El Nio La Nia and the Southern Oscillation

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ENSO is probably the most widely recognized low-frequency cyclic event associated with atmospheric circulation. This phenomenon is a coupling of the atmosphere and ocean with a non-periodic recurrence of 2-5 or 3-7 years that has been suggested as the strongest source of natural variability in the Earth's climate system (Lockwood, 2001). It is particularly relevant for hydroclimate because it highlights the link between the climate system and the hydrologic cycle and the recurrence of floods and drought and other extreme conditions (Rajagopalan et al., 2000). In simple terms, El Niño is an oceanic circulation component consisting of a huge pool of anomalously warm water along the equator in the eastern Pacific Ocean. La Niña is another oceanic component identifiable as anomalously cool SSTs in the equatorial Pacific Ocean. The Southern Oscillation is an alteration of the trade wind circulation resulting in a reversal of flow over the tropical ocean. Typical ENSO events tend to develop during summer to early fall, mature during winter, and terminate the following spring (Deser and Wallace, 1990). In the following discussion, ENSO is used to refer to the general system that comprises both the El Niño warm phase and the La Niña cold episodes that together constitute SST extremes in the Central Pacific.

El Niño originated in reference to the local occurrence of a warm water current off the coast of Peru and Equador around Christmas and lasting for one to two months. Eventually, scientists began using the term El Niño in a broader context for describing the appearance of abnormally warm water across the entire equatorial Pacific lasting for a year or longer. El Niño events are so important in the year-to-year weather variability that signs of a developing event are routinely incorporated into long-range seasonal weather forecasts as well as in regional agricultural strategies.

8.6.1 ENSO development

The normal distribution of SSTs across the tropical Pacific Ocean includes a warm region in the western Pacific, mostly to the west of the International Date Line (IDL). Temperatures over most of this area exceed 28 °C. Further east, the ocean becomes colder and colder until the temperature along the coast of South America is less than 23 °C. The cold coastal water is due to upwelling that is especially active along the west coast of South America. Ocean temperatures decrease at higher latitudes along the coast. This climate state of the ocean is determined primarily by the stress placed on the ocean surface by winds that blow east to west (Fig. 8.3). Cold water is advected westward along the equator. The easterly winds weaken considerably as they approach the IDL and the waters of the western Pacific remain warm year-round.

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Fig. 8.3. Schematic illustration of normal conditions of sea surface temperatures and atmospheric circulation in the tropical Pacific Ocean. (Drawing courtesy of NOAA and the Pacific Marine Environmental Laboratory from their website at http://www. pmel.noaa.gov/.)
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Fig. 8.4. Schematic illustration of sea surface temperatures and atmospheric circulation in the tropical Pacific Ocean during an El Niño event. (Drawing courtesy of NOAA and the Pacific Marine Environmental Laboratory from their website at http:// www.pmel.noaa.gov/.)

Fig. 8.4. Schematic illustration of sea surface temperatures and atmospheric circulation in the tropical Pacific Ocean during an El Niño event. (Drawing courtesy of NOAA and the Pacific Marine Environmental Laboratory from their website at http:// www.pmel.noaa.gov/.)

The expansion of the area of warm water in the equatorial Pacific is now the indicator for an El Niño event. The anomalously warm water for 1982 was the largest warming in the last 100 years. The expansion of warm water is related to a weakening of the easterly flow of the atmosphere and a strengthening of westerly surface winds in response to changes in atmospheric circulation, ocean currents, and SST changes (Fig. 8.4). Warm water normally driven by the trade winds toward the western tropical Pacific drifts slowly eastward and may go as far poleward as British Columbia and central Chile. The ocean water drift is slow and may take several months to reach the west coasts of North and South

Fig. 8.5. Schematic illustration of sea surface temperatures and atmospheric circulation in the tropical Pacific Ocean during a La Niña event. (Drawing courtesy of NOAA and the Pacific Marine Environmental Laboratory from their website at http:// www.pmel.noaa.gov/.)

Fig. 8.5. Schematic illustration of sea surface temperatures and atmospheric circulation in the tropical Pacific Ocean during a La Niña event. (Drawing courtesy of NOAA and the Pacific Marine Environmental Laboratory from their website at http:// www.pmel.noaa.gov/.)

America. The tropical Pacific thermocline rises in the west and sinks in the east effectively cutting off upwelling along the coast of South America.

At times, an El Niño is followed by abnormally cold water along the coast of Peru and the equatorial Pacific. This cold water and the accompanying SST anomalies are basically opposite those observed during El Niño and they signal a La Niña event. La Niña begins with a period of unusually strong trade winds and vigorous upwelling over the eastern tropical Pacific (Fig. 8.5). Extremes of weather are often the opposite of those observed during El Niño. The atmosphere over the central equatorial Pacific tends to be less cloudy with less rainfall when the SSTs are colder than normal.

The Southern Oscillation is the surface pressure and atmospheric circulation component related to the warming of the equatorial Pacific Ocean that occurs with an El Niño event. The association between the warm water and an altered atmospheric circulation was first recognized in the 1950s, but variations in air pressure between the eastern and western tropical Pacific were recognized as early as the 1890s (Walker, 1923). The common depiction of the Southern Oscillation as an interannual fluctuation of atmospheric conditions over the tropical Pacific and Indian oceans that results in a reversal of winds over the equatorial Pacific Ocean obscures a complex interaction of processes. A significant facet of this circulation is a meridional component that supports a latitudinal transfer of energy and mass accounting for the ability of the tropical Southern Oscillation to have worldwide ramifications.

It remains unclear whether the ocean or atmosphere initiates the conditions that become an ENSO event. In the south equatorial Pacific, the normal atmospheric circulation mode is the longitudinal Hadley cell. However, near the coast of South America the winds blow offshore and result in an upwelling of water that is 5 °C colder than waters in the western Pacific. Air is stabilized by the cold water and cannot rise and join the normal ascending motion of the Hadley cell circulation. The stable air flows westward between the Hadley cell circulation of the two hemispheres forming the southeast trade winds across the South Pacific (see Fig. 8.3). After gaining heat and moisture as it crosses the ocean, the air ascends over the western Pacific, sea-level pressure is low, and rainfall is plentiful. Some of the rising air flows eastward to complete a cell of zonal atmospheric circulation known as the Walker circulation (Bjerknes, 1969). The atmosphere over the eastern tropical Pacific at this time is cold and dry.

At intervals of 1 to 5 years, the Walker circulation weakens and reverses direction and the Hadley circulation intensifies. The trade winds are relaxed, the zone of warm surface water and heavy precipitation shifts eastward, and sea-level pressure rises in the west while it falls in the east (see Fig. 8.4). The eastward shift of the region of heavy precipitation brings plentiful rainfall to the eastern tropical Pacific while inflicting drought on northern Australia and parts of southeastern Asia and Indonesia. The related seesaw variation in atmospheric mass and air pressure between the eastern and western tropical Pacific is known as the Southern Oscillation. This variation in atmospheric conditions is quantified by the Southern Oscillation Index (SOI), which is based on the difference in surface air pressure over Tahiti Island (17° S, 149° W) and Darwin, Australia (12° S, 131° E), compared to normal conditions. When the difference in pressure at these two stations is high the SOI is persistently negative, an El Niño event is in progress, and the low-pressure area near Darwin has moved eastward toward Tahiti. La Niña events are associated with an opposite array of conditions (see Fig. 8.5). The SOI is persistently positive, and atmospheric pressure is low in the western Pacific and high in the central Pacific near Tahiti (McCabe and Dettinger, 1999).

The Multivariate ENSO Index (MEI) evaluates both atmospheric and oceanic conditions to quantify ENSO events. A comprehensive data set incorporates air temperature, SSTs, sea-level pressure, zonal and meridional surface winds, and cloudiness to define the coupled ocean-atmosphere state. Spatial filtering, principal component analysis, and standardization are used to keep the MEI comparable (Wolter and Timlin, 1998). Positive MEI values represent El Niño events, and MEIs greater than 1 are significant events (Fig. 8.6). Negative MEIs indicate La Niña events and MEIs below —1 correspond to significant events. The MEI was developed for research purposes, but it has broad appeal because it specifically addresses the coupled ocean-atmosphere phenomenon and incorporates more information than other indices. Kiem and Franks (2001)

1950 1960

1970 1980 Date

1990 2000

Fig. 8.6. The monthly Multivariate ENSO Index (MEI) for January 1950 to August 2006 plotted as standardized departures from the 1950-93 reference period. (Data courtesy of NOAA and the Earth System Research Laboratory from their website at http://www.cdc.noaa.gov/people/klaus.wolter/MEI/mei.html.)

1950 1960

1970 1980 Date

1990 2000

Fig. 8.6. The monthly Multivariate ENSO Index (MEI) for January 1950 to August 2006 plotted as standardized departures from the 1950-93 reference period. (Data courtesy of NOAA and the Earth System Research Laboratory from their website at http://www.cdc.noaa.gov/people/klaus.wolter/MEI/mei.html.)

demonstrate that eastern Australia rainfall and runoff are better estimated using the MEI compared to estimates using other ENSO indices.

8.6.2 Global ENSO impact

ENSO is part of a global-scale variability in climate that occurs periodically, but not regularly enough to be predicted reliably. The global consequences can be traced to shifts in tropical rainfall which affect global wind patterns. The ITCZ in the Pacific is at about 8° N to 15° N. The South Pacific Convergence Zone (SPCZ) is a somewhat broader region of heavy rains extending southeastward from New Guinea towards the Polynesian region. The SPCZ is interrupted on its eastern margin by the relatively cold SSTs of the South Pacific dry zone, which is air influenced by the Humboldt Current flowing equatorward along the west coast of South America. Both convergence zones exist throughout the year, but they are notably stronger during their respective summer seasons. During warm ENSO events, both convergence zones shift equatorward and appear to become merged over their western portions near the IDL. The merged state of the convergence zones results in wetter than normal conditions along the equator with anomalously dry conditions in their usual positions. Drier than normal conditions further west affect a broad region of the tropics bordering the eastern Indian Ocean, such as Australia, Indonesia, and south Asia (Trenberth et al., 2002). Dense tropical rain clouds distort atmospheric flow aloft much as rocks distort the flow of a river, but on a horizontal scale of thousands of miles. Waves in the atmosphere determine the positions of monsoons, jet streams, and storm tracks. Rainfall areas over Indonesia and the far western Pacific move eastward into the central Pacific during an El Nino and the waves aloft are altered. Bjerknes (1969) proposed that an unusually warm equatorial Pacific Ocean would create anomalous zonal and meridional SST gradients over large spatial scales. These gradients would provide an enhanced input of thermal energy to the Hadley cells in that quadrant of the globe. This would in turn increase the poleward flux of angular momentum to the winter hemisphere jet streams through a more efficient meridional circulation. Ultimately, this would strengthen the mid-latitude westerlies and affect weather patterns downstream of the original disturbance. In this way, the El Nino warming in the equatorial Pacific can project teleconnected climatic anomalies to remote regions of the globe.

The year-to-year SST changes in the tropical Pacific are on the order of 0.5 °C to 1.0 °C, but ENSO affects climate and weather conditions in a number of locations in different ways. It is the dominant mode of interannual variability in global and hemispheric land precipitation (New et al., 2001). The El Nino signal is strongest in the eastern tropical Pacific Ocean, but it has far-reaching consequences throughout a large area of the tropics and into the mid-latitudes. ENSO has severe local impact along the west coast of South America, and its influence in middle latitudes shows up clearly during the winter (Kane, 1997; Hoerling and Kumar, 2003). ENSO is the single largest cause of global extreme precipitation events, but the seasonality is region dependent (Dai et al., 1997). Global annual average precipitation for 1979-2004 shows most variations are associated with ENSO and have no trend (Smith et al., 2006).

The strongest ENSO precipitation signal over North America affects the Gulf coast region of the United States and parts of northern Mexico, Texas, and the Caribbean islands where wetter than normal conditions occur during the winter. This signal is one of the most consistent extratropical teleconnections associated with ENSO. These signals are related to a strengthening of the subtropical jet over the Gulf of Mexico and are associated with an active storm track to the north. The southwestern United States is drier than normal during La Nina events. In general, opposite ENSO effects are observed in the northwestern United States, but the ENSO influence on United States West Coast precipitation largely depends on SST patterns in the central Pacific Ocean (Cayan and Webb, 1992; Hidalgo and Dracup, 2003). ENSO-related influences are most evident in months at the beginning and end of the winter season in the Upper Rio Grande basin of Colorado and New Mexico (Lee et al., 2004). The collective result of ENSO on annual precipitation in the United States is to increase precipitation variability by 5%-25% for stations in ENSO-influenced regions (Peel et al., 2002). In southern and eastern Mexico, ENSO is associated with reduced precipitation, but the ENSO signal is weaker (Mendoza et al., 2006). The ENSO influence on twentieth century precipitation in Europe and North Africa is strongest in the spring, and it displays three distinct periods. During the first and third periods, precipitation is enhanced in northern and central Europe and precipitation is reduced over the Iberian Peninsula and northwestern Africa. Precipitation during the second period is reversed with central and western Europe experiencing low rainfall and little rainfall influence evident over the Iberian Peninsula and northwestern Africa (Knippertz et al., 2003).

The North American runoff response to ENSO is a complex pattern with a negatively correlated northern region and four distinct positively correlated regions in the west, south, central, and east (Chen and Kumar, 2002). For the western United States, McCabe and Dettinger (2002) found that Niño-3 SSTs explained only a small percentage of the 1 April snowpack variability. In the Pacific Northwest region, advances in the timing of spring peak streamflow appear related to enhanced ENSO activity, which results in more precipitation occurring as rain instead of snow (Regonda et al., 2005). Such alterations in the moisture input may underlie changes in the probability of floods in a given year (Jain and Lall, 2001). However, spatial scale is a consideration as shown by Twine et al. (2005) who report no ENSO signal in streamflow for the Mississippi River at Vicksburg, Mississippi, but the presence of significant correlations in certain regions within the basin. Coulibaly and Burn (2005) found a strong ENSO signal in spring-summer and winter streamflow in both eastern and western regions of Canada. In southeastern South America, the Negro and Uruguay rivers have enhanced streamflow coincident with El Niño events (Robertson and Mechoso, 1998). The major influence of ENSO on the hydroclimate of the North Atlantic region is to initiate circulation changes that produce identifiable temperature anomalies in the British Isles and Europe (Pozo-Vazquez et al., 2001). Daily extreme winter temperatures in the United States are reduced during El Niño events and increased during La Niña and ENSO-neutral years (Higgins et al, 2002). At the global scale, ENSO mechanisms account for a 0.06 °C temperature increase for 1950-98 and contribute to the spatial character of the temperature lag relative to tropical Pacific Ocean SSTs (Trenberth et al., 2002).

Increasing evidence indicates that the strength, duration, and frequency of ENSO events have varied significantly over the past two centuries, and no two El Niño events are the same (Bonsal and Lawford, 1999; Rajagopalan et al., 2000; Verdon and Franks, 2006). May 1982 to July 1983 produced the largest area of warm water anomalies and was one of the most intense ENSO events of the twentieth century (see Fig. 8.6). The 1990-3 ENSO was one of the longest ENSO events in history, but it was of modest intensity. El Niño occurrences back to 1525 have been documented by Quinn and Neal (1992) using various proxy data sources.

ENSO provides a glimpse into the complex workings of the hydroclimatic system afforded by few other phenomena. Because it occurs every few years, the variability of the hydroclimate system components can be viewed with relative frequency. From these observations it is evident there is no single weather pattern associated with ENSO, and time variations of ENSO amplitude and period occur on decadal and longer time scales (Mokhov et al., 2004).

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