Stratospheric Planetary Wave Research Articles

Climatological features of mesosphere and lower thermosphere stationary planetary waves within ±40° latitude

Zonal and meridional wind measurements from the High Resolution Doppler Imager (HRDI) and Wind Imaging Interferometer (WINDII) instruments on the Upper Atmosphere Research Satellite (UARS) are used to construct monthly average stationary planetary wave (SPW) structures from 15 km to 110 km over the period December 1991 to September 1994. Because of viewing constraints of the instruments, intercomparisons between Northern Hemisphere (NH) and Southern Hemisphere (SH) structures and seasonal evolutions are mainly confined to ±40° latitude. The largest zonal wave number s = 1 amplitudes (10–25 m s−1) in the stratosphere and mesosphere at ±40° exist in the zonal wind fields and occur during September–January in the NH and July–October in the SH. SPW s = 2 structures are much less prevalent, especially above 80 km because of more restrictive filtering effects due to the mean winds. Above about 70 km, s = 1 wave amplitudes (∼5–10 m s−1) are more uniform, extending over all nonsummer months in both hemispheres up to 80–90 km, with similar amplitudes existing throughout the year at higher altitudes. Our interpretation is that the SPW above about 80–90 km during summer owe their existence to ducting from the winter hemisphere, in agreement with numerical simulations by Pogoreltsev and Sukhanova [1993]. The origin of SPW above about 80 km during winter at midlatitudes is not possible to determine unambiguously. Phase structures undergo a transition from vertical propagation with long vertical wavelengths (hundreds of kilometers) below ∼80 km to either evanescent behavior or an extreme shortening of vertical scale at higher altitudes. This change in phase behavior may be connected with negative values in refractive index Q for quasi‐geostrophic wave propagation, which occur over a narrow height region near 80 km during almost every month. This interruption of normal phase progression makes it impossible to distinguish the origin of the planetary wave oscillations from (1) those that “tunnel” through the Q < 0 region and (2) those that might be excited in situ by zonally asymmetric momentum deposition from gravity waves which have undergone filtering by the stratospheric planetary wave wind field. In addition, quantitative estimates of aliasing due to the diurnal tide indicate that it may be of sufficient magnitude to contaminate retrieved SPW structures between 80–90 km and 100–110 km, where local time coverages are incomplete. Comparisons are made between the observed SPW structure and those from the horizontal wind model [Hedin et al., 1993, 1996]. This model is found to provide a reasonable approximation to NH winter SPW s = 1 structures below 80 km, but severely underestimates the amplitudes of SPW in the SH.

Variability of the semidiurnal tide due to fluctuations in solar activity and total ozone

An attempt was made to investigate some variations of the semidiurnal tide observed in the mesosphere/lower thermosphere (MLT) region by meteor radar above the UK in the time interval 1 January 1989–6 May 1993, which are possibly due to the variability in the total ozone and solar activity. The total ozone is used here mainly as a proxy for stratospheric planetary wave information in the extra tropics. A positive correlation between the solar activity and the variability of the semidiurnal tide and total ozone was found for the investigated interval. During winter the semidiurnal amplitude modulations with periods: ∼10, ∼16 and 25–28 days were found to be present simultaneously with similar variations in the total ozone. They strengthen significantly during stratospheric warmings and the amplification of the stratospheric height wave 1. The amplitude modulations of the semidiurnal tide observed in the MLT region during winter are mainly produced by non-local coupling between the semidiurnal tide and planetary waves in the stratosphere. The influence of the planetary wave activity in the lower stratosphere on the variability of the semidiurnal tide observed in the MLT region indicates the non-negligible effects of the tropospheric tidal forcing. A strong response of the total ozone and the semidiurnal tide to solar radio flux variations on the time scale of the solar rotation period was found especially in the beginning of 1991. There is some provisional evidence for a response of the semidiurnal tide and the total ozone to the variations in the solar radio flux at intermediate periods of 50–80 days.

Interannual Variations of Planetary Waves in the Southern Hemisphere Stratosphere.

A statistical analysis is made of the interannual variability of stratospheric planetary waves in the Southern Hemisphere (SH) in late winter using NCEP/NCAR reanalysis data gathered over a 20 year period from 1979 to 1998. The study investigates the dynamical coupling between the planetary waves and the tropospheric circulation, by focusing on the activity of transient baroclinic disturbances. With the aid of EOF analysis applied to the 10 hPa monthly mean height anomalies and tropospheric circulation patterns, it is found that the year-to-year variation of quasi-steady planetary waves in late winter (September and October) is characterized by a longitudinal phase shift of zonal wavenumber 1 as well as by a variation in wave amplitudes. An eastward (westward) shift of the stratospheric planetary waves corresponds to a double-jet (single-jet) structure of the upper tropospheric (300 hPa) zonal winds. On the other hand, planetary wave amplitude variation is closely related to that of the meridional heat flux in the upper troposphere. These results are reinforced by the Eliassen-Palm flux diagnosis composed of the stratospheric EOF scores, and hence the effect of tropospheric transient waves on the forcing to the stratospheric planetary waves is stressed.

The Evolution of a Stratospheric Wave Packet

This work examines the extent to which a few basic concepts that apply to plane waves, for example, the refraction of waves up the gradient of the index of refraction, apply to stratospheric planetary waves. This is done by studying the relation between group velocity (Cg) and the wave activity velocity, which is defined as the Eliassen–Palm flux divided by the wave activity density (Va = F/A). It is shown that although in the limit of plane waves Va equals Cg, the two velocities are not equal for stratospheric waves, because of reflection, tunneling, and superposition. The use of conservation of wave activity to understand the spatial variations of wave structure is explored. This is done by defining a wave activity packet as part of the wave that moves with Va. Integral lines of Va are then used to keep track of the wave packet location and volume. In the idealized case of an almost-plane wave, conservation of wave activity leads to variations in the amplitude of the wave when it is refracted by the slowly varying basic state. This effect is related to changes in wave packet volume. The wave activity packet framework is used to examine the importance of the “volume effect” for explaining the spatial variations of stratospheric waves. The wave packet formulation is also used to study the evolution of a wave propagating from the troposphere to the stratosphere. It is shown that the consequence of the polar night jet being a leaky waveguide is that perturbations initially concentrate up into the waveguide and only later leak out to the equatorial region. This can explain the observed stratospheric wave life cycle of baroclinic growth followed by a barotropic stage. Finally, integral lines of Va are used to estimate vertical propagation timescales of an observed wave, and it is shown that this estimate is consistent with linear wave dynamics.

The Effect of Reflecting Surfaces on the Vertical Structure and Variability of Stratospheric Planetary Waves

The effects of an upper-stratospheric reflecting surface on the vertical structure of stratospheric planetary waves are considered. A diagnostic of the basic-state wave propagation characteristics, which is particularly useful for determining the existence and location of turning surfaces for meridional and vertical propagation, is developed. The diagnostic used is a more accurate indicator of wave propagation regions than the index of refraction because it diagnoses meridional and vertical propagation separately. The diagnostic is tested on a series of simple models, both steady state and time dependent. It is found that the stratospheric waveguide sets the meridional wavenumber of the waves, regardless of the characteristics of their tropospheric forcing, making it easier to understand the effects of damping and turning surfaces on the vertical structure of the waves. The diagnostic is then applied to observations of the Southern Hemisphere winter of 1996. It is shown that the differences in vertical wave structure between middle and late winter can be explained as a linear response to the seasonal evolution of the basic state, which involves a formation of a reflecting surface in late winter. It is also shown that on daily timescales wave‐mean flow interactions cause significant changes in the basic-state propagation characteristics for periods of a few days. These changes, along with the time variations in the forcing of the waves, are responsible for the observed daily timescale variations in wave structure. The fact that the observed evolution of the waves and the basic state are consistent with linear or quasi-linear wave theory (depending on the timescale looked at) supports the applicability of the theory, as well as the validity of the observations.

An Updated Climatology of the Troposphere–Stratosphere Configuration of the Met Office's Unified Model

Abstract In this paper results are presented from an improved version of the troposphere–stratosphere configuration of the Met Office Unified Model (UM). The new version incorporates a number of changes, including new radiation and orographic gravity wave parameterization schemes, an interannually varying sea surface temperature and sea ice climatology, and the inclusion of convective momentum transport. The UM climatology is compared with assimilated data and with results from a previous version of the UM. It is shown that the model cold biases in the January winter stratosphere and the January and July summer stratosphere are reduced, chiefly because the new radiation scheme is more accurate. The separation between subtropical and polar night jets in July is also better simulated. In addition, in the current version stratospheric planetary wave amplitudes in southern winter are less than half those in northern winter, which is in much better agreement with observations than the previous model version. D...

Diagnosing the Impact of Stratospheric Planetary Wave Breaking in a Linear Model

Abstract In the past, linear quasigeostrophic theory has proven successful in modeling the vertical and meridional propagation of stationary planetary waves in the stratosphere. Since in such models the wave solution does not sensitively depend on the wave damping, the latter was usually implemented as relaxation with a simple damping coefficient. As far as the damping is concerned, this is likely to be unrealistic, since it does not account for the locally enhanced dissipation arising from stratospheric Rossby wave breaking. In the present study, a parameterization for Rossby wave breaking (Garcia) is applied to obtain an improved representation of wave damping throughout the stratosphere. Although solving for the wave turns into a nonlinear problem, the model remains linear in the sense that both the basic-state zonal wind and the wave at the tropopause level are specified and kept fixed. The divergence of the Eliassen–Palm flux and the steady-state residual circulation are computed in order to diagnose...

Effective diffusivity as a diagnostic of atmospheric transport: 1. Stratosphere

The transport and mixing properties of the isentropic flow in the lower and middle stratosphere are analyzed by using observed winds to advect a tracer on isentropic surfaces in the range 400–850 K. The effective diffusivity diagnostic introduced by Nakamura and collaborators is applied to the tracer field in order to identify barriers to transport and mixing regions, and to follow their seasonal evolution. Large effective diffusivity corresponds to strong mixing, and small effective diffusivity corresponds to weak mixing, i.e., to barriers. The effective diffusivity shows, in the winter stratosphere of each hemisphere, the evolution of the vortex‐edge barrier and the midlatitude surf zone, and also the extent of any mixing within the vortex. At low latitudes in the stratosphere there is a region of low effective diffusivity whose latitudinal width varies with height, broadening substantially from 400 K to 550 K. The low values of effective diffusivity in this “tropical‐reservoir” region imply little isentropic transport into or out of it. There is a strong seasonal cycle to the reservoir, which has different forms at 400 K, 450–600 K, and above 650 K, determined by the relative influences of tropospheric synoptic eddies and stratospheric planetary waves. Comparison of effective diffusivity between the Northern Hemisphere winters 1996/1997 and 1997/1998 shows strong differences at low latitudes according to the phase of the quasi‐biennial oscillation (QBO). When there are QBO easterlies, there is a broad region of very low effective diffusivity at low latitudes. When there are QBO westerlies, there are very low values of effective diffusivity at low latitudes within the westerlies themselves but larger values at their edges.

The Tropopause in a 2D Circulation Model

The influence of baroclinic waves on tropopause height was studied with a 2D version of the standard climate 3D GCM ECHAM3. The main feature of the 2D version is a self-consistent parameterization of the transient tropospheric eddy heat and momentum fluxes. The mixing processes that result from the stratospheric planetary waves are prescribed according to climatological data. The model calculations were made with and without the parameterized eddy fluxes, separately revealing the effect of midlatitude tropopause lifting due to the tropospheric eddies and the effect of midlatitude tropopause steepening due to the stratospheric circulation. These effects are also demonstrated by the temperature and the lapse rate of the model results and, additionally, by the meridional distribution of trace gas constituents when the 2D model is run as a dynamical–chemical transport model. The results reveal the strong dependence of the tropopause height on the strength of the stratospheric circulation.

東西非一様な基本場における成層圏プラネタリー波の鉛直伝播

Vertical propagation of stratospheric planetary waves through a zonally asymmetric basic state is investigated in a simplified numerical model for understanding co-existence effects of zonal wavenumber 1 and 2. We compare wave solutions in a zonally asymmetric basic flow with those in a pure zonal flow. The differences are considered from a view of potential vorticity (PV) distribution, to find that the zonally asymmetric basic flow has an effect of the zonal modulation of PV induction by local westerlies associated with the wave. As a result, a component forced at the bottom boundary in the zonally asymmetric flow departs from that in the zonally symmetric flow, and components unforced appear. Detailed behaviors of each wave component in the zonally asymmetric flow depend on the phase relation between the distorted basic flow and the bottom forcing.

Dynamical Forcing of Stratospheric Planetary Waves by Tropospheric Baroclinic Eddies

Abstract The forcing of planetary wave variability in the stratosphere by synoptic-scale baroclinic eddies in the troposphere is considered. Simple forced–dissipative numerical experiments are performed in a primitive equation model using a deep hemispheric model domain. The flow is thermally relaxed toward zonally symmetric notional wintertime conditions. No zonally asymmetric thermal or topographic forcing is applied. All planetary-scale zonal asymmetry arises solely through the nonlinear wave–wave interaction of the baroclinic eddies in the troposphere. The numerical experiments indicate that realistic stratospheric planetary wave amplitudes and variability, comparable to those observed in the Southern Hemisphere, can be forced through this mechanism. No evidence is found in these simulations for planetary-scale disturbances arising through in situ instability in the stratosphere. The nonlinear tropospheric forcing mechanism in the numerical simulations is further investigated by reproducing the strato...

Middle atmosphere variability in the UK Meteorological Office Unified Model

AbstractThis paper describes the seasonal evolution of the stratospheric circulation simulated by a stratospheretroposphere configuration of the UK Meteorological Office (UKMO) Unified Model. Results are shown from a five‐year integration of the model. The model's simulation of the stratosphere and its seasonal evolution compares well with global analyses produced by the UKMO troposphere‐stratosphere data‐assimilation system. The contrast between the winter circulation in the two hemispheres is well simulated. The zonal‐mean winds show strong interannual variability in northern winter, while the southern hemisphere winter jet is much less variable. In northern wither the model spontaneously produces two major warmings and a number of minor warmings. In southern winter and spring the model reproduces well the break‐up of the polar vortex and elements of the flow regime that often precedes this break‐up. The model does, however, exhibit a number of shortcomings. Lack of conservation of potential vorticity prevents the model from capturing some of the ingredients of the flow regimes associated with stratospheric warmings and with the merger of anticyclones. There is a cold bias in the stratosphere throughout the year, with a maximum cold bias over the winter pole near the stratopause. This temperature bias appears to be due to a cooling bias in the long‐wave part of the radiation scheme. The model also has unrealistically strong planetary waves in the upper stratosphere, although amplitudes in the upper troposphere and lower stratosphere are in good agreement with those derived from the UKMO analyses. This paper discusses possible future model improvements that should alleviate these problems.

Climate Change and the Middle Atmosphere. Part III: The Doubled CO2Climate Revisited

Abstract The response of the troposphere–stratosphere system to doubled atmospheric CO2 is investigated in a series of experiments in which sea surface temperatures are allowed to adjust to radiation imbalances. The Goddard Institute for Space Studies (GISS) Global Climate Middle Atmosphere Model (GCMAM) warms by 5.1°C at the surface while the stratosphere cools by up to 10°C. When ozone is allowed to respond photochemically, the stratospheric cooling is reduced by 20%, with little effect in the troposphere. Planetary wave energy increases in the stratosphere, producing dynamical warming at high latitudes, in agreement with previous GCMAM doubled CO2 simulations; the effect is due to increased tropospheric generation and altered refraction, both strongly influenced by the magnitude of warming in the model’s tropical upper troposphere. This warming also results in stronger zonal winds in the lower stratosphere, which appears to reduce stratospheric planetary wave 2 energy and stratospheric warming events. ...

Transient Stratospheric Planetary Waves Generated by Tropospheric Forcing

In this paper the tropospheric-stratospheric system is modeled as a two-layer, quasigeostrophic atmosphere, and the modal and nonmodal solutions that arise from an impulse forcing within the troposphere are found. Regardless of the nature of the zonal flow, the troposphere can support disturbances over a broad range of wavenumbers, whereas the stratosphere solutions have significant amplitudes only at low wavenumbers. The results are applied to observational data given by a tropospheric-stratospheric analysis. It is shown that explosive baroclinic disturbances within the troposphere can generate transient waves within the stratosphere.

Global Variations of Zonal Mean Ozone during Stratospheric Warming Events

Eight years of Solar Backscatter Ultraviolet ozone data are examined to study zonal mean ozone variations associated with stratospheric planetary wave (warming) events. These fluctuations are found to he nearly global in extent, with relatively large variations in the tropics, and coherent signatures reaching up to 50° in the opposite (summer) hemisphere. These ozone variations are a manifestation of the global circulation cells associated with stratospheric warming events; the ozone responds dynamically in the lower stratosphere to transport, and pholochemically in the upper stratosphere to the circulation-induced temperature changes. The observed ozone variations in the tropics are of particular interest because transport is dominated by zonal-mean vertical motions (eddy flux divergences and mean meridional transport are negligible), and hence, substantial simplifications to the governing equations occur. The response of the atmosphere to these impulsive circulation changes provides a situation for robust estimates of the ozone-temperature sensitivity in the upper stratosphere.

What Causes the Seasonal Cycle of Stationary Waves in the Southern Stratosphere?

Stationary planetary waves in the southern stratosphere display a characteristic seasonal cycle. Previous research based on a one-dimensional model suggests that this behavior is mainly determined by seasonally varying transmission properties of the atmosphere with respect to wave propagation. The issue is investigated with the help of a hemispheric, linear, quasi-geostrophic model. It reproduces well some of the observed qualitative features and is internally consistent in the sense that its seasonal wave cycle can be explained in terms of varying wave transmission properties of the mean circulation. On the other hand, the model does not yield the observed seasonal cycle. Despite considerable sensitivity to modifications in the basic-state wind and dissipation parameterization, the model could not be reasonably fit to reproduce the observed seasonal cycle. Possible reasons for the model deficiency are put forward. In summary, even though suggestive, the present study is not entirely conclusive about the degree to which the observed cycle is determined by wave transmission properties alone.

The Effects of Transience on the Propagation of Stratospheric Planetary Waves

The propagation of planetary-scale Rossby waves in the stratosphere is investigated in a linear, time-dependent, primitive equation model. Two distinct maxima in the convergence of the Eliassen-Palm flux (EP flux) are found for steady and for transient waves in a variety of realistic northern hemisphere, winter season, zonal flows; one below the stratospheric polar jet and the other north of the zero wind line. These maxima appear at higher altitudes for wave 1 than for wave 2, especially in low latitudes. Different mechanisms cause the formation of these two maxima. The confinement of wave activity in high latitudes is mainly due to the dissipation of waves in that region under non-WKBJ conditions. The maximum in lower latitudes is related to the absorption of Rossby waves near their critical lines. For wave 2, the intensity of the high latitude maximum is sensitive to the transience of the wave forcing at the tropopause, while for wave 1 this sensitivity is reduced. For both waves the low latitude maximum shifts northward with increasing transience.

On the seasonal cycle of stratospheric planetary waves

On the seasonal cycle of stratospheric planetary waves

A Modification of the Atmospheric Energy Cycle

A modification is made of the conventional energy cycle by combining the eddy flux convergence and the mean meridional circulation terms in the mean momentum and heat equations. The combined terms are interpreted as the effective flux convergences in the extratropics where the steady state mean circulation is regarded as essentially being induced by eddies. In the presence of mean heating, the modified energy cycle is simpler and less misleading than the transformed energy cycle based on the transformed Eulerian-mean equations. This modification suggests that the major energy source of tropospheric planetary waves can be traced to the thermal generation of mean potential energy and that the stratospheric planetary wave is maintained by the total (mean plus eddy) vertical flux of energy from the troposphere, The conventional energy cycle of observed tropospheric planetary waves is, however, not as complicated as that of theoretical planetary waves in the quasi-nonacceleration condition. This is due to the fact that the observed tropospheric eddy heat flux convergence is counterbalanced by the mean heating and does not induce a large mean circulation in the steady state.

An Observational Study of High-Latitude Stratospheric Planetary Waves in Winter

Abstract Nimbus 4 and 5 radiances, representing temperatures in the upper and lower stratosphere, are analyzed to obtain power and coherence squared spectra at high latitudes during the winters of both hemispheres. The spectra for zonal wavenumbers 1–3 are presented, indicating a variety of wave features which exist in the upper stratosphere. An eastward moving wavenumber 1 feature with ∼4 day period is described in greater detail; the possible nature of this wave is briefly discussed. A second wave. observed only in the Southern Hemisphere wavenumber 2 field, is found to be westward moving with period near 2.3 days.