El Niño / Southern Oscillation and North American Land Birds El

B I R D I N G • D E C E M B E R 2 0 0 3606

IntroductionFrom the end of 1982 through thefirst few months of 1983, in theGalápagos Islands of the easterntropical Pacific Ocean, there was ahuge mortality of nesting seabirds.For example, populations of theGalápagos Penguin (Spheniscusmendiculus) and Flightless Cor-morant (Phalacrocorax harrisi) de-clined by 49% and 77%, respec-tively (Valle and Coulter 1987).Other species, such as Blue-footedBooby, Lesser Frigatebird, Magnifi-cent Frigatebird, and Brown Peli-can, also experienced high mortali-ties. Many individuals of these andother species managed to survive bymigrating away from the GalápagosIslands, abandoning their youngbirds to starve to death.

At first, this dramatic incidentpuzzled ornithologists. However, itquickly became evident that thismassive mortality and migration ofseabirds was due to the occurrenceof an exceptionally strong warmingof the waters in the eastern tropicalPacific Ocean. Such a pattern ofoceanic warming was a clear signalof the occurrence of an El Niño.Measurements of the sea surfacetemperature (hereafter, SST) subse-quently revealed that the 1982–1983El Niño was the strongest El Niñoup to that time during the 20th cen-tury. The ocean water in this regionis normally cold due to the up-

welling of deep water from far be-low the ocean surface. Accompany-ing this cold water is an abundanceof nutrients that supports a diverseecosystem. During an El Niño, thiscold water upwelling is suppressed.As a result, there is a large reduc-tion in available nutrients, leadingto a collapse in the populations ofthe fish and other marine speciesthat seabirds normally prey upon.

Twenty years ago, El Niño wasnot only unknown to the public;even most atmospheric scientistswere unaware of this phenomenon.The apparent intensity and dra-matic impact of the 1982–1983 ElNiño on worldwide weather, caus-ing destructive floods and devastat-ing droughts on a global scale,brought this phenomenon to theattention of atmospheric scientistsand the general population alikethroughout the world. In NorthAmerica, perhaps the most strikinginfluence of this El Niño was therash of severe storms that batteredthe coast of California.

Over the past two decades, sincethe 1982–1983 event, there havebeen many interesting scientificstudies on the impact of El Niño onNorth American birds. Most of thesestudies have focused on seabirds.Presumably this emphasis onseabirds stems from the observationthat during El Niño the SST in anarrow strip along much of the west

W E AT H E R A N D C L I M AT E

El Niño / Southern Oscillationand North American Land BirdsEl Niño / Southern Oscillationand North American Land Birds

by Steven B. Feldstein2217 Earth-Engineering Science Building

EMS Environment Institute

Pennsylvania State University

University Park PA 16802

[email protected]

El Niño / Southern Oscillationand North American Land Birds

W W W . A M E R I C A N B I R D I N G . O R G 607

coast of North America in-creases by several degreescentigrade. This SST in-crease not only leads to a re-duction in the nesting suc-cess of several West Coastseabird species (e.g., Wilson1991, Harding et al. 2003),but also coincides with nu-merous sightings of warm-water species, such as Black-vented Shearwater, Red-billed Tropicbird, ElegantTern, Xantus’s Murrelet, andCraveri’s Murrelet, at lati-tudes well to the north ofwhere they are normallyseen. These El Niño /seabird relationships will notbe further discussed in this article.

There has been much less re-search on the impact of El Niño onNorth American land birds—thefocus of this article—presumablydue to the more subtle and indirectlinks between El Niño and landbirds. In this article, I summarizethe results of several recent scien-tific studies on the impact of ElNiño on North American landbirds, and I briefly discuss a simpletechnique commonly used by at-mospheric scientists that mighthelp enhance our understanding ofthe relationship between El Niño

and North American land birds. Ialso discuss the science that de-scribes how El Niño affects theweather and climate over NorthAmerica. (For our purposes,weather is defined as the day-to-dayvariation in temperature, precipita-tion, and wind, whereas climate isdefined as the monthly or seasonalaverage of the weather.)

Studies of the Impact of ENSOon North American Land BirdsHere I summarize the findings ofthree recent papers that examinedthe relationship between ENSO and

North American land bird popula-tions (Sillett et al. 2000, Morrisonand Bolger 2002, Nott et al. 2002).

Sillett et al. (2000) examined de-mographic changes in Black-throatedBlue Warbler populations over sev-eral ENSO events (1986–1998), bothon their tropical winter quarters inJamaica and on their mid-latitudebreeding grounds in New Hampshire.Relationships between warbler sur-vival and fecundity to ENSO werefound. During El Niño events, whenthe rainfall is lower than average inJamaica, fewer adults were found tosurvive the winter. The opposite wasfound during La Niña events. The au-thors attributed these differences insurvival to the fluctuations in foodavailability, which they linked to theamount of rainfall. Further observa-tions suggested that this impact ofENSO extends into the breeding sea-son, as the fecundity (as measured bythe number of fledglings per nest and

The effects of El Niño on marine bird species, such as the GalápagosPenguin (above, with marine iguana, Amblyrhynchus cristatus) andFlightless Cormorant (right), are well known. Comparatively littlestudied, however, is the question of El Niño’s effects on land birdpopulations. This article addresses that question and proposes severalinvestigations that could be carried out by birders in North America.

Galápagos Islands; November 2002. © Jan Nachlinger.

Galápagos Islands; November 2002. © Gwen Kittell.

continued on page 610

Fig. 1. SST and surface wind anomalies during typical El Niño and La Niña events. The SST anomaly valuesare normalized to give a maximum value of 1.0. Image courtesy of © the University of Washington.

El Niño Southern OscillationEl Niño La Niña

tropical Pacific is warmer, there is an increase in evapora-tion. This leads to an increase in the strength and fre-quency of thunderstorms (see Fig. 3). When many ofthese thunderstorms occur at the same time and extendover a fairly broad area, the air is compressed in the verti-cal direction. This causes the excitation of so-called plane-tary-scale Rossby waves, which propagate northeastwardtoward North America (Hoskins and Karoly 1981,Sardeshmukh and Hoskins 1988, Held and Kang 1987).

To put it in technical terms, Rossby waves occur in flu-ids on rapidly rotating spheres, such as the atmosphereand ocean; the adjective ‘planetary scale’ refers to the verylarge horizontal scale of the Rossby waves, typically beinggreater than several thousand kilometers. To put it inmore familiar terms, consider the simple analogy of drop-ping a stone into a body of water. The behavior of thestone is analogous to that of the thunderstorm, since thestone depresses the surface of the water, which in turn ex-cites surface-water waves. As we all have experienced,these waves take on the shape of concentric circles, whichcontinue to propagate in an outward direction until theydissipate due to friction. The planetary-scale Rossbywaves behave in a similar manner, as they propagate awayfrom their source region in the eastern and central tropi-cal Pacific toward North America. For further discussion,see <www.cdc.noaa.gov/ENSO/enso.description.html>.

Among the many differences between surface-waterwaves and Rossby waves, there are two that are crucial forour discussion: (1) Surface-water waves generated by thestone may travel tens of meters before being dissipated,whereas Rossby waves can easily travel half way aroundthe globe; and (2) these surface-water waves usually havea wavelength of just a few centimeters, whereas theRossby waves have a wavelength that is typically severalthousand kilometers.

At almost all times there is some Rossby wave propaga-tion from the eastern and central tropical Pacific towardNorth America. Both the El Niño and the La Niña SSTanomalies alter the characteristics of this wave propaga-tion. As described above, during an El Niño, there is anincrease in the thunderstorm activity in the eastern andcentral tropical Pacific. This leads to the more frequentexcitation by thunderstorms of large-amplitude Rossbywaves that propagate to North America. The oppositecharacteristics occur during a La Niña event. Because ofthe colder SSTs in the eastern tropical Pacific during a LaNiña, there is less evaporation and thus a reduction in thethunderstorm activity (see Fig. 3) and a correspondingdecrease in the frequency and amplitude of the Rossbywaves that propagate to North America. Returning to ouranalogy of dropping stones, we consider the scenariowhere stones are continuously being dropped into the

There is much confusion in the popular media as towhat is meant by the term El Niño. The standard

scientific definition is that El Niño represents a warm-ing of the sea surface in the eastern and central tropi-cal Pacific Ocean (between the Date Line and theSouth American coast). These characteristics are illus-trated in Fig. 1, which shows typical SST anomaly val-ues during an El Niño event. (Throughout this article,the term anomaly will refer to the difference betweenthe value of a variable during a particular month andthe value of that variable averaged over many years forthat month.) For an average El Niño, the SST warmsby one or two degrees centigrade, with a three- orfour-degree warming during a more extreme event. Inthose years when an El Niño event takes place, a well-organized SST anomaly first appears between June andAugust. The SST anomaly gradually increases instrength, attaining its largest amplitude usually by Oc-tober. By December, the SST anomaly is typically de-caying, and by April it is usually only weakly present(Larkin and Harrison 2002).

The El Niño SST warming described above is part ofan irregular cycle that repeats every three to seven years.During the opposite side of the cycle, known as La Niña,there is a cooling of the SST in the eastern and centraltropical Pacific (Fig. 2 shows a listing of the most recentEl Niño and La Niña events). The spatial pattern of thiscold SST anomaly is similar to that for El Niño, exceptfor the change in the sign (+ or –) of the anomaly (seeFig. 1). The growth and decay of the La Niña SST anom-aly tends to occur over similar months as those for ElNiño (Larkin and Harrison 2002).

In order to reflect the observation that changes to theatmosphere coincide with an El Niño or La Niña, scien-tists coined the name El Niño / Southern Oscillation(hereafter, ENSO) for this cycle. The El Niño part ofthis name represents the oceanic part of the cycle, i.e.,the warm and cold SST anomalies described above, andthe Southern Oscillation refers to the atmospheric con-tribution, which takes the form of a seesaw fluctuationin the sea-level pressure between Tahiti and Darwin,Australia. As first observed more than 70 years ago byWalker and Bliss (1932), during an El Niño event theaverage sea-level pressure is reduced at Tahiti and is en-hanced at Darwin, Australia. The opposite changes insea-level pressure occur during a La Niña event. Al-though the name ENSO is firmly engrained within thescientific community, this name is unfortunately some-what misleading. This is because this name may incor-rectly suggest that El Niño is much more importantthan La Niña.

During an El Niño, because the water in the eastern


E L N I Ñ O A N D B I R D S

Science of El Niño / Southern Oscillation (ENSO)


body of water. The analogy to El Niño would be to dropthe stones more frequently and to use larger stones. Thisincreases the frequency of excitation of the surface-waterwaves and also increases their amplitude. The correspon-ding analogy to La Niña would be to drop these stonesless often and to use smaller stones. This analogy is basedon the property that the changes in thunderstorm activityover the eastern and central tropical Pacific during ElNiño and La Niña events have a stronger influence onNorth American climate than do the thunderstormchanges over the western tropical Pacific (see Fig. 3).

The planetary-scale Rossby waves that reach North Amer-ica from the eastern tropical Pacific during an El Niño or LaNiña have their greatest influence during the winter season.One may wonder why the impact of El Niño and La Niña isgreatest during the winter, since the El Niño and La NiñaSST anomalies typically reach their maximum amplitudeduring October. This question has been addressed in the at-mospheric science literature. Without going into much de-tail, one factor is the presence of a jet, i.e., a narrow belt ofvery strong winds, over the central subtropical PacificOcean. This jet provides an energy source which amplifiesthe Rossby waves that are propagating from the eastern trop-ical Pacific toward North America (Simmons et al. 1983,Branstator 1985a, Branstator 1985b). In addition, the mid-latitude storms associated with day-to-day weather interactwith the planetary-scale Rossby waves in a manner that fur-ther increases their amplitude (Kok and Opsteegh 1985,Held and Kang 1987). Both of these processes have theirstrongest influences during the winter season. Therefore,these two processes more than compensate for the reductionin the amplitude of the SST anomaly during the winter.

Schematic diagrams (Fig. 4) illustrate the average NorthAmerican climate during both El Niño and La Niñaevents, for the months of January through March. The dif-ferences between the El Niño and La Niña climates in Fig.4 are due mostly to the influences of the planetary-scaleRossby waves excited by the El Niño and La Niña SSTanomalies. Keeping in mind that during an average winterthe observed winds are intermediate between those shownin the two schematic diagrams in Fig. 4, we see that oneimpact of these planetary-scale Rossby waves is to dis-place the Pacific jet stream southward during an El Niñoand northward during a La Niña. Other features to note inFig. 4 are that the jet stream takes on a west-to-east orien-tation across the southern states during El Niño and amore wavy structure across the northern states during LaNiña. Another important difference is that the polar jetstream is stronger and more extensive during La Niña.

Distinct differences between the El Niño and La Niñaanomalous temperature fields can also be seen in Fig. 4.For example, during El Niño, an extensive warming is

present across the northern U.S., southern and westernCanada, and southern Alaska. A widespread cooling inmore or less a similar region takes place during LaNiña. Furthermore, there is cooling across the southernU.S. during El Niño, and a warming in the same regionduring La Niña. These temperature differences can beunderstood from the property that the narrow belt ofwinds that makes up the jet stream separates cold air tothe north and warm air to the south. Thus, when the jetstream shifts southward, as during an El Niño, thesouthern U.S. becomes colder (Fig. 4). Similar argu-ments can account for all of the anomalous warm andcold regions shown in Fig. 4.

The relationship between the anomalous precipitationin North America and the El Niño and La Niña SSTanomalies is rather subtle, because the planetary-scaleRossby waves indirectly influence the precipitation. Be-fore I describe how this happens, it is first important toemphasize that these Rossby waves propagate veryslowly and can persist for about one month (Feldstein2000). Because these waves—or equivalently, the jetstreams influenced by corresponding El Niño and LaNiña events—evolve much more slowly than do mid-latitude storm systems, these jet streams behave as ifthey were not changing with time during the lifetime ofthe storm system. This perspective allows us to regardthe storm systems as being steered by jet streams influ-enced by El Niño or La Niña. As a result, during ElNiño, storm systems originating in the mid-latitudeNorth Pacific are steered by the jet stream toward Cali-fornia and then farther eastward toward the Gulf states,bringing additional rain to this region. This contrastswith La Niña, in which the jet stream steers the stormsystems toward the Pacific Northwest and across thenorthern states. This results in more rain and snow inthose states, and in a much drier southern U.S.

It is important to emphasize that the Northern Hemi-sphere response to ENSO, shown in Fig. 4, representsan average over many separate El Niño and La Niñaevents. During an individual El Niño or La Niña event,the climate can be strikingly different from that shownin Fig. 4. The reason is that there are a whole host ofother important physical processes that contribute tothe variability of North American climate. The strongimpact of these various phenomena implies that it isvery difficult to make a linkage between the NorthAmerican climate and ENSO during any individual ElNiño or La Niña event. It is only when an average ismade over many El Niño or La Niña events that an at-mospheric pattern emerges which is both statisticallysignificant to a reasonable degree and which isamenable to investigation.



610

their mass at fledging) was higherfollowing La Niña events and lowerafter El Niño events. These variationsin fecundity were found to be relatedto fluctuations in the populationsof their lepidopteran larvae prey,which were significantly corre-lated with ENSO.

Morrison and Bolger (2002)investigated the reproductivesuccess of Rufous-crowned Spar-rows in the coastal sage-scrub ofsouthern California. Three con-secutive breeding seasons wereexamined. These included 1997,when the rainfall total in south-ern California was close to itslong-term average, and 1998 and1999, which coincided with ElNiño and La Niña events, peri-ods during which the rainfall insouthern California was wellabove and below average, respec-tively. A dramatic difference inthe breeding success was ob-served between the El Niño andLa Niña events. For example, duringthe 1998 El Niño event, there wasan average of 5.1 fledglings per pairfor the breeding season. This con-trasted with a value of 0.8 fledglingsper pair for the 1999 La Niña breed-ing season. Careful observationsshowed that these differences in fe-cundity were due to an increase (ordecrease) in both the brood size andthe number of fledged nests per pairduring the El Niño (or La Niña)

events. The authors also examinedthe primary factors that may ac-count for these fecundity differ-ences. Their findings suggested that

the poor reproduction during the LaNiña event was due to low levels ofavailable food, such as seeds andarthropods. For the El Niño event,the level of available food was closeto that of the previous “average”year, suggesting that a factor otherthan food availability may be moreimportant in accounting for the highfecundity. The observations of theauthors indicated that there was areduction in nest predation by

snakes early in the breeding season.They suggested that the cool, rainyweather during the El Niño eventmay have resulted in a lower levelof activity for these snakes.

Nott et al. (2002) examined therelationship between ENSO and thereproductive success of 16 speciesof neotropical migrants that winterin western Mexico. Nine years(1992–2000) of data, which spantwo El Niño and one La Niñaevents, were used. To examine re-productive success, the authorsused banding data from 33 Moni-toring Avian Productivity and Sur-vivorship (MAPS) stations in thePacific Northwest. (MAPS was cre-ated by The Institute for Bird Popu-lations of Point Reyes Station, Cali-fornia, for the purpose of monitor-ing the population dynamics ofNorth American land bird species.There are many opportunities forbirders to contribute to research inbird population dynamics by volun-teering to work at MAPS stations.)With their banding data, the au-thors calculated the ratio of thenumbers of young birds to adults.This ratio defined their reproductiveindex, which is used to measure thereproductive success of the neo-tropical migrants. When all 16species were averaged, the authorsfound highly significant linear cor-relations between the reproductiveindex and an index that measures

Fig. 2. Recent El Niño and La Niña events. The vertical lines denote the summer season. Typical ENSO events begin in the summer season of one year and end duringthe spring season of the following year. Figure prepared by Virginia Maynard.

continued from page 607

Several recent studies have pointed to possible effects of El Niño/ Southern Oscillation (ENSO) on population fluctuations in NorthAmerican land birds. For example, breeding success in Rufous-crowned Sparrows that breed in coastal California may ulti-mately be determined by ENSO-influenced variation in precipita-tion. Anza-Borrego State Park, California; March 2003. © Bob Steele.


ENSO. Their results suggest thatafter El Niño (or La Niña) events,neotropical migrant species have in-creased (or decreased) reproductivesuccess. The authors suggest thatthis relationship may be related inpart to changes in the prey biomassin western Mexico, which is likelyto increase during the wetter ElNiño years and decrease during thedrier La Niña years. They also spec-ulate that these changes in the preybiomass may influence the physicalcondition of neotropical migrants atthe time they arrive on their breed-ing grounds. Furthermore, their ex-amination of the strength and direc-tion of the wind showed that dur-ing El Niño events the winds aremore southerly at the altitude atwhich the neotropical migrants fly.As the authors point out, these en-hanced southerly winds during ElNiño events may aid the northwardmigration, which will also cause thebirds to be in better physical condi-tion when they arrive on theirbreeding grounds.

Composite AnalysisThere is a popular and simple re-search technique that is used by at-mospheric scientists and that couldlikely be adopted for examiningENSO / bird population relation-ships. This technique is known ascomposite analysis and is used toinvestigate the properties of numer-ous atmospheric variables, such aswind, temperature, precipitation,and ozone. When using compositeanalysis for ENSO research, atmos-pheric scientists select all of thoseyears for which either an El Niñoor La Niña event has taken placeand then take an average of thevariable under investigation. Forexample, if one is interested in thetemperature field associated with ElNiño, one would take an average of

the temperature field for all El Niñoevents. This approach will yield amap, or a spatial pattern, of the ElNiño temperature field. These cal-culations need to include a carefulanalysis of statistical significance,because one can make physicallymeaningful claims about averagesonly over many El Niño or La Niñaevents, not about a single event.

This technique is illustrated by the

following example. Suppose wewished to investigate whether there isa relationship between the winter ir-ruption of Snowy Owls and La Niña(from Fig. 4, we can see that duringLa Niña there is more snow and coldweather over the western part of theSnowy Owl’s winter range). For thisinvestigation, we could calculate thecomposite Snowy Owl populationwith either Christmas Bird Count or

Fig. 3. Thunderstorm location, SST, and vertical winds during typical El Niño (top) and La Niña (bottom)events. Image provided as a public service by © the National Oceanic and Atmospheric Administration.



Winter Bird Population Survey data.This analysis would result in a mapof the Snowy Owl population as it isassociated with La Niña. Calcula-tions of statistical significance wouldthen be performed to evaluate theextent to which we could claim thatthere is a relationship between LaNiña and Snowy Owl irruptions.

A perusal of North American Birds(and its predecessors, AmericanBirds and Field Notes) for several El

Niño and La Niña events suggests tome that there may be numerousother interesting ENSO / bird popu-lation relationships. For example, insouthern Florida, it is my impres-sion that there tends to be an in-crease in waxwings, robins, andnorthern finches during El Niño anda decrease in the same species dur-ing La Niña. Furthermore, in Texasand California, it seems that duringEl Niño events waterfowl tend to be

dispersed over wideareas, and during LaNiña events the wa-terfowl are concen-trated in fewer bodiesof water. Presumablythis is because of theincrease in rainfall inthose states during ElNiño and the corre-sponding decrease inrainfall during LaNiña. However, it isalso apparent fromNorth American Birdsand its predecessorsthat there is muchcase-to-case variabil-ity in bird popula-tions from one ElNiño or La Niñaevent to the next.

The strength of thecomposite techniqueis that it allows oneto find statisticallysignificant patternsof bird populationsthat are related toENSO. However, thiscomposite analysisshould not be usedto make causalclaims of how ENSOalters bird popula-tions. To addresscausal relationships,it is necessary to use

more sophisticated techniques thatinclude additional observations anddatasets, such as those used in thethree papers described in the previ-ous section.

ConclusionsENSO has a very large impact onthe climate of North America. Notsurprisingly, recent research hasshown that ENSO also has poten-tially important influences on the

Fig. 4. Temperature anomalies and wind during typical El Niño and La Niña events, for the months of January through March.Image provided as a public service by © the National Oceanic and Atmospheric Administration.


population dynamics of NorthAmerican land birds. Presumably,future research will reveal a wholehost of additional relationships be-tween North American land birdpopulations and ENSO.

Among the atmospheric andoceanic phenomena that fluctuatewith time periods greater than oneyear and less than ten years, ENSOis the most dominant. However,there are other phenomena thatmake a large contribution to vari-ability on these time scales. Theseinclude the North Atlantic Oscilla-tion and Pacific / North American“teleconnection” patterns (Wallaceand Gutzler 1981, Barnston andLivezey 1987, Feldstein 2000). Intheir study of the impact of ENSOon North American land birds,Nott et al. (2002) also show thatthe North Atlantic Oscillation hasan important impact on the repro-ductive success of land bird speciesthat breed in the Pacific Northwestand winter throughout the westernU.S. It is likely that future researchwill find many other interesting re-lationships between land bird pop-ulations and these two teleconnec-tion patterns.

AcknowledgmentsI would like to thank SukyoungLee, Bernd Haupt, and Peter Gentfor their helpful comments on thisarticle. I would also like to thankDavid DeSante and Philip Nott forthe useful ENSO / bird informationthat they provided.

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Branstator, G. 1985a. Analysis of

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CITY STATE ZIP

For a free brochure and East Coast map, or toobtain a birding permit, call (757) 331-2960 ormail to Chesapeake Bay Bridge-Tunnel, Dept.B-1003, Cape Charles, VA 23310.www.cbbt.com

(757) 331-2960

A landmark and sanctuaryfor migrating birds.

A spectacular 17.6-mile drive for motorists.

And one of the Seven Engi-neering Wonders of the

Modern World.

CHANGING MIGRATION PATTERNS

SINCE 1964.

CHANGING MIGRATION PATTERNS

SINCE 1964.

Documents

El Niño / Southern Oscillation and North American Land Birds El