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Anomalously strong westward winds during the first phase of the equatorialmesospheric semi-annual oscillation (MSAO) have been attributed to unusualfiltering conditions producing exceptional gravity-wave fluxes. We test thishypothesis using meteor-radar measurements made over Ascension Island(8° S, 14° W). An anomalous wind event in 2002 of −85.5 ms−1 occurred simultaneously with the momentum fluxes of high-frequencygravity waves reaching the largest observed westward values of −29 m2 s−2 and strong westward wind accelerations of −510 ms−1 day−1. However, despite this strong wave forcing during theevent, no unusual filtering conditions or significant increases inwave-excitation proxies were observed. Further, although strong westwardwave-induced accelerations were also observed during the 2006 MSAO firstphase, there was no corresponding simultaneous response in westward wind. Wethus suggest that strong westward fluxes/accelerations of high-frequencygravity waves are not always sufficient to produce anomalous first-phasewestward MSAO winds and other forcing may be significant

annales geophysicae

Gravity-wave momentum fluxes in the mesosphere over Ascension Island (8° S, 14° W) and the anomalous zonal winds of the semi-annual oscillation in 2002