Variability Of Planetary Waves Research Articles

Short- and long-term stationary planetary wave variability in the middle atmosphere in contemporaneous satellite and reanalysis data

Short- and long-term variabilities of stationary planetary waves (SPW) in the middle atmosphere are investigated from temperatures obtained from two satellites (Formosa Satellite-3/Constellation Observing System for Meteorology, Ionosphere and Climate (FORMOSAT-3/COSMIC) and Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere Ionosphere Mesosphere Energetics Dynamics (TIMED)) and a reanalysis dataset (ERA-Interim). COSMIC data analysis with a smaller window size enables investigation of short-term variabilities and wavelet analysis shows the presence of 30 to 120 days oscillations in the stratosphere during different years and are attributed to wave-mean flow interactions. This result in observational data is seen only due to the use of COSMIC data and its unique sampling pattern that enabled the extraction of short-term variability. Additionally, annual and quasi-biennial features are also observed. SABER analysis also shows similar long-term variations. The annual variation in both datasets shows linearly increasing spectral power indicating that the stratosphere is increasingly becoming chaotic and irregular and the dynamics are becoming stronger and expanding equatorward. Vertically upward propagation of SPW1 and SPW2 into the mesosphere is hindered during Sudden Stratospheric Warmings (SSW) due to the prevalent westward winds and smaller mesospheric amplitudes are observed. During no SSW years, larger SPW1 and SPW2 amplitudes are seen in the mesosphere. A positive correlation is observed between COSMIC SPW amplitudes and gravity wave (GW) potential energy (Ep) while the longitudes of maximum amplitudes are out of phase.

Local stratopause temperature variabilities and their embedding in the global context

Abstract. The stratopause is by definition the transition between the stratosphere and mesosphere. During winter the circulation at mid-latitudes and high latitudes in the stratosphere is mainly driven by quasi-stationary planetary waves (PWs), while the circulation in the mesosphere is mainly driven by gravity waves (GWs). The question arises of whether PWs or GWs dominate the variability of the stratopause. The most famous and dramatic variability of the middle atmosphere is a sudden stratospheric warming (SSW) generated by PWs interacting with the polar vortex. A similar phenomenon but smaller in magnitude and more regional is stratopause temperature enhancements (STEs) initially observed by local measurements and generated by breaking PWs. Thus it seems that PWs dominate the variability of the stratopause. In this study we want to quantify to which extent quasi-stationary PWs contribute to the stratopause variability. To do that we combine local lidar observations at Kühlungsborn (54∘ N, 11∘ E) and Andenes (69∘ N, 16∘ E) with global MERRA-2 reanalysis data bringing the local variability of the stratopause into the global context. Therefore we compare the temperature time series at Kühlungsborn and Andenes at 2 hPa, the altitude where STEs maximize, with characteristics (amplitude and phase) of PWs with wave numbers 1, 2 and 3. We found that for Kühlungsborn and Andenes 98 % of the local day-to-day variability of the stratopause can be explained by the variability of PWs with wave number 1, 2 and 3. Thus, the winter stratopause day-to-day variability is highly dominated by the variability of PWs.

Modulation of air‐sea fluxes by extratropical planetary waves and its impact during the recent surface warming slowdown

AbstractIt is widely accepted that natural decadal variability played a major role in the slowdown in global warming observed in the 21st century, with sea surface cooling in the tropical Pacific recognized as a major contributor. However, the warming pause was most pronounced during boreal winter, with Northern Hemisphere flow anomalies also playing a role. Here we quantify the contribution of extratropical heat exchanges by comparing geopotential and temperature anomalies simulated by ensembles of seasonal forecasts with similar ocean temperature but different heat fluxes north of 40°N, as a result of planetary wave variability. We show that an important part of heat flux anomalies is associated with decadal variations in the phase of a specific planetary wave pattern. In model simulations covering the last three decades, this variability pattern accounts for a decrease of 0.35°C/decade in the post‐1998 wintertime temperature trend over northern continents.

How Does Interhemispheric Coupling Contribute to Cool Down the Summer Polar Mesosphere?

Abstract Interhemispheric coupling is commonly associated with events of high planetary wave activity in the winter stratosphere triggering a heating of the polar mesopause region in the opposite hemisphere. Here, a more fundamental role that this mechanism plays in the absence of planetary wave variability is highlighted. This study focuses directly on the mesospheric part of the coupling chain, which is induced by the gravity wave drag in the winter mesosphere. To investigate the effect that the winter residual flow has on the summertime high-latitude upwelling, the Kühlungsborn Mechanistic General Circulation Model (KMCM) is used to compare a control simulation to runs where the parameterized gravity waves are removed from the winter hemisphere. The model response in the summer mesosphere reveals that the winter mesospheric residual circulation fosters a net (and substantial) cooling of the summer polar mesopause. These results offer an extension of the current view of interhemispheric coupling: from a mode of internal variability to a constant, gravity wave–driven phenomenon that is modulated by planetary wave activity.

Projected intensification of subseasonal temperature variability and heat waves in the Great Plains

AbstractCompared to changes in the climatological mean temperature, we have less confidence in how much and by what mechanisms temperature variability may be affected by anthropogenic climate change. Here based on a 30‐member climate change projection from an earth system model, we find that summertime subseasonal temperature variability in the U.S. Great Plains is enhanced by approximately 20% in 2070–2100 relative to 1980–2010. In particular, daily temperature departures from the new climatologies during future heat waves are on average 0.6°C warmer than are the corresponding departures under present‐day conditions. Although in both periods heat waves in the Great Plains tend to be associated with planetary wave events, the amplification of future heat waves does not appear to be induced by changes in planetary wave variability in the midlatitudes. Instead, in this experiment the strengthening appears to be primarily caused by enhanced local land‐atmosphere feedbacks resulting from a warmer/drier future climate.

Wavelet analysis of polar vortex variability over the twentieth century

AbstractRecent increases in extreme weather events in the Northern Hemisphere have been linked to amplified planetary waves. Changes in planetary wave properties are linked to changes in climate; hence, finding a mechanism that links the planetary wave variability under climatic forcing and midlatitude blocking events has engendered a great deal of interest. In this study, wavelet analysis is applied to time series of planetary wave phase speeds at high latitudes as a first step to assessing the potential for identifying these mechanisms in the observational record. A circumpolar multiannual cycle increase is found but signals also demonstrate a complex westward propagation pattern with periods of intense wave variability. Significant correlations between wavelet power at all time scales and albedo, snow cover, atmospheric ozone levels, and surface air temperature are demonstrated.

Contrasting meridional structures of stratospheric and tropospheric planetary wave variability in the Northern Hemisphere

The meridional structures of stratospheric and tropospheric planetary wave variability (PWV) over the Northern Hemisphere (NH) extratropics were investigated and compared using reanalysis data. By performing the spherical double Fourier series expansion of geopotential height data, the horizontal structures of PWV at each vertical level could be examined in the two-dimensional (2D) wavenumber (zonal and meridional wavenumbers) space. Comparing the amplitudes of wave components during the last three decades, the results suggested that the structures of PWV in the NH troposphere significantly differ from the stratospheric counterparts. The PWV in the troposphere shows multiple meridional wave-like structures, most pronounced for the meridional dipole; while in contrast, PWV in the stratosphere mainly shows large-scale zonal wave patterns, dominated by zonal waves 1 and 2, and have little wave-like fluctuation in the latitudinal direction. The dominant patterns of the NH PWV also show contrasting features of meridional structure between the stratosphere and the troposphere. As represented in the 2D wavenumber space, the leading two empirical orthogonal functions of PWV in the stratosphere largely exhibit the zonal wave 1 pattern, while those in the troposphere clearly show meridional wave-like structures and are dominated by the dipole. The refractive index was derived based on the zonal mean basic state to qualitatively interpret the observational findings. The results suggested that the basic state in the NH troposphere is much more favourable for latitudinally propagating stationary waves than the stratosphere. The difference in meridional structure between stratospheric and tropospheric planetary waves can be well captured in a linear baroclinic model with the observed zonal mean basic state. Furthermore, both theoretical and modelling analyses demonstrated that the fact that zonal wave patterns are preferred in the NH stratosphere may be partly attributable to the vertical curvature of the stratospheric zonal mean basic state.

The neutral dynamics during the 2009 sudden stratosphere warming simulated by different whole atmosphere models

The present study compares simulations of the 2009 sudden stratospheric warming (SSW) from four different whole atmosphere models. The models included in the comparison are the Ground‐to‐topside model of Atmosphere and Ionosphere for Aeronomy, Hamburg Model of the Neutral and Ionized Atmosphere, Whole Atmosphere Model, and Whole Atmosphere Community Climate Model Extended version (WACCM‐X). The comparison focuses on the zonal mean, planetary wave, and tidal variability in the middle and upper atmosphere during the 2009 SSW. The model simulations are constrained in the lower atmosphere, and the simulated zonal mean and planetary wave variability is thus similar up to ∼1 hPa (50 km). With the exception of WACCM‐X, which is constrained up to 0.002 hPa (92 km), the models are unconstrained at higher altitudes leading to considerable divergence among the model simulations in the mesosphere and thermosphere. We attribute the differences at higher altitudes to be primarily due to different gravity wave drag parameterizations. In the mesosphere and lower thermosphere, we find both similarities and differences among the model simulated migrating and nonmigrating tides. The migrating diurnal tide (DW1) is similar in all of the model simulations. The model simulations reveal similar temporal evolution of the amplitude and phase of the migrating semidiurnal tide (SW2); however, the absolute SW2 amplitudes are significantly different. Through comparison of the zonal mean, planetary wave, and tidal variability during the 2009 SSW, the results of the present study provide insight into aspects of the middle and upper atmosphere variability that are considered to be robust features, as well as aspects that should be considered with significant uncertainty.

The role of synoptic eddies in the tropospheric response to stratospheric variability

The tropospheric response to sudden stratospheric warmings (SSWs) is analyzed in an idealized model setup regarding the respective roles of planetary‐scale and synoptic‐scale waves. The control model run includes a full interactive wave spectrum, while a second run includes interactive planetary‐scale waves but only the time‐mean synoptic‐scale wave forcing from the control run. In both runs, the tropospheric response is characterized by the negative phase of the respective tropospheric annular mode. But given their different latitudinal structure, the control run shows the expected response, i.e., an equatorward shift of the tropospheric jet, whereas the response in the absence of interactive synoptic eddies is characterized by a poleward jet shift. This opposite jet shift is associated with a different planetary wave variability that couples with the zonal flow between the stratosphere and the surface. These results indicate that the synoptic eddy feedback is necessary for the observed tropospheric response to SSWs.

The influence of atmospheric tide and planetary wave variability during sudden stratosphere warmings on the low latitude ionosphere

Numerical simulations are performed for a sudden stratosphere warming (SSW) under different atmospheric tide and planetary wave forcing conditions to investigate the tidal variability in the mesosphere and lower thermosphere (MLT). The influence of variability of different tides in the MLT on generating perturbations to the low latitude ionosphere is also investigated. Significant changes are found to occur in the migrating semidiurnal solar (SW2) and lunar (M2) tides as well as in the westward propagating nonmigrating semidiurnal tide with zonal wave number 1 (SW1). The changes in the zonal mean atmosphere that occur during SSWs lead to an enhancement in the SW2 and M2 tides. The vertical wavelength of the SW2 is also changed, resulting in phase variability in the SW2 at a constant altitude. Significant enhancements in the SW1 are found to occur only in the presence of additional planetary wave forcing, and this demonstrates that nonlinear planetary wave‒tide interactions lead to the enhanced SW1 during SSWs. The amplitude and phase variability of the SW2 is found to be capable of producing temporal variability in the vertical plasma drift velocity that is similar to the observed variability. Changes in the M2 during SSWs can contribute up to an additional ∼30% of the total ionosphere variability; however, the overall influence of the lunar tide is found to be dependent upon the phase of the moon relative to the timing of the SSW. Although the influence is relatively minor, the SW1 also contributes to the low latitude ionosphere variability during SSWs. The simulation results for the vertical plasma drift velocity and total electron content (TEC) further illustrate that significant longitude variability occurs in the ionosphere response to SSWs.

A Zonal Wavenumber 3 Pattern of Northern Hemisphere Wintertime Planetary Wave Variability at High Latitudes

Abstract A prominent pattern of variability of the Northern Hemisphere wintertime tropospheric planetary waves, referred to here as the Wave3 pattern, is identified from the NCEP–NCAR reanalysis. It is worthy of attention because its structure is similar to the linear trend pattern as well as the leading pattern of multidecadal variability of the planetary waves during the past half century. The Wave3 pattern is defined as the second empirical orthogonal function (EOF) of detrended December–February mean 300-hPa meridional wind V300 and denotes a zonal shift of the ridges and troughs of the climatological flow. Although its interannual variance is roughly comparable to that of EOF1 of V300, which represents the Pacific–North America (PNA) pattern, its multidecadal variance is nearly twice as large as that of the PNA. Wave3 is not completely structurally or temporally distinct from the northern annular mode (NAM) but, for some attributes, the linkage of the observed trend to Wave3 is clearer than to NAM. The prominence of the Wave3 pattern is further supported by attributes of many climate models that participated in phase 3 of the Coupled Model Intercomparison Project (CMIP3). In particular, in the Community Climate System Model, version 3 (CCSM3), the Wave3 pattern is present as EOF3 of V300 in both a fully coupled integration and a stand-alone atmospheric integration forced by climatological sea surface temperatures. Its existence in the latter experiment indicates that the pattern can be produced by atmospheric processes alone.

Possible solar forcing of interannual and decadal stratospheric planetary wave variability in the northern hemisphere: An observational study

The variability of stratospheric planetary waves and their possible connection with the 11-year solar cycle forcing have been investigated using annual-mean temperatures for the period of 1958–2001 derived from two reanalysis data sets. The significant planetary waves (wavenumbers 1–3) can be identified in the northern mid-high latitudes (55–75°N) in the stratosphere using this data. Comparisons with satellite-retrieved products from the Microwave Sounding Unit (MSU) confirm the significant planetary wave variability seen in the reanalyses. A planetary wave amplitude index (PWAI) is defined to indicate the strength of the stratospheric planetary waves. The PWAI is derived from Fourier analysis of the temperature field for wavenumbers 1–3 and averaged over 55–75°N latitude and the 70–20 hPa layers. The results include two meaningful inter-annual oscillations (2- and 8-year) and one decadal trend (16-year) that was derived from wavelet analysis. The stratospheric temperature structure of the wave amplitudes appear associated with the Arctic Oscillation (AO) which explicitly changed with the PWAI. The temperature gradients between the polar and mid-high latitudes show opposite tendencies between the top-10 strong and weak wave regimes. The variation of the planetary wave amplitude appears closely related to the solar forcing during the recent four solar cycles (20–23). The peak of the 2-year oscillation occurs synchronously with solar minimum, and is consistent with the negative correlation between the PWAI and the observed solar UV irradiance. The UV changes between the maxima and minima of the 11-year solar cycle impact the temperature structure in the middle-lower stratosphere in the mid-high latitudes and hence influence the planetary waves. During solar maximum, the dominant influence appears to be exerted through changes in static stability, leading to a reduction in planetary wave amplitude. During solar minimum, the dominant influence appears to be exerted through changes in the meridional temperature gradient and vertical wind shear, leading to an enhancement of planetary wave amplitude.

Variability of planetary waves as a signature of possible climatic changes

The long-term variability of stationary and traveling planetary waves in the lower stratosphere has been investigated using the data of NCEP/NCAR reanalysis. The results obtained show that during the last decades winter-mean amplitude of the stationary planetary wave with zonal wave number 1 (SPW1) increases at the higher middle latitudes of the Northern Hemisphere. It has been suggested that the observed increase in the SPW1 amplitude should be accompanied by the growth in the magnitude of the stratospheric vacillations. The analysis of the SPW1 behavior in the NCEP/NCAR data set supports this suggestion and shows a noticeable increase with time in the SPW1 intra-seasonal variability. The amplitudes of the long-period normal atmospheric modes, the so-called 5-, 10- and 16-day waves, diminish. It is supposed that one of the possible reasons for this decrease can be a growth of radiative damping rate caused, for instance, by the increase of CO 2 . To investigate a possible climatic change of the middle atmosphere dynamics caused by observed changes in the tropospheric temperature, two sets of runs (using zonally averaged temperature distributions in the troposphere typical for January 1960 and 2000) with the middle and upper atmosphere model (MUAM) have been performed. The results obtained show that on average the calculated amplitude of the SPW1 in the stratosphere increased in 2000 and there is also an increase of its intra-seasonal variability conditioned by nonlinear interaction with the mean flow. This increase in the amplitudes of stratospheric vacillations during the last four decades allows us to suggest that stratospheric dynamics becomes more stochastic.

Interannual variability of planetary waves in the ozone layer at 65° S

Planetary wave activity in the ozone layer was analysed using the Total Ozone Mapping Spectrometer (TOMS) satellite total ozone measurements. The area under study is southern latitude 65° S, the closest to the location of the Vernadsky station (65°15′ S, 64°16′ W). The time–longitude variations in the total ozone during August–December 1979–2003 are presented. The five‐month average distributions of the total ozone along the latitude circle of 65° S show the steady features of quasi‐stationary zonal distribution caused by superposition of the total wave numbers. The structure of wave number 1 is dominant. Interannual variations and long‐term trends of the total wave number amplitude and longitudinal position of the wave ridge (high ozone) and trough (low ozone) are described. Due to the wave amplitude increase the difference between low and high ozone in the opposite longitudinal sectors reached ca 30% in the early 2000s. During 1979–2003 the wave ridge position was relatively stable with the mean longitude of 152±15° E; however, the wave trough position drifted eastward from 55° W to the Greenwich meridian with the mean velocity of the long‐term drift of 23.6±7.2° per decade. Deviation of the quasi‐stationary wave characteristics relative to the mean tendency took place in 1988 and 2002.

Seasonal, interannual and short‐term variability of planetary waves in Met Office stratospheric assimilated fields

AbstractMet Office stratospheric assimilated data have been used to study the variability of travelling planetary waves in the upper stratosphere during the last decade (1992–2001). To distinguish the various travelling and quasi‐standing waves, a new space‐time filtering method has been developed. The results of this analysis show that longer‐period planetary waves (10<T<20 days) have significant amplitudes mainly during winter in both hemispheres. Westward‐propagating waves are dominant in the northern hemisphere, but in the southern hemisphere eastward‐ and westward‐propagating waves have comparable amplitudes and sometimes eastward‐propagating waves are dominant. It is suggested that the main forcing of longer‐period travelling waves is an internal variability of the quasi‐stationary planetary waves in the tropo/stratosphere, and that the substantial activity of eastward‐propagating waves in the southern hemisphere is associated with seasonal changes of the general circulation of the middle atmosphere. In the shorter‐period part of the wavelet spectra (T<10 days), westward‐propagating waves are dominant and the observed maxima in amplitude spectra can be identified with the first symmetric normal atmospheric modes, the so‐called 5‐ and 4‐day waves. These waves, present in the stratosphere during most of the year, increase in amplitude during seasonal transitions. Analysis of the behaviour of wavelet amplitudes at different pressure levels shows that these normal‐mode waves propagate upward from the lower atmosphere and their amplitudes increase with altitude. There exists also a substantial long‐period modulation of the 5‐day wave amplitude with quasi‐period of about 30 days. Copyright © 2004 Royal Meteorological Society

QBO influences on the variability of planetary waves in the equatorial mesopause region

MF radar observations of hourly mean zonal and meridional winds at 86 km over Tirunelveli (8.7°N, 77.8°E), India, over a period of five years (1993–1997) are utilized to study the possible influence of stratospheric quasibiennial oscillation (QBO) on the vertical propagation of planetary-scale waves. The radiosonde winds at Singapore are used to identify the phase of the stratospheric QBO at 30 hPa. The study reflects that the interannual variability of the planetary waves may be explained partly by the direction of the stratospheric QBO winds. Limitations of the present work are: (i) the QBO cycles considered are not many and (ii) the observed waves are presumed to originate from below and propagate through the stratospheric QBO wind system.

The 11-year solar cycle, the Sun's rotation, and the middle stratosphere in winter. Part II: Response of planetary waves

The effect of the 11-year solar cycle on the response of planetary wavenumbers 1 and 2 at 10 and 30 hPa in winter to solar activity oscillations on the time scale of the Sun's rotation (27.2 day) is discussed in terms of statistical spectral analysis. The three oscillations studied are the 27.2 d (period of the Sun's rotation), 25.3 d (periodicity caused by modulation of the 27.2 d stratospheric response by annual atmospheric variation), and 54.4 d (doubled period of the solar rotation). A significant effect of the 11-year solar cycle is found for the 54.4 d periodicity in planetary wavenumber 1, and for the 27.2 and 25.3 d periodicities in planetary wavenumber 2. The effect of the 11-year solar cycle is expressed in the evident differences between the amplitudes of responses of planetary waves at maximum and minimum of the solar cycle: the amplitudes are much larger at high than at low solar activity. The 11-year modulation of planetary wave activity is most pronounced at mid-latitudes, mainly at 40–60°N, where the observed variability of planetary waves is large. The results obtained are in good agreement with results of the recent modeling study by Shindell et al. (Science 284 (1999) 305).

Internal Variability of the Troposphere–Stratosphere Coupled System Simulated in a Simple Global Circulation Model

Internal variability of a troposphere‐stratosphere coupled system is investigated in a series of numerical experiments with a simple global circulation model under a perpetual winter condition. In order to examine the relative importance of forced planetary waves in the interaction with the zonal mean zonal flow and baroclinic disturbances, amplitude of a sinusoidal surface topography of zonal wavenumber 1 or 2 is swept as an experimental parameter; 1000-day integrations are performed for 110 combinations of the external parameters. Intraseasonal variability of the extratropical winter stratosphere in the parameter sweep experiment of the zonal wavenumber-1 topography is classified into four regimes, dependent on the topographic amplitude: (I) a nearly radiative equilibrium state; (II) small undulations of the polar vortex; (III) intermittent breakdowns of the polar vortex, or occurrence of stratospheric sudden warming events; and (IV) a usually weak and warm state of the polar vortex. The behavior of planetary waves in the stratosphere is characterized by linear propagation in regime I, and by quasi-linear, or weakly nonlinear, wave‐mean flow interaction in regime II. In regimes III and IV, on the other hand, it is in a highly nonlinear state, reflecting the occurrence of sudden warming events and the usually distorted polar vortex, respectively. The Northern Hemisphere winter stratosphere corresponds to regime III in the intraseasonal variations, while the southern counterpart is close to regime II. A lag-correlation analysis shows that the dynamical linkage between the stratosphere and the troposphere is also dependent on the regimes. The vertical linkage is primarily the upward control in regime I; planetary waves are generated by nonlinear interaction of baroclinic disturbances in the troposphere and propagate into the stratosphere. In regime II, stratospheric variations are largely confined in the stratosphere, although planetary waves are generated in the troposphere. The linkage in regime III, on the other hand, is inevitably two-way. Planetary wave variability has upward influence from the troposphere to the stratosphere as well as the zonal mean zonal wind shows preconditioning in the troposphere before stratospheric sudden warming events. Downward propagation of signals to the upper troposphere are also seen in the zonal mean temperature in high latitudes; it is higher than normal at the final stage of the sequence of sudden warming events. The linkage is also two-way in regime IV.

Mesospheric planetary waves at northern hemisphere fall equinox

Northern hemisphere planetary waves are strong in the winter and weak in the summer, and they go through a fast transition around equinox. This transition is studied here using NCAR Thermosphere‐Ionosphere‐Mesosphere‐Electrodynamics general circulation model (TIME‐GCM) simulations with 1997 National Centers for Environmental Prediction (NCEP) analysis. The planetary wave variability during the transition and its effect on the temperature and winds in the mesosphere are examined. The simulated planetary wave structure agrees with climatological studies, and the fast transition of the planetary waves is captured by the model. The wave variability produces large temperature changes in the upper atmosphere above local stations in middle and high latitudes. The qualitative behavior of the model is in excellent agreement with recent observations of a major perturbation in OH mesospheric temperatures from Ft. Collins (Taylor et al., 2001), although the smaller calculated magnitude suggests that the planetary wave amplitude might be underestimated by the model.

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...