Abstract
Abstract The transition between the middle atmosphere and the thermosphere is known as the MLT region (for mesosphere and lower thermosphere). This area has some characteristics that set it apart from other regions of the atmosphere. Most notably, it is the altitude region with the lowest overall temperature and has the unique characteristic that the temperature is much lower in summer than in winter. The summer-to-winter-temperature gradient is the result of adiabatic cooling and warming associated with a vigorous circulation driven primarily by gravity waves. Tides and planetary waves also contribute to the circulation and to the large dynamical variability in the MLT. The past decade has seen much progress in describing and understanding the dynamics of the MLT and the interactions of dynamics with chemistry and radiation. This review describes recent observations and numerical modeling as they relate to understanding the dynamical processes that control the MLT and its variability. Results from the Whole Atmosphere Community Climate Model (WACCM), which is a comprehensive high-top general circulation model with interactive chemistry, are used to illustrate the dynamical processes. Selected observations from the Sounding the Atmosphere with Broadband Emission Radiometry (SABER) instrument are shown for comparison. WACCM simulations of MLT dynamics have some differences with observations. These differences and other questions and discrepancies described in recent papers point to a number of ongoing uncertainties about the MLT dynamical system.
Highlights
The region of transition from the middle atmosphere to the thermosphere is known as the MLT
Since the radiative heating is relatively weak in the MLT, gravity wave processes can make a substantial contribution to the global mean heating/cooling
The heating changes include both the absorption of radiation by water vapor and latent heat release. They found that the heating changes during 1997–1998 were sufficient to account for a maximum in the diurnal tide amplitude that was observed in low latitudes during that period
Summary
The region of transition from the middle atmosphere to the thermosphere is known as the MLT (mesosphere and lower thermosphere). WACCM is a comprehensive global climate model that extends from the Earth’s surface into the lower thermosphere It includes interactive chemistry with realistic surface concentrations and trends of natural and anthropogenic trace gases. Model results shown here are from extended integrations using WACCM version 3.5 This version does not include interactions between the atmosphere and the oceans; instead, sea surface temperatures are specified using observations from recent decades. The topics covered in this paper are the characteristics of the large-scale winds and temperatures, the balances that maintain the basic state, the description of waves and their interactions, the dynamical coupling between the MLT and the rest of the middle atmosphere, and the response of the MLT dynamical environment to external forcing by anthropogenic composition changes and solar variability. A review of whole atmosphere models by Akmaev (2011) discusses current understanding of the role of the MLT in coupling between atmospheric regions
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