Articles | Volume 34, issue 2
Regular paper
03 Mar 2016
Regular paper |  | 03 Mar 2016

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

Andrew C. Moss, Corwin J. Wright, Robin N. Davis, and Nicholas J. Mitchell

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.

Short summary
Gravity waves are fundamental to the dynamics of the mesosphere. In some years very strong winds are observed in the first phase of the MSAO. It has been proposed that this is due to filtering of ascending gravity waves. We report the first gravity-wave momentum flux observations from the Ascension Island (8° S, 14° W) meteor radar and show that anomalous fluxes were observed during one such event in 2002. Analysis of the underlying winds suggests the wave-filtering hypothesis is not valid.