Atmospheric Chemistry and Physics, vol. 13, issue 18 (2013) pp. 9515-9523
Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension Island (8 degrees S, 14 degrees W) have been measured at heights of approximately 80-100 km by a meteor radar. The results presented in this study are from the interval October 2001 to December 2011. In all years, the monthly-mean meridional winds display a clear annual oscillation. Typically, these winds are found to be southward during April-October, when they reach velocities of up to about -23 m s(-1), and northward throughout the rest of the year, when they reach velocities up to about 16 m s(-1). The monthly-mean zonal winds are generally westward throughout most of the year and reach velocities of up to about -46 m s(-1). However, eastward winds are observed in May-August and again in December at the lower heights observed. These eastward winds reach a maximum at heights of about 86 km with velocities of up to about 36 m s(-1), but decay quickly at heights above and below that level. The mesospheric semi-annual oscillation (MSAO) is clearly apparent in the observed monthly-mean zonal winds. The winds in first westward phase of the MSAO are observed to be much stronger than in the second phase. The westward phase of the MSAO is found to maximise at heights of about 84 km with typical first-phase wind velocities reaching about -35 m s(-1). These meteor-radar observations have been compared to the HWM-07 empirical model. The observed meridional winds are found to be generally more southward than those of the model during May-August, when at the lower heights observed the model suggests there will be only weakly southward, or even northward, winds. The zonal monthly-mean winds are in generally good agreement, although in the model they are somewhat less westward than those observed. Throughout the observations there were eight occasions in which the first westward phase of the MSAO was observed. Strikingly, in 2002 there was an event in which the westward winds during the first phase of the MSAO were much stronger than normal and reached velocities of about -75 m s(-1). This event is explained in terms of a previously proposed mechanism in which the relative phasing of the stratospheric quasi-biennial oscillation (SQBO) and the MSAO allows an unusually large flux of gravity waves of large westward phase speed to reach the mesosphere. It is the dissipation of these gravity waves that then drives the MLT winds to the large westward velocities observed. It is demonstrated that the necessary SQBO-MSAO phase relationship did indeed exist during 2002, but not during the other years observed here. This demonstration provides strong support for the suggestion that extreme zonal-wind events during the MSAO result from the modulation of gravity-wave fluxes.
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