Propagation of Waves Exited by Localized Episodic Heating in the Tropics and Their Effect on the Middle Atmosphere

Comparison between two QBO Phases

Abstract

Propagation of waves excited by localized episodic heating in the tropical troposphere and their effect on the middle atmosphere are investigated numerically with a global primitive-equation model in which a realistic radiation scheme for the middle atmosphere is incorporated. Equinoctial initial states with two opposite phases of the quasi-biennial oscillation (QBO) are used for comparison of the propagation and the effect of the waves.
Time evolutions of the responses of the equinoctial initial states are not much different from the linear responses of a resting atmosphere obtained by Horinouchi and Yoden (1996). If the duration of the heating is small (less than about a day), Eliassen-Palm (EP) flux in the middle atmosphere is mainly due to gravity waves including Kelvin waves, while if the duration is large (more than about a day), it is mainly due to Kelvin waves, Rossby waves, and Rossby-gravity waves.
In the westerly-shear phase of the QBO, westerly acceleration comparable to or a little smaller than that required by the QBO is obtained for wide range of heating parameters, while in the easterly-shear phase, realistic easterly acceleration cannot be obtained if the heating events have large time and horizontal scales (more than about a day and a few thousand kilometers). Gravity waves propagating into the low- and mid-latitude mesosphere are affected by the QBO. The difference in the divergence of the EP flux due to gravity waves may explain the observed quasi-biennial variations in the low- and mid-latitude mesosphere.
Excitation of the global normal mode 5-day wave is sensitive to the QBO phase. Since such a QBO modulation has not been observed, this result implies that the source of the 5-day wave in the real atmosphere is not or not only in the tropics, but a considerable portion of the wave is excited outside the tropics.