Midnight Temperature Maximum (MTM) in Whole Atmosphere Model (WAM) Simulations
University of Colorado Boulder United States
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Discovered almost four decades ago, the midnight temperature maximum MTM with typical magnitudes of 50-100 K has been regularly observed by satellite and ground-based instruments in the tropical upper thermosphere. Although several mechanisms have been suggested to explain the phenomenon, previous attempts to reproduce it with comprehensive thermosphere-ionosphere models have been unsuccessful. First long-term simulations with the Whole Atmosphere Model WAM reveal the presence of a realistically prominent MTM and reproduce the salient features of its daily, seasonal, and latitudinal variability. Preliminary analysis indicates that the feature may be traced down to the lower thermosphere, where it is manifested primarily in the form of an upward propagating terdiurnal tidal wave. Its spectrum expands to higher-order zonal wavenumbers and frequencies and its phase advances to near midnight higher up in the thermosphere. Possible mechanisms generating this wave may involve nonlinear interactions between other tidal harmonics originating in the middle and lower atmosphere. Our results thus suggest that the MTM is yet another phenomenon driven by dynamical links between the lower and upper atmosphere and ionosphere.