El Niño

Pacific Ocean sea level in July 1997. Click on the picture for a larger version and credits.

During the fall of 1997 the El Niño phenomenon, and alleged consequences thereof, received substantial media coverage. In particular, a lot of attention was directed towards the Indonesian forest fires and their veil of smoke covering large areas in Southeast Asia. Other incidents was also linked to El Niño: high typhoon activity, beach erosion and rising prices of coffee are just a few examples. Due to all this publicity, "El Niño" became the new term of this season in our vocabulary, even at the remote latitudes and longitudes of Norway.

El Niño - "The Christ Child"

The story of El Niño begins on the eastern margins of the Pacific Ocean. For centuries, Peruvian fishermen have known that the usually cold and nutrient rich waters from time to time become exceptionally warm, accompanied by collapsing fish stocks. At the same time, torrential rain and flooding of the rivers of the Andes occur. This abnormal situation returns every 3-7 years and, since the event usually peaks around Christmas, the fishermen named the phenomenon "El Niño" ("el niño" is Spanish for boy child). For a long time, El Niño was considered to be a weather phenomenon local to the countries of the western part of South America. Only early in the 20th century did scientists begin to realize that a relation exists between El Niño and monsoon conditions in Southeast Asia.

The northeast monsoon in the western Pacific Ocean usually begins around the end of August, and lasts for a few months. The monsoon carries moist air and, consequently, heavy rain. Sir Gilbert Walker, who was the head of the Meteorological Service in India, discovered that weak Pacific monsoons and small amounts of precipitation frequently coincided with El Niño events. By examination of sea level pressure records from Tahiti (in the central Pacific) and Darwin, Australia, he found that the pressure difference between these meteorological stations oscillates in (anti-)phase with El Niño. This oscillation is now known as the Southern Oscillation. The normal situation is that the pressure over the equatorial Pacific Ocean is higher in the east than in the west. During El Niño this pressure difference is significantly reduced (or even reversed). For this reason, pressure anomalies have been used to identify El Niño events. In recent years the use of anomalies in sea surface temperature or sea level in the eastern Pacific Ocean for defining El Niño events has gained popularity, mostly because oceanic quantities are much less subject to "noise" (high frequency natural variability on a time scale of weeks to a few months) than their atmospheric counterparts.

Jack Bjerknes and the explanation of El Niño

Sir Walker established a statistical relationship between El Niño, weak monsoons and the Southern Oscillation. However, the dynamics of the ocean-atmosphere system related to El Niño remained unexplained for decades. It was the Norwegian-born UCLA professor Jack Bjerknes who managed to fit the pieces together towards the end of the 1960s.

Vertical crossection along the equator of temperature in a neutral (non-El Niño) situation. Click on the picture for a larger version and credits.

In the tropical regions of the oceans, the water masses consist of a wind mixed surface layer (the mixed layer) separated from the cold water of the deep ocean by a well-defined thermocline. The easterly trade winds pull the water masses of the surface layer from America towards Asia in the Pacific Ocean. The mixed layer then becomes thick to the west, and the sea surface rises slightly so that the wind drag on the surface of the ocean is partially compensated by a pressure force acting in the opposite direction. Thus, in the western Pacific Ocean wind mixing is not deep enough for entrainment of colder water through the thermocline to take place, and the water piled up to the west becomes very warm.

The situation in the eastern region of the tropical Pacific Ocean is the exact opposite: The mixed layer is thin, allowing cold, nutrient rich waters to be entrained from beneath and provide food in abundance for the large fish stocks off the Peruvian coast.

During El Niño the trade winds relax, or even reverse, in the central and western Pacific so that the force that indirectly holds the thermocline down in the west is reduced, or disappears entirely. In this way the thermocline is released from its normal inclination in the east/west direction, and the resulting motion can as a good approximation be described as an internal Kelvin wave trapped in the Pacific equatorial belt. By the action of pressure forces this enormous thermocline wave, which will span the equatorial Pacific Ocean, propagates eastwards until it reaches the American continent. At this point, the upwelling of nutrient-rich deep water off the coast of Peru will be suppressed, and the conditions for life in the ocean will be dramatically impaired until the normal situation is restored (many months later).

When the surface water temperature rises in the Eastern Pacific under El Niño conditions, evaporation increases correspondingly and an unusually large amount of precipitation in the adjacent areas results. At the same time the atmospheric convection pattern over the tropical Pacific Ocean will be shifted. The ocean can be somewhat cooled on the Asian side of the Pacific, and Southeast Asia monsoon rains, which commonly occurs in August-September, will fail due to reduced evaporation from adjacent ocean areas and changes in the circulation of the atmosphere. Southeast Asia thus suffers droughts during El Niño, while parts of Latin America are hit by heavy rainfalls and accompanying flooding.

Predicting El Niño

The trapped internal Kelvin wave in the Pacific Ocean moves slowly (around 2 m/s) eastwards. It takes about two months for the depression of the thermocline in the western parts of the Pacific to reach the opposite coast. By using an adequate set of observations it is possible to predict an El Niño event well ahead of its dramatic consequences. There is a network of oceanographic buoys called the TAO-array along the equatorial Pacific that makes continual observations of temperature, currents and wind. These observations are sent via satellite to a ground station. Data from the buoys and illustrations are made available on a public domain Internet server. Satellite observations are used to monitor sea level, wind, and sea surface temperature. Through these observations an El Niño event can be detected at an early stage, and the ensuing development can be tracked literally on a day to day basis. A more quantitative method for predicting El Niño events is to use numerical models for the ocean and atmosphere circulation.

Pacific Ocean temperature anomaly (snapshot from an animation). Click on the picture for the full animation and credits.

In April and May 1997 there were unambiguous signs that an El Niño was developing, and in June all atmospheric indications confirmed this. The event of 1997 was in other words very well predicted. According to some indicators this was the strongest El Niño of the century. Each El Niño episode has a unique signature. The 1997 episode differed from earlier events by its intensity, its early development, and its long duration (significant warm anomalies of sea surface temperature remained well into the first half of 1998). This may be seen in an animation of the vertical distribution of heat in the equatorial Pacific. At the time of writing (early August, 1998) there are indications that the circulation in the Pacific Ocean is switching to the opposite extreme, with anomalously cold surface water in the east.

Impact on the weather in other parts of the world

There is much speculation as to how El Niño affects seasonal weather elsewhere in the world. the media, it has been asserted that, due to El Niño, this winter in Norway will be extremely mild and that there will be a lot of precipitation throughout the season. Such assertions tend to blame El Niño for virtually anything. It is correct that large-scale phenomena in the ocean and atmosphere will have an influence on the weather and climate far away from the areas where the phenomena actually occur. As an example, it is documented that the southwestern U.S. receives anomalously large amounts of precipitation during El Niño winters. Further, the position of the polar front over North America is often shifted in a certain fashion in the winters of El Niño events.

At the same time one should be aware of the fact that ocean and atmosphere conditions elsewhere will also have an impact on the seasonal weather. Moreover, even though some regions of the world experienced a wet and mild winter during the last El Niño, it is not granted that the seasonal weather will develop similarly everywhere after the 1997-1998 event. The atmosphere and ocean form a system that interacts in a very complex way, and this makes it hard to decide what importance the El Niño phenomenon will have on the winter at our latitudes. A general connection between El Niño and weather in Norway has not been established.

El Niño does not only cause changes that are experienced as negative. The best example of this is perhaps that Atlantic Ocean tropical hurricane frequency drops significantly during El Niño events. Professor William Gray at the Colorado State University, an authority within seasonal forecasting of tropical storms in the Atlantic Ocean, predicted a high storm activity in the 1997 hurricane season: eleven tropical storms, whereof seven hurricanes (three very strong). This prediction, which was publicized in April that year, was predominantly based on the fact that the Atlantic Ocean was warmer than usual at the time. It was therefore very interesting to monitor the storm activity in the tropical Atlantic Ocean during the summer and fall of 1997.

In July 1997 it looked as though Gray had been too conservative; there were four tropical storms this month, whereof two hurricanes ("Bill" and "Danny"). After that, this part of the atmosphere was considerably more calm than usual: no storms in August, one hurricane in September ("Erika") and two storms in October. In sum this gives seven tropical storms in the Atlantic Ocean, whereof three hurricanes - this is considerably fewer than Gray's prediction. An assertion that El Niño led to a tranquil Atlantic Ocean after July in the hurricane season of 1997 should nevertheless be documented beyond these facts. Also, a reminder that one extreme hurricane can do more damage than ten "normal" hurricanes is appropriate: "Andrew", the most devastating hurricane ever to occur in the Atlantic Ocean raged during the El Niño event of 1992.

El Niño and global climate change

As mentioned above, there are accounts of El Niño dating back several hundred years.El Niño is a phenomenon that is caused by natural variations in ocean and atmosphere, and not by anthropogenic (man-made) changes. Occurrences of El Nino are nevertheless of interest for the research on anthropogenic climate change because the air-sea exchange of CO₂ is partially determined by the ocean temperature. Observed concentrations of the atmospheric CO₂ content can thus undergo incidental changes in connection with protracted El Niño episodes.

All the same, one often reads that the frequency of El Niño events has increased during the last decade, and that this is due to a global heating of the atmosphere. In a speech on emergency management in October 1997, Vice President Al Gore declared that we are experiencing an increase in both the frequency and intensity of El Niño events, and that this change may be caused by an increased supply of green house gases to the atmosphere. It is important to realize that no such link has been established for the present day climate. Frightening scenarios associated with global climate change are popular matter in the media. The threat of global anthropogenic climate change is a serious problem that our global society confronts today, but one must also be aware of the fact that there is a large natural variability in weather and climate. In general one should therefore be cautious in asserting connections between this type of event and global climate change.

By Arne Melsom and Øyvind Sætra, Section for Oceanography.
Information on R&D activities in physical oceanography at the institute is available here.

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