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Atmospheric and hydrospheric sciences
The dynamics of the mesosphere and lower thermosphere: a brief review
Vincent R A
Atmospheric tides, Gravity waves, Planetary waves, Middle atmosphere, Mesosphere, Lower thermosphere, Wave coupling
The 2006 major sudden warming. Top: time-height cross-section of temperatures during the major SSW in late January 2006. Note the descent and disappearance of the stratopause at the peak of the warming and its subsequent reappearance and descent from the mesosphere. Bottom: the time variation of the zonal-mean zonal wind at 10 hPa and 60 N. The blue line shows the wind during the stratwarm, while the red line depicts the climatalogical seasonal cycle. Adapted from Figure one of Yamazaki et al. (2012).
Stratospheric warmings occur when large-scale planetary waves break in the middle atmosphere (10-100 km) in winter. This figure shows that the whole middle atmosphere is affected, so that the EW (zonal) winds and the temperature structure temporarily revert to summer-like conditions. The zonal means winds in the stratosphere (blue line in bottom figure) become westward (negative) for a short time while those above 70 km (the mesosphere) become eastward. In turn, this change in the prevailing wind affects the vertical propagation of atmospheric gravity waves and changes their impact on the middle atmosphere. The effects of stratospheric warmings can also be observed in the ionosphere, impacting on radiowave propagation.
The dynamics of the mesosphere-lower thermosphere (MLT) (60 to 110 km) is dominated by waves and their effects. The basic structure of the MLT is determined by momentum deposition by small-scale gravity waves, which drives a summer-to-winter pole circulation at the mesopause. Atmospheric tides are also an important component of the dynamics of the MLT. Observations from extended ground-based networks, satellites as well as numerical modelling show that non-migrating tidal modes in the MLT are more important than previously thought, with evidence for directly coupling into the thermosphere/ionosphere. Major disturbances lower in the atmosphere, such as wintertime sudden stratospheric warmings, temporarily disrupt the circulation pattern and thermal structure of the MLT. In the equatorial mesosphere, gravity wave driving leads to oscillations in the zonal wind on semiannual time scales, although variability on quasi-biennial time scales is also apparent. Planetary-scale waves such as the quasi-two-day wave temporarily dominate the dynamics of the summertime MLT, especially in the southern hemisphere. Impacts may include short-term changes to the thermal structure and physics of the high-latitude MLT. Here, we briefly review the dynamics of the MLT, with a particular emphasis on developments in the past decade.