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Natural Disasters

A strong El Niño

2015 brought us a very strong El Niño event. Its impact was felt in many places and such strong events could occur more frequently in future.

02.03.2016

From March 2015, the climate phenomenon first nicknamed “the Christ child” – El Niño – by Peruvian fishermen developed into one of the strongest events registered since records began in 1950. If we measure the ocean portion of this phenomenon, following the weekly mean sea surface temperature across what is known as the Niño 3.4 region (see pages 20/21), the largest deviation from the mean value in the climate reference period (1981–2010) up to the end of 2015 was 3.1°C. That is even greater than the deviation in the 1997/98 event, which was considered the “El Niño of the century” (Fig. 1).

2015 brought us a very strong El Niño event. Its impact was felt in many places and such strong events could occur more frequently in future.

However, the changes in atmospheric circulation that accompanied the ocean changes in the strong events of 1982/83 and 1997/98 were more intense than in the current episode. El Niño is a climate phenomenon that couples ocean and atmosphere. For a comprehensive record of the intensity of the event, the most practical approach is therefore to summarise the various ocean and atmospheric variables in a single index. This was attempted with the Multivariate ENSO Index (MEI) defined by Wolter and Timlin, which incorporates sea-level air pressure, the north to south and west to east wind components, sea surface temperature, air surface temperature, and cloud cover in the tropical Pacific region. This analysis shows that the 2015 El Niño event, up to and including December, is the third strongest event since 1950 (Fig. 2). Typically in El Niño events, a trend of warmer sea surface temperatures is registered in the eastern equatorial area of the Pacific, with the development peaking around the end of the year. As a result, towering rain clouds associated with warm sea surface temperatures are displaced into the central and eastern regions of the equatorial Pacific. This means that it becomes unusually dry in the west of the tropical Pacific, in other words along the coast of (north)east Australia as far as Southeast Asia, while central and eastern areas, close to Ecuador and northern Peru tend to experience unusually high rainfall. We already described other typical effects in the Topics Geo 2014 issue, which you can download from our client portal connect.munichre.com.

2015 brought us a very strong El Niño event. Its impact was felt in many places and such strong events could occur more frequently in future.

After the start of the New Year, the run-up phase to the peak (onset year) is typically followed by a regression to neutral conditions (decay year). In most cases, significant El Niño events in the context of this sequence reach their peak around the end of the year, and in many cases, then switch sign in the decay year; in other words, in the second half of the year, they transition into La Niña events, the cold sister of El Niño. Here, the effects are, to a degree, the opposite of El Niño: the trade wind drives warm water to the coastlines of the western tropical Pacific, bringing increased rainfall to the region – in other words, to northeastern Australia, Indonesia and Southeast Asia. Conversely, it tends to be dry in the eastern part of the tropical Pacific and along the equatorial coasts of South America, while the ocean cools significantly. While it is still unknown at the start of 2016 whether this will develop into a La Niña phase, there is at least an increased likelihood of that happening.

Change in cyclone activity

One of the most prominent teleconnection events from strong El Niño episodes, and one that was once again in evidence in 2015, is a change in tropical cyclone activity in each of the ocean basins. In the North Atlantic, hurricane activity typically declines, because the atmospheric conditions are less favourable for the formation and development of tropical cyclones, especially in the tropical west. One reason for this is the more powerful wind shear resulting from a stronger easterly airflow at high altitude in conjunction with slightly stronger trade winds from easterly directions close to the sea surface. Air also descends from high altitude, leading to localised warming and drier conditions, thus inhibiting convection, a fundamental process in the physics of tropical cyclones. Primarily because of these effects, the Accumulated Cyclone Energy (ACE) in this season was only 60% of the mean value over the climate reference period 1981–2010 (Klotzbach, 2015). In contrast, the level of activity of severe cyclones in the eastern North Pacific is exceptionally high – likewise a typical effect of El Niño. The ACE there was 219% of the mean value for the normal period. The reason for this increase is that, during substantial El Niño episodes, wind shear tends to be below average, while sea surface temperatures are above average. Both these factors promote the development of severe storms. The El Niño conditions also produced a similar result in the western part of the North Pacific: an unusual number of severe storms developed there, because their locations of formation were shifted east towards the warmer water, and closer to the equator. Consequently, the storms moved over relatively warm sea surfaces for longer periods, where they were able to reach high intensities under light wind shear. The Accumulated Cyclone Energy (ACE) in 2015 was 161% of the mean value of the climate reference period 1981–2010.

Notable effects in 2015

The video shows the regional precipitation-related teleconnection events which occur with a typical high-intensity El Niño. Loss events are also noted that correspond to these categories and were recorded up to the end of 2015. It should, however, be remembered that El Niño teleconnection events can overlap with other climate phenomena, such as the phases of the Indian Ocean Dipole. Because of these individual conditions, each El Niño event has its own characteristics. The losses can only be aggregated after the event has come to an end in 2016.

In what way does a strong El Niño impact the macroeconomy? Some countries may suffer a short-term reduction in real GDP growth, as has been observed in Indonesia, South Africa and Australia, for example. This is partly due to reduced agricultural yields as heat and drought take their toll. Indonesia, for example, has experienced reduced yields for coffee, cocoa and palm oil. The production and export of nickel (used in steel production) can also suffer as low water levels have a major impact on hydropower facilities and river transportation. This can push up the global price of foodstuffs and metals in particular. However, there are also countries that enjoy a brief boost to GDP growth during El Niño events, for example the US. This is because there are fewer hurricanes, and changes in temperature or rainfall can increase the output of certain agricultural products such as soybeans. US neighbours Canada and Mexico also benefit (Cashin et al., 2015).
As well as the illustrated loss effects, there were other noticeable repercussions. One of the most important is that the El Niño event contributed to the high global mean temperature in 2015, the warmest year since records began. On the environmental front, according to the US weather service NOAA, the excessive warming of the seas triggered the third registered global episode of coral bleaching after 1998 and 2010. Under environmental stress caused by increased temperatures, coral sheds algae that normally live symbiotically in its tissues, with the result that it takes on a bleached appearance. In addition, in losing the algae, it also loses its main source of food and becomes susceptible to disease. If this condition persists over a period of months, the coral then dies. The reef structures then degrade rapidly, their coastal protective function against storms quickly declines, the habitat for fish and other environmentally and economically important species disappears, and local tourism loses visitors. The event began in mid-2014 in the North Pacific and then began to have an effect on the South Pacific and the Indian Ocean. Hawaii has now been badly affected, and the islands of the Caribbean are also at risk. Researchers expect this event to continue in 2016.

Strong El Niño events will become more frequent

Strong El Niño events like that in 2015/16 may occur much more frequently this century than they did in the 20th century if the observed pace of climate change continues (business-as-usual scenario). This is the conclusion of a study conducted by leading ENSO researchers (Cai et al., 2014). According to its projections, intensive El Niño events that occurred every 20 years, or less often, in the period 1891–1990, will be experienced twice as frequently in the period 1991–2090. The main reason for this is the relatively strong warming of the eastern equatorial Pacific that would occur with continued climate change. This would mean that the level of warming required there for the formation of a strong El Niño phase would become increasingly easy to achieve. The criterion used here for an extreme El Niño event is not the extent of the anomaly in sea surface temperature, but rather the consequent anomaly in precipitation of at least 5 mm/day in the Niño 3 zone. This effect in the atmosphere also takes into account the long-range atmospheric teleconnection patterns associated with extreme events. If, following the COP21 resolutions in Paris, emissions rise by less than the business-as-usual scenario, this would mean that the increase in extreme El Niño events will reduce accordingly.

Qualified predictability

It is important for risk management purposes that a variability in climate such as El Niño can be predicted, within limits, roughly six to eight months in advance (see Topics Geo 2014). At the same time, the origins of these events are dependent on processes in much shorter time scales that are in some cases difficult to predict. The models are therefore imprecise in terms of temporal dynamics or the maximum intensity achieved by an event. Since roughly the end of April 2015, the ensemble average from the international prediction models listed by the International Research Institute for Climate and Society indicated a maximum expected intensity close to the upper end of the moderate range (Niño 3.4 index ≈1.5), and a strong event was then finally forecast from May 2015, although with a much lower amplitude than actually developed.

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