ENSO and Droughts

The ENSO events have worldwide consequences. Many researchers have studied the relationships between ENSO events and weather anomalies around the globe to determine whether links between ENSO events and droughts exist. ENSO events perturb the atmosphere around the tropics to varying degrees, generating anomalous climatic patterns at the regional and local levels. As a result, possible connections between an ENSO event and, for example, drought in northeast Brazil, Australia, and Africa can be identified (figure 3.2).

Not all anomalies, even in ENSO years, are due to ENSO phenomenon. In fact, statistical evidence shows that ENSO accounts for only a small fraction of the interannual variance in rainfall. In many ENSO-affected places such as eastern and southern Africa, the ENSO accounts at most for about 50% of the variance in rainfall (Ogallo, 1994), but many of the more extreme anomalies, such as severe droughts, flooding and hurricanes, have strong teleconnections to ENSO events. (A "teleconnection" refers to a link between two events [e.g., ENSO, drought, flood] that are separated by large distances.)

Warm

Warm

Warm

Warm

Wet and cool

Wet_Dry

Warm ami warm

Warm

Figure 3.2 The main world regions impacted by the warm phase or ENSO (from Ropelewski and Halpert [1987]).

The 1982-83 El Niño was one of the strongest events in the recorded history. Virtually every continent was affected by this event. Some 10002000 deaths were blamed on the event and the disasters that accompanied it. The extreme drought in the Midwest Corn Belt of the United States during 1988 has been linked to the "cold event" of 1988 that followed the ENSO event of 1986-87 (Reibsame et al., 1990).

During an ENSO event, drought can occur virtually anywhere in the world. First, the shift of the deep convection areas to the east over the Pacific Ocean brings dry spells over much of the western Pacific countries, and then the global atmospheric anomalies (due to ENSO) induce droughts in places far from the Pacific Basin. The strongest connections between ENSO and intense drought can be found in Australia, India, Indonesia, the Philippines, parts of east and south Africa, the Western Pacific Basin islands (including Hawaii), Central America, and various parts of the United States.

Droughts occur due to below-normal precipitation, and a strong relationship exists between ENSO events and regional precipitation patterns around the globe (Ropelewski and Halpert, 1987). In northeastern South America, from Brazil up to Venezuela, El Niño brings heavy rains over Peruvian coast of South America. In addition, ENSO events affect regions in the lower latitudes, especially in the equatorial Pacific bordering tropical areas. The relationships in the mid-latitudes do not seem to be as pronounced or consistent. El Niño affects regional precipitation and results in wet or dry seasons. Indirectly, it produces significant impacts including agricultural droughts. For instance, during the 1998 El Niño, crop shortfalls from the worst drought ever recorded in Indonesia forced the government to import an unprecedented 6 million tons of rice to stave off social instability in the country. The Philippines, affected by the same drought, had to import more than 2 million tons of rice to meet the demand. Some agricultural experts predicted that rice shortages would increase in Asia and threaten regional stability. But by early 1999, the La Niña event brought heavy rainfall to much of Southeast Asia, and rice harvests in Indonesia and the Philippines recovered. Indonesia, the world's largest rice importer, bought less than half as much rice as it imported in 1998, while the Philippines enjoyed its highest rice stocks in 15 years. Major rice exporters such as Thailand and Vietnam reaped windfall profits from the 1997-98 El Niño as the export price of rice nearly doubled. But Thailand's rice exports declined in 1999, and rice prices plummeted. Global rice trade, normally averaging 18-20 million tons per year, expanded to nearly 27 million tons in 1998 (U.S. Department of Agriculture, 1999).

ENSO and Drought in Oceania

Australia and Oceania are severely affected by the lack of rains attributed to the warm phase of ENSO (Stone and Auliciems, 1992). Usually, El Niño events result in reduced rainfall across eastern and northern Australia, particularly during winter, spring, and early summer. However, the precise nature of the impact differs markedly from one event to another, even with similar changes and patterns in the Pacific Ocean. For example, the 1982-83 and 1997-98 events were both very strong, but their impacts in Australia were completely different. Eastern and southern Australia were gripped by severe droughts during 1982-83, while southern Australia experienced heat-wave conditions and bushfires, and virtually two-third of eastern Australia recorded extremely low rainfall. But in 1997, average or above-average rainfalls were common in May, and a dry spell over winter was broken by widespread, heavy rains in September.

In Papua New Guinea, the largest developing country in the southwest Pacific region, El Niño has been linked to the severe droughts of 1896, 1902,1914, 1972, and 1982 (Barr et al., 2001). During the 1997-98 El Niño, the lives of about 80,000-300,000 people were at risk due to the prolonged droughts, and rice cultivation was severely affected, forcing the government to import rice to insure the food availability. The1997-98 El Niño also caused drought in Fiji Islands, reducing sugarcane production by 50% (Kaloumaira et al., 2001).

ENSO and Drought in Southeast Asia

The ENSO produces a profound impact on climate and weather in southeast Asia (Indonesia, Brunei, Malaysia, Philippines, Vietnam, Myanmar, Cambodia, Laos, and Thailand) by causing large-scale changes in the atmospheric circulation in the region (Sirabaha and Caesar, 2000). In the Philippines, for instance, El Niño significantly impacts the main rainfall patterns. Due to the 1997-98 event, a dry spell with severe drought affected

68% of the country, compared to only 28% in 1972 and 16% in 1982. Water supply reduction affected thousands of hectares of rice and cornfields, bringing the agricultural output to the lowest level in 20 years.

In Vietnam, drought occurred in 1998, causing a total economic loss of about 5,000 billion Vietnamese dong. The lack of water increased the area affected by the saltwater intrusion in the Mekong Delta, one of the main agricultural areas of Vietnam. As a result, more than 4000 people almost starved in the mountain and central regions of Vietnam.

ENSO and Drought in Latin America

In Latin America, the onset of an El Niño event is associated with heavy rains along the Pacific coast because onshore winds carry more moisture as a result of warmer oceans, and with droughts in northeast Brazil (Ro-pelewski and Halpert, 1987; Grimm et al., 2000). In the 19th century, droughts during the 1877-79 period caused a famine in which 500,000 people may have died, and many others migrated. During 1982-83, yields of crops in this region of Brazil fell by more than 50%, and 28 million people and more than 1400 municipalities were affected (Rebello, 2000).

El Niño also causes droughts in other regions of Latin America including Colombia, Venezuela, and Mexico (Magaña and Quintanar, 1997; Poveda and Mesa, 1997). The drying up of reservoirs frequently disrupts hydroelectric energy supplies and drinking water supplies for cities such as Bogota, Colombia. Reduced rainfall from June to September hurt crops in an area stretching south from Mexico through Central America to Colombia and east to the Caribbean and northern Brazil.

ENSO and Droughts in Africa

In southern Africa, the frequency of drought is on the rise. Between 1988 and 1992, more than 15 drought events affected at least 1% of the population of this continent, compared to fewer than 5 such events between 1963 and 1967. This trend can be tied in part to the increased population growth and cultivation of marginal lands, and to some extent to the ENSO-related anomalies. Cane et al. (1994) noted a strong statistical relationship between Zimbabwean maize yields and sea surface temperatures in the equatorial Pacific. These studies generally show that, during ENSO episodes, large areas of southern Africa tend to experience drier than normal conditions. Between 1875 and 1978, there were 24 ENSO events, 17 of which corresponded to a decline in rainfall at least by 10% from the long-term median over this area (Rasmussen, 1987). Looking specifically at Zimbabwe's rainfall records for the 20th century, the trend indicates a clear increase in the number of below-average rainfall years and, since the 1960s, a more severe decline in rainfall in such years (Glantz et al., 1997). Figure 3.3 shows Zimbabwe's rainfall record from 1980 to 1992. It should be noted that water is the major limiting factor in Zimbabwe's

» Annual Rainfall - Long Term Mean

Figure 3.3 Zimbabwe's yearly average rainfall records (from Glantz et al., 1997).

» Annual Rainfall - Long Term Mean

Figure 3.3 Zimbabwe's yearly average rainfall records (from Glantz et al., 1997).

agricultural production, given that its climate is fairly dry. In any given year, rainfall is generally not plentiful enough to allow adequate crop production throughout the country (Bratton, 1987).

Other regions in Africa have significant connections between ENSO and droughts. In Ethiopia, the major drought occurred following an El Niño that decreased the main June—September rainfall but boosted the small February—March rainfall. Drought is the dominant ENSO-related disaster in Ethiopia, which has led to the deaths of many people and animals during 1957-58, 1964-65, 1972-73, and 1983-84 (Tsegay et al., 2001).

ENSO and Droughts in the United States

In North America, particularly the United States, the impacts of ENSO are most dramatic in the winter. The variability induced by ENSO typically brings drought or rainy conditions to specific regions (Ropelewski and Halpert, 1986). In the Great Basin area of the western United States, during ENSO years, above-normal precipitation was recorded for the April-October season for 81% of the years. For the same percentage of the ENSO years in the southeastern United States and northern Mexico, above-normal precipitation was recorded during March—October. 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 deaths. U.S. crop losses from the 1982-83 El Niño were projected to be in the neighborhood of $10-12 billion.

In the Southwest, El Niño years have a fairly consistent pattern of increased rainfall, with drought during La Niña. These impacts are also felt further inland, but the extent and magnitude is much more variable outside the Southwest.

Although El Niño events appear to have brought summer drought to the southeastern United States in the past 40-50 years, this pattern was not seen (and even reversed) during the late 19th and early 20th centuries. In fact, there seem to be substantial decadal changes in the impacts of ENSO on drought in the United States over the past 150 years (Cole et al., 1998).

ENSO-Related Data

An ENSO event usually continues for 12-18 months. If we could use ENSO-related data to monitor the development of an ENSO, it would be possible to predict regional precipitation and hence drought. Usually researchers make use of single "descriptor indices" to monitor an ENSO event. These indices are simple, easy to handle, and they capture relevant information about the events.

There are two main groups of ENSO indices: simple and composite. The simple indices are derived from single properties of either the atmosphere or the ocean. The most popular of such indices is the SOI, as defined earlier in this chapter. Surface temperature anomalies at different regions can be used to define these indices. Toward this end, the tropical Pacific has been divided into a number of regions such as Niño 1, 2, 3, 4, and 3.4 (which encompasses part of both regions 3 and 4). Niño 1 is the area defined by 80°-90° W and 5°-10° S, Niño 2 by 80°-90° W and 0°-5° S, Niño 3 by 90°-150° W and 5° N-5° S, Niño 4 by 150° W-160° E and 5° N-5° S, and Niño 3.4 by 120° W-170° W and 5° N-5° S (figure 3.4).

In addition, some research or monitoring centers all over the world use other more specific indices such as the outward long-wave radiation (OLR) to monitor the shift of the deep convection over the western Pacific, the zonal wind component at 850 Hpa level as a measure of the intensity of the trade winds, and the temperatures of the subsurface ocean layers. Simple indices may fail to capture a coupled ocean—atmosphere event, and their selection could lead to wrong results.

The composite (or coupled) indices combine characteristics of the ocean and atmosphere. The most successful index in this group is the multivari-ate ENSO index (MEI). The MEI is a composite index using sea-surface temperatures, surface air temperatures, sea-level pressure, zonal east-west surface wind, meridional north-south surface wind, and the total amount of cloudiness over the tropical Pacific. Positive MEI values are related to warm-phase or El Niño events and negative values to the cool-phase or La Niña events. The MEI can be understood as a weighted average of the main ENSO features, and it is calculated as the first unrotated principal component (PC) of all six observed fields combined. This is accomplished by normalizing the total variance of each field first and then performing the extraction of the first PC on the variance matrix of the combined fields

(Wolter and Timlin, 1993). To keep the MEI comparable, all seasonal values are standardized with respect to each season and to the 1950-93 reference period. Further information about MEI and data sets can be found on the NOAA Web site (http://www.cdc.noaa.gov/ENSO/enso.meLindex. html; http://www.cdc.noaa.gov/~kew/MEl/table.html).

Recently, the Cuban Meteorological Service began to use a composite ENSO index, IE:

where MSSTA is the three-month moving average of the sea surface temperature anomaly for El Niño 3 region, MSOI is the three-month moving average of the SOI index, and S is 1 or —1 depending on if the MSOI is positive or negative, respectively. Further information on the IE is available on a Web site (http://www.met.inf.cu). This composite index has proved better for the ENSO impact studies for Cuba than the traditional single indices (Cardenas and Naranjo, 1999).

By monitoring data for past events and the data for the months leading up to an event, scientists can use numerical models to help predict and/or simulate ENSO events. The dynamic coupled nature of the new models has allowed for prediction of ENSO events a year or more in advance. 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 countries. As prediction models improve, the role of ENSO advisories for planning in the mid-latitude countries will increase.

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