WALKER CIRCULATION K-M Lau, NASA/Goddard Space Flight Center, Greenbelt, MD, USA S Yang, NOAA/NWS/NCEP, Climate Prediction Center, Camp Springs, MD, USA Copyright 2002 Elsevier Science Ltd. All Rights Reserved. doi:10.1006/rwas.2002.0450 Lau, K-M NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA Yang, S NOAA/NWS/NCEP, Climate Prediction Center, Camp Springs, MD 20746, USA Introduction The term Walker Circulation was first introduced in 1969 by Professor Jacob Bjerknes, referring to the large-scale atmospheric circulation along the longi- tude–height plane over the equatorial Pacific Ocean. The Walker Circulation features low-level winds blowing from east to west across the central Pacific, rising motion over the warm water of the western Pacific, returning flow from west to east in the upper troposphere, and sinking motion over the cold water of the eastern Pacific. Since Bjerknes’s introduction of the Walker Circulation, there have been reports of similar east–west circulation cells spanning different longitudinal sectors along the Equator. Today, the Walker Circulation generally refers to the totality of the circulation cells as shown in Figure 1. Bjerknes originally named the Pacific east–west circulation the Walker Circulation because he consid- ered it the key part of Sir Gilbert Walker’s Southern Oscillation (see El Nino and the Southern Oscillation: Observation (0148)). He interpreted the Walker Cir- culation as an atmospheric circulation driven by the gradient of sea surface temperature along the Equator and suggested that the characteristics of the Walker Circulation were largely determined by the coupling between the tropical atmosphere and oceans. Bjerknes’s work on the Walker Circulation marked an important milestone toward our basic understand- ing of the dynamics of zonal atmosphere–ocean coupling along the equatorial Pacific Ocean. Although his results were based on very limited data, Bjerknes’s original conjecture that the year-to-year variation of the Walker Circulation is closely tied to that of the Southern Oscillation and El Nin ˜ o has been confirmed by a large number of observational and modeling studies during the several decades since his first report. Climatology and Variability Annual Mean Thanks to the advance in satellite observations and improved assimilation of observations into global general circulation models, we have now a much more detailed and quantitative description of the Walker Circulation. We know that the tropical wind is made up of rotational and divergent components. The former is directly related to the effects of the rotation of the Earth and the latter to the overturning circula- tion, driven by atmospheric heating processes. The Walker Circulation and associated overturnings in the equatorial plane should refer only to the divergent component of the wind. Figure 2A shows the annual climatology (the mean state of all months) of the overturning circulations along the equatorial plane as streamlines constructed from the divergent zonal and vertical winds. It can be seen that the major rising High tropospheric isobaric surface Low tropospheric isobaric surface INDIAN PACIFIC ATLANTIC 0° 90° E 180° 90° W 0° Longitude 0450-F0001 Figure 1 Schematic view of the east–west atmospheric circulation along the longitude–height plane over the Equator. The cell over the Pacific Ocean is referred to as the Walker Circulation. (Adapted from Webster (1983).) 0450-P0005 0450-P0010 0450-P0015 0450-P0020 WALKER CIRCULATION 1 rwas.2002.0450 26/4/02 13:03 Ed:: M. SHANKAR No. of pages: 6 Pgn:: seetharama
Transcript
WALKER CIRCULATION
K-M Lau, NASA/Goddard Space Flight Center,Greenbelt, MD, USA
S Yang, NOAA/NWS/NCEP, Climate Prediction Center,Camp Springs, MD, USA
Copyright 2002 Elsevier Science Ltd. All Rights Reserved.
doi:10.1006/rwas.2002.0450
Lau, K-MNASA/Goddard Space Flight Center,Greenbelt, MD 20771, USA
Yang, SNOAA/NWS/NCEP, Climate Prediction Center, CampSprings, MD 20746, USA
Introduction
The term Walker Circulation was first introduced in1969 by Professor Jacob Bjerknes, referring to thelarge-scale atmospheric circulation along the longi-tude–height plane over the equatorial Pacific Ocean.The Walker Circulation features low-level windsblowing from east to west across the central Pacific,rising motion over the warm water of the westernPacific, returning flow from west to east in the uppertroposphere, and sinking motion over the cold waterof the eastern Pacific. Since Bjerknes’s introduction ofthe Walker Circulation, there have been reports ofsimilar east–west circulation cells spanning differentlongitudinal sectors along the Equator. Today, theWalker Circulation generally refers to the totality ofthe circulation cells as shown in Figure 1.
Bjerknes originally named the Pacific east–westcirculation the Walker Circulation because he consid-ered it the key part of Sir Gilbert Walker’s SouthernOscillation (see El Nino and the Southern Oscillation:
Observation (0148)). He interpreted the Walker Cir-culation as an atmospheric circulation driven by thegradient of sea surface temperature along the Equatorand suggested that the characteristics of the WalkerCirculation were largely determined by the couplingbetween the tropical atmosphere and oceans.
Bjerknes’s work on the Walker Circulation markedan important milestone toward our basic understand-ing of the dynamics of zonal atmosphere–oceancoupling along the equatorial Pacific Ocean. Althoughhis results were based on very limited data, Bjerknes’soriginal conjecture that the year-to-year variation ofthe Walker Circulation is closely tied to that of theSouthern Oscillation and El Nino has been confirmedby a large number of observational and modelingstudies during the several decades since his first report.
Climatology and Variability
Annual Mean
Thanks to the advance in satellite observations andimproved assimilation of observations into globalgeneral circulation models, we have now a much moredetailed and quantitative description of the WalkerCirculation. We know that the tropical wind is madeup of rotational and divergent components. Theformer is directly related to the effects of the rotationof the Earth and the latter to the overturning circula-tion, driven by atmospheric heating processes. TheWalker Circulation and associated overturnings in theequatorial plane should refer only to the divergentcomponent of the wind. Figure 2A shows the annualclimatology (the mean state of all months) of theoverturning circulations along the equatorial plane asstreamlines constructed from the divergent zonal andvertical winds. It can be seen that the major rising
Hightroposphericisobaricsurface
Lowtroposphericisobaricsurface
INDIAN PACIFIC ATLANTIC
0° 90° E 180° 90° W 0°Longitude
0450-F0001 Figure 1 Schematic view of the east–west atmospheric circulation along the longitude–height plane over the Equator. The cell over the
Pacific Ocean is referred to as the Walker Circulation. (Adapted from Webster (1983).)
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branch of the Walker Circulation is found over thewestern Pacific and maritime continent, with a max-imum in the upper troposphere (300–200 hPa) overIndonesia (115–1201E). Awestward tilt with height ofthe ascending motion is also apparent. Over theeastern Pacific, there is a broad region of subsidence,with maximum descent in the coastal region ofEcuador (B801W). Connecting the ascending and
descending branches are low-level easterlies and upperlevel westerlies over the central and eastern Pacificwith strong low-level convergence near 1601E.
As is evident in Figure 2A, the Walker Circulationalso includes secondary circulation cells whose risingmotions appear over the land regions of SouthAmerican and Africa, with compensating subsidenceover the Atlantic and the Indian Ocean. Compared to
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0450-F0002 Figure 2 The equatorial east–west atmospheric circulation calculated from the reanalysis of data from the US National Centers for
Environmental Prediction and National Center for Atmospheric Research. Shown are the climatologies of all months (top panel), January
(middle panel), and July (bottom panel) for the period 1949–99. The streamlines where vertical motions have been multiplied by 30 times
are constructed from the divergent components of winds. Areas of strong upward and downward motions (in meters per second) are
shaded.
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the Pacific branch of the Walker Circulation, thesecells cover smaller longitude ranges and tend to haveweaker vertical motions in the annual mean climatol-ogy. Also closely linked to the Walker Circulation arethe east–west circulations away from the equatorialregion in the subtropics, associated with the large-scale monsoon circulation and the convergence of thetrade winds. Some meteorologists consider thesecirculations part of the family of Walker cells.
Because of the prevailing subsidence, the atmos-phere over the tropical eastern Pacific is highly stable(see Instability: Conditional/Convective (0174)). Thisstability is unfavorable to and limits the occurrence ofdeep clouds and precipitation. In contrast, over thewarm pool (see Ocean Circulation: General Processes(0276)) of the western Pacific and Indonesian region,the air is unstable and deep convective clouds andheavy precipitation occur frequently. A significantportion of latent heat associated with cumulus con-vection (see Thermodynamics: Moist (unsaturated)Air (0405)) that drives the global atmospheric circu-lation is released in the ascending region of the WalkerCirculation over the warm pool. In addition, theWalker Circulation is associated with low sea levelpressure in the west and high pressure in the east. Thebasin-wide pressure gradient is the main driving forcefor the low-level zonal wind of the Walker Circulation.Consistent with the distribution of surface pressureand sea surface temperature, the lower troposphere isrelatively warm in the ascending branch and cold in itsdescending branch. This means that the WalkerCirculation is a thermally direct circulation, whichconverts available potential energy to kinetic energy ofatmospheric motions.
Seasonal Variations
The location and intensity of the Walker Circulationundergo large fluctuations on seasonal to interannualtime scales. Also affecting the Walker Circulation areintraseasonal fluctuations associated with the Maddenand Julian Oscillation (see Tropical Meteorology:Intra-seasonal Oscillation (Madden-Julian Oscilla-tion) (0415)). The seasonal variation of the WalkerCirculation is a reflection of the east-west swaying ofthe large-scale circulation in the tropical atmospherein response to thermal contrasts between sea and landinduced by the annual cycle of incoming solar radia-tion. In January (Figure 2B), the ascending branch ofthe Walker Circulation is very pronounced over theIndo-Pacific and maritime continent region (60–1201E) and the descending branch over the easternPacific (150–901W). The South America/Atlantic cellis strong, with pronounced rising motion over thenorth-eastern Brazil (B601W). However, the Africa/
Indian Ocean cell is not very well established inJanuary. In July (Figure 2C), the Walker Circulationintensifies and becomes well defined, with the risingbranch shifted eastward and concentrated near 1501Eand the sinking branch over the eastern Pacific (120–901W). The strengthening of the Walker Circulation isconsistent with the seasonal development of the coldwater over the equatorial eastern Pacific and theincreased sea surface temperature gradient across thePacific. Mid-tropospheric sinking motion is foundover the eastern Indian Ocean. This sinking motion isconnected by low-level westerly wind blowing fromthe Indian Ocean toward the western Pacific, joiningthe rising motion there. The overturning is completedby the returning upper-level easterlies over the IndianOcean. The Indian Ocean cell is associated with thedevelopment of the South Asian monsoon (see Mon-soon: Overview (0235)). In July, the overturningmotion over the South America/Atlantic sector issuppressed.
Interannual Variability
Figure 3A shows the Walker Circulation duringJanuary 1998 when an El Nino event was at its peak.The Walker Circulation and accompanying east–westcirculations differed significantly from the climatolo-gy. Rising motions prevailed at almost all longitudes.In particular, strong ascent in the mid-tropospherereplaced climatological descending motion over thecentral and eastern Pacific, where the water wasanomalously warm due to El Nino. The WalkerCirculation was weakened and became less organized.On the contrary, during January 1999 when a La Nina,or reverse El Nino, was at its peak, the WalkerCirculation was enhanced and became very pro-nounced, with well-defined rising and sinking branch-es (Figure 3B). While the Walker Circulation andSouth American/Atlantic cell intensified, the IndianOcean cell weakened compared to the climatology (seeMonsoon: ENSO – Monsoon Interactions (0237)).Changes in the Walker Circulation between El Ninoand La Nina years shown above are accompanied bychanges in cloud and rainfall patterns. When theWalker Circulation weakens during an El Nino, cloudsand precipitation increase over the central-easternPacific and decrease over the western Pacific. Whenthe Walker Circulation strengthens during a La Nina,clouds and precipitation are enhanced over the west-ern Pacific and Indonesian region.
Ocean–Atmosphere Coupling
As Bjerknes postulated, the Walker Circulation and itsvariations are strongly coupled to fluctuations of the
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tropical sea surface temperature across the entirePacific basin. A climatological Walker Circulationwith strong surface easterlies maintains an equilibri-um state in the tropical atmosphere and ocean inwhich the western Pacific is characterized by highersea level, deeper thermocline, higher sea surfacetemperature, lower atmospheric surface pressure,and increased precipitation relative to the easternPacific. During an El Nino, a relaxation of the lowerlevel easterlies, signaling a weakening of the WC, is
accompanied by weaker upwelling in the easternPacific, leveling of the thermocline, and reduction ofsea surface temperature gradient across the Pacific.During a La Nina, changes of the opposite sign occur.
The apparently self-sustaining oscillations of theWalker Circulation stem from the interplay of variousfeedback processes associated with strong coupling ofthe tropical atmosphere and oceans. Warming of thewestern Pacific increases water vapor, latent heat, andupward motion in the atmosphere, producing deep
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0450-F0003 Figure 3 The equatorial east–west atmospheric circulation, calculated from the reanalysis of data from the US National Centers for
Environmental Prediction and National Center for Atmospheric Research, for January 1998 (peak of an El Nino; top panel (A) and January
1999 (peak of a La Nina; panel (B)). The streamlines where vertical motions have been multiplied by 30 times are constructed from the
divergent components of winds. Areas of strong upward and downward motions (in meters per second) are shaded.
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clouds and heavy precipitation. However, the in-creased clouds and precipitation reduce the incomingsolar radiation, causing the sea surface to cool, thuslimiting the further development of atmosphericconvection. This process tends to slow down theWalker Circulation and arrest the warming. On theother hand, the warm sea surface temperature andstrong upward atmospheric motions will lead tostronger surface easterlies that induce further warmwater in the western Pacific and cool water in theeastern Pacific through increased upwelling. Theincreased east–west sea surface temperature gradientcan lead to a stronger Walker Circulation. This processwas put forward by Bjerknes as an increase of theWalker Circulation also provides for ‘an increase ofthe east–west temperature contrast that is the cause ofthe Walker Circulation in the first place.’
Impacts on World Weather and Climate
The Walker Circulation regulates global exchange ofmomentum, heat, and water vapor within the tropicsvia massive overturning motions. In doing so, it playsan important role in the balance of atmospheric energyin the equatorial region and in determining thecharacteristics of weather and climate in the tropics.The strongest atmospheric impacts associated with thefluctuations of the Walker Circulation are found overtropical and subtropical regions around the Pacificrim. During an El Nino, the weakening WalkerCirculation causes widespread drought (see Drought(0037)) in Indonesia/maritime continent, drought innortheastern Brazil, severe floods (see Flooding(0151)) in Peru and Ecuador, and in south-easternBrazil and northern Argentina. During a La Nina, theWalker Circulation intensifies and leads to rainfallanomalies with reverse sign compared to El Nino. TheWalker Circulation also represents the fundamentallink between the changes in sea surface temperature inthe eastern Pacific and the variability of the Asian–Australian monsoon. The mechanisms that are re-sponsible for the interactions between the monsoonand El Nino Southern Oscillation have been attribut-ed, in part, to the changes in the Walker Circulation.Although individual case may vary, in the summerpreceding the peak phase of El Nino, which usuallyoccurs in the northern winter, the Walker Circulationis weakened and shifted eastward owing to reducedeast–west sea surface temperature gradient across thePacific ocean. This suppresses broad-scale convectionover the western Pacific and eastern Indian Ocean andleads to weaker South Asian monsoon. There is someevidence that changes in land use, such as deforesta-tion in Brazil and in Indonesia, may also cause long-
term changes in the Walker Circulation due to changesin land temperature, and therefore east–west thermalgradient along the tropical belt.
Besides modulating tropical weather and climate,the Walker Circulation is important as a driver ofenergy exchange between the tropics and higherlatitudes. Bjerknes envisioned an interaction betweenthe Walker Circulation and the Hadley Circulation inthe form of an inverse variation between the twocirculations. Bjerknes stated that ‘when the cold waterbelt along the Equator is well developed, the air aboveit will be too cold and heavy to join the ascendingmotion in the Hadley circulations. Instead, the equa-torial air flows westward between the Hadley circu-lations of the two hemispheres to the warm westPacific.’ Such an interaction leads to changes inextratropical westerly jet streams (see Jet Streaks(0187)) and teleconnection patterns such as thePacific/North American pattern. These changes arethe causes for severe weather and climate anomalies inthe Asian–Pacific–American regions.
Summary
The Walker Circulation comprises east–west atmos-pheric circulation cells along the equatorial belt. ThePacific branch of the Walker Circulation consists ofeasterly winds at the lower troposphere, westerlywinds at the upper troposphere, rising motion over thewestern Pacific, and subsidence over the easternPacific. The Walker Circulation is characterized by apronounced seasonal cycle and interannual variabil-ity, and is an integral component of the El Nino-Southern Oscillation climate system. Fluctuations ofthe Walker Circulation can lead to extreme weatherconditions in different parts of the world.
See Also
Drought (0037). El Nino and the Southern Oscillation:Observation (0148). Flooding (0151). Instability: Condi-tional/Convective (0174). Jet Streaks (0187). Monsoon:ENSO – Monsoon Interactions (0237); Overview (0235).Ocean Circulation: General Processes (0276). Ther-modynamics: Moist (unsaturated) Air (0405). TropicalMeteorology: Intra-seasonal Oscillation (Madden-JulianOscillation) (0415).
Further Reading
Bjerknes J (1966) A possible response of the atmosphericHadley circulation to equatorial anomalies of oceantemperature. Tellus 4: 820–829.
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Krishnamurti TN (1971) Tropical east–west circulationsduring the northern summer. Journal of AtmosphericScience 28: 1342–1347.
Krishnamurti TN, Kanamitsu M, Koss WJ, Lee JD (1973)Tropical east–west circulations during the northernwinter. Journal of the Atmospheric Sciences 30: 780–787.
Troup AJ (1965) The ‘southern oscillation.’ QuarterlyJournal of Royal Meteorological Society 91: 490–506.
Lau KM, Bua W (1998) Mechanisms of monsoon–SouthernOscillation coupling: Insights from GCM experiments.Climate Dynamics 14: 759–779.
Walker GT (1923) Correlation in seasonal variations ofweather, VIII: a preliminary study of world weather.Memoires of the Indian Meteorological Department,Calcutta, 24(4): 75–131.
Walker GT (1924) Correlation in seasonal variations ofweather, IX: A further study of world weather. Memoires
of the Indian Meteorological Department, Calcutta,24(9): 275–332.
Walker GT (1928) World weather III. Memories of the RoyalMeteorological Society, London, 2(17): 97–106.
Walker GT, Bliss EW (1932) World weather V. Memoires ofthe Royal Meteorological Society, London, 4(36): 53–84.
Wallace JM, Gutzler DS (1981) Teleconnections in thepotential height field during the Northern Hemispherewinter. Monthly Weather Review 109: 784–812.
Webster PJ (1983) The large scale structure of the tropicalatmosphere. In: Hoskins and Pearce (eds) GeneralCirculation of the Atmosphere, pp. 235–275. London:Academic Press.
Webster PJ, Yang S (1992) Monsoon and ENSO: selectivelyinteractive systems. Quarterly Journal of Royal Meteor-ological Society 118: 877–926.
Zhou J, Lau KM (2001) Principal modes of interannual anddecadal variability of summer rainfall over South Amer-ica. International Journal of Climatology 21: 1623–1644.
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