SPRINGTIME REDUCTION IN MF MESOSPHERIC ECHO OCCURRENCE

Radar echoes from the upper mesosphere recorded by the dynasonde at Ha lley (76 degrees S, 27 degrees W), Antarctica, over four years have be en studied to reveal the diurnal and seasonal characteristics of echo occurrence. In addition to the expected daily variation, a distinct se asonal change in the number and altitude of the echoes is apparent. Th e prevalence of echoes during the winter months is followed, in all fo ur years studied, by a period during the Antarctic spring when echo oc currence falls markedly for a period lasting for about three weeks. Th is dearth of echoes coincides with the breakdown of the polar vortex c irculation pattern in the lower atmosphere providing strong evidence o f the neutral dynamical coupling between the stratosphere and mesosphe re.