HR: 1400h
AN: SA53B-03    [Abstracts]
TI: A Mechanism for Seasonal and Interannual Variability of the Migrating Diurnal Tide
AU: * Ortland, D A
EM: ortland@nwra.com
AF: NorthWest Research Associates, 14508 NE 20th St, Bellevue, WA 98006, United States
AB: The amplitude of the migrating diurnal tide in the mesosphere and lower thermosphere varies seasonally with a semi-annual pattern that peaks during equinox, often with largest amplitude during March. The amplitude is also significantly correlated with the quasi-biennial oscillation (QBO) within the stratosphere. This talk will present mechanistic model simulations that suggest that these amplitude variations are controlled directly by the structure of the zonal mean background near the stratopause, where the semi-annual oscillation (SAO) is strongly modulated by the QBO. The model experiments employ background zonal-mean winds and temperatures from the UARS reference atmosphere that extend from surface to the thermosphere and represent the variability observed from 1992-1995 by UARS. Tidal heating is calculated from monthly NVAP water vapor and UARS ozone measurements for the same period. Model tide amplitudes calculated using only tropospheric heating have only minor variability due to seasonal changes in the heating and mean wind structures. However, the tide amplitude variations observed by HRDI are reproduced by the model when the ozone tidal heating is also included. It is found that the zonal mean background in the ozone heating source region significantly affects the tidal response to this heating. The modulation of the ozone tide by variations in the background can be explained in terms of the dependence of the Green's functions for the tidal equations on the static stability. The SAO- modulated ozone tide alternates between constructive and destructive interference with the tide response to troposphere heating, producing significant amplitude modulation of the combined response in the MLT.
DE: 3332 Mesospheric dynamics
DE: 3367 Theoretical modeling
DE: 3389 Tides and planetary waves
SC: SPA-Aeronomy [SA]
MN: 2007 Joint Assembly