Dynamics, Transport, and Waves

Contact Gloria Manney

Dynamics and Transport are among the fundamental processes controlling the composition of the middle atmosphere (stratosphere and mesosphere) and its connections with the lower atmosphere (troposphere). Dynamics comprises the fundamental physical processes determining the characteristics and evolution of a fluid (the atmosphere) on rotating sphere externally forced by solar radiation. The study of atmospheric dynamics uses observations and mathematical models to characterize and explain the evolution of temperatures and winds, as well as products derived from them (e.g., geopotential heights, streamfunctions, vorticity and potential vorticity). An important aspect of this behavior is characterization of wave motions. Of particular importance in the middle atmoshpere are planetary scale (1 to 3 cycles around a latitude circle) waves that are forced by upper tropospheric weather systems or can be generated internally by instabilities in the background wind fields, and gravity waves.

In the stratosphere, the dominant feature of the circulation is the winter polar vortex, consisting of a band of strong winds westerly winds roughly encircling the pole (the polar night jet) that forms as a result of the fundamental balance between the Coriolis forcing and radiative forcing (air moves upward and poleward as a result of solar heating in the tropics, is deflected eastward by the Coriolis force, and descends in the polar winter because of very low temperatures there resulting from the absence of sunlight).

One of the most dramatic dynamical phenomena in the middle atmosphere is the midwinter stratospheric sudden warming (SSW). SSWs occur frequently (historically about once every two years on average) but unpredictably as a result of waves propagating from the upper troposphere under conditions that result in the waves breaking -- depositing their momentum -- in the upper stratosphere and leading to a breakdown of the winter stratospheric polar vortex. A similar process results in the "final warming" by which the polar vortex breaks down in spring, but SSWs are notable in that they disrupt and reverse the circulation in midwinter, leading to a period of easterly winds followed by a recovery to westerlies and re-establishment of the polar vortex. The most dramatic SSWs (three of which have occurred very recently, in Jan 2004, 2006, and 2009) (Manney et al, 2005, JGR; 2008, JGR, ACP; 2009, ACP, GRL) result in a complete disruption of the typical middle atmosphere high-latitude temperature structure (minimum near the tropopause, ~8-10km, maximum at the stratopause, ~50km), such that the conventional distinction between stratosphere and mesosphere is rendered somewhat meaningless (Manney et al 2008, JGR; 2009 GRL, and references therein). While SSWs are thought of as being controlled primarily by planetary scale wave motions, recent work has shown that gravity waves are also important, especially the re-establishment of the vortex after strong SSWs (e.g., Siskind et al, 2007).

The evolution of the circulation and wave motions in the middle atmosphere have also been shown to extend through the mesosphere (Lee et al, 2009, GRL, and references therein), and to affect the weather patterns in the upper troposphere, especially during extreme events such as SSWs. Dynamical processes and wave motions are also important in the tropics: Phenomena such as the annual cycle of upward transport of water vapor (and other trace gases, the so-called "tape-recorder" effect), the Quasi-biennial and semiannual oscillations, and Kelvin wave motions play a large role in determining the structure of the equatorial middle atmosphere.

The dynamics of the polar middle atmosphere is instrumental in determining the extent and timing of polar processing and ozone loss, since those processes are strongly dependent on temperature and vortex containment . In addition, dynamics play a large role in determining the composition of the middle atmosphere via transport processes. The relative roles of transport and chemistry in determining the distribution of trace gases depend on the time scales for chemical and dynamical changes; these time scales vary dramatically with season, altitude, latitude and species. Measurements of trace gases with long chemical lifetimes (e.g., N2O, in some regions CO, H2O, O3) provide information to quantify transport processes; this information is critical for assessing the ability of models to correctly reproduce atmospheric transporteg, (eg, Jin et al, 2009, ACP; Manney et al, 2009, ACP) and for calculations aimed at separating the contribution of chemical and dynamical processes to changes in atmospheric composition, especially for ozone (e.g., Manney et al, JAS, 1995a, b; Singleton et al, 2007, JGR; and references therein).

Prior to datasets from Aura MLS (and other instruments on that satellite), measurements of many species (including long-lived tracers) in the middle atmosphere were sporadic and limited in both spatial and temporal coverage. Aura MLS now provides measurements of long-lived tracers from the upper troposphere through the mesosphere with global daily coverage. CO, HNO3, O3, and H2O measurements are useful in the upper troposphere/lower stratosphere as in many situations they behave as tracers of transport there. N2O is a long-lived tracer useful in studying transport in the lower through the middle stratosphere. CO and H2O provide information on transport in the upper stratosphere and into the mesosphere. Measurements from Aura MLS and other recent satellite instruments have been and continue to be instrumental in improving our understanding of middle atmosphere dynamics and transport.

Further Reading

General

• Labitzke and van Loon, "The Stratosphere" (Springer, 1999)

Advanced

• Andrews, Holton, and Leovy, "Middle Atmosphere Dynamics"

Review Papers

• Shepherd, T.G., The Middle Atmosphere, JASTP, 2000
• Shepherd, T.G., Transport in the middle atmosphere, JMSJ, 2007
• Shepherd, T.G., Dynamics, Stratospheric Ozone, and Climate Change, Atmosphere-Ocean, 2008
• Baldwin et al, The quasi-biennial oscillation, Rev. Geophys., 2001
• Fritts and Alexander, Gravity Wave Dynamics and Effects in the Middle Atmosphere, Rev. Geophys., 2003