Theory of the Mesopause Semiannual Oscillation

A semiannual oscillation in monthly mean wind has been observed in the upper mesosphere over Ascension Island (8°S) and Kwajalein (9°N). It is suggested that the selective transmission of gravity and Kelvin waves through the lower-level stratopause semiannual oscillation is responsible for this “mesopause” semiannual oscillation. No in situ semiannual forcing is required at the mesopause. The theoretical model developed here also illustrates the importance of the time-mean component of the mean zonal flow as it affects wave propagation through the equatorial middle atmosphere.