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| Titel |
Mean winds in the MLT, the SQBO and MSAO over Ascension Island (8° S, 14° W) |
| VerfasserIn |
K. A. Day, N. J. Mitchell |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1680-7316
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| Digitales Dokument |
URL |
| Erschienen |
In: Atmospheric Chemistry and Physics ; 13, no. 18 ; Nr. 13, no. 18 (2013-09-27), S.9515-9523 |
| Datensatznummer |
250085714
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| Publikation (Nr.) |
 copernicus.org/acp-13-9515-2013.pdf |
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| Zusammenfassung |
| Mean winds in the mesosphere and lower thermosphere (MLT) over Ascension
Island (8° S, 14° 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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