Recent observational and theoretical studies of the global properties of small-scale atmospheric gravity waves have highlighted the global effects of these waves on the circulation from the surface to the middle atmosphere. The effects of gravity waves on the large-scale circulation have long been treated via parametrizations in both climate and weather-forecasting applications. In these parametrizations, key parameters describe the global distributions of gravity-wave momentum flux, wavelengths and frequencies. Until recently, global observations could not define the required parameters because the waves are small in scale and intermittent in occurrence. Recent satellite and other global datasets with improved resolution, along with innovative analysis methods, are now providing constraints for the parametrizations that can improve the treatment of these waves in climate-prediction models. Research using very-highresolution global models has also recently demonstrated the capability to resolve gravity waves and their circulation effects, and when tested against observations these models show some very realistic properties. Here we review recent studies on gravitywave effects in stratosphere-resolving climate models, recent observations and analysis methods that reveal global patterns in gravity-wave momentum fluxes and results of very-high-resolution model studies, and we outline some future research requirements to improve the treatment of these waves in climate simulations.

The response of stratospheric climate and circulation to increasing amounts of greenhouse gases (GHGs) and ozone recovery in the twenty-first century is analyzed in simulations of 11 chemistry–climate models using near-identical forcings and experimental setup. In addition to an overall global cooling of the stratosphere in the simulations (0.59 ± 0.07 K decade−1 at 10 hPa), ozone recovery causes a warming of the Southern Hemisphere polar lower stratosphere in summer with enhanced cooling above. The rate of warming correlates with the rate of ozone recovery projected by the models and, on average, changes from 0.8 to 0.48 K decade−1 at 100 hPa as the rate of recovery declines from the first to the second half of the century. In the winter northern polar lower stratosphere the increased radiative cooling from the growing abundance of GHGs is, in most models, balanced by adiabatic warming from stronger polar downwelling. In the Antarctic lower stratosphere the models simulate an increase in low temperature extremes required for polar stratospheric cloud (PSC) formation, but the positive trend is decreasing over the twenty-first century in all models. In the Arctic, none of the models simulates a statistically significant increase in Arctic PSCs throughout the twenty-first century. The subtropical jets accelerate in response to climate change and the ozone recovery produces a westward acceleration of the lower-stratospheric wind over the Antarctic during summer, though this response is sensitive to the rate of recovery projected by the models. There is a strengthening of the Brewer–Dobson circulation throughout the depth of the stratosphere, which reduces the mean age of air nearly everywhere at a rate of about 0.05 yr decade−1 in those models with this diagnostic. On average, the annual mean tropical upwelling in the lower stratosphere (∼70 hPa) increases by almost 2% decade−1, with 59% of this trend forced by the parameterized orographic gravity wave drag in the models. This is a consequence of the eastward acceleration of the subtropical jets, which increases the upward flux of (parameterized) momentum reaching the lower stratosphere in these latitudes.

Recent studies using comprehensive middle atmosphere models predict a strengthening of the Brewer–Dobson circulation in response to climate change. To gain confidence in the realism of this result it is important to quantify and understand the contributions from the different components of stratospheric wave drag that cause this increase. Such an analysis is performed here using three 150-yr transient simulations from the Canadian Middle Atmosphere Model (CMAM), a Chemistry–Climate Model that simulates climate change and ozone depletion and recovery. Resolved wave drag and parameterized orographic gravity wave drag account for 60% and 40%, respectively, of the long-term trend in annual mean net upward mass flux at 70 hPa, with planetary waves accounting for 60% of the resolved wave drag trend. Synoptic wave drag has the strongest impact in northern winter, where it accounts for nearly as much of the upward mass flux trend as planetary wave drag. Owing to differences in the latitudinal structure of the wave drag changes, the relative contribution of resolved and parameterized wave drag to the tropical upward mass flux trend over any particular latitude range is highly sensitive to the range of latitudes considered. An examination of the spatial structure of the climate change response reveals no straightforward connection between the low-latitude and high-latitude changes: while the model results show an increase in Arctic downwelling in winter, they also show a decrease in Antarctic downwelling in spring. Both changes are attributed to changes in the flux of stationary planetary wave activity into the stratosphere.

Nearly all chemistry-climate models (CCMs) have a systematic bias of a delayed springtime breakdown of the Southern Hemisphere (SH) stratospheric polar vortex, implying insufficient stratospheric wave drag. In this study the Canadian Middle Atmosphere Model (CMAM) and the CMAM Data Assimilation System (CMAM-DAS) are used to investigate the cause of this bias. Zonal wind analysis increments from CMAM-DAS reveal systematic negative values in the stratosphere near 608S in winter and early spring. These are interpreted as indicating a bias in the model physics, namely, missing gravity wave drag (GWD). The negative analysis increments remain at a nearly constant height during winter and descend as the vortex weakens, much like orographic GWD. This region is also where current orographic GWD parameterizations have a gap in wave drag, which is suggested to be unrealistic because of missing effects in those parameterizations. These findings motivate a pair of free-running CMAM simulations to assess the impact of extra orographic GWD at 608S. The control simulation exhibits the cold-pole bias and delayed vortex breakdown seen in the CCMs. In the simulation with extra GWD, the cold-pole bias is significantly reduced and the vortex breaks down earlier.Changes in resolved wave drag in the stratosphere also occur in response to the extra GWD, which reduce stratospheric SH polar-cap temperature biases in late spring and early summer. Reducing the dynamical biases, however, results in degraded Antarctic column ozone. This suggests that CCMs that obtain realistic column ozone in the presence of an overly strong and persistent vortex may be doing so through compensating errors.

Abstract. High-horizontal-resolution temperature data from the Microwave Limb Sounder (MLS) are analyzed to obtain information about high intrinsic frequency gravity waves in the stratosphere. Global climatologies of temperature variance at solstice are computed using six years of data. A linear gravity wave model is used to interpret the satellite measurements and to infer information about tropospheric wave sources. Globally uniform sources having several different spectral shapes are examined and the computed variances are filtered in three-dimensional space in a manner that simulates the MLS weighting functions. The model is able to reproduce the observed zonal mean structure, thus indicating that the observations reflect changes in background wind speeds and provide little information about the latitudinal variation of wave sources. Longitudinal variations in the summer hemisphere do reflect source variations since the modeled variances exhibit much less variation in this direction as a consequence of the zonal symmetry of the background winds. A close correspondence between the MLS variances and satellite observations of outgoing-longwave radiation suggests that deep convection is the probable source for these waves. The large variances observed over the tip of South America in winter are most certainly linked to orographic forcing but inferences about wave sources in Northern Hemisphere winter are difficult to make as a result of the high degree of longitudinal and temporal variability in the stratospheric winds. Comparisons of model results using different source spectra suggest that the tropospheric sources in the subtropics in summer have a broader phase speed spectrum than do sources at middle latitudes in winter.

Thermospheric winds measured by the Wind Imaging Interferometer (WINDII) on the upper atmosphere research satellite are analyzed for migrating solar tides. The data cover a 2‐year period commencing February 1992 and are obtained from the atomic oxygen O(1S) 557.7‐nm emission, which provides observations of the 90‐ to 200‐km altitude range during daytime and the 90‐ to 110‐km range at night. The subtropical lower thermosphere is dominated by the diurnal propagating tide which exhibits a vertical wavelength of approximately 22 km, grows in amplitude up to 95 km, and decays rapidly above where molecular diffusion greatly reduces the vertical shears. Although the phase remains fairly uniform throughout the year, a pronounced semiannual oscillation is observed in the diurnal tide amplitude. At both 20°N and 20°S the meridional and zonal wind components attain their maximum values at equinox of approximately 70 and 40 m/s, respectively, while the solstitial minima are nearly a factor of 2 smaller. At 35°N the diurnal tide semiannual amplitude oscillation is still present in the lower thermosphere, but above 100 km it is replaced by an annual cycle with a maximum in July and August. This contrasts with 35°S where the July/August peak is absent and the semiannual oscillation extends to 110 km. At midlatitudes the zonal and meridional winds are of similar magnitude, and no significant hemispheric asymmetries in amplitudes are observed. In the lower thermosphere the semidiurnal tide amplitude exhibits an annual oscillation, with maximum values of 30 to 40 m/s occurring in June/July near 100 km at 35°N, 35°S, and the equator. A bimodal structure in the seasonal variation of the semidiurnal phase is observed. This feature is characterized by rapid equinoctial transitions and is particularly well defined at the equator. Examination of the equatorial middle thermosphere indicates that the semidiurnal tide attains its maximum amplitude at 140 km and exhibits a vertical wavelength of approximately 60 km. These findings indicate the predominance of the antisymmetric (2,3) Hough mode in the tropics.

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The separate effects of ozone depleting substances (ODSs) and greenhouse gases (GHGs) on forcing circulation changes in the Southern Hemisphere extratropical troposphere are investigated using a version of the Canadian Middle Atmosphere Model (CMAM) that is coupled to an ocean. Circulation-related diagnostics include zonal wind, tropopause pressure, Hadley cell width, jet location, annular mode index, precipitation, wave drag, and eddy fluxes of momentum and heat. As expected, the tropospheric response to the ODS forcing occurs primarily in austral summer, with past (1960-99) and future (2000-99) trends of opposite sign, while the GHG forcing produces more seasonally uniform trends with the same sign in the past and future. In summer the ODS forcing dominates past trends in all diagnostics, while the two forcings contribute nearly equally but oppositely to future trends. The ODS forcing produces a past surface temperature response consisting of cooling over eastern Antarctica, and is the dominant driver of past summertime surface temperature changes when the model is constrained by observed sea surface temperatures. For all diagnostics, the response to the ODS and GHG forcings is additive; that is, the linear trend computed from the simulations using the combined forcings equals (within statistical uncertainty) the sum of the linear trends from the simulations using the two separate forcings. Space-time spectra of eddy fluxes and the spatial distribution of transient wave drag are examined to assess the viability of several recently proposed mechanisms for the observed poleward shift in the tropospheric jet.