Nonorographic Gravity Wave Research Articles

A Study of Multiple Tropopause Structures Caused by Inertia–Gravity Waves in the Antarctic

Abstract Multiple tropopauses (MTs) defined by the World Meteorological Organization are frequently detected from autumn to spring at Syowa Station (69.0°S, 39.6°E). The dynamical mechanism of MT events was examined by observations of the first mesosphere–stratosphere–troposphere (MST) radar in the Antarctic, the Program of the Antarctic Syowa MST/Incoherent Scatter (IS) Radar (PANSY), and of radiosondes on 8–11 April 2013. The MT structure above the first tropopause is composed of strong temperature fluctuations. By a detailed analysis of observed three-dimensional wind and temperature fluctuation components, it is shown that the phase and amplitude relations between these components are consistent with the theoretical characteristics of linear inertia–gravity waves (IGWs). Numerical simulations were performed by using a nonhydrostatic model. The simulated MT structures and IGW parameters agree well with the observation. In the analysis using the numerical simulation data, it is seen that IGWs were generated around 65°S, 15°E and around 70°S, 15°E, propagated eastward, and reached the region above Syowa Station when the MT event was observed. These IGWs were likely radiated spontaneously from the upper-tropospheric flow around 65°S, 15°E and were forced by strong southerly surface winds over steep topography (70°S, 15°E). The MT occurrence is attributable to strong IGWs and the low mean static stability in the polar winter lower stratosphere. It is also shown that nonorographic gravity waves associated with the tropopause folding event contribute to 40% of the momentum fluxes, as shown by a gravity wave–resolving general circulation model in the lower stratosphere around 65°S. This result indicates that they are one of the key components for solving the cold-bias problem found in most climate models.

Case studies of nonorographic gravity waves over the Southern Ocean emphasize the role of moisture

AbstractTwo case studies of nonorographic gravity waves are carried out for wave events that occurred over the Southern Ocean in November 2005. Mesoscale simulations were carried out with the Weather and Research Forecast model. The simulated waves were compared to observations from superpressure balloons of the Vorcore campaign and from the High Resolution Dynamic Limb Sounder satellite. Satisfactory agreement is found, giving confidence in the estimations of wave parameters and amplitudes. For the amplitudes, both the model and observations provide a lower bound, for different reasons. Waves are found in the lower stratosphere with horizontal wavelengths of the order of 150–200 km in the horizontal, 5–8 km in the vertical, corresponding to intrinsic frequencies between 5 and 10 f, where f is the Coriolis parameter. Although the tropospheric flow is very different between the two cases, there are features which are common and appear significant for the gravity waves: these include intense localized updrafts associated with convection in the troposphere and a displaced polar vortex inducing strong winds in the stratosphere above the frontal region. Relative to theoretical expectations, the simulations emphasize the role of moisture. Intrinsic frequencies are significantly higher than those expected for waves produced by dry spontaneous generation from jets. To quantify the contribution of moisture, dry simulations were carried out, yielding momentum fluxes over oceanic regions that were 2.5 times weaker. Identification of the generation mechanisms in these complex flows calls for further study, and these should include moisture and a realistic stratospheric jet.

Intermittency in a stochastic parameterization of nonorographic gravity waves

AbstractA multiwave stochastic parameterization of nonorographic gravity waves (GWs), representing GWs produced by convection and a background of GWs in the midlatitudes, is tuned and tested against momentum fluxes derived from long‐duration balloon flights. The tests are done offline using data sets corresponding to the Southern Ocean during the Concordiasi campaign in 2010. We also adopt the limiting constraint that the drag produced by the scheme resembles that produced by a highly tuned spectral GW parameterization, the so‐called Hines scheme. Our results show that the parameterization can reproduce the momentum flux intermittency measured during the campaign, which is relevant since it strongly impacts on the vertical distribution of the GW drag. We also show that, at the altitude of the balloon flights, the momentum flux intermittency is in good part due to the GW sources: filtering by the background winds only becomes effective at much higher altitude. These results are based on bulk formulae for the GW momentum flux that could be used to replace our background GWs by GWs produced by fronts. Finally, the GW energy spectra built out of the stochastic scheme by averaging over a large ensemble of realizations are comparable to the classical vertical spectra of GWs, used today in globally spectral schemes. This indicates that multiwave and spectral schemes can be reconciled once a stochastic approach is used.

Structure, Energy, and Parameterization of Inertia–Gravity Waves in Dry and Moist Simulations of a Baroclinic Wave Life Cycle

Abstract The impact of moisture on inertia–gravity wave generation is assessed for an idealized unstable baroclinic wave using the Weather Research and Forecasting Model (WRF) in a channel on the f plane. The evolution of these waves in a moist simulation is compared with a dry simulation. The centers of action for inertia–gravity wave activity are identified as the equatorward-moving upper-level front and the poleward-progressing upper-level jet–surface front system. Four stratospheric wave packets are found, which are significantly more intense in the moist simulation and have slightly higher frequency. They are characterized by their structure and position during the baroclinic wave life cycle and are related to forcing terms in jet, front, and convection systems. By exploring the time series of mass and energy, it is shown that the release of latent heat leads to a change in enthalpy, an increase in the eddy kinetic energy, and an intensification of the inertia–gravity wave energy. The ratio of the inertia–gravity wave energy to the eddy kinetic energy is estimated to be about 1/200 for the moist simulation, which is 3 times larger than that for the dry simulation. An empirical parameterization scheme for the inertia–gravity wave energy is proposed, based on the fast large-scale ageostrophic flow associated with the jet, front, and convection. The diagnosed stratospheric inertia–gravity wave energy is well captured by this parameterization in six WRF simulations with different moisture and resolutions. The approach used to construct the parameterization may serve as a starting point for state-dependent nonorographic gravity wave drag schemes in general circulation models.

Effects of vertical resolution and nonorographic gravity wave drag on the simulated climate in the Community Atmosphere Model, version 5

AbstractHorizontal resolution of general circulation models (GCMs) has significantly increased during the last decade, however these changes were not accompanied by similar changes in vertical resolution. In our study, the Community Atmosphere Model, version 5 (CAM5) is used to study the sensitivity of climate to vertical resolution and nonorographic gravity wave drag. Nonorographic gravity wave drag is typically omitted from low‐top GCMs, however as we show, its influence on climate can be seen all the way to the surface. We show that an increase in vertical resolution from 1200 to 500 m in the free troposphere and lower stratosphere in CAM5 improves the representation of near‐tropopause temperatures, lower stratospheric temperatures, and surface wind stresses. In combination with nonorographic gravity waves, CAM5 with increased vertical resolution produces a realistic Quasi‐Biennial Oscillation (QBO), has an improved seasonal cycle of temperature in the extratropics, and represents better the coupling between the stratosphere and the troposphere.

What Is the Source of the Stratospheric Gravity Wave Belt in Austral Winter?

Abstract During austral winter, and away from orographic maxima or “hot spots,” stratospheric gravity waves in both satellite observations and Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data reveal enhanced amplitudes in a broad midlatitude belt extending across the Southern Ocean from east of the Andes to south of New Zealand. The peak latitude of this feature slowly migrates poleward from 50° to 60°S. Wave amplitudes are much weaker across the midlatitude Pacific Ocean. These features of the wave field are in striking agreement with diagnostics of baroclinic growth rates in the troposphere associated with midlatitude winter storm tracks and the climatology of the midlatitude jet. This correlation suggests that these features of the stratospheric gravity wave field are controlled by geographical variations of tropospheric nonorographic gravity wave sources in winter storm tracks: spontaneous adjustment emission from the midlatitude winter jet, frontogenesis, and convection.

Internal gravity waves from atmospheric jets and fronts

For several decades, jets and fronts have been known from observations to be significant sources of internal gravity waves in the atmosphere. Motivations to investigate these waves have included their impact on tropospheric convection, their contribution to local mixing and turbulence in the upper troposphere, their vertical propagation into the middle atmosphere, and the forcing of its global circulation. While many different studies have consistently highlighted jet exit regions as a favored locus for intense gravity waves, the mechanisms responsible for their emission had long remained elusive: one reason is the complexity of the environment in which the waves appear; another reason is that the waves constitute small deviations from the balanced dynamics of the flow generating them; i.e., they arise beyond our fundamental understanding of jets and fronts based on approximations that filter out gravity waves. Over the past two decades, the pressing need for improving parameterizations of nonorographic gravity waves in climate models that include a stratosphere has stimulated renewed investigations. The purpose of this review is to present current knowledge and understanding on gravity waves near jets and fronts from observations, theory, and modeling, and to discuss challenges for progress in coming years.

Net influence of an internally generated quasi-biennial oscillation on modelled stratospheric climate and chemistry

Abstract. A Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM) simulation with strong tropical non-orographic gravity wave drag (GWD) is compared to an otherwise identical simulation with near-zero tropical non-orographic GWD. The GEOSCCM generates a quasi-biennial oscillation (QBO) zonal wind signal in response to a tropical peak in GWD that resembles the zonal and climatological mean precipitation field. The modelled QBO has a frequency and amplitude that closely resembles observations. As expected, the modelled QBO improves the simulation of tropical zonal winds and enhances tropical and subtropical stratospheric variability. Also, inclusion of the QBO slows the meridional overturning circulation, resulting in a generally older stratospheric mean age of air. Slowing of the overturning circulation, changes in stratospheric temperature and enhanced subtropical mixing all affect the annual mean distributions of ozone, methane and nitrous oxide. Furthermore, the modelled QBO enhances polar stratospheric variability in winter. Because tropical zonal winds are easterly in the simulation without a QBO, there is a relative increase in tropical zonal winds in the simulation with a QBO. Extratropical differences between the simulations with and without a QBO thus reflect the westerly shift in tropical zonal winds: a relative strengthening of the polar stratospheric jet, polar stratospheric cooling and a weak reduction in Arctic lower stratospheric ozone.

A Comparison between Gravity Wave Momentum Fluxes in Observations and Climate Models

AbstractFor the first time, a formal comparison is made between gravity wave momentum fluxes in models and those derived from observations. Although gravity waves occur over a wide range of spatial and temporal scales, the focus of this paper is on scales that are being parameterized in present climate models, sub-1000-km scales. Only observational methods that permit derivation of gravity wave momentum fluxes over large geographical areas are discussed, and these are from satellite temperature measurements, constant-density long-duration balloons, and high-vertical-resolution radiosonde data. The models discussed include two high-resolution models in which gravity waves are explicitly modeled, Kanto and the Community Atmosphere Model, version 5 (CAM5), and three climate models containing gravity wave parameterizations, MAECHAM5, Hadley Centre Global Environmental Model 3 (HadGEM3), and the Goddard Institute for Space Studies (GISS) model. Measurements generally show similar flux magnitudes as in models, except that the fluxes derived from satellite measurements fall off more rapidly with height. This is likely due to limitations on the observable range of wavelengths, although other factors may contribute. When one accounts for this more rapid fall off, the geographical distribution of the fluxes from observations and models compare reasonably well, except for certain features that depend on the specification of the nonorographic gravity wave source functions in the climate models. For instance, both the observed fluxes and those in the high-resolution models are very small at summer high latitudes, but this is not the case for some of the climate models. This comparison between gravity wave fluxes from climate models, high-resolution models, and fluxes derived from observations indicates that such efforts offer a promising path toward improving specifications of gravity wave sources in climate models.

Dynamical Control of the Mesosphere by Orographic and Nonorographic Gravity Wave Drag during the Extended Northern Winters of 2006 and 2009

Abstract A version of the Canadian Middle Atmosphere Model (CMAM) that is nudged toward reanalysis data up to 1 hPa is used to examine the impacts of parameterized orographic and nonorographic gravity wave drag (OGWD and NGWD) on the zonal-mean circulation of the mesosphere during the extended northern winters of 2006 and 2009 when there were two large stratospheric sudden warmings. The simulations are compared to Aura Microwave Limb Sounder (MLS) observations of mesospheric temperature and carbon monoxide (CO) and derived zonal winds. The control simulation, which uses both OGWD and NGWD, is shown to be in good agreement with MLS. The impacts of OGWD and NGWD are assessed using simulations in which those sources of wave drag are removed. In the absence of OGWD the mesospheric zonal winds in the months preceding the warmings are too strong, causing increased mesospheric NGWD, which drives excessive downwelling, resulting in overly large lower-mesospheric values of CO prior to the warming. NGWD is found to be most important following the warmings when the underlying westerlies are too weak to allow much vertical propagation of the orographic gravity waves to the mesosphere. NGWD is primarily responsible for driving the circulation that results in the descent of CO from the thermosphere following the warmings. Zonal-mean mesospheric winds and temperatures in all simulations are shown to be strongly constrained by (i.e., slaved to) the stratosphere. Finally, it is demonstrated that the responses to OGWD and NGWD are nonadditive because of their dependence and influence on the background winds and temperatures.

Gravity wave variations during elevated stratopause events using SABER observations

AbstractElevated stratopauses formed at ~80–90 km altitude during the recovery phase of stratospheric sudden warmings in February 2006 and 2009. These likely occurred in response to changes in the downward circulation due to gravity waves (GWs) and/or planetary waves in the mesosphere and the lower thermosphere (MLT). However, the physical mechanisms are not fully understood, due in part to the lack of global GW observations in the MLT. This study presents global‐scale GW observations in the MLT during elevated stratopause events using Thermosphere, Ionosphere, Mesosphere Energetics Dynamics (TIMED)‐Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) temperature observation, which provide a better insight into the formation of an elevated stratopause. During the downward movement of elevated stratopause events in 2006 and 2009, GWs were suppressed below ~60 km and enhanced above ~60 km at high latitudes compared to non‐elevated stratopause years (2005 and 2007). Global SABER GW observations indicate that the regions of GW enhancement propagate from low‐mid latitudes to high latitudes in association with the equatorward shift of the polar night jet during elevated stratopause events. Ray‐tracing simulations show enhancements of the poleward propagation of GWs during elevated stratopause events as well as continuous propagation of non‐orographic GWs within high latitudes. Therefore, our results suggest that meridional propagation of GWs from lower to higher latitudes, which is typically not included in global‐scale models, plays an important role in determining GW variations and thus the downward movement of an elevated stratopause, in addition to non‐orographic GWs originating at high latitudes.

Stationary waves in the wintertime mesosphere: Evidence for gravity wave filtering by stratospheric planetary waves

Quasi‐stationary planetary‐scale waves in the wintertime mesosphere and lower thermosphere (MLT) are thought to be forced in part by drag imparted by gravity waves that have been modulated by underlying stratospheric waves. Although this mechanism has been demonstrated numerically, there have been very few observational studies that examine wave driving as a source of planetary waves in the MLT. This study uses data from EOS Aura and TIMED between 2005 and 2011 to examine the momentum budget of MLT wintertime planetary waves. Monthly averages for January indicate that the dynamics of zonal wave number 1 are determined from a three‐way balance among the Coriolis acceleration, the pressure gradient force, and a momentum residual term that reflects wave drag. The MLT circulations in January 2005, 2006, 2009, and 2011 are qualitatively consistent with a simple model of wave forcing by drag from gravity waves that have been modulated by stratospheric planetary waves. MLT winds during these years are also consistent with analyses from a high‐altitude operational prediction model that includes parameterized nonorographic gravity wave drag. The importance of wave drag for the MLT momentum budget suggests that the gradient wind approximation is inadequate for deriving planetary‐scale winds from global temperature measurements. Our results underscore the need for direct global wind measurements in the MLT.

The Stratospheric Extension of the Canadian Global Deterministic Medium-Range Weather Forecasting System and Its Impact on Tropospheric Forecasts

Abstract A new system that resolves the stratosphere was implemented for operational medium-range weather forecasts at the Canadian Meteorological Centre. The model lid was raised from 10 to 0.1 hPa, parameterization schemes for nonorographic gravity wave tendencies and methane oxidation were introduced, and a new radiation scheme was implemented. Because of the higher lid height of 0.1 hPa, new measurements between 10 and 0.1 hPa were also added. This new high-top system resulted not only in dramatically improved forecasts of the stratosphere, but also in large improvements in medium-range tropospheric forecast skill. Pairs of assimilation experiments reveal that most of the stratospheric and tropospheric forecast improvement is obtained without the extra observations in the upper stratosphere. However, these observations further improve forecasts in the winter hemisphere but not in the summer hemisphere. Pairs of forecast experiments were run in which initial conditions were the same for each experiment but the forecast model differed. The large improvements in stratospheric forecast skill are found to be due to the higher lid height of the new model. The new radiation scheme helps to improve tropospheric forecasts. However, the degree of improvement seen in tropospheric forecast skill could not be entirely explained with these purely forecast experiments. It is hypothesized that the cycling of a better model and assimilation provide improved initial conditions, which result in improved forecasts.

The impact of gravity wave drag on mesospheric analyses of the 2006 stratospheric major warming

[1] The 2006 Arctic stratospheric sudden warming was an unusual event in which the stratopause disappeared after the peak of the warming, only to reform at very high altitudes above 75 km. The stratopause then descended during February and March 2006, returning to climatological values. This event, which coupled the troposphere, stratosphere, and mesosphere, has been used to assess the quality of the upper stratospheric and mesospheric analyses produced with the Canadian Middle Atmosphere Model's data assimilation system. By comparing with simulations in which Sounding of the Atmosphere Using Broadband Emission Radiometry temperatures were assimilated, mesospheric analyses obtained without assimilating any observations above 45 km were shown to capture the timing and amplitude of the elevated stratopause event. Mesospheric analyses of temperature and winds were sensitive to the use of a nonorographic gravity wave drag scheme, but this sensitivity was reduced when mesospheric temperatures were assimilated. Moreover, a realistic nonorographic gravity wave drag can somewhat compensate for the lack of mesospheric measurements. On the other hand, the descent of mesospheric NOx after the peak of the stratospheric warming was found to be sensitive to the presence of nonorographic gravity wave drag even when mesospheric temperatures were assimilated, suggesting that eddy forcing is not fully constrained by mesospheric observations in our data assimilation system.

A new perspective on gravity waves in the Martian atmosphere: Sources and features

Using a Mars general circulation model (GCM), we investigate the characteristics of gravity waves in the atmosphere of Mars. Fluctuations due to internal gravity waves at tidal periods, such as diurnal, semidiurnal, and terdiurnal tides, are predominant in the lower atmosphere. In particular, the diurnal and semidiurnal tides having large zonal wave numbers (s > 10) are significant. Fluctuations associated with nontidal gravity wave components in the lower atmosphere are negligibly small except for the stationary orographic component. Nontidal gravity waves grow in amplitude in the middle atmosphere, while tidal components remain the dominant form of gravity waves. In the upper atmosphere (>80 km), nontidal gravity wave components and fluctuations due to the diurnal and semidiurnal tides having large zonal wave numbers are equally important. We estimate the global distributions of the gravity wave energy at various altitudes and attempt to define various sources. Our results show that most nontidal gravity wave energy is closely related with the diurnal amplitude except in the lower atmosphere where association with orography predominates. This suggests that nonorographic gravity waves are generated by the breaking and dissipation of the diurnal tide. On the other hand, nontidal gravity wave energy in the lower atmosphere peaks in mountainous regions and points to their source as being of orographic nature. We also enumerate the major differences between gravity waves on Mars and Earth as simulated by global models.

New Gravity Wave Treatments for GISS Climate Models

AbstractPrevious versions of GISS climate models have either used formulations of Rayleigh drag to represent unresolved gravity wave interactions with the model-resolved flow or have included a rather complicated treatment of unresolved gravity waves that, while being climate interactive, involved the specification of a relatively large number of parameters that were not well constrained by observations and also was computationally very expensive. Here, the authors introduce a relatively simple and computationally efficient specification of unresolved orographic and nonorographic gravity waves and their interaction with the resolved flow. Comparisons of the GISS model winds and temperatures with no gravity wave parameterization; with only orographic gravity wave parameterization; and with both orographic and nonorographic gravity wave parameterizations are shown to illustrate how the zonal mean winds and temperatures converge toward observations. The authors also show that the specifications of orographic and nonorographic gravity waves must be different in the Northern and Southern Hemispheres. Then results are presented where the nonorographic gravity wave sources are specified to represent sources from convection in the intertropical convergence zone and spontaneous emission from jet imbalances. Finally, a strategy to include these effects in a climate-dependent manner is suggested.

Explicitly Stochastic Parameterization of Nonorographic Gravity Wave Drag

Abstract A straightforward methodology is presented for converting the deterministic multiwave parameterizations of nonorographic gravity wave drag, currently used in general circulation models (GCMs), to stochastic analogs that use fewer waves (in the example herein, a single wave) within each grid box. Deterministic discretizations of source-level momentum flux spectra using a fixed spectrum of many waves with predefined phase speeds are replaced by sampling these source spectra stochastically using waves with randomly assigned phase speeds. Using simple conversion formulas, it is shown that time-mean wave-induced drag, diffusion, and heating-rate profiles identical to those from the deterministic scheme are produced by the stochastic analog. Furthermore, in these examples the need for bulk intermittency factors of small value is largely obviated through the explicit incorporation of stochastic intermittency into the scheme. When implemented in a GCM, the single-wave stochastic analog of an existing deterministic scheme reproduces almost identical time-mean middle-atmosphere climate and drag as its deterministic antecedent but with an order of magnitude reduction in computational expense. The stochastically parameterized drag is also accompanied by inherent variability about the time-mean profile that forces the smallest space–time scales of the GCM. Studies of mean GCM kinetic energy spectra show that this additional stochastic forcing does not lead to excessive increases in dynamical variability at these smallest GCM scales. The results show that the expensive deterministic schemes currently used in GCMs are easily modified and replaced by cheap stochastic analogs without any obvious deleterious impacts on GCM climate or variability, while offering potential advantages of computational savings, reduction of systematic climate biases, and greater and more realistic ensemble spread.

A case study of an elevated stratopause generated in the Whole Atmosphere Community Climate Model

[1] The Whole Atmosphere Community Climate Model (WACCM) is used to study the influence of gravity waves on the generation and evolution of an elevated stratopause following a sudden stratospheric warming (SSW). By comparing WACCM simulations of two Arctic winters, where one is dynamically undisturbed and one is disturbed, we find that intense planetary wave activity during a SSW drives the reversal of the zonal mean wind in the stratosphere. This alters the penetration of eastward propagating, non-orographic gravity waves into the mesosphere, which determine the extent of cooling in the lower mesosphere and upper stratosphere through the adiabatic effects of the gravity wave-driven residual circulation, and play a crucial role in the reformation of the elevated stratopause in the lower mesosphere. Eventually, the forcing due to gravity waves returns to normal wintertime values as the stratospheric zonal wind recovers, and is then associated with the warming and lowering of the elevated stratopause, by wave induced diabatic descent. We find that SSW followed by an elevated stratopause is a climatologically robust phenomenon in free running WACCM with characteristics closely resembling recently observed events in the Arctic.

Improved Middle Atmosphere Climate and Forecasts in the ECMWF Model through a Nonorographic Gravity Wave Drag Parameterization

AbstractIn model cycle 35r3 (Cy35r3) of the ECMWF Integrated Forecast System (IFS), the momentum deposition from small-scale nonorographic gravity waves is parameterized by the Scinocca scheme, which uses hydrostatic nonrotational wave dynamics to describe the vertical evolution of a broad, constant, and isotropic spectrum of gravity waves emanating from the troposphere. The Cy35r3 middle atmosphere climate shows the following: (i) an improved representation of the zonal-mean circulation and temperature structure; (ii) a realistic parameterized gravity wave drag; (iii) a reasonable stationary planetary wave structure and stationary wave driving in July and an underestimate of the generation of stationary wave activity in the troposphere and stationary wave driving in January; (iv) an improved representation of the tropical variability of the stratospheric circulation, although the westerly phase of the semiannual oscillation is missing; and (v) a realistic horizontal distribution of momentum flux in the stratosphere. By contrast, the middle atmosphere climate is much too close to radiative equilibrium when the Scinocca scheme is replaced by Rayleigh friction, which was the standard method of parameterizing the effects of nonorographic gravity waves in the IFS prior to Cy35r3. Finally, there is a reduction in Cy35r3 short-range high-resolution forecast error in the upper stratosphere.

Case studies of the mesospheric response to recent minor, major, and extended stratospheric warmings

We have studied the mesospheric response to two recent stratospheric warmings by performing short‐term forecasts at medium (1.5°) and high (0.5°) spatial resolution under different gravity wave drag (GWD) scenarios. We validated our models with our high‐altitude analysis that extends from 0 to 90 km. For the minor warming of January 2008, reduced upper‐level orographic GWD weakened the downward residual circulation and cooled the mesosphere. Parameterized nonorographic GWD increased the simulated mesospheric cooling. For the prolonged major warming of 2006, heavily attenuated orographic GWD led to pronounced cooling near 50 km. During the extended phase of this event, an unusually strong westerly polar vortex reformed in the lower mesosphere, which allowed westward propagating nonorographic gravity waves to reach the mesosphere and break, with net westward accelerations of over 50 m s−1 d−1. This, in turn, forced a strong residual circulation, yielding descent velocities over 2 cm s−1 between 65°N and 85°N, consistent with previous reports of enhanced downward transport of trace constituents. The resulting adiabatic heating, as evidenced by the unusually vertically displaced stratopause at 80 km, is likely a direct consequence of this enhanced gravity wave driven descent. High‐resolution simulations without parameterized GWD were closer to the analysis than medium‐resolution simulations with parameterized orographic GWD only, but still did not fully simulate the mesospheric thermal response. Specifically, the 80 km temperature enhancement was still underestimated in these simulations. This suggests that higher spatial resolution is needed to adequately resolve extratropical gravity wave momentum fluxes.