Sunday, December 21, 2014

National Drought Mitigation Center

ENSO and Drought Forecasting

El Niño Defined

Every two to seven years off the western coast of South America, ocean currents and winds shift, causing water temperatures to warm and displacing the nutrient-rich cold water that normally wells up from deep in the ocean. The invasion of warm water disrupts both the marine food chain and the economies of coastal communities that are based on fishing and related industries. Because the phenomenon peaks around the Christmas season, the fishermen who first observed it named it El Niño (“the Christ Child”). In recent decades, scientists have recognized that El Niño is linked with other shifts in global weather patterns. (For a colorful look at El Niño with real-time graphics, check out NOAA’s El Niño Theme Page.)

Frequency

The return period of the El Niño event varies from 2 to 7 years. The intensity and duration of the event are also varied and hard to predict. Typically, it lasts anywhere from 14 to 22 months, but it can be much longer or shorter. El Niño often begins early in the year and peaks between the following November and January, but no two events behave in the same way. El Niño is by no means a new phenomenon, and researchers are still working to determine whether global warming would intensify or otherwise affect El Niño. Evidence of the events goes back hundreds of years. This “proxy”, or indirect, climatic data can be found in the form of tree ring analysis, sediment or ice cores, coral reef samples, and even historical accounts from early settlers.

El Niño and the Southern Oscillation

The Southern Oscillation, a “seesaw of atmospheric pressure between the eastern equatorial Pacific and Indo–Australian areas” (Glantz et al., 1991), is closely linked with El Niño. During an El Niño–Southern Oscillation (ENSO) event, the Southern Oscillation is reversed. Generally, when pressure is high over the Pacific Ocean, it tends to be low in the eastern Indian Ocean, and vice versa (Maunder, 1992). It is measured by gauging sea-level pressure in the east (at Tahiti) and west (at Darwin, Australia) and calculating the difference. This is then put into an index called the Southern Oscillation Index (SOI) or Tahiti–Darwin Index. High negative values of the SOI represent an El Niño, or “warm event”. ENSO events are those in which a Southern Oscillation extreme and an El Niño occur together. El Niño and Southern Oscillation often occur together, but also happen separately. The Climate Prediction Center shows a plot of the SOI.

High positive values of the SOI indicate a La Niña, or “cold event”. La Niña is the counterpart of El Niño and represents the other extreme of the ENSO cycle. In this event, the sea surface temperatures in the equatorial Pacific drop well below normal levels and advect to the west while the trade winds are unusually intense rather than weak. La Niña years often (but not always) follow El Niño years. A listing of El Niño and La Niña years since the turn of the century is found here.

ENSO is perhaps a better term than El Niño for purposes of understanding global weather patterns, as it turns out that the shifts in sea surface temperatures (SSTs) off the west coast of South America are just one part of the coupled interactions of atmosphere, oceans, and land masses. The term Southern Oscillation refers to the atmospheric component of the relationship, and El Niño represents the oceanic property in which sea surface temperatures are the main factor.

Teleconnections

ENSO occurrences are global climate events that are linked to various climatic anomalies. Not all anomalies, even in ENSO years, are due to ENSO. In fact, statistical evidence shows that ENSO can account at most for about 50% of the interannual rainfall variance in eastern and southern Africa (Ogallo, 1994), but many of the more extreme anomalies, such as severe droughts, flooding, and hurricanes, have strong teleconnections to ENSO events. Teleconnections are defined as atmospheric interactions between widely separated regions (Glantz, 1994). Many researchers are studying the relationships between ENSO (and La Niña) events and weather anomalies around the globe to determine whether links exist. Understanding these teleconnections can help in forecasting droughts, floods, and tropical storms (hurricanes).

Estimates of the economic impacts of the 1982–83 El Niño, perhaps the strongest event in recorded history, conservatively exceeded $8 billion worldwide, from droughts, fires, flooding, and hurricanes (NOAA, 1994). Some 1,000–2,000 deaths have been blamed on the event and the disasters that accompanied it. Virtually every continent felt the impacts of this strong event. Incidentally, the extreme drought in the Midwest Corn Belt of the United States during 1988 has been inconclusively linked to the “cold event”, or La Niña, of 1988 that followed the ENSO event of 1986–87.

ENSO and U.S. Droughts

In North America, particularly the United States, the impacts of El Niño are most dramatic in the winter. El Niño produces winters that are generally mild in the northeast and central United States and wet over the south from Florida to Texas. Alaska and the northwestern regions of Canada and the United States can be abnormally warm. This might be a result of the forcing caused by a Pacific–North American (PNA) pattern that is typified by a high pressure ridge over northwestern North America and a low pressure trough in the southeastern United States. This serves as an upper-level steering mechanism for moisture and temperature at the surface. Once the pattern is entrenched, regions under the ridge can expect little in the way of precipitation while those in the trough can’t turn it off and are prone to frequent flooding.

Ropelewski and Halpert (1986) studied North American precipitation and temperature patterns associated with ENSO conditions and concluded the following. In the Great Basin area of the western United States, above-normal precipitation was recorded during ENSO years in 81% of the cases for the “season” that runs from April to October. In the southeastern United States and northern Mexico, above-normal precipitation was also recorded for 81% of the cases for the “season” that began in October of the ENSO year and concluded in March of the following year. For temperature anomalies during ENSO conditions in North America, the U.S. Pacific Northwest, western Canada, and parts of Alaska showed warmer temperatures in 81% of the years while the southeastern United States showed below-normal temperatures around 80% of the time. This would seem consistent with a typical PNA atmospheric pattern. During stronger events, the United States experiences flooding and severe storms in some regions and droughts and heat waves in other areas. Hurricane activity is usually minimal in the Atlantic Ocean, sparing the coastal areas from the Gulf of Mexico to the northeast. In the coastal west, the displacement of the jet stream can bring abnormally large amounts of rain and flooding to California, Oregon, and Washington. During the summer, heat waves and below-normal precipitation bring drought, crop failures, and even death. U.S. crop losses from the 1982–83 El Niño were projected to be in the neighborhood of $10–12 billion (Wilhite et al., 1987).

ENSO and Drought Around the World

During an ENSO event, drought can occur virtually anywhere in the world, although researchers have found the strongest connections between ENSO and intense drought in Australia, India, Indonesia, the Philippines, Brazil, parts of east and south Africa, the western Pacific basin islands (including Hawaii), Central America, and various parts of the United States. Drought occurs in each of the above regions at different times (seasons) during an event and in varying degrees of magnitude.

Ropelewski and Halpert also looked at the link between ENSO events and regional precipitation patterns around the globe (1987). Northeastern South America from Brazil up to Venezuela shows one of the strongest relationships. In 17 ENSO events, this region had 16 dry episodes. It is not uncommon to find the rain forests burning during these dry periods. Other areas from their study also showed a strong tendency to be dry during ENSO events. In the Pacific basin, Indonesia, Fiji, Micronesia, and Hawaii are usually prone to drought during an event. Virtually all of Australia is subjected to abnormally dry conditions during ENSO events, but the eastern half has been especially prone to extreme drought. This is usually followed by bush fires and a decimation of crops. India has also been subjected to drought through a suppression of the summer monsoon season that seems to coincide with ENSO events in many cases. Eastern and southern Africa also showed a strong correlation between ENSO events and a lack of rainfall that brings on drought in the Horn region and areas south of there. Another region they found to be abnormally dry during warm events was Central America and the Caribbean Islands.

Thus, ENSO events seem to have a stronger influence on regions in the lower latitudes, especially in the equatorial Pacific and bordering tropical areas. The relationships in the mid-latitudes aren’t as pronounced, nor are they as consistent in the way wet or dry weather patterns are influenced by El Niño. The intensity of the anomalies in these regions is also more inconsistent than those of the lower latitudes. NOAA’s Climate Prediction Center has short papers on the typical impacts associated with ENSO and La Niña episodes.

Can We Predict ENSO?

If we can understand some of the teleconnections discussed above, it can lead us to some general predictive capabilities via numeric computer models that can help us determine and conclude when conditions are favorable for the onset of an event. Numeric models try to emulate processes (and dynamic relationships) that occur in nature using sets of numbers and equations. But once an event is underway, forecasting its duration and intensity are difficult at best.

In Ropelewski and Halpert’s (1987) study on global precipitation patterns and ENSO events, they found that the consistency and magnitude of the precipitation relationships to ENSO events could serve as a practical utility for forecasting precipitation in certain regions (and seasons) once it was determined that an event was in progress. This can serve as a broad-brush approach for given regions, with the understanding that expanses within any given area will not behave in the exact same manner from event to event.

NOAA has established and now operates an array of moored buoys in the equatorial Pacific Ocean. These buoys measure temperature, currents, and winds in this region on a daily basis. The data is available to scientists around the world in real time, enabling them to use the data for both research and forecasting. This network is very valuable in that the first stages of an ENSO event occur in this region. By monitoring data from past episodes and the data from the months leading up to an episode, scientists can use numerical models (similar to but not as reliable as those used in weather forecasting) to help them predict and/or simulate ENSO events. The predictive models are becoming more sophisticated and more effective in many respects thanks in part to the expanded data sets that are available for the equatorial Pacific region. The dynamic coupled nature of the new models has allowed for prediction of ENSO events a year or more in advance.

ENSO forecasts help countries anticipate and mitigate droughts and floods, and are very useful in agricultural planning. Countries that are in latitudes with strong El Niño connections to weather patterns, such as Brazil, Australia, India, Peru, and various African nations, use predictions of near-normal conditions, weak El Niño conditions, strong El Niño conditions, or a La Niña to help agricultural producers select crops most likely to be successful in the coming growing season. In countries or regions with a Famine Early Warning System (FEWS) in place, ENSO forecasts can play a key role in mitigating the impacts of flood or drought that can lead to famine. Famine, like drought, is a slow-onset disaster, so forewarning may enable countries to greatly reduce, if not eliminate, its worst impacts.

ENSO advisories are used to a lesser extent in planning in North America and other extratropical countries, because the links between ENSO and weather patterns are less clear in these areas. As prediction models improve, the role of ENSO advisories in planning in mid-latitude countries will increase. The Climate Prediction Center is responsible for issuing ENSO advisories. For the latest information on the status of ENSO, go to the ENSO Diagnostic Advisory.

References

Glantz, M.(ed.) 1994. Usable Science: Food Security, Early Warning, and El Niño. Proceedings of the Workshop on ENSO/FEWS, Budapest, Hungary, October 1993. UNEP, Nairobi; and NCAR, Boulder, Colorado.

Glantz, M.; R. Katz; and N. Nicholls (eds.). 1991. Teleconnections Linking Worldwide Climate Anomalies. Cambridge University Press, Cambridge.

Glantz, M.; R. Katz; and M. Krenz (eds.). 1987. Climate Crisis. UNEP, Nairobi; and NCAR, Boulder, Colorado.

Maunder, W.J. 1992. Dictionary of Global Climate Change. Chapman and Hall, New York.

NOAA. 1994. El Niño and Climate Prediction—Reports to the Nation on Our Changing Planet. A publication of the University Corporation for Atmospheric Research pursuant to National Oceanic and Atmospheric Administration Award No. NA27GP0232–01. UCAR, Boulder, Colorado.

Ogallo, L.A. 1994. Validity of the ENSO-Related Impacts in Eastern and Southern Africa. In M. Glantz (ed.). Usable Science: Food Security, Early Warning, and El Niño, pp. 179–184. Proceedings of the Workshop on ENSO/FEWS, Budapest, Hungary, October 1993. UNEP, Nairobi; and NCAR, Boulder, Colorado.

Philander, S.G. 1990. El Niño, La Niña, and the Southern Oscillation. Academic Press, San Diego, California.

Ropelewski, C.F.; and M.S. Halpert. 1987. Global and regional scale precipitation patterns associated with the El Niño–Southern Oscillation. Monthly Weather Review 115:1606–1626.

Ropelewski, C.F.; and M.S. Halpert. 1986. North American precipitation and temperature patterns associated with the El Niño–Southern Oscillation (ENSO). Monthly Weather Review 114:2352–2362.

Wilhite, D.A.; D.A. Wood; and S.J. Meyer. 1987. Climate-related impacts in the United States during the 1982–83 El Niño. In M. Glantz, R. Katz, and M. Krenz (eds.). Climate Crisis, pp. 75–78. UNEP, Nairobi; and NCAR, Boulder, Colorado.

For more information, please contact Mark Svoboda, NDMC Climate/Water Resources Specialist.

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