University of Iowa, Iowa City, Iowa 52242

… Background

… Typical Effects of an El Niño

… Tracking and Forecasting El Niño events

… References

The Southern Oscillation is the name given to the variability of the global circulation patterns in the tropical regions of the globe, specifically the Indo-Pacific Walker circulation. This oscillation is caused by differences in the ba rometric pressures between the eastern Pacific near the coast of South America and the Indian Ocean. The normal patterns exist due to slightly higher atmospheric pressure in the eastern Pacific Ocean, which leads to predominating trade winds from east to west. The result of this pressure gradient in normal conditions is the warm, tropical waters present near Indonesia are essentially ëpiledí and retained in the west. This pilin g is significant, as the sea surface is normally over a foot and a half higher in Indonesia than in Ecuador, a massive amount of warm water.

The resulting tongue of unseasonably warm water extending across the Pacific Ocean can have devastating effects on global weather patterns (these effects are called "teleconnections") and ecosystems depending on its severity. The coupling between the trade wind relaxation (the Southern Oscillation) and the warming of the waters in the eastern Pacific (El Niño) have led to the exten sive use of the acronym ENSO (El Niño Southern Oscillation) to describe the event.

With this cyclical relationship in mind, it is easy to see how a major disruption in the normal location of warm sea surface temperatures (SSTís) can have an effect on global weather patterns.

As previously mentioned, the effects of an ENSO event range from very localized to far-reaching global implications. On the local scale, the economy of Peru can be severely disrupted by a significant ENSO event. As a coastal nation, Peru relies heavily on the fishing industry to sustain its otherwise agrarian based economy. During non-ENSO years, the waters off the coast of Peru are cooler and full of the necessary nutrients for sustaining important marine life. Especially important is the position of the thermocline, or the depth at which there is a separation between the surface layers and the deep layers of the ocean. The sub-thermocline waters contain the bulk of the nutrients needed in the upper ocean layers, which are supplied through a process called upwelling. During an ENSO event, this thermocline is suppressed to deeper levels due to the presence of a large quantity of warmer water at the surface. The warm waters from the west are nutrient poor, and due to the suppression of the thermocline the natural upwelling does not occur. As a result, thousands of marine species that normally inhabit this area of the Peruvian coastline either die of starvation or relocate. The effect on the local fisheries is devastating, but there is also a global economic affect: one of the primary catches in Peru is the anchovy, which is a primary component in many types of agricultural fertilizers as well as some animal feeds. As the price of these products increase, so do all agricultural products and livestock around the world.

The marine ecosystem of coastal Peru is not the only ecosystem that has reported biological effects. Marine life as far north as the coast of North America, where a coastal water warming is also usual for an EN SO event, has also been affected in the past. Specifically, the migration of unusual species such as mackerel, which tend to feed extensively on juvenile salmon, also has a direct effect on the local salmon fisheries. In addition, it has also been reporte d that the seasonal migration patterns of salmon off the coast of North America are very much influenced by the presence of warmer waters as a result of an equatorial ENSO event.

The onset of a severe ENSO event upsets the global atmospheric circulation patterns to the point that some geographical locations do not receive the rainfall that is usual, and vice versa. Much of this is due to the shifting of the global jet streams, streams of rapid winds that traverse the globe and deliver weather systems as well as separate warm and cold air masses on the earthís surface. A prime examp le of ENSO induced drought is Australia, which often experiences severe lack of precipitation and increased brushfires during an ENSO event. Due to the displacement of the oceanic energy source near Indonesia in an ENSO year, there is also an increased te ndency for occurrence of drought and subsequent brushfire. Researchers have also found stronger correlation for drought in India, the Philippines, Brazil, parts of east and South Africa, the Western Pacific Basin Islands (including Hawaii), Central Americ a, and various parts of the United States. One of the most severe ENSO years to data was that of 1982-83, during which there were widespread areas of drought occurring at different stages of the event.

The other side of the coin is equally viable during an ENSO event, with many areas of the world experiencing increased precipitation and subsequent flooding. The Great Flood of 1993 was a severe example of ENSO reaching into the Northern Hemisphere. As mentioned previously, the global circulation disruptions often change the positions of the jet streams that pass over the earth. During the summer of 199 3, such a shift occurred, positioning the west to east jet stream directly over the Midwestern United States, forming a weather front convergence zone. The repositioned jet stream drew warm, wet air from the Gulf of Mexico northward, where it subsequently collided with cold, dry air masses from Canada. The result of these repeated collisions was the so-called "train-effect"óthunderstorms forming in the same place and moving over the same area over and over again without relief. The end results were billio ns of dollars in property damage and lost crops.

 Tracking and Forecasting ENSO Events

The next indicators are the sea surface temperatures that, as previously mentioned, are normally warm in the west and cooler in the eastern Pacific. Specifically, the part of the ocean 0 to 10 degrees south and 90 to 80 degrees west is monitored continually by the Tropical Atmosphere and Ocean (TAO) array of buoys. This array of buoys also takes continual measurements of the third important indicator: wind speed and direction. When the east to west winds diminish or reverse, an ENSO event is likely occurring.

While the TAO buoys supply necessary real-time data for the prediction of the severity and duration of an ENSO event, these data have not proved useful for use in computer modeling of the event in the hope of future predictions. This is largely due to the irregular nature of the ENSO events, occurring anywhere from 2 to 10 years apart. The reasons for this irregularity are not well understo od, but are hypothesized to stem from atmospheric weather of varying timescales or perhaps the global or local seasonal cycles. However, more sophisticated models are being developed that explore, with some success, the dependence on these parameters in o rder to try and reproduce the actual ENSO irregularity.

While the future ENSO events are not very predictable at present, the early detection of current events provided by the TAO array has proved to be extremely useful in the global mitigation of effects. The benefits to the economy of Peru are a prime example of the utility of these short-term forecasts. Two of the primary crops in Peru are rice and cotton, on the extremes of the precipitation requirement spectrum. On years forecasted to be drier than normal, more cotton is planted; in years forecasted as wetter, more rice is planted. The result is elimination of a lot of severe variability in the economy of Peru. Other countries with strong cl imatic correlation to ENSO have taken similar initiatives, such as Australia, Brazil, India, and some African countries. While tropical nations clearly have the most to gain due to the strength of the correlation, many other nations are taking this initia tive as well. In the United States, for instance, it has been estimated that between $200 and $300 million dollars could be saved annually by successful agricultural and disaster insurance planning based on the ENSO.

 In addition to the financial benefits, there are also great benefits to human health if the ENSO can be accurately forecast. During an ENSO event, a reas that would not normally be warm become unseasonably so, and as a result disease borne vectors can spread to areas where they might not normally reach. Diseases like malaria can then spread very rapidly through populations that are tragically unprepared, having a significant toll on human life. If these weather conditions conducive to this occurrence could be predicted, then the potentially affected peoples could be suitab ly prepared through education, vaccination, etc.

 References

• http://www.crseo.ucsb.edu/geos/13.html
• http://www.dir.ucar.edu/esig/use_tx.html
• http://www.cdc.noaa.gov/~cas/ENSO/Movie/movie.gif
• http://covis1.atmos.uiuc.edu/guide/El-Nino/thrm.html
• http://www.usatoday.com/weather/tg/wjstream/wjstream.htm
• http://www.ncr.usace.army.mil/ncrpa/fl-2.htm
• http://www.pmel.noaa.gov/toga-tao/realtime.html
• http://www.pmel.noaa.gov/toga-tao/el-nino-report.html
• http://earth.agu.org/revgeophys/battis01/node18.html
• http://www.noaa.gov/ogp/ENsoarcl2.html