The Coming Storm – Winslow Homer, 1901. Source
‘El Niño’ – a Spanish phrase describing a global climate phenomenon. How did this phrase, which translates to ‘the child’ or ‘the boy’, come to define the ever changing tropical Pacific temperatures, winds and rainfall? I’ll present a short history of scientific discovery, describe the main feedbacks involved and end with an outlook for 2014.
The journey begins in 17th century Peru. Along the Peruvian coast the Humboldt Current flows northward. It forces upwelling, bringing cold, nutrient rich waters and anchovies to the shores. Fishing is productive and the local communities flourish. The fishermen (very professionally) keep track of their total catch, over time they notice a yearly decline starting around December. The decreased catch coincides with a reversal of the coastal current and subsequent warming of the waters. The change to the ocean came to be known as ‘El Niño’, the boy, written in capitals to mean The Boy, Jesus Christ, it was Christmas time after all [1,2]. Once every two to seven years El Niño would be more extreme: fishing would drop to zero, while the country was subject to severe rainfall and flooding. Happy Christmas.
Scientific recognition of these events came much later. In the late 19th century interest in forecasting climate anomalies grew, motivated by concerns about agriculture and food security. Sir Charles Todd drew intercontinental weather charts, covering Australia and parts of Asia. In 1893 he noted the tendency for simultaneous droughts to occur in India and Australia. A severe Indian famine was the reason for Sir Gilbert Walker to start investigating weather data. In the 1920s he described the recurring oscillation of atmospheric pressure in the Pacific and Indian Oceans and its correlation with temperature and rainfall patterns [e.g. 3]. The first person to relate these advances in atmospheric science to oceanography was Hendrik Berlage Jr. in 1934 [4]. However, it wasn’t until 1969 when Jacob Bjerknes published a physical mechanism that could explain the coupled anomalies of sea surface temperatures, wind and rainfall in the Pacific [5].
In its ‘normal’ climatological state the equatorial Pacific has an asymmetrical sea surface temperature pattern (see figure below). This asymmetry is caused by the surface winds over the Pacific. The easterly trade winds push ocean water westward and create a tilted thermocline in the ocean interior. Upwelling in the east creates a cold tongue, whereas surface water travelling west warms up and forms a large warm pool. The atmosphere responds to the different surface temperatures. Differential surface heating creates a pressure gradient that intensifies the trade winds. On the travel westwards air picks up moisture and above the high surface temperatures, where conditions favour convection, thunderstorms cause rainfall. The reversal of the Peruvian coastal currents is merely a side effect of these main features.
Schematic of the equatorial Pacific ocean-atmosphere system under normal conditions (left) and during El Niño conditions (right). The ocean and atmosphere interact through the Bjerknes feedback between sea surface temperatures and trade winds.
However, as the Peruvian fishermen know, these features are constantly changing. A burst of westerly winds can start an El Niño event. Warm surface water is pushed eastward and forces the thermocline downward. At the same time, the zonal (east-west direction) oceanic current reverses. The atmosphere will respond to the new surface temperature pattern by weakening the trade winds, which forces the convective activity eastward, away from Indonesia and Australia. In this example the changes in the wind are both the cause and the result of changing temperatures, a classic example of the chicken and the egg. One cannot exist without the other; El Niño is the result of feedbacks between ocean and atmosphere.
TAO buoy. Source
Due to the significant impacts El Niño has on global weather patterns, natural hazards, food security, economic growth and cricket scores [6], forecasting events has received a lot of scientific attention. After the strong, unexpected and disastrous 1982/83 El Niño, 70 moorings were installed in the tropical Pacific Ocean as part of the Tropical Atmosphere Ocean (TAO) project. These moorings have since monitored the upper 300 metres of the ocean, providing real-time data through satellite connections. Even though some of the moorings have failed in the past two years [7], the data from the TOA array provides forecasters and climate models with vital information.
A typical El Niño event does not exist; each event has had its own length, strength and mechanisms for onset and decay. To determine whether we are in a neutral state or in an extreme state different indices have been developed. NOAA’s operational definitions are based on sea surface temperatures in the ‘Niño-3.4’ region (5°N-5°S, 120°W-170°W). The 3-month running mean sea surface temperature anomaly in the region has to exceed 0.5°C for five consecutive months. Additionally, there has to be an appropriate atmospheric response over the equatorial Pacific Ocean, i.e. a weakening of the trade winds and convective activity further eastward.
Since March of this year the sea surface temperatures have increased by about 1°C in the central Pacific. Likewise the upper-ocean is warmer than normal. In contrast, the trade winds are close to their normal strength and deep convective storms are triggered mostly in their climatological position. The lack of a clear atmospheric response to the warmer ocean is the reason why the situation is currently El Niño-neutral. However, climate models predict a further warming of the Pacific Ocean over the summer. The last forecast by NOAA´s Climate Prediction Center (issued June 23th) states the chance of El Niño is 70% for this summer and reaches 80% this fall (see figure below). The ENSO Alert System has therefore moved from ‘not active’ to ‘El Niño Watch’ [8,9]. For Europe this does not improve the skill of seasonal forecasts much. However, Indonesia and Australia can expect drier than normal conditions, droughts even. In the east Pacific it will be warm and wet. And for Peru, a lack of fish this Christmas.
NOAA’s Climate Prediction Centre probabilistic El Niño outlook (updated: 5 June 2014). Lines show the climatological probability of El Niño (red), Neutral (green) and La Niña (blue). Bars indicate the projected probability for three month periods in 2014/2015. Source
References
[1] K. E. Trenberth (1997): The definition of El Niño. Bulletin of the American Meteorological Society, 78, pp. 2771-2777.
[2] S. G. Philander (2006): Our affair with El Niño: how we transformed an enchanting Peruvian current into a global climate hazard. Princeton University Press, 288 pp.
[3] G. T. Walker (1924): Correlation in seasonal variations of weather, IX. A further study of world weather. Memoirs of the India Meteorological Department, 24, pp. 275-333.
[4] H. A. M. Snelders (2013): Berlage [jr.], Hendrik Petrus (1896-1968).
[5] J. Bjerknes (1969): Atmospheric teleconnections from the equatorial Pacific. Monthly Weather Review, 97, pp. 163–172.
[6] M. M. Joshi (2009): Could El Niño Southern Oscillation affect the results of the Ashes series in Australia? Weather, 64, pp. 178-180.
[7] J. Tollefson (2014): El Niño monitoring system in failure mode. Nature News, 23 January 2014.
[8] NOAA Climate Prediction Center (5 June 2014): El Niño/Southern Oscillation diagnostic discussion.
[9] NOAA Climate Prediction Center (23 June 2014): ENSO: Recent evolution, current status and predictions.
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Karin van der Wiel
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