Charles McLandress

Satellite observations of thermospheric tides: Results from the Wind Imaging Interferometer on UARS

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.

Chemistry-climate model simulations of twenty-first century stratospheric climate and circulation changes

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 6 0.07 K decade 21 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 21 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 twentyfirst 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

On the importance of gravity waves in the middle atmosphere and their parameterization in general circulation models

Abstract This tutorial paper discusses the problem of parameterizing unresolved gravity waves in general circulation models (GCMs) of the middle atmosphere. For readers who are unfamiliar with middle atmosphere dynamics a review of the basic dynamics of both the large-scale circulation and internal gravity waves is presented. A fairly detailed and physically-based description is given of several gravity wave drag (GWD) schemes that are currently employed in middle atmosphere GCMs. These include the parameterizations of McFarlane (1987) , Medvedev and Klaassen (1995) , and Hines, 1997a , Hines, 1997b , which are used in the Canadian Middle Atmosphere Model, as well as the parameterization of Fritts and Lu (1993) , which is used in the TIME-GCM. Results from a mechanistic model and the two above mentioned GCMs are presented and discussed. This paper is not intended as a review of all GWD parameterizations nor is it meant as a quantitative comparison of the schemes that have been chosen.

Simulated Anthropogenic Changes in the Brewer–Dobson Circulation, Including Its Extension to High 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.

Separating the Dynamical Effects of Climate Change and Ozone Depletion. Part II: Southern Hemisphere Troposphere

Abstract 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...

Microwave Limb Sounder observations of gravity waves in the stratosphere: A climatology and interpretation

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.

Combined mesosphere/thermosphere winds using WINDII and HRDI data from the Upper Atmosphere Research Satellite

This paper examines the combined mesospheric and thermospheric (50 to 200 km) longitudinally averaged winds measured by the wind imaging interferometer (WINDII) and the high-resolution Doppler imager (HRDI) onboard the Upper Atmosphere Research Satellite. The data analyzed cover 2 years from February 1992 to February 1994 and consist of both day and nighttime WINDII winds obtained from the O(1S) green line emission and mesosphere/lower thermosphere daytime HRDI winds from the O2 atmospheric band. The combination of the WINDII and HRDI data sets is first justified by comparing all the data in the lower-thermosphere overlap region for days and orbits when both instruments were observing the same volume of atmosphere. This comparison shows good agreement between the two instruments. An analysis of the combined WINDII and HRDI winds during equinox and solstice periods is then performed. The amplification with height of the diurnal tide at equinox and its subsequent decay in the lower thermosphere is clearly demonstrated by the observations. The corresponding background (i.e., diurnal mean) zonal wind component exhibits a broad region of easterlies at lower latitudes in the upper mesosphere and lower thermosphere and westerlies at midlatitudes. Above 120 km the mean winds revert to easterlies in the zonal component and a two-celled equator to pole meridional circulation. The solstice circulation is highly asymmetric about the equator in accordance with the interhemispheric difference in solar heating. The reversal of the mesospheric jets as well as the summer to winter hemisphere meridional flow in the middle thermosphere are clearly shown. At solstice a significantly weaker and more hemispherically asymmetric propagating diurnal tide is also evident.

The Seasonal Variation of the Propagating Diurnal Tide in the Mesosphere and Lower Thermosphere. Part II: The Role of Tidal Heating and Zonal Mean Winds

A linear mechanistic tidal model is used to understand the mechanisms responsible for the seasonal variation of the propagating diurnal tide in the mesosphere and lower thermosphere simulated in the Canadian Middle Atmosphere Model (CMAM). The linear model uses a spectral approach to represent the horizontal structure of the tidal perturbations and employs dissipative processes that do not depend on season. By constraining the model with the zonal mean zonal winds, zonal mean temperatures, and tidal heating from the CMAM, the relative role of each of these terms is assessed. The linear model is able to reproduce all of the important tidal features found in the CMAM, in particular the semiannual amplitude variation in the lower thermosphere at low latitudes that is seen in observations. From this analysis the effects of both heating and mean winds are found to be responsible for the seasonal variation of the tidal amplitude, while variations in the tidal phase are attributed solely to changes in the mean winds. The strong sensitivity of the tide to the mean winds is the novel result of this study. This sensitivity is attributed to latitudinal shears in the zonal mean easterlies in the summer mesosphere. Although these shears occur on an annual basis, their impact on tidal amplitudes in the lower thermosphere is semiannual as a result of the 6-month shift in seasons between the two hemispheres. Simulations using observational datasets from the Committee on Space Research (COSPAR) International Reference Atmosphere (CIRA) and the High Resolution Doppler Imager (HRDI) reveal significant differences in the resulting tidal structure from that obtained using the CMAM winds, and point to possible deficiencies in these datasets.

The Seasonal Variation of the Propagating Diurnal Tide in the Mesosphere and Lower Thermosphere. Part I: The Role of Gravity Waves and Planetary Waves

Abstract The seasonal variation of the propagating diurnal tide in the mesosphere and lower thermosphere is examined using results from a 2-yr simulation of the extended version of the Canadian Middle Atmosphere Model (CMAM). The CMAM is shown to be able to reproduce not only the observed semiannual amplitude variation of the tide in the lower thermosphere but also more subtle features such as amplitude maxima that are stronger in March/April than in September/October, a 4- to 6-h shift in phase between winter and summer in the Northern Hemisphere, and a weaker seasonal variation of phase in the Southern Hemisphere. Part I of this two-part series of papers investigates the importance of two of the mechanisms that have been proposed to explain the observed variation of tidal amplitude, namely, 1) interactions with small-scale gravity waves and 2) interactions with planetary-scale waves like the quasi–2-day wave. Analysis of the tidal momentum and thermodynamic budgets shows that the direct effects of param...

A Robust Mechanism for Strengthening of the Brewer-Dobson Circulation in Response to Climate Change: Critical-Layer Control of Subtropical Wave Breaking

AbstractClimate models consistently predict a strengthened Brewer–Dobson circulation in response to greenhouse gas (GHG)-induced climate change. Although the predicted circulation changes are clearly the result of changes in stratospheric wave drag, the mechanism behind the wave-drag changes remains unclear. Here, simulations from a chemistry–climate model are analyzed to show that the changes in resolved wave drag are largely explainable in terms of a simple and robust dynamical mechanism, namely changes in the location of critical layers within the subtropical lower stratosphere, which are known from observations to control the spatial distribution of Rossby wave breaking. In particular, the strengthening of the upper flanks of the subtropical jets that is robustly expected from GHG-induced tropospheric warming pushes the critical layers (and the associated regions of wave drag) upward, allowing more wave activity to penetrate into the subtropical lower stratosphere. Because the subtropics represent the...