Mean winds and tides in the mesosphere and lower thermosphere above Halley, Antarctica

Abstract

An imaging Doppler interferometer (IDI) at Halley, Antarctica (76°S, 26°W) has been used to record near continuous mean winds in the mesosphere/lower thermosphere since December 1996. Monthly mean winds are calculated between 75 and 105 km for comparison with the HWM93 model winds. Below 95 km the zonal mean winds are 5–10 m s −1 eastward weakening to 0 m s −1 in summertime. Between 95 and 105 km a wintertime eastward wind (0–5 m s −1) strengthens to 17 m s −1 in summer. Above 95 km the meridional wind is northward (5–10 m s −1) in winter turning strongly southward (11 m s −1) in summertime, with weaker northward winds below this strengthening around equinox. Excellent agreement is found between the IDI-measured zonal winds and the model except for mid-summer when the model winds between 75 and 95 km are much stronger westwards than those observed. IDI meridional winds are found to agree well with the model across the entire height range in mid-summer and mid-winter with a bias of around 8 m s −1 northwards found around the equinoxes. A wavelet analysis of the entire data set shows the inter- and intra-annual variation in waves with periods between 12 h and 30 days as well as their relative strength as a function of height between 75 and 105 km. Strong 12 and 24 h components are observed in both the zonal and meridional winds increasing in amplitude with height and peaking in summer. The phase of the 24 h wave is seen to vary from one year to the next though on average it is evanescent during the summer months with increased phase scatter during winter. The 12 h wave is vertically propagating all seasons and demonstrates a strong phase transition around equinox in both the zonal and meridional components. The nature of the 12 h wave is more consistent with that seen at South Pole and Scott Base/McMurdo than at lower latitudes suggesting that the non-migrating zonal wavenumber s=1 12 h wave extends at least 15° north of pole in the southern hemisphere.