Observation of low frequency Kelvin waves in the mesosphere

This paper presents evidence for a quasi-stationary Kelvin wave of zonal wa venumber 1 that is forced in situ in the upper mesosphere and propagates up ward from there. Although large scale quasi-stationary longitudinal variati ons are common in the mesopause equatorial winds, these variations often do not have a structure associated with a vertically propagating mode. Howeve r, during the periods of easterly winds that occur near the equinoxes in as sociation with the mesopause semi-annual oscillation (MSAO), a Kelvin wave propagates away from the presumed altitude of forcing. The momentum transpo rt by the Kelvin wave is small and does not make a significant contribution to the SAO wind evolution.