Zonal and meridional flow

Zonal and meridional flowMain

Community hub

8 pages, 0 posts

0 subscribers

Recent from talks

Be the first to start a discussion here.

Recent from talks

Be the first to start a discussion here.

Contribute something

About hubMembersContent overviewUpdatesRules

Main reference articles

Zonal and meridional flow

View on Wikipedia

from Wikipedia

This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed.
Find sources: "Zonal and meridional flow" – news · newspapers · books · scholar · JSTOR (May 2011) (Learn how and when to remove this message)

Zonal and meridional flow are directions and regions of fluid flow on a globe. Zonal flow follows a pattern along latitudinal lines, latitudinal circles or in the west–east direction.^[1] Meridional flow follows a pattern from north to south, or from south to north, along the Earth's longitude lines, longitudinal circles (meridian) or in the north–south direction.^[2] These terms are often used in the atmospheric and earth sciences to describe global phenomena, such as "meridional wind", or "zonal average temperature".

In the context of physics, zonal flow connotes a tendency of flux to conform to a pattern parallel to the equator of a sphere. In meteorological term regarding atmospheric circulation, zonal flow brings a temperature contrast along the Earth's longitude. Extratropical cyclones in zonal flows tend to be weaker, moving faster and producing relatively little impact on local weather.

Extratropical cyclones in meridional flows tend to be stronger and move slower. This pattern is responsible for most instances of extreme weather, as not only are storms stronger in this type of flow regime, but temperatures can reach extremes as well, producing heat waves and cold waves depending on the equator-ward or poleward direction of the flow.

For vector fields (such as wind velocity), the zonal component (or x-coordinate) is denoted as u, while the meridional component (or y-coordinate) is denoted as v.

In plasma physics, "zonal flow" means poloidal, which is the opposite from the meaning in planetary atmospheres and weather/climate studies.

Notes

[edit]

^ "Zonal". Glossary of Meteorology. American Meteorological Society. Retrieved 12 January 2018.
^ "Meridional". Glossary of Meteorology. American Meteorological Society. Retrieved 12 January 2018.

Revisions and contributors Edit on Wikipedia Read on Wikipedia

View on Grokipedia

from Grokipedia

In atmospheric science, zonal and meridional flows refer to the primary directional patterns of large-scale air movements in the Earth's atmosphere. Zonal flow is characterized by a dominant east-west component parallel to lines of latitude, with minimal north-south motion, often manifesting as a relatively straight jet stream that separates polar and tropical air masses while preserving consistent temperature gradients across regions.^[1]^[2] Conversely, meridional flow features pronounced north-south components along lines of longitude, resulting in a wavy or amplified jet stream that promotes mixing between high- and low-latitude air, thereby enhancing the poleward transport of heat and moisture.^[3]^[4] These flow regimes are central to the general circulation of the atmosphere, governing the distribution of energy and influencing global weather patterns. Zonal dominance typically yields milder, more predictable conditions by limiting meridional exchanges, whereas meridional patterns can drive extreme events such as heatwaves, cold snaps, and storms through amplified undulations in the jet stream.^[2]^[4] The interplay between them arises from thermal contrasts between equator and poles, modulated by factors like topography, ocean-atmosphere interactions, and seasonal cycles, with meridional circulations—such as the Hadley, Ferrel, and polar cells—playing a key role in balancing radiative imbalances by advecting heat northward.^[5] Variations in these flows also contribute to phenomena like blocking highs and contribute to interannual climate variability, underscoring their importance in both short-term forecasting and long-term climate modeling.^[6]

Fundamental Concepts

Definition of Zonal Flow

Zonal flow refers to fluid motion in the atmosphere or oceans that is predominantly directed in the west-east direction, parallel to lines of latitude and the equator. This pattern contrasts with more variable or meridional components, emphasizing a largely uniform progression along longitudinal coordinates.^[1] Geometrically, zonal flow is characterized by velocity vectors that align primarily with the longitudinal axis, exhibiting minimal deviation in the latitudinal direction. This results in streamlines that closely follow circles of constant latitude, approximating east-west trajectories with limited north-south excursions.^[2] The term "zonal" originates from the concept of "zone," derived from the Latin zona meaning "belt" or "girdle," which historically described latitudinal belts or divisions on Earth's surface in geography and early climatology.^[7] A classic example of zonal flow is the prevailing westerly winds in mid-latitudes, where air masses move predominantly from west to east between approximately 30° and 60° latitude in both hemispheres, forming broad, nearly straight jet streams.^[8]

Definition of Meridional Flow

Meridional flow describes large-scale fluid motion primarily in the north-south direction, aligned parallel to lines of longitude that extend from pole to pole. This type of flow emphasizes the component of velocity that varies with latitude, representing transport along meridional planes. In atmospheric and oceanic contexts, it plays a key role in redistributing heat, momentum, and mass between equatorial and polar regions.^[9] The term "meridional" originates from "meridian," referring to the geographical lines of longitude that run north-south on Earth's surface, distinguishing this flow from east-west patterns. Geometrically, the flow vector in meridional circulation is predominantly directed along the latitudinal axis, with minimal variation in the longitudinal direction, often visualized in cross-sections perpendicular to latitude circles. This representation highlights how meridional components drive poleward or equatorward advection in rotating fluid systems.^[4] Illustrative examples of meridional flow include the Hadley cells in the tropical atmosphere, where warm air rises near the equator, flows poleward in the upper troposphere, sinks in the subtropics around 30° latitude, and returns equatorward near the surface, facilitating the transport of heat and moisture toward higher latitudes. Another example is the meridional component of the Brewer-Dobson circulation in the stratosphere, involving ascent in the tropics, poleward flow aloft, descent over the polar regions (associated with the polar vortex), and equatorward return flow, which helps distribute trace gases like ozone. These instances underscore meridional flow's role in balancing global circulation patterns, contrasting with zonal flows that dominate east-west transport.^[4]^[10]

Components in Fluid Dynamics

Zonal and Meridional Wind Components

In fluid dynamics, particularly within atmospheric and oceanic sciences, the horizontal wind velocity vector

\vec{V}

is decomposed into orthogonal zonal and meridional components to analyze directional flows on a rotating sphere. The vector is expressed as $\vec{V} = u \hat{i} + v \hat{j}$

=ui^+vj^, where $u$ represents the zonal (east-west) component, positive eastward along parallels of latitude, and $v$ represents the meridional (north-south) component, positive northward along meridians of longitude. This decomposition simplifies the study of circulation by separating longitudinal and latitudinal motions.^[11] The components are defined in a spherical coordinate system, with longitude $\lambda$ (increasing eastward from 0 to $2\pi$ ) and latitude $\phi$ (ranging from $-\pi/2$ at the South Pole to $\pi/2$ at the North Pole). Here, $u$ quantifies variations in the east-west direction, while $v$ captures north-south advection, enabling the representation of complex wind fields in global models and observations. The Earth's radius $a$ scales the gradients in these coordinates.^[11] Under the geostrophic approximation, which balances Coriolis and pressure gradient forces in large-scale flows, the wind components derive from the horizontal momentum equations neglecting acceleration terms. The zonal component arises from $f u = -\frac{1}{a} \frac{\partial \Phi}{\partial \phi},$ yielding $u \approx -\frac{1}{f a} \frac{\partial \Phi}{\partial \phi}$ , where $\Phi$ is the geopotential height and $f = 2 \Omega \sin \phi$ is the Coriolis parameter ( $\Omega$ is Earth's angular velocity). Similarly, the meridional component follows from $- f v = -\frac{1}{a \cos \phi} \frac{\partial \Phi}{\partial \lambda},$ giving $v \approx \frac{1}{f a \cos \phi} \frac{\partial \Phi}{\partial \lambda}$ . These relations indicate that zonal winds strengthen with latitudinal geopotential gradients, while meridional winds intensify with longitudinal variations, providing a diagnostic tool for balanced flows.^[12]^[13] Practically, $u$ and $v$ are resolved from direct measurements using cup or sonic anemometers at surface stations, which record total wind speed $U$ and direction $\theta$ (meteorological convention: from north, clockwise), then compute $u = -U \sin \theta$ and $v = -U \cos \theta$ to align with the coordinate conventions. Satellite platforms, such as those equipped with scatterometers (e.g., ASCAT on MetOp satellites), infer vector winds from ocean surface roughness via radar backscatter, directly retrieving $u$ and $v$ components at resolutions up to 25 km over global oceans. These methods ensure accurate separation of components for data assimilation in weather models.^[14]^[15]

Zonal Mean Calculations

Zonal averaging in atmospheric science involves computing the mean value of a meteorological quantity, such as temperature or wind speed, by integrating over all longitudes at a fixed latitude. This is mathematically expressed as

\bar{X}(\phi) = \frac{1}{2\pi} \int_0^{2\pi} X(\lambda, \phi) \, d\lambda

, where

X(\lambda, \phi)

represents the quantity as a function of longitude

\lambda

and latitude

\phi

.^[16] The primary purpose of zonal mean calculations is to eliminate longitudinal variations, thereby isolating the latitudinal structure of the atmosphere and facilitating the analysis of globally symmetric circulation patterns.^[17] This approach is crucial for studying large-scale features of the general circulation, such as meridional temperature gradients and mean flow regimes, by filtering out local asymmetries caused by topography or transient weather systems.^[17] For instance, zonal mean temperature profiles reveal the characteristic tropospheric lapse rate, where temperature decreases with height at an average rate of approximately 6.5 K/km in the lower troposphere, transitioning to more stable conditions aloft.^[18] These profiles, derived from reanalysis data or observations, highlight equator-to-pole cooling and seasonal shifts in thermal structure, providing key insights into radiative-convective equilibrium.^[18] A key limitation of zonal mean calculations is their reliance on an assumption of zonal symmetry, which may not hold in regions dominated by large-scale wave perturbations where longitudinal variability significantly distorts the averaged fields.^[17] In such cases, the method can oversimplify the dynamics, necessitating complementary analyses of eddy contributions. The zonal means of the east-west wind component

u

and north-south component

v

are often computed this way to assess overall flow symmetry, as detailed in discussions of wind components.^[16]

Patterns in Atmospheric Circulation

Zonal Flow Characteristics

Zonal flow in the atmosphere is characterized by strong, unidirectional westerly winds that flow predominantly parallel to lines of latitude, exhibiting minimal meandering and forming straight, latitudinally confined patterns. These winds are particularly prominent in the subtropics and mid-latitudes, where they achieve high speeds due to the alignment of the flow with the underlying thermal wind balance driven by meridional temperature contrasts.^[19] In the Northern Hemisphere, the subtropical jet typically peaks around 30°N, while the mid-latitude jet is situated further poleward near 50°N, both contributing to a largely symmetric, east-west circulation that dominates the mean atmospheric state.^[20] The vertical structure of zonal flow reveals its maximum intensity in the upper troposphere, particularly at pressure levels between 200 and 300 hPa, where wind speeds can exceed 50 m/s under typical conditions. This core strength diminishes toward the surface due to frictional effects and increases in static stability, resulting in weaker near-surface winds that are still westerly but less organized. Above the tropopause, the flow transitions into the stratosphere, maintaining zonal characteristics but with reduced variability compared to tropospheric levels.^[21]^[19] Seasonal variations in zonal flow are driven by changes in meridional temperature gradients, which are strongest during winter in each hemisphere, leading to more pronounced and intensified westerly jets. In the Northern Hemisphere winter, for instance, the mid-latitude jet strengthens and shifts equatorward slightly, enhancing the overall zonal dominance. This contrasts with summer, when reduced gradients weaken the flow and allow greater meridional influences.^[20]^[19] A notable observational example of persistent zonal flow is the polar night jet in the stratosphere, a circumpolar westerly wind maximum centered around 60° latitude that forms during polar winter due to radiative cooling. This jet, with peak speeds often reaching 80 m/s or more at 5 hPa levels, acts as a stable barrier to meridional mixing and exemplifies the unidirectional, latitude-parallel nature of zonal circulation in the upper atmosphere.^[22]

Meridional Flow Characteristics

Meridional flow in the atmosphere is characterized by north-south directed motion, which contrasts with the predominantly east-west progression of zonal flow. This component often manifests through amplified planetary waves, creating alternating ridges of high pressure and troughs of low pressure that facilitate cross-latitudinal transport of air masses. Unlike the rapid zonal winds in jet streams, meridional flows typically exhibit slower overall speeds, as the Coriolis effect and thermal wind balance constrain their intensity, with typical meridional wind components being an order of magnitude weaker than zonal ones in midlatitudes.^[23]^[24] In terms of horizontal structure, meridional flow is particularly enhanced in regions of baroclinic instability, where north-south temperature gradients drive the growth of synoptic-scale eddies. These instabilities generate perturbation meridional winds that advect warm air poleward and cold air equatorward, playing a crucial role in the exchange of heat between the equator and poles. This process converts available potential energy into kinetic energy, sustaining midlatitude storm tracks and maintaining the poleward heat flux essential for balancing global radiative imbalances.^[25]^[24] Temporally, meridional flow displays greater variability compared to the more steady zonal circulation, with episodes of strengthening linked to specific atmospheric regimes. It intensifies during blocking highs, where persistent ridges disrupt normal westerly flow, leading to prolonged meridional patterns that last from days to weeks and cause regional weather extremes. Similarly, during El Niño events, enhanced meridional flow emerges as part of a more wavy general circulation, accelerating north-south exchanges near coastal regions and altering jet stream configurations.^[23]^[26] A key example of organized meridional circulation is the Hadley cell, a tropical overturning feature extending from the equator to about 30° latitude. In this cell, air rises in the Intertropical Convergence Zone near the equator, flows poleward in the upper troposphere (exporting energy and angular momentum), subsides in the subtropics, and returns equatorward near the surface via trade winds (importing moisture). This structure exemplifies how meridional flow drives large-scale heat redistribution, with the upper branch's poleward motion interacting briefly with zonal jets to influence subtropical dynamics.^[27]^[24]

Meteorological and Climatic Implications

Impact on Weather Systems

Zonal flow, characterized by straight and persistent jet streams, facilitates the rapid progression of extratropical cyclones across mid-latitudes, resulting in weaker storm systems that produce milder weather conditions and fewer instances of extreme precipitation. In such regimes, the uniform westerly winds limit the development of deep troughs and ridges, constraining cyclone intensification and leading to faster-moving systems with reduced baroclinic energy release. This configuration minimizes the potential for prolonged stagnation of weather patterns, thereby dampening the severity of associated fronts and convective activity.^[28] In contrast, meridional flow introduces wavy, amplified patterns in the jet stream, enabling slower-moving and deeper extratropical cyclones that can stall and intensify, thereby promoting extreme weather events such as heat waves, cold outbreaks, and heavy rainfall episodes. The undulating flow allows for enhanced poleward transport of warm air and equatorward incursions of cold air, fostering conditions ripe for blocking highs and intense low-pressure systems that persist over regions, exacerbating local anomalies. These patterns often lead to divergent weather outcomes, with trapped heat or cold air masses contributing to record-breaking temperatures or severe storms.^[4]^[29] The underlying mechanism involves modulation of upper-level divergence and convergence by the flow type, where zonal regimes suppress baroclinicity through stronger background westerlies that inhibit eddy growth and reduce the meridional temperature gradient's influence on wave amplification. Meridional configurations, however, enhance baroclinicity by promoting differential vorticity advection and Eliassen-Palm flux divergences that sustain wave perturbations, allowing for greater release of potential energy into kinetic forms that drive intense weather. This dynamic interplay governs the scale and persistence of synoptic-scale features.^[28]^[30] A notable case illustrating meridional flow's impact is the 2010 Russian heat wave, where an amplified blocking high persisted over Euro-Russia from late June to early August, driven by quasi-stationary Rossby waves and a split jet stream that locked warm air in place, resulting in temperatures exceeding 40°C, widespread wildfires, and over 15,000 deaths. This event highlighted how meridional blocking can decouple regional weather from broader zonal circulation, leading to prolonged extremes far beyond typical variability.^[31]^[32]

Role in Climate Variability

Zonal flow tends to dominate in stable climatic regimes, promoting more predictable patterns and reduced interannual variability through enhanced east-west atmospheric transport. This is particularly evident during La Niña phases of the El Niño-Southern Oscillation (ENSO), where trade winds intensify, strengthening the zonal circulation over the equatorial Pacific and maintaining cooler sea surface temperatures that suppress anomalous weather disruptions.^[33] Such conditions correlate with diminished global temperature fluctuations and more consistent precipitation regimes in tropical regions, as the reinforced zonal winds limit meridional heat exchanges.^[34] In contrast, meridional flow amplifies during periods of heightened climate variability, facilitating north-south exchanges that drive oscillatory modes and long-term anomalies. The North Atlantic Oscillation (NAO), a key atmospheric teleconnection pattern, exemplifies this by modulating surface buoyancy fluxes in deep convection sites like the Labrador and Irminger Seas, thereby influencing the Atlantic Meridional Overturning Circulation (AMOC).^[35] Positive NAO phases enhance westerly winds and dense water formation, leading to strengthened AMOC and decadal-scale temperature anomalies across the North Atlantic basin, with lagged responses amplifying regional variability over 10–30 years.^[35] This meridional dominance contributes to broader hemispheric shifts, such as altered storm tracks and precipitation patterns, underscoring its role in sustaining multi-decadal climate oscillations.^[36] Paleoclimate reconstructions, including ice core proxies from Greenland and marine sediment records, reveal that zonal flow was prevalent during glacial maxima, such as the Last Glacial Maximum (LGM) around 21,000 years ago, supporting efficient meridional heat transport despite overall cooling. Model simulations indicate a stronger and narrower Atlantic jet stream with more zonal orientation, reducing eddy activity by 15–30% and minimizing storminess, which aligned with proxy evidence of stable, cold conditions over ice-covered regions.^[37] This zonal regime facilitated poleward heat fluxes comparable to modern levels (peaking at approximately 6 PW in the Northern Hemisphere), helping to balance radiative imbalances under expanded ice sheets.^[37] Under projected global warming scenarios, IPCC assessments suggest a potential increase in meridional circulation influences, driven by shifts in annular modes and enhanced mid-latitude variability through the 21st century. In high-emission pathways like SSP5-8.5, the Southern Annular Mode (SAM) is expected to trend more positive, with poleward jet shifts implying greater meridional wave activity and precipitation variability over land regions (medium confidence).^[38] Similarly, Northern Hemisphere projections indicate low-confidence changes in the jet stream, potentially including weak poleward shifts that may foster meridional amplification and exacerbate decadal anomalies. Recent studies as of 2025 suggest ongoing trends toward increased planetary wave resonance and jet stream narrowing in the Northern Hemisphere, consistent with Arctic amplification.^[38]^[39]

Applications and Extensions

Use in Numerical Modeling

In general circulation models (GCMs) of the atmosphere, zonal and meridional flows are explicitly resolved through the prognostic equations for the horizontal wind components, denoted as

u

(zonal velocity) and

v

(meridional velocity), on a discretized grid typically in spherical coordinates. These components are advected and forced by terms including pressure gradients, Coriolis effects, friction, and subgrid-scale parameterizations for processes like turbulence and convection, allowing the simulation of large-scale circulation patterns. In more idealized setups, such as axisymmetric models that assume longitudinal invariance, the flow simplifies to a zonal-mean state where meridional circulation cells (e.g., Hadley cells) interact with prescribed zonal winds, facilitating targeted studies of mean circulation dynamics without eddy variability.^[40]^[41]^[42] Diagnostic indices derived from model outputs provide quantitative measures of zonal and meridional flow strengths, often benchmarked against reanalysis datasets. For instance, the Southern Annular Mode (SAM) index, calculated as the leading empirical orthogonal function of sea-level pressure anomalies south of 20°S, serves as a key diagnostic for meridional circulation intensity and zonal jet variability in the Southern Hemisphere, with positive phases indicating strengthened westerlies and poleward-shifted circulation. This index is routinely computed from reanalysis products like ERA5, which assimilates observations to produce gridded fields of

u

and

v

, enabling model evaluation of circulation trends and teleconnections. Such diagnostics reveal how models capture mode amplitudes, with SAM variability linked to meridional heat and momentum transports.^[43]^[44]^[45] Despite advances, CMIP6 GCMs exhibit biases in zonal flow representation, including overestimation of midlatitude westerly jet strengths in the Southern Hemisphere, which can suppress meridional eddy activity and lead to underestimated frequencies of extreme weather events like blocking highs. These biases arise from deficiencies in cloud-radiative feedbacks and ocean-atmosphere coupling, resulting in excessive zonal momentum convergence and reduced poleward heat transport compared to observations. For example, in coupled simulations, the amplified zonal flow contributes to equatorward-biased storm tracks, diminishing the simulation of meridional incursions that drive precipitation extremes.^[46]^[47]^[48] Model validation often involves direct comparisons of simulated meridional heat fluxes—computed as the covariance of

v

and temperature perturbations—with satellite-derived estimates, which provide global coverage of radiative and turbulent fluxes at the top-of-atmosphere and surface. Satellites like CERES measure incoming solar and outgoing longwave radiation, allowing inference of meridional energy transports that models must reproduce within 10-20% accuracy for climate sensitivity assessments. Discrepancies, such as underestimated eddy contributions in GCMs, highlight needs for improved subgrid parameterizations.^[49]^[50]

Contexts Beyond the Atmosphere

In oceanic circulation, zonal flows manifest as predominantly east-west currents driven by trade winds and Coriolis effects, such as the North Equatorial Countercurrent, which flows eastward beneath the westward North Equatorial Current in the Pacific Ocean at depths of 100-200 meters and speeds up to 0.5 m/s. This countercurrent exemplifies zonal transport, facilitating nutrient upwelling and influencing equatorial productivity without significant latitudinal displacement. In contrast, meridional flows dominate in large-scale gyres, where poleward components redistribute heat and salinity; for instance, the Gulf Stream carries warm water northward along the western North Atlantic boundary at velocities exceeding 2 m/s, forming part of the subtropical gyre's clockwise circulation and contributing to the Atlantic Meridional Overturning Circulation.^[51] These meridional gyres, spanning thousands of kilometers, contrast with zonal currents by emphasizing north-south advection, which modulates regional climates through heat transport equivalent to 1-2 petawatts.^[52] Extending to planetary atmospheres, zonal flows appear as alternating east-west jet streams, notably on Jupiter, where multiple prograde and retrograde jets encircle the planet at latitudes up to 60°, with peak speeds of 100-150 m/s near the equator, sustained by shallow-layer dynamics and Rossby wave interactions.^[53] These jets form banded cloud structures, differing sharply from Venus's circulation, where a robust meridional overturning cell—analogous to a Hadley circulation—dominates, with equatorward flow in the upper atmosphere and poleward return in the lower layers, driving subsidence at high latitudes and maintaining a retrograde superrotation of 100 m/s at cloud tops.^[54] On Venus, this single-cell meridional pattern, extending from the surface to 70 km altitude, contrasts Jupiter's multi-jet zonal regime by prioritizing latitudinal heat redistribution over azimuthal banding.^[55] In plasma physics, particularly tokamaks for magnetic confinement fusion, zonal flows refer to azimuthally symmetric, poloidally symmetric E × B drifts with finite radial wavenumber and zero frequency, generated by nonlinear Reynolds stress from drift-wave turbulence, differing fundamentally from atmospheric zonal flows that are large-scale azimuthal jets driven by thermal convection and Coriolis forces.^[56] These plasma zonal flows, with radial scales of millimeters to centimeters (comparable to ion gyro-radii), suppress turbulent transport by shearing apart eddies, enhancing plasma confinement, whereas atmospheric counterparts operate on planetary scales without magnetic influences.^[56] The distinction arises from tokamak geometry, where zonal flows align poloidally around the magnetic axis, contrasting the toroidal, geostrophically balanced atmospheric flows.^[56] Geophysical extensions apply these concepts to Earth's mantle convection, where zonal flows emerge as large-scale toroidal (azimuthal) components in the deep interior, organizing into banded patterns amid upwelling plumes and downwelling slabs, as evidenced by seismic tomography revealing low-velocity zones aligned zonally in the lower mantle.^[57] These zonal bands, driven by viscous coupling between the core and mantle, facilitate lateral shear and influence plate tectonics by modulating stress fields, with flow speeds on the order of centimeters per year, in contrast to the dominant meridional poloidal circulation that drives vertical mass transport.^[57] Such structures persist over geological timescales, linking core dynamics to surface deformation without direct analogy to atmospheric winds.^[58]

Info Pages

Talk Pages

Special Pages

Zonal and meridional flow

Recent from talks

Recent from talks

Contribute something

Contribute something

Media Pages

Timelines

Articles

Notes collections

Notes

Notes

Days in Chronicle

Zonal and meridional flow

See also

Notes