1Centre for Space, Atmospheric and Oceanic Science, Department of Electronic and Electrical Engineering, The University of Bath, Bath, BA2 7AY, UK
2Met Office, Exeter, EX1 3PB, UK
Abstract. Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8° S and 14° W) have been investigated using meteor radar wind observations. The results presented in this study are from the interval October 2001 to December 2011.
There is a clear annual oscillation in the monthly-mean meridional winds. The monthly-mean meridional winds observed over Ascension Island at meteor heights are found to be southward during April–October, reaching velocities up to about −23 m s−1 and northward the rest of the year, reaching velocities up to about 16 m s−1. The monthly-mean zonal winds are generally westward through most of the year, reaching velocities up to about −46 m s−1. However, there are eastward winds in May–August and again in December in the lower heights that the radar observes. These winds maximises at heights of about 86 km reaching velocities up to about 36 m s−1 and decays quickly above and below. The Mesospheric Semi-Annual Oscillation (MSAO) is clearly observed in the monthly-mean zonal winds. The first westward phase of the winds is much stronger than the second. The first westward phase of the MSAO was found to maximise at heights of about 84 km and to in general reach amplitudes of about −35 m s−1.
We have compared the HWM-07 model to our observations. Our observed meridional winds are generally more southward than those of the model at meteor heights in the southern hemispheric winter, whereas HWM-07 suggests that in this season only weakly southward, or even northward flows occur at the lower heights. The zonal monthly-mean winds are in general agreement but somewhat less westward than observed by the radar.
In one of the eight events in which the first westward phase of the MSAO was observed, the strongest westward winds reached about −75 m s−1, compared to the mean of about −35 m s−1 for other events. We explain this observation in terms of a mechanism which has been previously proposed by others. In this the relative phasing of the Stratospheric Quasi-Biennial Oscillation (SQBO) and the MSAO allow an unusually large flux of gravity waves with westward phase speed to reach the mesosphere. The dissipation of these waves then drives the MLT winds to large westward velocities. We demonstrate that the necessary phase relationship existed during the event we observed in 2002 and not during other times. This provides strong support for the suggestion that those extremes in zonal flow are a~result of modulated gravity-wave fluxes.