Academic literature on the topic 'El Niño/La Niña Weather Patterns'

El Niño events are the most powerful of natural climate variations, rearranging weather patterns around the world and triggering countless extreme events. This chapter gives an overview of El Niño, its history and its physics, including its important effect on the Pacific jet stream. El Niño is the main source of information for the science of seasonal forecasting, which is also introduced here.

One of the most prominent aspects of our weather and climate is its variability. This variability ranges over many time and space scales, from small-scale weather phenomena such as wind gusts, localized thunderstorms, and tornadoes, to larger-scale weather features such as fronts and storms and to prolonged climate features such as droughts, floods, and fluctuations occurring on multiseasonal, multiyear, and multidecade time scales. Some examples of these longer time-scale fluctuations include abnormally hot and dry summers, abnormally cold and snowy winters, a series of abnormally mild or exceptionally severe winters, and even a mild winter followed by a severe winter. In general, the longer time-scale variations are often associated with changes in the atmospheric circulation that encompass areas far larger than a particular affected region. At times, these persistent circulation features affect vast parts of the globe, resulting in abnormal temperature and precipitation patterns in many areas. During the past several decades, scientists have discovered that important aspects of interannual variability in global weather patterns are linked to a naturally occurring phenomenon known as the El Niño / Southern Oscillation (ENSO) cycle. The heart of ENSO lies in the tropical Pacific, where there is strong coupling between variations in ocean surface temperatures and the circulation of the overlying atmosphere. The terms El Niño and La Niña represent opposite extremes of the ENSO cycle, and they cause very different rainfall outcomes, as illustrated in Figure 2-1. Before describing the oceanic and atmospheric characteristics of the ENSO cycle, it is necessary to describe the average climatic conditions and how they vary throughout the year. Interannual climate variability is often measured by comparing the observed conditions to the long-term mean conditions. The mean state of the tropical Pacific Ocean is identified by both its surface and its subsurface characteristics, each of which exhibits considerable evolution across the eastern half of the tropical Pacific during the course of the year. Throughout the year, the ocean surface is warmest in the west and coldest in the east. The largest difference between the two regions is observed during September and October, when temperatures in the eastern Pacific reach their annual minimum.

The El Nino–Southern Oscillation (ENSO) is one of the most important contributors to interannual variability on Earth (Diaz and Markgraf 2000). It is an aperiodic phenomenon that tends to reoccur within the range of 2 to 7 years, and it is manifest by the alternation of extreme warm (El Niño) and cold (La Niña) events. There is also evidence (Allen 2000) that the aperiodic ENSO phenomenon must be considered in conjunction with climate fluctuations at decadal to multidecadal time frames that may modulate ENSO’s lower frequency variability. Numerous studies show global climatic impacts associated with the ENSO phenomenon. Further, there is considerable evidence to indicate that ENSO impacts the climate of both middle and high latitudes, and a recent analysis (figure S.1, discussed below) provides a global picture of warm versus cold ENSO conditions. Consequently, it is not surprising that many LTER sites, from the Arctic to Antarctic, show evidence of ENSO-related fluctuations in environmental variables. The quasi-quintennial timescale of variability is second only to seasonal variability in driving worldwide weather patterns. Consequently, an important theme in part II is the worldwide influence of ENSO-related climate variability and the teleconnected spatial patterns of this variability. Also, a common theme for several ecosystems discussed in this section is their high sensitivity to small climatic changes that are subsequently amplified and cascaded through the system. For example, the narrow temperature threshold for an ice-to-water phase change may create a pronounced nonlinear ecosystem response to what is a relatively small temperature shift (as demonstrated for the McMurdo Dry Valleys). Or alternatively, this narrow temperature threshold may shift a sea ice–dominated ecosystem (Palmer LTER) to a more oceanic marine ecosystem by reducing the seasonality and magnitude of the sea ice habitat. Such nonlinear amplifications of small climatic changes can increase the ecological response and make it more detectable within the natural background of variability. We explore these themes here. To illustrate the global footprint of ENSO variability, composites of yearly averaged El Niño and La Niña conditions for surface air temperature (SAT) and sea surface temperature (SST, Reynolds and Smith 1994) were generated.

Almost all the studies performed during the past century have shown that drought is not the result of a single cause. Instead, it is the result of many factors varying in nature and scales. For this reason, researchers have been focusing their studies on the components of the climate system to explain a link between patterns (regional and global) of climatic variability and drought. Some drought patterns tend to recur frequently, particularly in the tropics. One such pattern is the El Niño and Southern Oscillation (ENSO). This chapter explains the main characteristics of the ENSO and its data forms, and how this phenomenon is related to the occurrence of drought in the world regions. Originally, the name El Niño was coined in the late 1800s by fishermen along the coast of Peru to refer to a seasonal invasion of south-flowing warm currents of the ocean that displaced the north-flowing cold currents in which they normally fished. 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 (Bjerknes, 1969; Wyrtki, 1975; Alexander, 1992; Trenberth, 1995; Nicholson and Kim, 1997). The recurring period of El Niño varies from two to seven years. The intensity and duration of the event vary too and are hard to predict. Typically, the duration of El Niño ranges from 14 to 22 months, but it can also be much longer or shorter. El Niño often begins early in the year and peaks in the following boreal winter. Although most El Niño events have many features in common, no two events are exactly the same. The presence of El Niño events during historical periods can be detected using climatic data interpreted from the tree ring analysis, sediment or ice cores, coral reef samples, and even historical accounts from early settlers.

Regional variations in South America’s weather and climate reflect the atmospheric circulation over the continent and adjacent oceans, involving mean climatic conditions and regular cycles, as well as their variability on timescales ranging from less than a few months to longer than a year. Rather than surveying mean climatic conditions and variability over different parts of South America, as provided by Schwerdtfeger and Landsberg (1976) and Hobbs et al. (1998), this chapter presents a physical understanding of the atmospheric phenomena and precipitation patterns that explain the continent’s weather and climate. These atmospheric phenomena are strongly affected by the topographic features and vegetation patterns over the continent, as well as by the slowly varying boundary conditions provided by the adjacent oceans. The diverse patterns of weather, climate, and climatic variability over South America, including tropical, subtropical, and midlatitude features, arise from the long meridional span of the continent, from north of the equator south to 55°S. The Andes cordillera, running continuously along the west coast of the continent, reaches elevations in excess of 4 km from the equator to about 40°S and, therefore, represents a formidable obstacle for tropospheric flow. As shown later, the Andes not only acts as a “climatic wall” with dry conditions to the west and moist conditions to the east in the subtropics (the pattern is reversed in midlatitudes), but it also fosters tropical-extratropical interactions, especially along its eastern side. The Brazilian plateau also tends to block the low-level circulation over subtropical South America. Another important feature is the large area of continental landmass at low latitudes (10°N–20°S), conducive to the development of intense convective activity that supports the world’s largest rain forest in the Amazon basin. The El Niño–Southern Oscillation phenomenon, rooted in the ocean-atmosphere system of the tropical Pacific, has a direct strong influence over most of tropical and subtropical South America. Similarly, sea surface temperature anomalies over the Atlantic Ocean have a profound impact on the climate and weather along the eastern coast of the continent. In this section we describe the long-term annual and monthly mean fields of several meteorological variables.

Agriculture is the mainstay of Kenya’s economic development and accounts for about 30% of the country’s gross domestic product, 60% of export earnings, and 70% of the labor force. This sector is the largest source of employment (Government of Kenya, 1995). More than 85% of the population survives in one way or the other on agricultural activities (crops and livestock). Agriculture in Kenya is mainly rain-fed, with little irrigation. About 46% of the rural population live below the poverty line, with 70% of them below food poverty line. Like many parts of the tropics, the majority of agricultural activities in Kenya are rain dependent. Small-scale farmers, pastoralists, and wildlife are most often affected by drought, with crops withering and livestock as well as wildlife dying. Drought of more than one season overwhelms the social fabric, as crops, livestock, wild animals, and humans die. Such droughts affect pastoral communities (e.g., the Masai in Kenya and Tanzania) by killing livestock and game animals, forcing these communities to invade the nearby towns and cities to find remnants of patches of grass still left there or grass growing at the roadsides. The death of game animals affects ecotourism. Interannual climate variability that often leads to the recurrence of climate extremes such as droughts has far-reaching impacts on agricultural production. Figure 18.1 shows below-normal rainfall during different years that are often associated with droughts in Kenya. These rainfall deficits are caused by the anomalies in the circulation patterns that can extend from local or regional to very large scales. Some patterns that are responsible for spatial and temporal distribution of rainfall in Kenya include the Intertropical Convergence Zone (ITCZ), subtropical anticyclones, monsoonal wind systems, tropical cyclones, easterly/westerly wave perturbations, subtropical jet streams, East African low-level jet stream, extratropical weather systems, teleconnection with El Niño/Southern Oscillation (ENSO), and quasi-biennial oscillation (Ogallo, 1988, 1991, 1994). In addition, complex physical features such as large inland lakes, mountains, and complex orographic patterns (e.g., the Great Rift Valley) influence rainfall patterns. Lake Victoria in western Kenya is also one of the largest freshwater lakes in the world and has its own strong circulation patterns in space and time.

The El Niño–Southern Oscillation (ENSO) is a coupled ocean–atmosphere phenomena that has a worldwide impact on climate. An aperiodic phenomena that reoccurs every 2 to 7 years, the ENSO is second only to seasonal variability in driving worldwide weather patterns. As Greenland notes in chapter 6, the term “quasi-quintennial” is chosen to recognize that climatic events other than ENSO-related events might occur at this timescale, although it is widely recognized that ENSO contributes the lion’s share of the higher frequency variability in paleorecords of the past several thousand years. In this section, we consider variability with cycles of 2 to 7 years and the resulting ecological response. Although we emphasize the ENSO timescale in this section, there is growing evidence that this phenomena is neither spatially nor temporally stable over longer time periods. Indeed, Allan (2000) suggests the ENSO climatic variability must be viewed within the context of climate fluctuations at decadal to interdecadal timescales, which often modulate the higher frequency ENSO variability. As a consequence, results in this and the next section often display overlapping patterns of variability, and their separation is not sharply defined. An important theme in this section is the worldwide influence of ENSO-related climate variability. Greenland (chapter 6) provides an LTER network overview with an analysis of ENSO-related variability of temperature and precipitation records for many LTER sites from the Arctic to the Antarctic. He discusses the general nature of ENSO and its climatic effects, summarizes previous climate-related work in the LTER network, and provides a cross-site analysis of the correlations between the Southern Oscillation Index (SOI) and temperature and precipitation at LTER sites. His results are consistent with the expected patterns of the geography of ENSO effects on the climate. Greenland’s cross-site analysis provides the basis for studying climate variability and ecosystem response within the context of the series of framework questions that form an underlying theme for this volume. Brazel and Ellis (chapter 7) provide an excellent analysis of climate-related parameters within the context of ENSO indices. Reporting on the Central Arizona and Phoenix (CAP) LTER urban-rural ecosystem, these authors provide a comprehensive analysis linking water-related parameters to climate forcing, as indicated by these indexes. Their studies show a strong connection between ENSO and winter moisture in Arizona, perhaps making it possible to forecast impending conditions.

El Niño 97-98 will be remembered as one of the strongest ever recorded (Glantz, 1999). For the first time, climate anomalies associated with the event were anticipated by scientists, and this information was communicated to the public and policy makers to prepare for the “meteorological mayhem that climatologists are predicting will beset the entire globe this winter. The source of coming chaos is El Niño . . .” (Brownlee and Tangley, 1997). Congress and government agencies reacted in varying ways, as illustrated by the headlines presented in Figure 7-1. The link between El Niño events and seasonal weather and climate anomalies across the globe are called teleconnections (Glantz and Tarlton, 1991). Typically, during an El Niño cycle hurricane frequencies in the Atlantic are depressed, the southeast United States receives more rain than usual (chapter 2), and parts of Australia, Africa, and South America experience drought. Global attention became focused on the El Niño phenomenon following the 1982-1983 event, which, at that time, had the greatest magnitude of any El Niño observed in more than a century. After El Niño 82-83, many seasonal anomalies that had occurred during its two years were attributed, rightly or wrongly, to its influence on the atmosphere. As a consequence of the event, societies around the world experienced both costs and benefits (Glantz et al., 1987). Another lasting consequence of the 1982-1983 event was an increase in research into the phenomenon. One result of this research in the late 1990s has been the production of forecasts of El Niño (and La Niña) events and the seasonal climate anomalies associated with them. This chapter discusses the use of climate forecasts by policy makers, drawing on experiences from El Niño 97-98, which replaced the 1982-1983 eventas the” climate event of the century.” The purpose of this chapter is to draw lessons from the use of El Niño -based climate forecasts during the 1997-1998 event in order to improve the future production, delivery, and use of climate predictions. This chapter focuses on examples of federal, state, and local responses in California, Florida, and Colorado to illustrate the lessons.

Ecological disturbances at Long-Term Ecological Research (LTER) sites are often the result of extreme meteorological events. Among the events of significance are tropical storms, including hurricanes, and extratropical cyclones. Extratropical storms are low-pressure systems of the middle and high latitudes with their attendant cold and warm fronts. These fronts are associated with strong, horizontal thermal gradients in surface temperatures, strong winds, and a vigorous jet stream aloft. These storms and their attendant fronts generate most of the annual precipitation in the continental United States and provide the lifting mechanisms for thunderstorms that, on occasion, spawn tornadoes. Off the United States West and East Coasts, extratropical storms generate winds, wind waves, wind tides, and long-shore currents that rework coastal sediments, alter landscape morphology, and change the regional patterns of coastal erosion and accretion (Dolan et al. 1988). Although extratropical storms do not match hurricanes in either precipitation intensity or in the strength of the winds generated, they are much larger in size and have a more extensive geographic impact. On occasion, extratropical storms will intensify at an extraordinary rate of 1 millibar (mb) per hour for 24 hours or more. Such storms are classed as “bomb” and are comparable to hurricanes. Extratropical storms occur in all months of the year but are most frequent and more intense in winter when the north-south temperature contrast is large and dynamic support for storm intensification from the stronger jet stream aloft is great. In this chapter, we will explore the history of storminess for those LTER sites in the continental United States at which more than a century of data on storms and their storm tracks are readily available. Specifically, we will look at the record of changes in storminess at both the regional and national scales. During the 1990s, significant storms along the U.S. West Coast and droughts and fires in Florida in an El Niño year led to a hypothesis that El Niño and La Niña conditions were associated with a modulation in the frequency of storms. In addition, it has been suggested that the frequency of El Niño and La Niña events and, by inference, storminess, has increased during the past century.