El Niño and the Southern Oscillation: Observation☆

Nan Chen, University of Wisconsin–Madison, Madison, WI, United StatesSulian Thual, Fudan University, Shanghai, ChinaShineng Hu, Scripps Institution of Oceanography, La Jolla, CA, United States

© 2019 Elsevier Inc. All rights reserved.

Introduction 1The ENSO History and Discovery 1The Basic ENSO Mechanisms 1Global Impacts 2Observation Network 2Monitoring the ENSO 4ENSO Evolution in the Observation Era 5ENSO Diversity 5ENSO and Climate Change 5Further Reading 7

Introduction

The El Niño Southern Oscillation (ENSO) is the most prominent interannual climate variability on Earth with large ecological andsocietal impacts. The term El Niño refers to warming of the tropical Pacific Ocean occurring every 2–7 years, while the opposite coldphase is known as La Niña. Anomalous warming or cooling conditions are associated with a large-scale east-west sea level pressureseesaw, termed the Southern Oscillation, which represents the atmospheric manifestation of the coupled ENSO phenomenon. TheENSO cycle has considerable irregularity in amplitude, duration, temporal evolution, and spatial structure. Since the ENSO has asignificant global impact, understanding the mechanisms of the ENSO and the ENSO diversity is of practical importance. Theunderstanding and prediction of the ENSO is supported by a large network of observations as well as a vast hierarchy of models ofincreasing complexity.

The ENSO History and Discovery

The name El Niño, from the Spanish for “the little boy,” refers to the Christ child, because the phenomenon is usually noticedaround Christmas time in the Pacific Ocean, where a warm current was observed around the area along the Peruvian coast. La Niña,similarly, means “the little girl.” The term El Niño was originated in the 19th century with fishermen in Ecuador and Peru, althoughat that time no one would imagine that this warm current had a counterpart at the other end of the Pacific.

Due to the worst famine in late 1800s caused by the failure of the monsoon rains, Sir Gilbert Walker was elected as the DirectorGeneral of the meteorological observatory to predict Asian monsoon fluctuations. This allowed him to sort through world weatherrecords for the sea level pressure swing between South America and India–Australia. Then he discovered the “Southern Oscillation,”which is the large-scale changes in sea level pressure across Indonesia and the tropical Pacific, and he also found that many globalclimate variations were correlated with the Southern Oscillation. However, he did not recognize that it was linked to changes in thePacific Ocean or El Niño.

In the late 1960s, Bjerknes noticed that there is a direct thermal circulation in the atmosphere along the Pacific due to the fact thatthe sea surface temperature (SST) at the eastern end of the Pacific is remarkably colder than that in the western Pacific. The cool dryair above the cold eastern equatorial Pacific waters flows westward along the surface toward the warm west Pacific. There, the air isheated and supplied with moisture from the warm water. This systematic equatorial circulation associated with the zonal pressuregradient was named the “Walker Circulation” by Bjerknes. Bjerknes and others then realized that the changes in the ocean and theatmosphere were connected and the hybrid term ENSO was born.

The Basic ENSO Mechanisms

Fig. 1A shows a sketch of the normal conditions in the equatorial Pacific. In the ocean, the warm and light water parcels are ingeneral located right under the surface and above the cold and dense waters of the deep ocean. This volume of warm water,representing the thermal content, is separated from the colder waters by a layer called the thermocline. The tropical Pacific holds themost important reserve of warm waters of the planet, known as the warm pool, with a deep thermocline and warm sea surface

☆Change History : December 2018. The present chapter has been updated by Nan Chen, Sulian Thual and Shineng Hu. The structure and content have beenentirely modified, including all sections/figures/tables. No figure/table from previous edition have been reused.

Earth Systems and Environmental Sciences https://doi.org/10.1016/B978-0-12-409548-9.11766-X 1

Fig. 1 Schematic representation of (A) normal, (B) El Niño, and (C) La Niña conditions. Warmer colors correspond to warmer waters. Adapted from McPhaden, M.J.et al. (1998). The tropical ocean-global atmosphere observing system: A decade of progress. Journal of Geophysical Research 103(C7): 14169–14240.

2 El Niño and the Southern Oscillation: Observation

temperatures (>28�C). The normal conditions in the tropical Pacific atmosphere consist of a zonal Walker circulation withconvection in the western Pacific, subsidence in the eastern Pacific, and surface trade winds blowing toward the west at the equator.The trade winds force westward ocean surface currents that advect the warm waters to the west. Meanwhile, in the east (the so-called“cold tongue” region) the advected warm surface waters are replaced by subsurface cold water due to equatorial upwelling. Notethat ocean and atmosphere circulations reinforce each other, which is called the “Bjerknes feedback” after its discovery: the Walkercirculation increases the enhanced zonal gradient of temperatures by advecting warm waters to the west and upwelling cold watersin the east, while the zonal gradient of temperatures increases the Walker circulation by favoring convection in the west andsubsidence in the east.

Nowadays, the basic mechanisms of the El Niño and La Niña are relatively well understood in terms of departures from thenormal conditions in the tropical Pacific. Fig. 1B shows a schematic representation of the El Niño conditions in the tropical Pacificthat occur when the average ocean and atmospheric circulations are destabilized. The trade winds weaken allowing the warm watersto invade the eastern region, which then results in a further decrease of the zonal Walker circulation. Fig. 1C shows a schematicrepresentation of the La Niña conditions, during which conversely both the zonal gradient of sea surface temperature and the zonalWalker circulation increase, reinforcing each other.

Fig. 2 shows sea surface temperature (SST) in the equatorial Pacific during the major El Niño event of 1997/98 as well as themajor La Niña event of 1988/89. Consistent with the basic mechanisms depicted above, the El Niño event is characterized by aneastward expansion of warm SSTs (above 28�C) toward the eastern Pacific. In order to better characterize the state of the equatorialPacific, it is useful to consider anomalies that is, departures from average conditions, as also shown in Fig. 2. This showsmore clearlythe anomalous warming in the eastern Pacific during El Niño as well as the cooling during La Niña.

Global Impacts

Even though the ENSO is a phenomena defined as warming or cooling of water temperatures in the equatorial Pacific, the impactsof El Niño and La Niña can be dramatic on a global scale, known as the teleconnections.

Fig. 3 shows how El Niño commonly affects weather patterns around the world in boreal winter. In the normal years, the centerof convection and rainfall is usually located over the western Pacific. However, when El Niño occurs, the location will move towardthe eastern Pacific. Near the equator, this results in increased rainfall over South America and conversely drying over Asia and themaritime continent. The relocation of the center of convection also disrupts the atmospheric circulation patterns that connect thetropics with the middle latitudes, which in turn modifies the mid-latitude jet streams. By modifying the jet streams, ENSO can affecttemperature and precipitation across the United States and other parts of the world.

El Niño is also associated with the global warming hiatus. Recent research indicates that weak El Niño activity from 1998 until2013, rather than a pause in long-term global warming, was the root cause for slower rates of increased global mean temperature.On the other hand, the global mean temperature increased dramatically during strong El Niño years, such as 1982, 1987, 1998,and 2016.

Due to those global impacts, numerous sectors of activity depend on accurate predictions of the ENSO, including agricultural,electrical or forestry management as well as fishery, transport, tourism, and even financial markets. The El Niño may result inextreme weather events such as floods in Peru and Ecuador, forest fires in Indonesia and Papua New Guina, or decreased hurricaneactivity in the Atlantic. Those global impacts explain the considerable efforts made in recent decades to observe, understand andpredict the phenomenon.

Observation Network

The observation network that continuously monitors ENSO activity is sketched in Fig. 4. This network has become remarkablydenser in recent decades, notably with the launch of satellite allowing for global coverage of surface conditions (e.g., sea surface

Fig. 3 Sketch of El Nino global impacts during boreal winter (December–January–February). NWS/NCEP Climate Prediction Center.

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Fig. 2 Sea surface temperature (�C) during December 1997 (El Niño) and December 1999 (La Niña). Bottom repeats the figures for SST anomalies. Data is fromthe ERSST reanalysis V5.

El Niño and the Southern Oscillation: Observation 3

temperatures, sea level, etc.). Complementary to this, a set of stations collects in situ data, including fixed buoys along the equatorwhich allow to observe the ocean thermal content and subsurface variability. This system is complemented by drifting buoys andautonomous systems that pair some of the merchant ships.

Historical reconstructions also provide a view of ENSO’s past variability. Such measures include historical records, tropical coralisotopic data, Andean glacier cores, tree rings and sedimentary deposits, etc. The reconstructions show the existence of the ENSO

Fig. 4 Distribution of in-situ measurements of the ENSO observation system. Adapted from McPhaden, M.J. et al. (1998). The tropical ocean-global atmosphereobserving system: A decade of progress. Journal of Geophysical Research 103(C7): 14169–14240.

4 El Niño and the Southern Oscillation: Observation

activity in the Pacific at least back from several hundred to thousand years. For example, ENSO variability would have been very lowat the beginning and middle of the Holocene (<5000 years). It would have declined during the warm medieval period (10–12thcentury) compared to the period of the Little Ice Age (17–18th century). Moreover, thanks to data assimilation methods, thereanalysis products make it possible to objectively combine the historical and current observations with the dynamics of the modelsto simulate the ENSO variability. These products offer global coverage (especially in the subsurface), which to some extentovercomes the disproportionate distribution of existing measures.

Monitoring the ENSO

To characterize the state of the Pacific at a given moment, scientists use representative indices of anomalies of the large scale, such asthe Niño 3.4-SST or the Southern Oscillation Index (SOI) as shown in Fig. 5.

The Niño 3.4-SST index measures the surface water temperature averaged along the equator (170W–120W, 5N–5S). It is positiveduring El Niño events and negative during La Niña ones. It is clear from the Niño 3.4-SST index that the ENSO cycle consists ofalternating periods of anomalously warm El Niño conditions and cold La Niña conditions every 2–7 years in the tropical Pacific,with considerable irregularity in amplitude, duration, temporal evolution and spatial structure of these events. Typically El Niñoevents are identified when the Niño 3.4-SST index is above theþ0.5�C threshold while La Niña events are identified when the indexis below �0.5�C.

The SOI index is another widely used ENSO index that measures the difference in surface air pressure between the east (Tahiti)and the west (Darwin) of the Pacific. The pressure increases in the west with the intensity of the convection and decreases in the eastwith the intensity of the subsidence, thus the SOI index measures the intensity of the atmospheric Walker circulation. The SOI andNiño 3.4-SST index are strongly anticorrelated due to the weakening of the Walker circulation during El Niño events.

In addition to Niño 3.4-SST index and SOI, sea surface temperature in other areas of the tropical Pacific, thermocline depth, sealevel atmospheric pressure and the combinations of different fields are also widely used to form different ENSO indices.

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Fig. 5 Temporal evolution of Nino3.4 SSTA and SOI indices. Data source: NOAA/NCDC ERSST.v4 data.

El Niño and the Southern Oscillation: Observation 5

ENSO Evolution in the Observation Era

Fig. 6 shows the spatiotemporal evolutions of several observed fields near the equator over the last few decades. These observedvariables are zonal winds at the ocean surface, sea surface temperatures and thermocline depth (identified here as the depth of the20�C isotherm). The fields are averaged near the equator (from 2S to 2N in degrees of latitude) and therefore the resulting variablesdepend only on longitude and time. The equatorial Pacific extends from around 120E (Australia–Indonesia) to 80W (SouthAmerican coast) in degrees longitude. All fields are anomalies in this figure, which is obtained by removing the average seasonalcycle (i.e., climatology) computed through averaging over the years.

During El Niño events the eastern Pacific warms up, the trade winds weaken (which appears here as positive zonal windanomalies) and the thermocline is becomes deepened. Major El Niño events happened in 1982/83, 1997/98 and 2015/16. Somemoderate El Niño events occurred in 1987, 1992, 2003 and 2007. La Niña events on the contrary are marked by a colder than usualeastern Pacific, stronger trade winds and a shallower thermocline. The major La Niña events happened in 1983, 1999, 2017,following strong El Niño events.

ENSO Diversity

As discussed in the previous sections, the traditional El Niño involves anomalous warm SST in the equatorial eastern Pacific ocean,where its atmospheric response is the eastward shift of the anomalous Walker circulation. Although the differences amongtraditional ENSO events have been known for many years, a renewed interest in the ENSO diversity is stimulated by a differenttype of El Niño that has been frequently observed in recent decades and is called the central Pacific (CP) El Niño or El Niño Modoki(a Japanese word that means similar but different). The CP El Niño is characterized by warm SST anomalies confined to the centralPacific, flanked by colder waters to both east and west. Such zonal SST gradients result in an anomalous two-cell Walker circulationover the tropical Pacific, with a strong convection locating in the central Pacific. See Fig. 7 for a comparison of the traditional El Niñoin the eastern Pacific in December 1997 and the CP El Niño in December 2009.

On the other hand, according to the strength of the events, the traditional El Niño can also be categorized into weak, moderateand strong events. The latter is also known as “super El Niño.” The three El Niño events happened during years 1982–83, 1997–98and 2015–16 are the typically super El Niños. Yet, the one occurred during 2015–16 is quite different from the other two. In fact,this most recent super El Niño event was quite remarkable with the occurrence of a failed El Niño in 2014–15 favoring a subsequentsuper El Niño in 2015–16. Due to its unique chronology, this event (or sequence of events) is a delayed super El Niño.

Meanwhile, in 2016/2017 a very intense increase in sea surface temperatures, extreme rainfalls and floods occurred locally alongthe South American coast despite the otherwise La Niña conditions in the rest of the Pacific, which was referred to as coastal (or fareastern Pacific) El Niño.

Fig. 6 also illustrates the strong diversity of El Niño events, including the increase of the number of CP El Niños in the past fewdecades.

ENSO and Climate Change

The intrinsic modulation of ENSO can also affect the multidecadal background state. In fact, the increasing amplitude of El Niño inthe central Pacific can contribute to the well-observed multidecadal warming in this region, which enhances the zonal SST gradient.

Fig. 6 Spatiotemporal patterns of observed fields in the equatorial Pacific averaged near the equator (5N–5S), as a function of longitude from 120E to 80W andtime (in years). The fields include zonal winds (m s�1), SST (�C), and thermocline depth (m). All fields are anomalies. Data is from the NCEP/NCAR and GODASrenalysis.

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Fig. 7 SST anomalies during December 1997 (EP El Niño) and December 2009 (CP El Niño), as a function of longitude and latitude. Data is from the ERSSTreanalysis V5.

6 El Niño and the Southern Oscillation: Observation

El Niño and the Southern Oscillation: Observation 7

In addition, it has been noticed that the equatorial thermocline steepened in moving from 1980 to 1999 (when eastern Pacific (EP)events prevailed) to 2000–10 (when CP events prevailed). However, given the substantial remaining uncertainty in the anthropo-genic impacts and of ENSO’s interactions with the mean state, it is not clear to what extent observed ENSOmodulation is a cause ora consequence of either anthropogenic or intrinsic decadal-scale changes in the equatorial Pacific mean state.

Further Reading

Allan R, Lindesay J, and Parker D (1996) El Niño southern oscillation & climatic variability. CSIRO publishing.Capotondi A, et al. (2015) Understanding ENSO diversity. Bulletin of the American Meteorological Society 96.6: 921–938.Diaz HF, Diaz HF, and Markgraf V (eds.) (1992) El Niño: Historical and paleoclimatic aspects of the southern oscillation. Cambridge University Press.McPhaden MJ, et al. (1998) The tropical ocean-global atmosphere observing system: A decade of progress. Journal of Geophysical Research 103(C7): 14169–14240.Philander SGH (1990) El Niño, La Niña, and the southern oscillation. New York: Academic Press.Sarachik ES and Cane MA (2010) The El Niño-southern oscillation phenomenon. Cambridge University Press.