Eddy Momentum Flux Research Articles

The effect of orography on the global atmospheric angular momentum and the general circulation

Orographic features affect the atmospheric circulation not only locally but also globally. In this paper, the latter is quantified in terms of the global relative angular momentum in a series of numerical experiments using the Portable University Model of the Atmosphere (PUMA). Two effects of an orographic barrier on the atmospheric relative angular momentum are identified. On the one hand, an orographic barrier acts to break the zonal symmetry of the circulation. Because of the symmetry breaking, the energy inputed is stored not only in the time-mean part, but also in the time-varying part of the zonal flow, leading to a reduction in the mean and an enhancement in the variance of the relative angular momentum. On the other hand, orographic features lead also to changes in the eddy momentum fluxes, which are necessary to balance the mountain torque. These eddy fluxes induce an increase of global relative angular momentum through a strengthening and an equatorward shift of the zonal flow. The second effect prevents a further decrease in the global relative angular momentum with an increase in the height of the barrier. Zusammenfassung Die Orographie der Erdoberfl¨

Tropical Zonal Momentum Balance in the NCEP Reanalyses

Abstract The seasonal cycle of the zonal-mean zonal momentum balance in the Tropics is investigated using NCEP reanalysis data. It is found that the climatological stationary waves in the tropical upper troposphere, which are dominated by the equatorial Rossby wave response to tropical heating, produce an equatorward eddy flux of westerly momentum in the equatorial belt. The resulting westerly acceleration in the tropical upper troposphere is balanced by the advection of easterly momentum associated with the cross-equatorial mean meridional circulation. The eddy momentum fluxes and the cross-equatorial flow both tend to be strongest during the monsoon seasons, when the maximum diabatic heating is off the equator, and weakest during April–May, the season of strongest equatorial symmetry of the heating. The upper-level Rossby wave pattern exhibits a surprising degree of equatorial symmetry and follows a similar seasonal progression. Solutions of the nonlinear shallow water wave equation also show a predominantly equatorially symmetric response to a heat source centered off the equator.

Baroclinic Multiple Zonal Jets on the Sphere

Abstract Multiple zonal jets are investigated with a two-level primitive equation model on the sphere in which both baroclinicity and planetary radius are varied. As in the case for a two-layer quasigeostrophic model on a β-plane channel, it is found both that the Rhines scale successfully predicts the meridional scale of the multiple zonal jets, and that these jets are maintained in part by an eddy momentum flux divergence associated with slow baroclinic waves at the interjet minimum. A scaling analysis suggests that njets∝ (a/θm)1/2, with the constraints ζe ≡ 8 sin2f (θm/▵θ ) > 1 and njets ≥ 1, where njets is the number of the jets, a the planetary radius, θm one-half of the pole-to-equator potential temperature difference, ξe the supercriticality of the two-layer Phillips model, Δθ the potential temperature difference between the two levels, and ϕ the latitude. The number of jets simulated by the model agrees with this scaling, provided that Ljet ≤ a, where Ljet is the jet scale. In model runs with a large planet where multiple zonal jets exist, the time–mean eddy heat flux is found to be consistent with the diffusive picture of Held and Larichev. In contrast, for the model runs with the planetary size equal to that of Earth, baroclinic adjustment is found to be more relevant. These results are consistent with the finding that in the large-planet (Earth-like) model runs, the jet/eddy scale is smaller than (comparable to) the corresponding planetary radius.

The Role of Wave Packets in Wave–Mean Flow Interactions during Southern Hemisphere Summer

Abstract In this study, reanalysis data produced by the European Centre for Medium-Range Weather Forecasts for 14 Southern Hemisphere (SH) summer seasons have been analyzed. All cases of hemispheric transient eddy kinetic energy (TEKE) maxima have been identified, and the evolution of the local energetics and planetary-scale flow anomalies accompanying these TEKE growth/decay episodes are composited. The longitude–time evolution of the composite energetics shows the clear signature of a wave packet propagating eastward at a group velocity of about 27° longitude per day and undergoing a life cycle of growth and decay, with the energetics within a volume close to the wave packet center dominating the hemispheric mean energetics. When individual cases are examined, 52% are found to resemble the composite and have the energetics life cycle dominated by the evolution of a single wave packet, and an additional 21% are found to be dominated by the evolution of two wave packets having similar amplitudes. Only the remaining 27% can be regarded as having experienced TEKE growth and decay throughout much of the hemisphere. The zonal mean flow and eddy feedback anomalies (i.e., reduction in the meridional temperature gradient due to the effects of the eddy heat fluxes, as well as increase in the barotropic shear due to a narrowing of the midlatitude jet through the effects of the eddy momentum fluxes) associated with the cases dominated by the evolution of a single wave packet are also found to be dominated by anomalies close to the wave packet center. The fact that hemispheric wave growth/decay is often dominated by the evolution of a single wave packet has interesting dynamical consequences when the climatological basic flow is not zonally symmetric. When a wave packet propagates over regions of enhanced baroclinicity, it can extract more energy from the mean flow via baroclinic conversion, leading to its preferential growth. On the other hand, when a wave packet propagates over regions of weak baroclinicity, baroclinic conversion is suppressed; hence any packet growth must be due to other processes. By examining the location of wave packet peaks when hemispheric TEKE is at a maximum, it is observed that hemispheric mean TEKE peaks much more frequently when the dominant wave packet is located downstream of the region with strongest baroclinicity. In addition, the growth in TEKE for these cases is usually dominated by an increase in baroclinic conversion. In contrast, for the small number of cases in which the hemispheric mean TEKE maximum occurs when the dominant wave packet is located downstream of the region with weakest baroclinicity, the growth of the hemispheric TEKE is instead dominated by a reduction in barotropic dissipation.

Zonal Momentum Budget of the Madden–Julian Oscillation: The Source and Strength of Equivalent Linear Damping

Abstract Linear, dissipative models with resting base states are sometimes used in theoretical studies of the Madden–Julian oscillation (MJO). Linear mechanical damping in such models ranges from nonexistent to strong, since an observational basis for its source and strength has been lacking. This study examines the zonal momentum budget of a composite MJO over the equatorial western Pacific region, constructed using filtering and regression techniques from 15 yr (1979–93) of daily global reanalysis data. Two different reanalyses (NCEP–NCAR and ERA-15) give qualitatively similar results for all major terms, including the budget residual, whose structure is consistent with its interpretation as eddy momentum flux convergence (EMFC) in convection. The results show that the MJO is a highly viscous oscillation, with a 3–5-day equivalent linear damping time scale, in the upper as well as lower troposphere. Upper-level damping is mainly in the form of large-scale advection terms, which are linear in MJO amplitude but involve horizontal and vertical background flow. Specifically, the leading terms are the advection of time-mean zonal shear by MJO vertical motion anomalies and advection of MJO wind anomalies by time-mean ascent. This upper-level damping in the western Pacific is mostly confined between 10°N and 10°S. In contrast, zonal wind damping in the lower troposphere involves EMFC (budget residual) and zonal mean linear meridional advection. Stated another way, the strong upper-level damping necessitates upper-level geopotential height gradients to maintain the observed zonal wind anomalies over the time scales implied by the MJO’s low frequency. The existence of the background flow thus tends to shift MJO temperature perturbations westward so that the warm anomaly ahead (east) of the convective center is shifted back into the convection. This shifting effect is fully realized only for anomalies with a period much longer than the 3–5-day damping time.

The Transient Atmospheric Response to Midlatitude SST Anomalies

Abstract To study the transient atmospheric response to midlatitude SST anomalies, a three-layer quasigeostrophic (QG) model coupled to a slab oceanic mixed layer in the North Atlantic is used. As diagnosed from a coupled run in perpetual winter conditions, the first two modes of SST variability are linked to the model North Atlantic Oscillation (NAO) and eastern Atlantic pattern (EAP), respectively, the dominant atmospheric modes in the Atlantic sector. The two SST anomaly patterns are then prescribed as fixed anomalous boundary conditions for the model atmosphere, and its transient responses are established from a large ensemble of simulations. In both cases, the tendency of the air–sea heat fluxes to damp the SST anomalies results in an anomalous diabatic heating of the atmosphere that, in turn, forces a baroclinic response, as predicted by linear theory. This initial response rapidly modifies the transient eddy activity and thus the convergence of eddy momentum and heat fluxes. The latter transforms the baroclinic response into a growing barotropic one that resembles the atmospheric mode that had created the SST anomaly in the coupled run and is thus associated with a positive feedback. The total adjustment time is as long as 3–4 months for the NAO-like response and 1–2 months for the EAP-like one. The positive feedback, in both cases, is dependent on the polarity of the SST anomaly, but is stronger in the NAO case, thereby contributing to its predominance at low frequency in the coupled system. However, the feedback is too weak to lead to an instability of the atmospheric modes and primarily results in an increase of their amplitude and persistence and a weakening of the heat flux damping of the SST anomaly.

Mechanisms of ENSO-forcing of hemispherically symmetric precipitation variability

Summary The patterns of precipitation anomalies forced by the El Nino-Southern Oscillation during Northern Hemisphere winter and spring are remarkably hemispherically symmetric and, in the mid-latitudes, have a prominent zonally symmetric component. Observations of global precipitation variability and the moisture budget within atmospheric reanalyses are examined to argue that the zonally symmetric component is caused by interactions between transient eddies and tropically-forced changes in the subtropical jets. During El Nino events the jets strengthen in each hemisphere and shift equatorward. Changes in the subtropical jet influence the transient eddy momentum fluxes and the eddy-driven mean meridional circulation. During El Nino events eddy-driven ascent in the mid-latitudes of each hemisphere is accompanied by low level convergence and brings increased precipitation. This changes in the transient eddy and stationary eddy moisture fluxes almost exactly cancel each other and, in sum, do not contribute to the zonal mean precipitation anomalies. Propagation of anomalous stationary waves disrupts the zonal symmetry. Flow around the deeper Aleutian Low and the eastward extension of the Pacific jet stream supply the moisture for increased precipitation over the eastern North Pacific and the western seaboard of the United States while transient eddy moisture convergence supplies the moisture for increased precipitation over the southern United States. In each case increased precipitation is fundamentally caused by anomalous ascent forced by anomalous heat and vorticity fluxes.

On the maintenance of thermal wind balance and equatorial superrotation in Titan's stratosphere

Titan's atmospheric winds, like those on Venus, exhibit superrotation at high altitudes. Titan general circulation models have yielded conflicting results on whether prograde winds in excess of 100 m s −1 at the 1 mbar level are possible based on known physical processes that drive wind systems. A comprehensive two-dimensional (2D) model for Titan's stratosphere was constructed to systematically explore the physical mechanisms that produce and maintain stratospheric wind systems. To ensure conservation of angular momentum in the limit of no net exchange of atmospheric angular momentum with the solid satellite and no external sources and sinks, the zonal momentum equation was solved in flux form for total angular momentum. The relationships among thermal wind balance, meridional circulation, and zonal wind were examined with numerical experiments over a range of values for fundamental input parameters, including planetary rotation rate, radius, internal friction due to wave stresses, and net radiative drive. The magnitude of mid-latitude jets is most sensitive to a single parameter, the planetary rotation rate and results from the conversion of planetary angular momentum to relative angular momentum by the meridional circulation, whereas the strength of meridional circulation is mainly determined by the magnitude of the radiative drive. For Titan's slowly rotating atmosphere, the meridional temperature gradient is vanishingly small, even when the radiative drive is enhanced beyond reasonable magnitudes, and can be inferred from zonal winds in gradient/thermal wind balance. In our 2D model large equatorial superrotation in Titan's stratosphere can be only produced through internal drag forcing by eddy momentum fluxes, which redistribute angular momentum within the atmosphere, while still conserving the total angular momentum of the atmosphere with time. We cannot identify any waves, such as gravitational or thermal tides, that are sufficiently capable of generating the required eddy forcing of >50 m s −1 Titan-day −1 to maintain peak prograde winds in excess of 100 m s −1 at the 1 mbar level.

Equatorial Superrotation and the Factors Controlling the Zonal-Mean Zonal Winds in the Tropical Upper Troposphere

Abstract The response of the zonal-mean zonal winds in the tropical upper troposphere to thermal forcing in the Tropics is studied using an idealized general circulation model with 18 vertical levels and simplified atmospheric physics. The model produces a conventional general circulation, with deep easterly flow over the equator, when integrated using zonally invariant and hemispherically symmetric boundary conditions, but persistent equatorial superrotation (westerly zonal-mean flow over the equator) is obtained when steady longitudinal variations in diabatic heating are imposed at low latitudes. The superrotation is driven by horizontal eddy momentum fluxes associated with the stationary planetary wave response to the applied tropical heating. The strength of the equatorial westerlies is ultimately limited by vertical steady eddy momentum fluxes, which are downward in the tropical upper troposphere, and by the zonally averaged circulation in the meridional plane, which erodes the mean westerly shear via vertical advection. The transition to superrotation can be prevented by specifying offsetting zonally invariant heating and cooling anomalies on either side of the equator to create a “solstitial” basic state with a single dominant Hadley cell straddling the equator. Superrotation is restricted in the solstitial climate because the strength of the mean meridional overturning is enhanced, which increases the efficiency of vertical advection, and because the cross-equatorial flow in the upper troposphere provides an easterly zonal acceleration that offsets some of the momentum flux convergence associated with tropical eddy heating. The cross-equatorial flow aloft also reduces the stationary planetary wave response in the summer hemisphere. These results suggest that hemispheric asymmetry in the mean meridional circulation is responsible for maintaining the observed mean easterly flow in the tropical upper troposphere against the westerly torques associated with tropical wave sources.

Abrupt Transition to Strong Superrotation in an Axisymmetric Model of the Upper Troposphere

Abstract Abrupt transitions to strongly superrotating states have been found in some idealized models of the troposphere. These transitions are thought to be caused by feedbacks between the eddy momentum flux convergence in low latitudes and the strength of the equatorial flow. The behavior of an axisymmetric shallow-water model with an applied tropical torque is studied here to determine if an abrupt transition can be realized without eddy feedbacks. The upper-tropospheric layer is relaxed to a radiative equilibrium thickness, exchanging mass and thus momentum with the nonmoving lower layer. For low values of the applied torque, the circulation is earthlike; however, for larger values, an abrupt transition to a strongly superrotating state can occur. In some cases, the system remains superrotating as the torque is subsequently decreased. A simple analytical model is used to better understand the system. The bifurcation is caused by a feedback between the applied torque and the strength of the Hadley cell. As the torque increases, the strength of the cell decreases, reducing the damping caused by momentum transfer from the lower layer.

Effects of Environmentally Induced Asymmetries on Hurricane Intensity: A Numerical Study

Abstract The influence of uniform large-scale flow, the beta effect, and vertical shear of the environmental flow on hurricane intensity is investigated in the context of the induced convective or potential vorticity asymmetries in the core region with a hydrostatic primitive equation hurricane model. In agreement with previous studies, imposition of one of these environmental effects weakens the simulated tropical cyclones. In response to the environmental influence, significant wavenumber-1 asymmetries develop. Asymmetric and symmetric tendencies of the mean radial and azimuthal winds and temperature associated with the environment-induced convective asymmetries are evaluated. The inhibiting effects of environmental influences are closely associated with the resulting eddy momentum fluxes, which tend to decelerate tangential and radial winds in the inflow and outflow layers. The corresponding changes in the symmetric circulation tend to counteract the deceleration effect. The net effect is a moderate weakening of the mean tangential and radial winds. The reduced radial wind can be viewed as an anomalous secondary radial circulation with inflow in the upper troposphere and outflow in the lower troposphere, weakening the mean secondary radial circulation.

The Signature of the Annular Modes in the Tropical Troposphere

Abstract The extratropical annular modes are coupled with a distinct pattern of climate anomalies that spans the circulation of the tropical troposphere. The signature of the annular modes in the tropical troposphere exhibits a high degree of equatorial symmetry. It is associated with upper-tropospheric zonal wind anomalies centered about the equator, midtropospheric temperature anomalies located ∼20°N and 20°S, and opposing mean meridional circulation anomalies that span the subtropics of both hemispheres. The linkages between the annular modes and the tropical circulation are only evident during the cold season months, and are most robust in association with the Northern Hemisphere annular mode (NAM). The coupling between the annular modes and the circulation of the tropical troposphere is consistent with forcing by waves originating at extratropical latitudes. Both annular modes are characterized by anomalies in the eddy momentum flux convergence at tropical latitudes that act to reinforce the changes in the tropical wind and temperature fields. The most pronounced tropical anomalies lag indices of the annular modes by ∼2 weeks and are found over the eastern tropical Pacific, where climatological westerlies permit extratropical waves to propagate into the deep Tropics. The linkages between the NAM and the tropical tropospheric circulation are most pronounced during the cold phase of the El Niño–Southern Oscillation cycle. The recent trend in the NAM is linearly congruent with a ∼0.1-K cooling of the tropical troposphere over the past two decades during the Northern Hemisphere winter season.

Velocity Statistics Inferred from the TOPEX/Poseidon-Jason-1 Tandem Mission Data

Using a parallel-track approach to estimate geostrophic surface velocities, an estimate of the statistics of ocean geostrophic surface currents and momentum stresses is provided on a 10 km along-track resolution from the first 49 repeat cycles (16 months) of the Jason-TOPEX/Poseidon tandem altimetric sea surface height (SSH) data. Results are compared with estimates obtained in a traditional way from along-track SSH data at crossover points and with in situ, Acoustic Doppler Current Profiler (ADCP) measurements obtained on board the VOS Oleander along a nominal path connecting Bermuda with the U.S. mainland. Agreements with the Oleander data are reasonable when simultaneous (in space and time) sampling is available. However, amplitudes of parallel-track geostrophic velocity variances are about 25% lower as compared to Oleander measurements which represent geostrophic and ageostrophic flow components. Estimates of velocity variances show clear signs of an anisotropic eddy field in the vicinity of all major current systems. At the same time estimates of Reynolds stresses and eddy momentum fluxes show a convergence of eddy momentum in all those regions, suggesting a forcing of the mean flow by the eddy field there.

Why Do Baroclinic Waves Tilt Poleward with Height?*

Theoretical and modeling studies of nongeostrophic effects in baroclinic waves predict that baroclinic waves should tilt poleward with height, with a larger tilt in total meridional wind than in geostrophic quantities. Regression analysis of NCEP–NCAR reanalysis 6-hourly data demonstrates that observed baroclinic waves do indeed tilt poleward with height, although the observed tilt is smaller than predicted by previous studies. The meridional ageostrophic wind enhances the poleward tilt of meridional wind perturbations, despite being smaller in amplitude than the meridional geostrophic wind by a factor of 5. An improved estimate of the structure of the meridional ageostrophic wind in baroclinic waves is calculated assuming force balance. Several important terms in this estimate have been left out of previous estimates of the meridional ageostrophic wind. Three terms in the improved estimate produce nearly all of the poleward tilt of the meridional wind: 1) the advection of geostrophic zonal wind perturbations by the mean zonal wind, 2) the convergence of the eddy momentum flux, and 3) the effect of friction. The poleward tilt with height of baroclinic waves explains why upper-level storm tracks tend to occur poleward of near-surface baroclinic regions, and may play a role in the midwinter suppression of the Pacific storm track.

A Mechanism and Simple Dynamical Model of the North Atlantic Oscillation and Annular Modes

A simple dynamical model is presented for the basic spatial and temporal structure of the large-scale modes of intraseasonal variability and associated variations in the zonal index. Such variability in the extratropical atmosphere is known to be represented by fairly well-defined patterns, and among the most prominent are the North Atlantic Oscillation (NAO) and a more zonally symmetric pattern known as an annular mode, which is most pronounced in the Southern Hemisphere. These patterns may be produced by the momentum fluxes associated with large-scale midlatitude stirring, such as that provided by baroclinic eddies. It is shown how such stirring, as represented by a simple stochastic forcing in a barotropic model, leads to a variability in the zonal flow via a variability in the eddy momentum flux convergence and to patterns similar to those observed. Typically, the leading modes of variability may be characterized as a mixture of ‘‘wobbles’’ in the zonal jet position and ‘‘pulses’’ in the zonal jet strength. If the stochastic forcing is statistically zonally uniform, then the resulting patterns of variability as represented by empirical orthogonal functions are almost zonally uniform and the pressure pattern is dipolar in the meridional direction, resembling an annular mode. If the forcing is enhanced in a zonally localized region, thus mimicking the effects of a storm track over the ocean, then the resulting variability pattern is zonally localized, resembling the North Atlantic Oscillation. This suggests that the North Atlantic Oscillation and annular modes are produced by the same mechanism and are manifestations of the same phenomenon. The time scale of variability of the patterns is longer than the decorrelation time scale of the stochastic forcing, because of the temporal integration of the forcing by the equations of motion limited by the effects of nonlinear dynamics and friction. For reasonable parameters these produce a decorrelation time of the order of 5‐10 days. The model also produces some long-term (100 days or longer) variability, without imposing such variability via the external parameters except insofar as it is contained in the nearly white stochastic forcing.

Impact of isopycnal mixing on the tropical ocean circulation

The sensitivity of tropical ocean dynamics to the ocean lateral mixing orientation is explored using a z‐coordinate climate‐type ocean general circulation model. Compared to a simulation using horizontal mixing on both tracers and dynamics (HOR), a rotation of the tracer tensor in which mixing occurs along isopycnals (ISOT) improves the structure of the off‐equatorial density field, which consequently enhances the off‐equatorial circulation through geostrophy. However, the dynamics at the equator in ISOT are degraded compared to observations, as the upper part of the Equatorial Undercurrent (EUC) is too deep and the South Equatorial Current (SEC) is too intense. Next, isopycnal diffusion on momentum is implemented (ISOMT). An examination of the momentum balance at the equator shows that this change in lateral diffusion direction significantly reduces the meridional diffusive flux of momentum at the top of the EUC. This intensifies the EUC, which, in turn, weakens the SEC along the equator through vertical diffusion. The equatorial degradations observed in ISOT disappear. Compared to ISOT and HOR, the separation of the two SEC branches and the equatorial current magnitude in the surface layers are much better reproduced. Moreover, in contrast to the results from ISOT and HOR, isopycnal momentum mixing allows the existence of both horizontal up‐gradient and down‐gradient eddy momentum fluxes, notably improving the vertical profile of the lateral viscosity term. The ISOMT experiment is therefore shown to be the closest to observations. These results suggest that isopycnal mixing should be used on both tracers and momentum to better simulate the observed eddy effects and the tropical circulation in climate models.

Mechanisms of Hemispherically Symmetric Climate Variability*

Inspired by paleoclimate evidence that much past climate change has been symmetric about the equator, the causes of hemispherically symmetric variability in the recent observational record are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset and numerical models. It was found that the dominant cause of hemispherically symmetric variability is the El Niño–Southern Oscillation. During an El Niño event the Tropics warm at all longitudes and the subtropical jets in both hemispheres strengthen on their equatorward flanks. Poleward of the tropical warming there are latitude belts of marked cooling, extending from the surface to the tropopause in both hemispheres, at all longitudes and in all seasons. The midlatitude cooling is caused by changes in the eddy-driven mean meridional circulation. Changes in the transient eddy momentum fluxes during an El Niño event force upper-tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the Coriolis torque. The eddy-driven ascent causes anomalous adiabatic cooling, which is primarily balanced by anomalous diabatic heating. Using a quasigeostrophic spherical model, forced by an imposed surface eddy disturbance of chosen wavenumber and frequency, it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude–height plane of transient eddies. Changes in zonal winds, and associated changes in the meridional gradient of potential vorticity, create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward. Tropical forcing of variability in the eddy-driven mean meridional circulation is another way, in addition to Rossby wave teleconnections, whereby the Tropics can influence extratropical climate. Unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle.

Cloud‐tracked winds for the first Mars Global Surveyor mapping year

We have measured winds using cloud motion in consecutive Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) wide‐angle global map swaths taken during the first mapping year (Ls 135°–360°–111°). We present a total of ∼11,200 wind vectors collected in the north polar region during Ls 135°–195° (late summer/early fall) and Ls 20°–55° (mid spring) and in the south polar region during Ls 337°–10° (late summer/early fall). For cases with good coverage, we also present the derived mean zonal and meridional winds and the associated eddies. The speed of the zonal winds in 60°N–70°N increases at ∼0.6 m/s/°Ls in late northern summer, and that in 60°S–70°S increases at a rate of ∼0.7 m/s/°Ls in late southern summer. The latitudinal distribution of zonal wind within 50°N–75°N from mid northern summer to early northern fall indicates that winds at higher latitudes are generally weaker than those at lower latitudes, but the rate of increase with time is faster at higher latitudes. There is a cyclonic gyre in the 90°W–0°–30°E sector in the north polar region. There are large‐scale waves in the weekly mean meridional wind and in the biweekly mean eddy momentum flux and eddy kinetic energy fields in the north polar region from mid to late summer. The cloud‐tracked winds in the north are generally consistent with winds calculated by general circulation model at the water condensation level derived from MGS Thermal Emission Spectrometer (TES) observations but appear stronger than the gradient winds derived from TES assuming no flow at the surface.

Multiple planetary flow regimes and the eddy forcing in Northern Hemisphere wintertime variability

A cluster algorithm is applied to the zonal mean zonal wind ([u]) from winter monthly means of the reanalyses of NCEP/NCAR. Four reproducible and robust clusters are found: two regimes represent the opposite phases the Northern Hemisphere Annular mode (NAM), and two regimes represent the opposite phases of a pattern which can be identified with the Cold Ocean‐Warm Land (COWL) pattern. This identification is accomplished by regressing the [u] on the COWL‐index derived from 500 hPa height data by Wu and Straus [2002]. Transformed Eulerian (or Eliassen‐Palm) eddy forcing analysis is used to examine the maintenance of these clusters. The results show that barotropic forcing by the eddy momentum fluxes appears to maintain the wind anomalies in the upper troposphere and balances the Coriolis acceleration associated with the anomalous transformed mean meridional circulation, while the Coriolis acceleration maintains the wind anomalies against friction in the lower troposphere. This mechanism holds for both NAM‐like and COWL‐like regimes.

The Influence of Large-Scale Eddy Flux Variability on the Zonal Mean Ozone Distribution

The influence of the variability of large-scale eddy heat and momentum fluxes on interannual and decadal changes in the zonal mean ozone distribution is investigated based on a two-dimensional circulation model with interactively coupled transport and chemistry. The large-scale eddy fluxes are prescribed according to observations, that is, in terms of monthly mean eddy mixing coefficients that are derived from ECMWF reanalysis 1979–93 (ERA-15). The results show that the eddy-forced changes in the lower-stratospheric residual circulation and eddy mixing processes induce coherent changes in temperature, ozone, and other chemical tracers. Significant ozone declines are found mainly in the lower stratosphere during winter, with maximum values of about −1.2 Dobson units (DU) per kilometer per decade at midlatitudes and more than −2 DU km−1 decade−1 at polar latitudes. The seasonal and vertical structure of the observed Northern Hemispheric ozone trend is captured in a remarkable way. It is also shown that, at midlatitudes, maximum declines in the zonal mean lower-stratospheric ozone are mainly related to the eddy mixing processes, and, at polar latitudes, to the induced changes in the lower-stratospheric temperatures.