Chapter Two – El Niño/Southern Oscillation And Selected .

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CHAPTER TWO o/Southern OscillationEl Ninand Selected EnvironmentalConsequencesTomasz NiedzielskiInstitute of Geography and Regional Development, University of Wroc1aw, Wroc1aw, PolandE-mail: troductionFundamentals of El Nieno/Southern OscillationWhat Triggers El Nieno/Southern Oscillation?El Nieno/Southern Oscillation in the PastEl Nieno/Southern Oscillation versus Selected Geophysical Processes and TheirPredictions5.1 Earth Orientation and ENSO5.2 Climatological and Hydrological ENSO Teleconnections5.3 Sea Level Change and ENSO5.3.1 Global and Local Mean Sea Level5.3.2 Site-Specific Sea Level7879879095961001061101126. Concluding RemarksAcknowledgmentsReferences114115116Abstract o/Southern Oscillation (ENSO) and selectedThe paper presents a review of El Ninenvironmental consequences at a range of spatial scales. The fundamentals of ENSO aresummarized in a descriptive way, and the reader is provided with the key facts from thehistory of ENSO research as well as with recent developments in understanding theoscillation. Subsequently, a discussion on a potential initial driving force that begins thewarm ENSO episode is given, and the inference is limited to the Quasi-Biennial Oscillation which may be controlled by solar forcing. Later, the insight into the ENSO historyis provided, with a scrutiny about the most recent phenomena and the ENSO variabilityover the geological time. The core section of the paper focuses on three environmentalconsequences of ENSO: irregular fluctuations of the Earth Orientation Parameters(EOPs), climatic and hydrologic teleconnections that allow migration of the ENSO signalto remote regions of the Earth (the teleconnections are explained using the specificEuropean example), and sea level change in the equatorial Pacific and Indian Oceans.Advances in Geophysics, Volume 55ISSN .002Ó 2014 Elsevier Inc.All rights reserved.77j

78Tomasz NiedzielskiThe instances explain that ENSO is a phenomenon that impacts the dynamics of theentire Earth and controls some geophysical and environmental parameters of theatmosphere and hydrosphere at regional and local scales.1. INTRODUCTIONEl Ni no/Southern Oscillation (ENSO) is said to be one of the mostpowerful climatic and oceanic oscillations in the Earth. A repeat cycle ofENSO varies from two to seven years, but departures from this interval arealso probable. Uneven recurrence time implies considerable problems inforecasting El Ni no episodes (warm ENSO phases) and La Ni na events (coldENSO phases). The occurrence of ENSO is associated with the equatorialPacific and Indian Ocean, however, its impact on the environment has alsobeen confirmed for remote regions located far from these oceans. A keyfeature that drives the dynamics of ENSO is a strong ocean–atmospherecoupling. The oscillation in question influences numerous geophysical andenvironmental processes, acting both in global and regional scales. Suchprocesses include for instance: fluctuations of climate and weather, sea levelchange, fluctuations of sea surface temperature, variations in the Earth’srotation rate, and changes in hydrologic regimes. Due to irregularity ofENSO and dissimilar magnitudes of individual events, suchENSO-impacted processes cannot be accurately predicted, particularlyduring El Ni no or La Ni na. Although a considerable development inunderstanding and forecasting ENSO episodes is the case, there is noagreement as to the initial driving force of ENSO. The initial forcecommences a number of consecutive and interrelated atmospheric andoceanic processes in the equatorial Pacific. El Ni no is usually preceded bya period of intensified tropical easterlies (trade winds) in the central equatorial Pacific after which, approximately one year before the occurrence ElNi no, these enhanced trade winds weaken (Wyrtki, 1975, 1979). Thisinitiates the transport of warm water from the Western Pacific Warm Pool(WPWP), the large warm pool of water where sea surface temperature(SST) is greater than 27.5 C, toward the eastern equatorial Pacific. Theoccurrence of the above-mentioned initial condition commences the warmphase of ENSO. It is rather difficult to unequivocally define the mainprocess that controls the critical shift of the tropical easterlies and itsconsequences. External reasons are sought in solar activity, however theinvestigations into this topic are still in progress. It is thus possible todistinguish two specific fields of research that can strengthen investigations

El Ni no/Southern Oscillation and Selected Environmental Consequences79into the ENSO impact on geophysical processes and their prognoses,namely: progress in detecting physical and statistical relationships between ENSOand the selected geophysical processes (mainly activities toward understanding, modeling and forecasting ENSO), seeking initial driving force of El Ni no and La Ni na phenomena (mainlyactivities toward improving the ENSO prediction accuracy throughincorporating the potential new facts on ENSO driving processes).The remainder of this paper consists of five sections. The subsequent oneincludes an extensive and descriptive overview of ENSO fundamentals, withan emphasis put on the phenomenological explanation. In the third section theforces that may trigger ENSO warm and cold episodes are selectively discussed.The fourth section concerns ENSO history, and covers both ENSO dynamicsin geological time and its activity in a few last centuries. The fifth part of thepaper provides the reader with the insight into the selected relationshipsbetween ENSO and global/regional geophysical processes, the latter related tothe variable Earth’s rotation rate, regional-scale atmospheric and hydrologicphenomena, and sea level change. The last section summarizes the paper. 2. FUNDAMENTALS OF EL NINO/SOUTHERNOSCILLATIONThis section is mainly based on two recent books on ENSO (Clarke, 2008;Sarachik & Cane, 2010) which led to a significant structuring of ourknowledge about the oscillation in question. I was inspired by the booksand was honored to be given an opportunity to prepare their reviews(Niedzielski, 2011a,b). The reader is also advised to study the classical bookon ENSO by Philander (1990).The notion of ENSO was defined in the second half of the twentiethcentury. Before that time researchers considered two independent elements,oceanic and atmospheric components, and no link between the two wasinferred. The two parts may shortly be characterized in the following way. Oceanic componentdEl Ni no (La Ni na) phenomenon which isdefined as warm (cold) oceanic current that may occasionally reachwestern coasts of South America. The current modifies SST in the waythat during El Ni no the strongest positive SST anomalies occur in theeastern equatorial Pacific, however, during La Ni na the most significantnegative SST anomalies occur in the central tropical Pacific and the

80Tomasz Niedzielskislightly weaker negative anomalies, but still detectable, may be observedin the eastern equatorial Pacific. Atmosphericdoscillation that controls the “see-saw” of atmosphericpressure observed over the equatorial Pacific, known as the SouthernOscillation, considered between Tahiti (Central Pacific) and Darwin(Australia).The El Ni no phenomenon, understood as the warm ocean current, wasobserved much earlier than the atmospheric component. The observation wasmade a few centuries ago by Peruvian fishermen who noticed that rapidincrease in the ocean temperature led to the lower number of fish in theirnets. Indeed, warm water contains less oxygen than cold water, and fisheswere forced to migrate toward the oxygen-rich places. In addition, thefishermen observed that the anomalous warming of the ocean water wasassociated with the increased rainfall. The excess of precipitation led to thetransformation of Peruvian coastal desserts (e.g., Sechura Desert) into pastures(e.g., in 1891) and the initiation of floods that washed out the nutrients fromthe slopes of the West Andes. The fishermen also observed that the anomalous warm current did not appear every year. However, they noticed that theoccurrence time was locked to the end of year, usually around Christmas.Because of the latter finding, in the nineteenth century the phenomenon wascalled El Ni no, which in Spanish means a little boy, and the name allowed theconnotation with the Baby Jesus and Christmas.Some elements of the atmospheric component were first observed byBlanford (1884), but its existence was empirically confirmed at the end ofthe nineteenth century by Hildebrandsson (1897). A comprehensivedescription of the “see-saw” of atmospheric pressure was produced in 1920sby Sir G. Walker. His work (Walker, 1923, 1924) provided statisticalevidences for the existence of correlations between atmospheric pressurefluctuations over the equatorial Pacific, driven by irregular changes of tradewinds, and rainfall in various regions of the Pacific and Indian Oceans. Theterm “Southern Oscillation” was first used by G. Walker to characterize theaforementioned “see-saw” of atmospheric pressure over the remote areas ofthe tropical Pacific.The above-mentioned oceanic and atmospheric components weresaid to be interrelated in the early 1930s by Leighly (1933), howeverthe hypothesis was not widely accepted until the 1950s. The scientificcommunity accepted the relationship between the components whenBerlage and de Boer (1960) identified a statistical correlation betweenSST anomalies and changes in atmospheric pressure in the eastern

El Ni no/Southern Oscillation and Selected Environmental Consequences81tropical Pacific. Although that association was not explained in terms ofphysical fundamentals, at that time the phenomenon was called El Ni no/Southern Oscillation, the name that unequivocally implied couplingbetween oceanic (El Ni no) and atmospheric (the Southern Oscillation)components.The researcher, whose contribution to the understanding of ocean–atmosphere coupling in ENSO was the most significant, was ProfessorJ. Bjerknes. He proposed the mechanism that led to the explanation of therelationship between SST anomalies and the dynamics of the tropical atmosphere (Bjerknes, 1969). As a tribute to G. Walker, the theoretical wind cellover the equatorial Pacific was named the Walker Circulation. This circulationis based on meaningful differences in SST values observed usually between theeastern and western tropical Pacific, the differences that influence atmosphericpressuredand hence air motiondover the equatorial Pacific. Assuming a fewsimplifications and incorporating several findings, which are known now butremained unknown to J. Bjerknes at that time, the Walker Circulation may beexplained in the following way (Figure 1 helps to understand the phenomenological description included in the itemized facts below). During normal conditions or during La Ni na the tropical easterliestend to transport cold and dry air (see below for explanation of the origin ofthis cold air *) and cold water (see below for explanation of the origin of thisFigure 1 Sketch of the Walker Circulation acting in the equatorial plane during normal a (a) and El Nin o (b).conditions and La Nin

82Tomasz Niedzielskicold water **) from the eastern tropical Pacific westward toward theWPWP. While traveling over warm waters of this pool, above which deepatmospheric convection occurs, this air is being heated up and its moistureincreases. As there exists atmospheric low over the WPWP, this warm andhumid air migrates upward toward the tropopause and, in the form of theconvection loop, is transported aloft toward east and descends colder anddrier in the eastern equatorial Pacific where atmospheric pressure is high (*).Stable or increased trade winds, that control water transport from east towest, intensify upwelling which lifts up cold water from the deep towardthe ocean surface in the eastern equatorial Pacific (**).Tropical easterlies form a specific setting of the thermocline (the oceanlayer that separates cold water from warm water) in the Pacific. Duringnormal conditions the thermocline in the eastern equatorial Pacific isshallow, located approximately tens of meters below sea surface, and in thevicinity of the WPWP the thermocline is deep and reaches 200 m below sealevel. This setting implies positive SST anomalies in the western tropicalPacific and the negative ones in the eastern equatorial Pacific. Changes inSST of the ocean lead to the above-mentioned heating of the air that istransported westward.During La Ni na episodes the Walker Circulation is strengthened due tothe positive feedback that is initiated by the increase in the velocity of tradewinds.Both in normal conditions and during La Ni na events in the easternequatorial Pacific, the high pressure center is observed. In contrast, thereexists the atmospheric low in the vicinity of the WPWP. During El Ni no the Walker Circulation weakens and its spatial extent ismodified. The reason behind it is probably a much earlier (one yearbefore El Ni no) increase in velocity of the south-east trade winds in thecentral Pacific and a subsequent rapid weakening of these winds (Wyrtki,1975, 1979). This fast decrease in velocity of the south-east trade windsin the central Pacific stops the transportation of water from the eastern towestern equatorial Pacific. Its initial reason is not entirely clear. It isknown that in the western tropical Pacific westerly winds are generatedand, as a result of the enhanced Kelvin waves (see below for explanation),push the WPWP eastward (Figure 2). Warm and humid zone of deepatmospheric convection, locked to the WPWP, migrates along with theconsiderable rainfall toward the east. The eastward transport of the watersdriven by Kelvin waves strengthened by westerly winds is concurrentlyeased by the ceased or weakened upwelling in the eastern equatorial

El Ni no/Southern Oscillation and Selected Environmental Consequences83Figure 2 Locations of centers of low and high atmospheric pressure during normal a and El Nin o (left column) and locations of WPWP in these conditionsconditions, La Nin(right column).Pacific. Long-term weakening of trade winds causes the thermocline todeepen and, as a consequence, large volume of warm water may bestored below the surface of the eastern tropical Pacific. The easternmostlocation of the WPWP is controlled by the magnitude of a given El Ni noepisode (Clarke, Wang, & Van Gorder, 2000). In the case of very strongwarm ENSO events, the WPWP may be moved to the western equatorial coasts of South and North America. For instance, during El Ni no1982/1983 (one of the strongest warm ENSO episodes over pastdecades) the eastern edge of WPWP reached 90 W. Its location is a keyelement in modeling of the Walker Circulation during El Ni no.During El Ni no the low-pressure center migrates along with theWPWP, shrinking the longitudinal extent of the Walker Circulation. Asa result of this shift in the central and eastern equatorial Pacific atmospheric lows are stabilized. In contrast, the spatially large center of highatmospheric pressure is build up in the western tropical Pacific (Figure 2).The weakened Walker Oscillation acts between the central and easternequatorial Pacific, and the stronger El Ni no becomes the spatially tighterWalker Circulation is.

84Tomasz NiedzielskiIt is worth noting that the components of the Walker Circulation areinterrelated, and the positive feedback controls their dynamics. Indeed, theintensified (weakened) activity of a given process is triggered by intensification (weakening) of its driving process. The Walker Circulation acts asa close loop, as a chain reaction, the fluctuations of which are controlled bythe irregular dynamics of trade winds. Recall that velocity fluctuations oftrade winds in the eastern equatorial Pacific are preceded by strong variationsof the south-eastern trades in the central Pacific (Wyrtki, 1975). However, itis difficult to unequivocally state which of the processes is initial as they forma cycle and are driven by the above-mentioned feedback (see pages 28–30 inthe book by Clarke (2008)). It is likely that weakening of the WalkerCirculation causes weakening of trade winds what subsequently causes nextelements of the aforementioned feedback. The causal relationships may besummarized as follows (in bold a potential initial phenomenon is emphasizeddbut it is uncertain whether this phenomenon is really an initial onebecause may be triggered by another one). Intensification (weakening) of the Walker Circulation / increase(decrease) in the velocity of trade winds / intensification (weakening)of upwelling in the eastern equatorial Pacific / increase (decrease) inthe SST difference between the eastern and western equatorialPacific / intensification (weakening) of the deep atmosphericconvection in the western tropical Pacific, concurrent slight westwardmotion of the WPWP (concurrent eastward migration of the WPWP),increase (decrease) in the velocity of eastward air motion aloft just belowthe tropopause, intensification (weakening) of downward motion ofcold air from tropopause to the sea surface of the easternequatorial Pacific / intensification (weakening) of the WalkerCirculation.Intensification of the Walker Circulation occurs when there is a shiftfrom El Ni no (or normal) conditions to La Ni na conditions. Conversely,weakening of the Walker Circulation takes place in the case of transformation from La Ni na (or normal) conditions into El Ni no conditions.To build a fully coherent and comprehensive picture of the phenomenological background of ENSO, J. Bjerknes needed to detect thegeophysical process that ceases the feedback. In other words, he wanted toknow what causes that the Walker Circulation stops weakening orstrengthening and, as a consequence, what drives the shift (from ElNi no / normal conditions / La Ni na into La Ni na / normal conditions / El Ni no). The numerical solutions are provided by coupled

El Ni no/Southern Oscillation and Selected Environmental Consequences85ocean–atmosphere models which are based on mutual triggering betweenthe ocean and atmosphere. The knowledge about ocean waves that areresponsible for water transport in the equatorial Pacific is critical for buildinga conceptual framework of the ocean–atmosphere coupling.Water transport in the equatorial Pacific is controlled by two specificwaves, the equatorial Kelvin waves and Rossby waves. The first ones actalong the Equator (lack of the Coriolis force) and remove excess of watermasses from the central tropical Pacific by transporting them eastward. Incontrast, Rossby waves are associated with the nonzero Coriolis force, andtheir biggest activity is along parallels 4 S and 4 N. Rossby waves areresponsible for shifting the zone of deficit of water from the central equatorialPacific westward. The two waves migrate at dissimilar speeds, Kelvin wavesneed 70 days to travel over the entire Pacific, whereas Rossby waves do thisthree times longer. Equatorial Kelvin waves (Rossby waves) may be reflectedfrom continents and the reflection results in their transformation into thereflected Rossby (reflected Kelvin) waves. After reflection, the transformedwaves inherit the sign of sea level anomalies from the waves before reflection.The latter setting is theoretical and holds for a single wind impulse that triggersthe system (IRI, 2010), hence the Kelvin/Rossby wave dynamics is morecomplex during El Ni no, La Ni na or even normal conditions (Kim & Kim,2002). In addition, the equatorial Kelvin waves, having reached the west coastof South America, are transformed into coastal Kelvin waves which transportthe water northward and southward along the shore.The developing knowledge about Kelvin and Rossby waves supportedthe above-mentioned numerical studies that aimed to model ENSO with itsintrinsic turnabout and ocean–atmosphere coupling. The first coupled modelswere proposed by Cane and Patton (1984) and Cane, Zebi

CHAPTER TWO El Nino/Southern Oscillation and Selected Environmental Consequences Tomasz Niedzielski Institute of Geography and Regional Development, University of Wroc1aw, Wroc1aw, Poland . Long-term weakening of trade winds causes the thermocline to deepen and, as a consequence, large volume of warm water may be .

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