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

Abstract. Anomalously strong westward winds during the first phase of the equatorial mesospheric semi-annual oscillation (MSAO) have been attributed to unusual filtering conditions producing exceptional gravity-wave fluxes. We test this hypothesis 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-frequency gravity 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 the event, no unusual filtering conditions or significant increases in wave-excitation proxies were observed. Further, although strong westward wave-induced accelerations were also observed during the 2006 MSAO first phase, there was no corresponding simultaneous response in westward wind. We thus suggest that strong westward fluxes/accelerations of high-frequency gravity waves are not always sufficient to produce anomalous first-phase westward MSAO winds and other forcing may be significant.