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Walker circulation

The Walker circulation is a zonal atmospheric circulation cell in the tropical troposphere, primarily over the equatorial Pacific Ocean, characterized by low-level easterly trade winds that transport warm surface water westward toward the Maritime Continent, where moist air rises, while cooler waters and sinking dry air prevail in the eastern Pacific.^[1] This thermally direct circulation is driven by sea surface temperature gradients, with high pressure and subsidence over the relatively cool eastern Pacific contrasting low pressure and convection over the warm western Pacific.^[2] Named after British meteorologist Gilbert Thomas Walker, who identified it in the early 20th century through analysis of pressure variations across the Indian and Pacific Oceans, the circulation was later formalized as part of the broader tropical atmospheric dynamics by Jacob Bjerknes in 1969.^[1] The structure of the Walker circulation involves surface winds flowing eastward to westward near the equator, an upper-level return flow from west to east, and vertical motion that connects these branches, forming a closed loop approximately 10–15 km in vertical extent.^[3] These trade winds enhance upwelling of cold nutrient-rich water along the eastern Pacific coasts, such as Peru and Ecuador, while piling warm water in the west, which fuels intense rainfall and thunderstorm activity over regions like Indonesia and northern Australia.^[2] The circulation's strength and position influence global weather patterns, including drought in the western Pacific during weak phases and flooding in the east during strong phases.^[3] Central to the El Niño-Southern Oscillation (ENSO) phenomenon, the Walker circulation weakens during El Niño events as trade winds relax, allowing the western warm pool to shift eastward, reducing western Pacific convection and enhancing it centrally, which leads to global temperature anomalies and altered precipitation.^[1] Conversely, La Niña conditions strengthen the circulation, intensifying easterlies, deepening the eastern cool pool, and amplifying convection over the Maritime Continent, often resulting in heightened tropical cyclone activity and cooler global temperatures.^[2] Variations in the Walker circulation thus play a pivotal role in interannual climate variability, affecting agriculture, fisheries, and extreme weather worldwide.^[3]

Historical Development

Gilbert Walker's Contributions

Gilbert Thomas Walker (1868–1958) was a British mathematician and meteorologist renowned for his pioneering statistical approaches to climate forecasting. Educated at Trinity College, Cambridge, where he was senior wrangler in the Mathematical Tripos of 1889, Walker initially focused on applied mathematics, including electrodynamics and aerodynamics, earning the Adams Prize in 1899 for his work on electromagnetic fields.^[4] His transition to meteorology occurred in 1903 when he joined the Indian Meteorological Department in Simla, and by January 1904, he was appointed Director General of Observatories in India, a position he held until his retirement in 1924.^[5] In this role, Walker transformed the Indian weather service by emphasizing empirical statistical methods to predict seasonal variations, driven by the urgent need to mitigate famines linked to monsoon failures, such as those in 1877 and 1899.^[4] Walker's research in India centered on correlating global weather patterns to improve monsoon rainfall forecasts, culminating in a series of influential publications known as "World Weather." Beginning with "Correlation in Seasonal Variations of Climate" in 1909, he expanded his analyses across 20 papers, examining relationships between pressure, temperature, and rainfall anomalies worldwide.^[5] These efforts addressed the limitations of contemporary physical models by substituting complex atmospheric dynamics with simpler statistical correlations, enabling practical seasonal predictions for India's agriculture-dependent economy.^[4] In 1924, Walker first coined the term "Southern Oscillation" to describe a global-scale fluctuation linking anomalous sea-level pressures across the Indian Ocean, Pacific, and Atlantic regions. This built on his earlier studies, such as those in 1908, 1918, and 1923, which sought objective long-range forecasts for Indian summer monsoon rainfall by identifying interconnected atmospheric patterns.^[6] He further elaborated on it in his 1937 publication, "World Weather VI," appearing in the Memoirs of the Royal Meteorological Society.^[7] Walker's methodology relied on computing correlation coefficients between long-term seasonal datasets of sea-level pressure, temperature, and rainfall from a global network of stations, often spanning 40 years or more.^[8] He analyzed data from numerous observatories, including up to 21 key sites for defining oscillation indices, to uncover systematic relationships, such as the inverse correlation where high pressure in the eastern Pacific coincided with low pressure in the western Pacific and Indian Ocean regions.^[8] This statistical approach, devoid of physical modeling at the time, highlighted lagged correlations—for instance, a +0.83 coefficient between summer and winter phases of the oscillation—allowing for predictive formulas incorporating variables like South American pressures.^[8] The key findings from Walker's analyses revealed zonal "seesaw" patterns in equatorial pressures, where opposing anomalies drove variability in the Indian monsoon, with low Indian Ocean pressures associated with stronger rainfall and vice versa.^[6] He identified the Southern Oscillation as one of three major pressure oscillations (alongside the North Atlantic and North Pacific Oscillations), establishing it as a primary driver of interannual monsoon fluctuations and providing empirical predictors verified in later studies.^[6] The atmospheric circulation pattern later known as the Walker circulation, embodying these zonal pressure dynamics, was named in Walker's honor in 1969 by Norwegian-American meteorologist Jacob Bjerknes during his investigations of equatorial air-sea interactions.^[9]

Early Observations and Naming

Early observations of atmospheric phenomena related to the Walker circulation date back to the 18th century, when English meteorologist George Hadley described the trade winds and equatorial easterlies as components of a large-scale meridional circulation driven by differential solar heating at the equator and Earth's rotation. In his 1735 paper, Hadley proposed that warm air rising near the equator creates low-level convergence, resulting in surface winds that acquire an easterly component due to the Coriolis effect, forming the northeast and southeast trade winds north and south of the equator, respectively.^[10] Although Hadley's model focused on the Hadley cell extending to the subtropics, it laid foundational insights into tropical wind patterns that later contributed to understanding zonal circulations.^[11] By the late 19th century, maritime expeditions provided empirical data on pressure variations across the tropical Pacific, highlighting east-west gradients. The HMS Challenger expedition (1872–1876) systematically recorded atmospheric pressure, temperature, and winds during its global voyage, including extensive traverses of the Pacific Ocean, contributing to early meteorological datasets.^[12] Concurrently, in the context of devastating Indian monsoon failures in the 1870s—such as the 1876–1878 famine that killed millions—meteorologist Henry Blanford, appointed as India's first Meteorological Reporter in 1875, observed connections between deficient winter snow cover in the Himalayas and subsequent summer rainfall shortfalls, attributing them to altered pressure patterns over Asia and Australia.^[13] Blanford's 1879 analysis of the high atmospheric pressure anomaly during 1876–1878 linked these events to broader regional weather disruptions but lacked a comprehensive zonal circulation framework.^[14] The terminological evolution began with Gilbert Walker's identification of correlated pressure oscillations across the tropics in the early 1920s, which he termed the "Southern Oscillation" in his 1924 publication based on statistical analyses of global weather records.^[15] This concept gained physical context decades later when Norwegian-American meteorologist Jacob Bjerknes explicitly named the "Walker circulation" in his 1969 paper, portraying it as a zonal overturning cell in the equatorial troposphere—with ascent over the western Pacific warm pool, upper-level westerlies, descent over the eastern Pacific, and surface easterlies—directly building on Walker's oscillation as an atmospheric teleconnection mechanism.^[16] Over time, the term "Walker cell" became standard in descriptions of tropical circulation cells, distinguishing it from meridional cells like the Hadley circulation while emphasizing its role in east-west exchanges.^[17]

Physical Mechanisms

Atmospheric Components

The Walker circulation features a pronounced zonal sea-level pressure gradient along the equatorial Pacific, with high pressure prevailing over the cooler waters of the eastern Pacific and low pressure over the warmer waters of the western Pacific and Indonesian region. This gradient arises from differential heating and drives the surface easterly trade winds, which flow westward across the equatorial Pacific.^[18]^[19] In the vertical, the circulation exhibits a distinct structure: low-level easterly winds dominate from the surface up to approximately 850 hPa, feeding into strong rising motion over the Maritime Continent where convergence occurs. At upper levels around 200 hPa, these air parcels diverge as westerly winds, completing the return flow, while subsidence and descending motion characterize the eastern Pacific, reinforcing the high-pressure system there. This configuration forms a closed zonal overturning cell with a meridional extent of about 10° latitude north and south of the equator.^[19]^[20] Thermodynamically, the circulation is powered by intense heating from elevated sea surface temperatures in the western Pacific warm pool, which promotes deep moist convection and latent heat release, establishing the ascent branch over the Maritime Continent. This diabatic heating creates the zonal temperature contrast that sustains the pressure gradient, with the warm, moist air rising to form extensive cloud systems and precipitation.^[18]^[19] The dynamics of the zonal winds are described by the momentum equation in the tropical atmosphere, where the zonal acceleration is given by

\frac{\partial u}{\partial t} = -\frac{\partial \phi}{\partial x} - (\mathbf{u} \cdot \nabla) u + f v,

with \phi as the geopotential height, \mathbf{u} the velocity vector, u the zonal wind component, v the meridional wind, and f the Coriolis parameter. In the near-equatorial tropics, the small f emphasizes the role of the zonal pressure gradient force -\partial \phi / \partial x in accelerating the easterlies, balanced primarily by nonlinear advection and frictional effects in the steady-state Walker cell.^[20]

Oceanic Components

The oceanic components of the Walker circulation are driven primarily by the interaction between equatorial winds and ocean dynamics, resulting in distinct zonal patterns of surface currents, vertical motions, and thermal structures across the tropical Pacific. Easterly trade winds force the South Equatorial Current (SEC), a westward-flowing surface current that transports warm, low-salinity water from the eastern to the western Pacific, leading to the accumulation of this water in the western Pacific warm pool.^[21] This warm pool, characterized by sea surface temperatures (SSTs) exceeding 28°C, spans the region from the date line to the Indonesian seas and serves as a key heat reservoir influencing the overall circulation.^[22] The surface westward transport is balanced by the subsurface Equatorial Undercurrent (EUC), an eastward-flowing jet beneath the thermocline that returns warm water to the eastern Pacific. In the eastern Pacific, the easterly winds induce divergence through Ekman transport, promoting upwelling of cooler subsurface waters along the coast of South America. This process is driven by offshore Ekman transport due to coastal wind stress and equatorial divergence, bringing cold water from below the shallow thermocline to the surface. The resulting upwelling cools SSTs in the eastern equatorial Pacific to approximately 20–25°C, creating a sharp zonal temperature gradient that reinforces the atmospheric pressure differences central to the Walker circulation.^[23]^[24] A critical feature maintaining this east-west SST contrast is the tilted thermocline, which deepens to about 200 m in the western Pacific warm pool but shallows to around 50 m in the east, allowing cold water to access the surface more readily in the upwelling zone.^[23] The meridional Ekman transport, given by M_y = \frac{\tau_x}{f} where \tau_x is the zonal wind stress (negative for easterlies) and f is the Coriolis parameter, drives divergence near the equator: southward transport north of the equator and northward south of it, sustaining the upwelling.^[25] This oceanic response feeds back into the system, as the cooler eastern SSTs promote atmospheric subsidence and high pressure, which in turn strengthen the easterly winds and perpetuate the circulation.^[26]

Regional and Global Impacts

Effects on Tropical Precipitation

The Walker circulation drives a zonal dipole in tropical precipitation patterns, with its ascending branch over the western Pacific promoting intense convection and heavy rainfall in the Maritime Continent region, including Indonesia. This upward motion of warm, moist air fuels deep convective activity, supporting monsoonal systems and frequent thunderstorms that deliver area-averaged monthly rainfall totals exceeding 200 mm under normal conditions. For instance, in Indonesia, typical monthly precipitation ranges from 180 to 280 mm year-round, largely sustained by this convective uplift.^[27]^[28]^[29] In contrast, the descending branch over the eastern Pacific induces subsidence that suppresses convection, resulting in arid conditions along coastal South America. This downdraft warms and dries the mid-troposphere, inhibiting cloud formation and rainfall, which contributes to the extreme aridity of regions like the Atacama Desert, where annual precipitation can be less than 2 mm in its hyperarid core. The subsidence strengthens the trade winds and enhances atmospheric stability, further limiting moisture availability.^[30]^[31] This zonal asymmetry manifests as a precipitation dipole, with wet conditions in the west and dry in the east, often quantified by minima in outgoing longwave radiation (OLR) over the western Pacific indicating robust deep convection. The dipole reflects the circulation's response to east-west sea surface temperature gradients, amplifying rainfall contrasts across the equatorial Pacific. Seasonally, the Walker circulation strengthens during boreal winter (December-February), enhancing the dipole due to reduced interference from the Hadley cells, while it weakens in boreal summer as meridional circulations dominate. This modulation by the El Niño-Southern Oscillation can intensify or reverse the dipole, as detailed in related analyses.^[28]^[32]

Influence on Global Weather Patterns

The Walker circulation exerts influence on mid-latitude weather through teleconnections involving the propagation of Rossby waves, primarily driven by upper-level divergence associated with its ascending branch over the western Pacific warm pool. This divergence acts as a Rossby wave source, exciting stationary Rossby waves that propagate poleward and eastward, perturbing the mid-latitude jet stream and altering storm tracks. In North America, these waves can weaken the jet stream, leading to amplified weather extremes such as heatwaves or cold outbreaks, while in Europe, they contribute to variability in blocking patterns and precipitation anomalies.^[33] A weakening of the Walker circulation is linked to reduced rainfall and increased drought risk in eastern Australia, as the eastward shift in convection suppresses moisture convergence over the region. Observational analyses show that periods of weak Walker circulation coincide with below-average seasonal precipitation, exacerbating soil moisture deficits and agricultural impacts, as seen in multi-year droughts where anomalous subsidence dominates. This teleconnection arises from the diminished trade winds and altered moisture transport, independent of direct tropical forcing.^[34] The strength of the Walker circulation modulates Atlantic hurricane activity via changes in vertical wind shear across the tropical North Atlantic. During phases of strong Walker circulation, reduced upper-level westerly winds and enhanced trade winds result in lower vertical wind shear, favoring the development and intensification of hurricanes by allowing more organized convection. Conversely, a weak Walker circulation amplifies shear through stronger upper-level divergence and opposing surface winds, which disrupts storm formation and leads to fewer and weaker hurricanes.^[35] Observational evidence reveals correlations between Walker circulation variability and the North Atlantic Oscillation (NAO), influencing European winter weather. A weak Walker circulation phase tends to coincide with a negative NAO, characterized by a southward-shifted jet stream, increased blocking highs over Scandinavia, and colder-than-average temperatures across central and eastern Europe. This linkage is mediated by stratospheric-tropospheric interactions and extratropical wave trains, with composite analyses showing enhanced cold air outbreaks during such periods.^[36]

Relation to Climate Variability

Connection to El Niño-Southern Oscillation

The Walker circulation plays a central role in the El Niño-Southern Oscillation (ENSO), representing the atmospheric component of this coupled ocean-atmosphere phenomenon. In the normal or neutral state, the Walker circulation is robust, characterized by strong easterly trade winds that drive a tilted thermocline, with deeper warm waters in the western Pacific and upwelling of cooler waters in the east, maintaining a zonal sea surface temperature (SST) gradient.^[37] During La Niña phases, this circulation intensifies, with enhanced easterlies further steepening the thermocline slope and amplifying the east-west SST contrast, leading to cooler eastern Pacific SSTs and suppressed convection there.^[38] Conversely, in El Niño conditions, the Walker circulation weakens significantly, resulting in slackened trade winds, a flattened thermocline across the equatorial Pacific, and a reversal of the SST gradient as warm waters shift eastward, promoting widespread convection over the central and eastern Pacific.^[1] A key mechanism linking the Walker circulation to ENSO dynamics is the Bjerknes feedback, a positive ocean-atmosphere interaction that amplifies perturbations in the system. Initially proposed by Jacob Bjerknes, this feedback operates as follows: weakened easterly winds reduce upwelling in the eastern Pacific, allowing SSTs to rise there; the warmer SSTs then enhance local convection, which further weakens the trade winds via altered Walker circulation patterns, reinforcing the initial warming and perpetuating the El Niño state.^[39] This process can be conceptually represented in a simplified mixed-layer heat budget equation for SST tendency:

\frac{dSST}{dt} \propto -\mathbf{u} \cdot \nabla SST + Q

where \mathbf{u} is the zonal wind velocity, \nabla SST is the SST gradient, and Q represents net surface heat flux; the advective term (- \mathbf{u} \cdot \nabla SST) diminishes during reduced trades, contributing to eastern warming.^[40] The feedback's strength influences the rapidity and intensity of ENSO transitions, with stronger coupling leading to more pronounced Walker circulation anomalies. ENSO exhibits an irregular cycle with a typical periodicity of 2 to 7 years, during which the Walker circulation alternates between strengthened and weakened states.^[41] La Niña transitions often involve a rebound strengthening of the Walker circulation, restoring the zonal SST gradient and easterlies after an El Niño event, though the exact timing and amplitude vary due to internal variability and external influences.^[37] This oscillation is quantified using the Southern Oscillation Index (SOI), originally developed by Gilbert Walker as a measure of equatorial pressure anomalies; it is calculated as the standardized difference in sea-level pressure between Tahiti (eastern Pacific) and Darwin, Australia (western Pacific), where negative SOI values indicate weakened Walker circulation during El Niño, and positive values signal strengthening during La Niña.^[8]^[42]

Role in Climate Change Projections

Climate model projections indicate a weakening of the Walker circulation under anthropogenic global warming, primarily driven by the reduction in zonal sea surface temperature (SST) gradients across the equatorial Pacific due to overall mean SST increases. According to the IPCC Sixth Assessment Report (AR6), there is high confidence that the Walker cell will weaken by the end of the 21st century in most coupled general circulation models, with CMIP6 simulations projecting a reduction in circulation strength of approximately 10-15% under high-emission scenarios like SSP5-8.5 by 2100.^[43]^[44] This weakening arises from thermodynamic effects, where enhanced atmospheric stability in a warmer climate suppresses convection and reduces the zonal pressure gradient that drives the circulation.^[43] The projected changes in the Walker circulation have significant implications for the El Niño-Southern Oscillation (ENSO), potentially leading to more frequent and intense extreme El Niño events. Models suggest that the weakened mean state of the circulation could enhance the Bjerknes feedback, shifting the Walker cell eastward during ENSO extremes and amplifying their impacts.^[45]^[46] The IPCC AR6 assesses medium confidence in an increased frequency of extreme El Niño occurrences due to greenhouse warming.^[45] Regionally, these shifts are expected to alter precipitation patterns, resulting in drier conditions over the Maritime Continent and wetter conditions in the eastern Pacific. A constrained analysis of CMIP6 models projects reduced rainfall over the Maritime Continent in future climates, exacerbating drought risks in Indonesia and surrounding areas due to suppressed ascent in the weakened Walker cell. Conversely, enhanced convection over the eastern Pacific could lead to increased precipitation and flood risks in regions like Peru and Ecuador, consistent with an El Niño-like mean state.^[43] However, recent observations as of 2024 indicate a strengthening trend in the Walker circulation, attributed to sea surface temperature pattern effects that temporarily counteract the thermodynamic weakening expected under global warming.^[47] Despite these projections, substantial uncertainties remain, stemming from model biases in simulating tropical convection and the spread in the CMIP6 ensemble regarding Walker circulation variance. For instance, inter-model differences in cloud feedbacks and resolution lead to a wide range of projected weakening rates, with some models showing minimal changes or even strengthening under certain SST patterns.^[43]^[48] These discrepancies highlight the need for improved representation of moist processes in future model generations to refine regional climate impacts.^[49]

Observations and Modeling

Measurement Techniques

The Walker Circulation Index (WCI) quantifies the strength of the Walker circulation through the difference in 500 hPa vertical velocity anomalies between the eastern equatorial Pacific (5°S–5°N, 160°–120°W) and the western equatorial Pacific (5°S–5°N, 120°–160°E), reflecting the circulation's east-west gradient.^[50] This index, originally proposed by Wang in 2002 using vertical velocity, captures weakening during El Niño events when the east-west contrast in ascent diminishes.^[51] Complementing the WCI, the Southern Oscillation Index (SOI) serves as a pressure-based measure of the Walker circulation's intensity, computed from standardized sea-level pressure differences between Darwin, Australia, and Tahiti, with negative values indicating weakened circulation during El Niño phases.^[8] The SOI, developed from Walker's early work, provides a long-term record dating back to the late 19th century and correlates strongly with zonal wind anomalies, offering an indirect proxy for circulation strength.^[52] Satellite observations have revolutionized the monitoring of the Walker circulation's precipitation dipole, where enhanced rainfall over the western Pacific warm pool contrasts with suppressed precipitation in the eastern Pacific. The Tropical Rainfall Measuring Mission (TRMM), operational from 1997 to 2017, used its Precipitation Radar and Microwave Imager to map tropical rainfall with high spatial resolution, revealing the dipole's anomalies tied to circulation shifts, such as increased western Pacific precipitation during La Niña-strengthened Walker phases.^[53] TRMM data have quantified the dipole's variability, revealing substantial rainfall contrasts across the equatorial Pacific during strong circulation events.^[54] Additionally, satellite altimetry missions like TOPEX/Poseidon (1992–2006) measured sea surface height anomalies to infer thermocline depth variations, which deepen in the west and shallow in the east under strong Walker forcing, with anomalies up to 50 cm linked to circulation intensity.^[55] These altimetry records track the associated upwelling in the eastern Pacific, providing dynamic height proxies for the circulation's oceanic component.^[56] Reanalysis products integrate diverse observations to produce comprehensive three-dimensional wind fields for assessing Walker circulation strength. ERA5, the fifth-generation European Centre for Medium-Range Weather Forecasts reanalysis spanning 1940 to present, assimilates satellite, radiosonde, and surface data to derive vertical wind profiles, enabling calculation of the mass streamfunction ψ at 500 hPa, where negative values indicate upward motion over the western Pacific and descent eastward, quantifying cell vigor with uncertainties below 10% in the tropics.^[57] Similarly, JRA-55, the Japanese 55-year Reanalysis from 1958 onward, offers high-resolution 3D winds through four-dimensional variational assimilation, supporting streamfunction diagnostics that reveal circulation weakening trends via reduced ψ extrema.^[58] Both products facilitate tracking of zonal wind patterns, with ERA5 showing superior performance in tropical divergence fields compared to predecessors.^[59] In-situ observations from the Tropical Ocean Global Atmosphere (TOGA)-Tropical Atmosphere Ocean (TAO) buoy array, deployed since 1985 and maintained by NOAA, provide direct measurements along the equatorial Pacific from 8°N to 8°S and 165°E to 95°W. The array's approximately 70 buoys record surface winds, sea surface temperatures (SSTs), and upper-ocean currents at hourly intervals, capturing Walker-related anomalies like easterly wind bursts exceeding 10 m/s during active phases.^[60] These data have been instrumental in validating reanalyses and satellites, revealing SST gradients of up to 4°C across the basin tied to circulation strength, with near-real-time transmission enabling ongoing monitoring.^[61] The TAO array's longevity ensures a continuous record through 2025, supporting quantification of interannual variability in the circulation's surface expressions.^[62]

Recent Trends and Future Outlook

Reanalyses indicate a debated long-term weakening of the Walker circulation since the mid-20th century, with model projections suggesting a rate of about 8% decline per degree of global warming, primarily attributed to greenhouse gas forcing that enhances atmospheric stability and reduces the zonal sea surface temperature gradient across the equatorial Pacific.^[63] This long-term trend, identified in studies using sea level pressure differences and zonal wind indices, has been confirmed by analyses in the 2020s, which show persistent slowdown despite natural variability, though recent studies as of 2023–2025 attribute multidecadal variability to competing effects of greenhouse gases (weakening) and aerosols (strengthening), with no consensus on net industrial-era change.^[64]^[65]^[65] Post-2000 observations reveal a complex pattern, with strengthening of the circulation from 1992 to 2011 driven by La Niña-like conditions and anthropogenic aerosols, followed by an overall slowdown and temporary weakening during the 2014-2016 El Niño event due to enhanced easterly trade wind disruptions.^[65] Recent data from 2000-2020 indicate insignificant but consistent weakening across multiple indices derived from reanalysis products.^[28] In 2023-2024, amid ongoing global warming, the circulation exhibited anomalous weakness during a subdued El Niño, characterized by muted westerly wind and rainfall anomalies in the tropical Pacific, highlighting amplified sensitivity to warming trends.^[66] Projections from CMIP6 models suggest the Walker circulation will weaken further in the 21st century under greenhouse gas forcing, with thermodynamic effects dominating to reduce its intensity by enhancing moist static energy gradients.^[67] This weakening is expected to contribute to greater variability in the circulation, potentially increasing the frequency and intensity of ENSO extremes, particularly strong El Niño events, as eastern Pacific sea surface temperature variability amplifies.^[68] Insights from CMIP6 emphasize that post-2010 data integration reveals higher uncertainty in these projections compared to earlier model generations, underscoring the need for updated observational constraints. Key research gaps include the requirement for improved vertical resolution in climate models to better capture the decay of the Walker cell's upper-tropospheric branch, as current coarse resolutions contribute to biases in simulating its strength and response to forcing.^[69] Enhanced model fidelity in vertical structure would reduce discrepancies between simulated and observed trends, particularly in projecting circulation variability under accelerated warming.

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Bjerknes hypothesized that a positive ocean-atmosphere feedback involving the Walker circulation is a cause of ENSO. (The. Walker circulation consists of the ...
[37]
[PDF] Understanding ENSO Physics—A Review
Bjerknes hypothesized that a positive ocean-atmosphere feedback involving the Walker circulation is a cause of. ENSO. An initial positive sea surface ...
[38]
Climate Prediction Center - ENSO FAQ - NOAA
Apr 26, 2012 · Items 1-6 above are associated with a weaker-than-average equatorial Walker Circulation. An equatorward shift and eastward extension of the ...
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El Niño / Southern Oscillation (ENSO)
The Southern Oscillation Index (SOI) is a standardized index based on the observed sea level pressure (SLP) differences between Tahiti and Darwin, Australia.
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Chapter 4 | Climate Change 2021: The Physical Science Basis
4.2), and a weakening of the Walker circulation in most models. Those models with a El Niño-like warming tend to project a strengthening of ENSO SST ...
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The weakening of the tropical circulation is caused by the lifting of ...
Aug 9, 2023 · Based on the CMIP6 model simulations we find that the tropical circulation weakens by about 10-15% over the twenty-first century. The analysis ...
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Chapter 11: Weather and Climate Extreme Events in a Changing ...
For AR6 regions, several studies suggest an increase in the frequency ... et al., 2014b: Increasing frequency of extreme El Niño events due to greenhouse warming.
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Future increase in extreme El Niño supported by past glacial changes
Sep 25, 2024 · Under future greenhouse warming, a weakened Walker circulation leads to stronger mechanical coupling and an enhanced Bjerknes feedback in the ...
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How Robust Is the Weakening of the Pacific Walker Circulation in ...
This study investigates the changes in the Pacific Walker circulation under anthropogenic forcing and the sensitivity of its weakening response to internal ...Missing: AR6 | Show results with:AR6
[45]
Chapter 7: The Earth's Energy Budget, Climate Feedbacks, and ...
This chapter assesses the present state of knowledge of Earth's energy budget: that is, the main flows of energy into and out of the Earth system.
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(PDF) Indices of Pacific Walker Circulation Strength - ResearchGate
Feb 13, 2023 · In this study, we review and compare 10 different indices measuring the strength of the PWC using data from the ERA5 reanalyses for the period ...<|control11|><|separator|>
[47]
A hypothesis and a case-study projection of an influence of MJO ...
The change in the zonal circulation pattern is also confirmed by both the Walker Circulation Index ... hPa zonal wind), and then these are projected onto the WH04 ...
[48]
Weakening of the Walker Circulation and apparent dominance of El ...
Sep 20, 2007 · The Walker Circulation weakens during El Niño years and strengthens during La Niña years [e.g., Philander, 1990].Abstract · Introduction · Results · Discussion and Conclusions
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https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-7/
[50]
The Effect of Atmospheric Heating From Precipitation Over the ...
... Walker circulation. We use precipitation rates from the Tropical Rainfall Measurement Mission (TRMM) satellite to estimate atmospheric heating due to ...
[51]
Local and remote forcing of decadal sea level and thermocline ...
Jan 31, 2013 · Beginning in the early 1990s, decadal variability of the equatorial Pacific trades forces thermocline variations that modify the sea level and ...
[52]
Summary | TOPEX/Poseidon - Ocean Surface Topography from Space
TOPEX/Poseidon measured ocean surface topography, sea levels, and currents, and studied El Niño, La Niña, and ocean circulation.Missing: thermocline Walker
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The ERA5 global reanalysis - Royal Meteorological Society (RMetS)
May 17, 2020 · This paper presents a comprehensive overview of the ERA5 global reanalysis from 1979 onwards, which is produced at ECMWF as part of the ...
[54]
(PDF) Structure of the Pacific Walker Circulation Depicted by the ...
Mar 18, 2022 · We used the zonal mass streamfunction (ZMS) over the equatorial Pacific to indicate the strength of the PWC. The PWC's average monthly or ...
[55]
Trends in the tropospheric general circulation from 1979 to 2022
Jul 21, 2022 · Several comparisons are made between ERA5 and the JRA-55 reanalysis and between ERA5 and the observations it assimilated. They show reassuring ...
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McPhaden et al. -- TOGA observing system -- 2. Overview
These elements include an island and coastal tide gauge network to provide sea level measurements; drifting buoy arrays to provide mixed layer velocity and SST ...Missing: present | Show results with:present
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[PDF] ENSO Research - Tropical Pacific Observing System
Hayes, S. P., and co-authors (1991): TOGA-TAO - a moored array for real-time measurements in the tropical Pacific Ocean. Bull. Amer. Meteor. Soc., 72, pp ...
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[PDF] THE HISTORY AND EVOLUTION OF PMEL
Ocean (TAO) buoy array that was devel- oped over a 10-year period from 1985 to. 1994 as part of the international Tropical. Ocean Global Atmosphere (TOGA) pro-.
[59]
Two Competing Drivers of the Recent Walker Circulation Trend
Nov 29, 2023 · The Pacific Walker circulation (PWC) weakens under global warming in climate change projections, supported by a global hydrological constraint.
[60]
What Caused the Observed Twentieth-Century Weakening of the ...
The WC weakened during the twentieth century, reaching record low levels in recent decades. This weakening is thought to be partly due to global warming.<|control11|><|separator|>
[61]
Forced changes in the Pacific Walker circulation over the ... - Nature
Aug 23, 2023 · The PWC strengthened from 1992 to 2011, suggesting a significant role for anthropogenic and/or volcanic aerosol forcing 5 , or internal variability.<|separator|>
[62]
The 2023/24 El Niño event exhibited unusually weak extratropical ...
Jul 28, 2025 · The 2023/2024 El Niño was characterized by strong SST warming but only weak positive rainfall and westerly wind anomalies in the tropical ...
[63]
Early emergence and determinants of human-induced Walker ...
Oct 24, 2024 · The Walker circulation is projected to weaken at different tropospheric levels in a warmer future climate (Fig. 1). At the surface, an increase ...Missing: percentage | Show results with:percentage
[64]
How Does El Niño–Southern Oscillation Change Under Global ...
Oct 22, 2020 · We find that the latest generation of climate models shows changes in ENSO in a warmer world. The future variability is increasing, especially in the eastern ...1 Introduction · 3.3 Enso Diversity · 3.4 Enso Teleconnections
[65]
[PDF] An overview of coupled GCM biases in the tropics
mostly stem from an overly strong Walker circulation related to excessive warm pool ... We note, however, that other reasons, such as vertical wind-shear, likely ...