shear. As a result, there is an abundance of food for the fish. The very warm surface water that has been pushed westwards along the Equator piles up like the thick end of a wedge off the coast of Indonesia. The rising masses of moisture-laden air in this region that result from the higher level of evaporation are released as rain over Indonesia and eastern Australia, and are accompanied by lower local air pressure. This situation contrasts with a sinking mass of dry air, accompanied by higher air pressure and cloud dispersion, that is experienced over the eastern equatorial Pacific and the tropical coast of South America. Behind this atmospheric pattern lies an orderly circulation system along the Equator, whereby air rises above the western edge of the equatorial Pacific, flows eastwards at high altitude, before dropping again and moving west over the ocean in the form of the trade winds (Walker circulation).

El Niño configures global weather and conditions

As contrasting configurations of the tropical ocean-atmosphere system, El Niño and La Niña represent a natural climate oscillation with global effects, particularly in tropical and subtropical regions. Accordingly, the occurrence of an El Niño event alters the probability of weather features and weather extremes arising in many regions of the world.

Experts believe that the weather phenomenon, El Niño, is very likely to occur this year. What will this mean for global weather? © Munich Re
What happens during a neutral phase
Normal conditions are referred to as the neutral phase: Strong trade winds from easterly directions push the warm surface water west as far as the coast of Indonesia, while in the process reducing the thickness of the warm equatorial surface layer in the east. Cold, nutrient-rich, deep water is then able to upwell into the surface layer off the coast of South America. The particularly warm water off the coast of Indonesia evaporates. The resulting rising masses of moisture-laden air result in increased precipitation over Indonesia and the neighbouring land areas. Conversely, dry air, accompanied by cloud dispersion, falls over the eastern tropical Pacific and the west coast of South America, producing dry weather conditions here.

But El Niño disrupts these diverse interactions between the ocean and the atmosphere: episodes of strong westerly winds occur in the western equatorial Pacific, while the trade winds die away, or can even change direction. The warm surface water on the western edge of the tropical Pacific that has piled up like a wedge off the coast of Indonesia drifts back again towards the eastern equatorial Pacific and South America, thereby adding to the thickness of the warm surface layer. The phenomenon peaks in activity around Christmas time, and this is where the name comes from: the anchovy fishers called it El Niño, the Spanish name for the Christ Child. But this Christ Child does not arrive bearing gifts: instead, the warm masses of water mean certain death for the fish. The cold, nutrient-rich water of the Humboldt current is now at a much greater depth and can no longer mix with the layers near the surface. As a result, the fish die and there is nothing for the fishers to catch. A large number of sea birds also disappear.

Experts believe that the weather phenomenon, El Niño, is very likely to occur this year. What will this mean for global weather? © Munich Re
What happens during an El Niño phase
During an El Niño phase, the trade winds weaken substantially, or can even blow in the reverse direction. Warm surface water drifts from Indonesia along an equatorial corridor as far as South America, increasing the thickness of the warm ocean surface layer there. The cold deep water is then no longer able to upwell into the top layer of water. The warm masses of water now evaporate in the eastern tropical Pacific and off the west coast of South America, resulting in heavy rain there. In Indonesia and neighbouring regions, however, the weather tends to be dry.
While Ecuador, northern Peru and southern Colombia struggle with the effects of heavy rain and possible flooding, very dry conditions prevail in parts of Southeast Asia, Indonesia, and in the north and east of Australia. But depending on the intensity it achieves, El Niño can also have an effect on regions much further afield: heavy rainfall in Southeast China, droughts in India and South Africa, torrential rain in California. According to experts, all these effects can be triggered by a substantial El Niño event. However, due to the many different factors involved, the intensity of the phenomenon and particularly its long-distance effects can only be predicted with a low level of precision. Many aspects of the question of how exactly an El Niño event is triggered are still the subject of research. But one thing is certain: a La Niña (in Spanish "the girl") phase generally follows hard on the heels of a substantial El Niño event. This weather phenomenon often occurs in the year immediately following an El Niño, and is actually a reverse El Niño. The trade winds now blow more strongly, the surface water in the eastern equatorial Pacific and off the coast of tropical South America becomes especially cold, while the water off the coast of Indonesia becomes especially warm. La Niña also influences global weather patterns. In comparison with El Niño, however, it tends to have the reverse effects: in places where there was heavy rain, for example, there is now dry weather. La Niña can last for much longer than its sibling. Indeed, its effects can sometimes be observed over a period of years. The US Weather Service defines a period as a La Niña episode when sea surface temperatures in the equatorial eastern Pacific stay at least 0.5 degrees Celsius beneath the longtime mean for five consecutive, overlapping 3-month periods.
Experts believe that the weather phenomenon, El Niño, is very likely to occur this year. What will this mean for global weather? © Munich Re
What happens during a La Niña phase
The overall pattern is similar to the neutral phase, the only difference being that the trade winds blow more strongly from easterly directions. The thicker layer of warm surface water that has been pushed by wind shear along the equator close to the coast of Indonesia to form the thick end of a wedge results in significantly increased precipitation along the coast of Indonesia and neighbouring regions, while extremely dry weather prevails in the eastern tropical Pacific and on the west coast of South America.

Both El Niño and La Niña can have a strong regional impact on the insurance industry, because the occurrence probability of extreme weather events is altered in some regions. To take just one example: During a substantial El Niño event, there is a significant increase in the probability of brush fires in Indonesia, bush fires in eastern Australia, and of losses from flooding and landslides on the slopes of the Andes in Ecuador, southern Colombia and northern Peru, and also in California. On the other hand, hurricane activity in the north Atlantic declines during El Niño phases, and even though the average number of severe typhoons in the northwest Pacific increases, they make landfall less often. Experts believe that a 2014/2015 El Niño event that achieves the status of substantial could be followed in the second half of 2015 by a La Niña event.

Is it possible to predict El Niño or La Niña?

Modern dynamic and statistical prediction models are able to predict the probability of an El Niño or La Nina event up to six months in advance with an impressive level of accuracy. However, it is not yet possible to predict the maximum intensity with any degree of accuracy. The long-term effects on other regions can only be roughly predicted because other factors are also in play. Many aspects of the question of how exactly an El Niño event is triggered are still the subject of research.

Experts believe that the weather phenomenon, El Niño, is very likely to occur this year. What will this mean for global weather? © Munich Re

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