Springtime reduction in MF mesospheric echo occurrence

Radar echoes from the upper mesosphere recorded by the dynasonde at Halley (76°S, 27°W), Antarctica, over four years have been studied to reveal the diurnal and seasonal characteristics of echo occurrence. In addition to the expected daily variation, a distinct seasonal change in the number and altitude of the echoes is apparent. The prevalence of echoes during the winter months is followed, in all four years studied, by a period during the Antarctic spring when echo occurrence falls markedly for a period lasting for about three weeks. This dearth of echoes coincides with the breakdown of the polar vortex circulation pattern in the lower atmosphere providing strong evidence of the neutral dynamical coupling between the stratosphere and mesosphere.