Tropospheric tides from 80 to 400 km: Propagation, interannual variability, and solar cycle effects

Recent observations and model simulations demonstrate unequivocally that non-Sun-synchronous (nonmigrating) tides due to deep tropical convection produce large longitudinal and local time variations in bulk ionosphere-thermosphere-mesosphere properties. We thus stand at an exciting research frontier: understanding how persistent, large-scale tropospheric weather systems affect the geospace environment. Science challenge questions include: (1) How much of the tropospheric influence is due to tidal propagation directly into the upper thermosphere? (2) How large is the interannual and the solar cycle variability of the tides and what causes them? These questions are addressed using solar maximum to solar minimum tidal wind and temperature analyses from the Thermosphere Ionosphere Mesosphere Electrodynamics and Dynamics (TIMED) satellite in the mesosphere/lower thermosphere (MLT), and from the Challenging Minisatellite Payload (CHAMP) satellite at ˜400 km. A physics-based empirical fit model is used to connect the TIMED with the CHAMP tides, i.e., to close the "thermospheric gap" of current spaceborne observations. Temperature, density, and horizontal and vertical wind results are presented for the important diurnal, eastward, wave number 3 (DE3) tide and may be summarized as follows. (1) Upper thermospheric DE3 tidal winds and temperatures are fully attributable to troposphere forcing. (2) A quasi-2-year 15-20% amplitude modulation in the MLT is presumably caused by the QBO. No perceivable solar cycle dependence is found in the MLT region. DE3 amplitudes in the upper thermosphere can increase by a factor of 3 in the zonal wind, by ˜60% in temperature and by a factor of 5 in density, caused by reduced dissipation above 120 km during solar minimum.