The quasi-biennial oscillation (QBO) dominates the variability of the equatorial stratosphere (∼16–50 km) and is easily seen as downward propagating easterly and westerly wind regimes, with a variable period averaging approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven by propagating waves with periods unrelated to that of the resulting oscillation. Although the QBO is a tropical phenomenon, it affects the stratospheric flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the strength of Atlantic hurricanes. The effects of the QBO are not confined to atmospheric dynamics. Chemical constituents, such as ozone, water vapor, and methane, are affected by circulation changes induced by the QBO. There are also substantial QBO signals in many of the shorter-lived chemical constituents. Through modulation of extratropical wave propagation, the QBO has an effect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratosphere affects surface weather patterns, providing a mechanism for the QBO to have an effect at the Earth's surface. As more data sources (e.g., wind and temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. This review covers the current state of knowledge of the tropical QBO, its extratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (∼0–16 km) and mesosphere (∼50–100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the field, as well as a source of information and references for specialists. The history of research on the QBO is discussed only briefly, and the reader is referred to several historical review papers. The basic theory of the QBO is summarized, and tutorial references are provided.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/1999RG000073

The quasi-biennial oscillation

[1] The global-scale wave model (GSWM) is used to investigate mesospheric and lower thermospheric migrating and nonmigrating diurnal tidal components that propagate upward from the troposphere, where they are excited by latent heat release associated with deep tropical convection. Our diurnal tidal forcing parameterization is derived from a 7-year database of global cloud imagery. The GSWM migrating response is sufficiently large to modulate the dominant radiatively excited migrating diurnal tide in the middle and upper atmosphere during every month of the year. Five additional nonmigrating diurnal components, the eastward propagating zonal wave numbers 2 and 3, the westward propagating zonal wave number 2, and the standing oscillations, also introduce significant longitudinal variability of the diurnal tide in these regions. The comparative importance of the nonmigrating components evolves from month to month and varies with tidal field. Our GSWM investigation suggests that other dynamical models must account for the tropospheric latent heat source in order to make realistic predictions of the diurnal tide in the middle and upper atmosphere.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2001JD001236

Migrating and nonmigrating diurnal tides in the middle and upper atmosphere excited by tropospheric latent heat release

Preface 1. Basic properties of radiation, atmospheres and oceans 2. Basic state variables 3. Interaction of radiation with matter 4. Formulation of radiative transfer problems 5. Approximate solutions of prototype problems 6. Accurate numerical solutions of prototype problems 7. Emission-dominated radiative processes 8. Radiative transfer in spectrally-complex media 9. Solar radiation driving photochemistry and photobiology 10. The role of radiation in climate Appendix 1. A primer on radiative transfer: absorption and scattering opacity Appendix 2. Stokes parameters, Poincare sphere, and the Mueller matrix Appendix 3. Nomenclature: glossary of symbols Appendix 4. Principle of reciprocity for the bidirectional reflectance Appendix 5. Isolation of the azimuth-dependence Appendix 6. The streaming term in spherical geometry Appendix 7. Reflectance and transmittance of the invariant intensity (I n2) Appendix 8. Scaling transformation for anisotropic scattering Appendix 9. Reciprocity, duality and effects of surface reflection Appendix 10. Removal of overflow problems in the intensity formulas.

Radiative Transfer in the Atmosphere and Ocean

https://physicstoday.scitation.org/doi/pdf/10.1063/1.1333301

The Horizontal Wind Model (HWM) has been updated in the thermosphere with new observations and formulation changes. These new data are ground-based 630 nm Fabry-Perot Interferometer (FPI) measurements in the equatorial and polar regions, as well as cross-track winds from the Gravity Field and Steady State Ocean Circulation Explorer (GOCE) satellite. The GOCE wind observations provide valuable wind data in the twilight regions. The ground-based FPI measurements fill latitudinal data gaps in the prior observational database. Construction of this reference model also provides the opportunity to compare these new measurements. The resulting update (HWM14) provides an improved time-dependent, observationally based, global empirical specification of the upper atmospheric general circulation patterns and migrating tides. In basic agreement with existing accepted theoretical knowledge of the thermosphere general circulation, additional calculations indicate that the empirical wind specifications are self-consistent with climatological ionosphere plasma distribution and electric field patterns.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2014EA000089

An update to the Horizontal Wind Model (HWM): The quiet time thermosphere

[1] Owing to their sampling properties in local solar time, both the TIMED and UARS spacecrafts provide the opportunity to quantitatively study the mesospheric thermal tides. The information though is limited because it takes the TIMED instruments 2 months to cover the full range of 24 hours, almost twice as long as UARS. We present here the results of an analysis, in which the seasonal and global-scale variations of amplitude and phase for the migrating diurnal tide are retrieved from atmospheric temperature and wind measurements on the TIMED (SABER, TIDI) and UARS (MLS, HRDI) satellites. Comparisons are made with results from the Numerical Spectral Model (NSM), which is characterized in part by the parameterization of small-scale gravity waves. At 60 km and above, the amplitude and phase variations of the diurnal tide from the NSM agree generally well with those derived from the SABER temperatures. In the SABER temperature amplitudes at around 60 km there are evident asymmetries in latitude with respect to the equator, which are not apparent in the MLS data near 55 km or in the NSM. (The asymmetry in the SABER temperatures disappears above 60 km.) For the meridional winds of the migrating diurnal tide derived from TIDI, the phase and relative amplitude variation with latitude and season are in qualitative agreement with the HRDI measurements and the NSM. The amplitudes from TIDI winds are systematically smaller, but this may in part be due to interannual variations. However, the results for the zonal winds from TIDI more often do not agree well with those from HRDI or the NSM.

/pdf/diurnal-variations-of-temperature-and-winds-inferred-from-58h47xnn53.pdf

Diurnal variations of temperature and winds inferred from TIMED and UARS measurements

[1] The High-Resolution Doppler Imager on the Upper Atmosphere Research Satellite has observed the O2 atmospheric band nightglow at extremely high spatial resolution of 50 km in the horizontal over much of Earth's surface. These observations of the oxygen nightglow are shown to be a good surrogate for atomic oxygen outflow from the thermosphere and subsequent recombination in the upper mesosphere. The distribution of enhanced airglow and recombination verify the previous observations that bright regions of the nightglow are associated with descending motion, where the tides are transporting atomic oxygen out of the thermosphere, and regions of decreased airglow are associated with ascending tidal motion. The detailed observations of the global airglow at a scale of 100 km present a much more complex picture, however. The tidally enhanced regions are extremely structured, with strong longitudinal and latitudinal variability in brightness and associated atomic oxygen recombination. These structured regions are persistent on a timescale of a day or more, with certain regions of the globe exhibiting enhanced airglow much of the time. These regions are present over the entire night side of the globe but are strongly modulated by the ascending and descending tidal motions. A theoretical study of these enhancements suggests that they are caused by the upward propagation of gravity waves, which break in the high mesosphere and induce strong vertical motion in the atmosphere. It is interesting that the primary cause of the increased transport of atomic oxygen is vertical motion rather than the conventional enhanced turbulence explanation. These gravity waves are associated with wind shear and strong convective activity in the troposphere and often appear in certain preferred locations where convective activity is expected. These observations have important implications for the basic processes by which momentum and energy are deposited in the upper mesosphere. We suggest that the processes must be modeled as stochastic events that dance around the base of the thermosphere, generally in climatologically preferred regions, rather than the relatively uniform distribution of gravity wave forcings that is being used in general circulation models up to this time.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2003JD003400

A global view of the molecular oxygen night airglow

Received 25 June 2009; revised 30 October 2009; accepted 5 November 2009; published 30 March 2010. [1] The TIMED Doppler Interferometer (TIDI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite has been measuring horizontal winds in the mesosphere and lower thermosphere (MLT) since 2002. Because of the high inclination of the TIMED orbit, TIDI measures the horizontal winds from pole to pole every orbit. This paper presents the first assessment of the spatial structure and temporal evolution of the nonmigrating semidiurnal tides over the Arctic determined from the TIDI wind measurements and a comparison of the structure of the nonmigrating semidiurnal tide between the Arctic and Antarctic. The nonmigrating semidiurnal tides were determined as a 60 day average based on the yaw cycles of the spacecraft. The nonmigrating semidiurnal tidal wind field over the Arctic comprises mainly the westward‐propagating zonal wave numbers1(W1)and3(W3)andstandingzonalwavenumber0(S0)modes.TheW1modeis the most prominent, maximizing above 90 km poleward of 60°N during the yaw interval ranging from mid‐March to mid‐May. While this mode exhibits a slight amplitude increase toward the North Pole during this interval, its phase is nearly constant with latitude. The S0 mode is enhanced over two yaw intervals ranging from mid‐January to mid‐May, but its amplitude decreases toward the North Pole. Compared to the W1 semidiurnal tide over the Antarctic, that over the Arctic is smaller in amplitude, of less extended duration, achieves maximum amplitudes at higher altitudes by ∼10 km, and exhibits a weaker amplitude increase toward the pole. These differences likely result from differences in excitation mechanisms and efficiency and/or in propagation conditions in the two responses for the nonmigrating semidiurnal tides between the Arctic and Antarctic.

/pdf/nonmigrating-semidiurnal-tide-over-the-arctic-determined-hopdkwqdf1.pdf

Nonmigrating semidiurnal tide over the Arctic determined from TIMED Doppler Interferometer wind observations

[1] Spatial structure and temporal evolution of the nonmigrating semidiurnal tidal components over Antarctica are determined by analyzing horizontal wind measurements in the Mesosphere and Lower Thermosphere (MLT) collected using TIMED Doppler Interferometer (TIDI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) satellite from 2002 to 2007. The data were organized into six specific intervals of approximately 60 days corresponding to the TIMED yaw periods. The results confirm the existence of a westward propagating zonal wave number 1 (W1) semidiurnal tidal component in the Antarctic MLT meridional wind field prior to the Austral summer solstice. This wave achieves a peak amplitude near 20 m s−1 at 90 km and is vertically stratified while extending latitudinally from the pole to 60°S. A similar structure is observed in the zonal wind field. However, the amplitude maximizes around the Austral summer solstice during the yaw period spanning 15 November to 15 January. In addition to a strong latitudinal gradient in amplitude, the W1 component also shows a vertical wavelength from 20 km near the pole to 40 km at 60°S. The amplitude and phase agree well with ground-based meteor radar observations from the South Pole. Evidence for significant though weaker standing (S0) and W3 components is also found. These components diminish in the vicinity of the pole and appear during the winter months with latitudinally restricted structures. The vertical wavelength of the S0 component during the summer is 25 km, similar to the W1 component. During the winter the wavelength of the S0 component becomes nearly evanescent.

Wilbert R. Skinner

Papers

Diurnal variations of temperature and winds inferred from TIMED and UARS measurements

A global view of the molecular oxygen night airglow

Nonmigrating semidiurnal tide over the Arctic determined from TIMED Doppler Interferometer wind observations

Structure of the nonmigrating semidiurnal tide above Antarctica observed from the TIMED Doppler Interferometer

A sequential estimation technique for recovering atmospheric data from orbiting satellites