Seasonal variations in the horizontal wind structure from 0-100 km above Rothera station, Antarctica (67° S, 68° W)

A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km

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

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

Regional variations of mesospheric gravity-wave momentum flux over Antarctica

Images of mesospheric airglow and radar-wind measurements have been combined to estimate the difference in the vertical flux of horizontal momentum carried by high-frequency gravity waves over two dissimilar Antarctic stations. Rothera (67° S, 68° W) is situated in the mountains of the Peninsula near the edge of the wintertime polar vortex. In contrast, Halley (76° S, 27° W), some 1658 km to the southeast, is located on an ice sheet at the edge of the Antarctic Plateau and deep within the polar vortex during winter. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2002 and 2003 at Rothera for comparison with the 2000 and 2001 results from Halley reported previously (Espy et al., 2004). These cross-correlation coefficients were combined with wind-velocity variances from coincident radar measurements to estimate the daily averaged upper-limit of the vertical flux of horizontal momentum due to gravity waves near the peak emission altitude of the Na nightglow layer, 90km. The resulting momentum flux at both stations displayed a large day-to-day variability and showed a marked seasonal rotation from the northwest to the southwest throughout the winter. However, the magnitude of the flux at Rothera was about 4 times larger than that at Halley, suggesting that the differences in the gravity-wave source functions and filtering by the underlying winds at the two stations create significant regional differences in wave forcing on the scale of the station separation

Seasonal variations in the horizontal wind structure from 0-100 km above Rothera station, Antarctica (67° S, 68° W)

A medium frequency spaced-antenna radar has been operating at Rothera station, Antarctica (67° S, 68° W) for two periods, between 1997-1998 and since 2002, measuring winds in the mesosphere and lower thermosphere. In this paper monthly mean winds are derived and presented along with three years of radiosonde balloon data for comparison with the HWM-93 model atmosphere and other high latitude southern hemisphere sites. The observed meridional winds are slightly more northwards than those predicted by the model above 80 km in the winter months and below 80 km in summer. In addition, the altitude of the summer time zero crossing of the zonal winds above the westward jet is overestimated by the model by up to 8 km. These data are then merged with the wind climatology obtained from falling sphere measurements made during the PORTA campaign at Rothera in early 1998 and the HWM-93 model atmosphere to generate a complete zonal wind climatology between 0 and 100 km as a benchmark for future studies at Rothera. A westwards (eastwards) maximum of 44 ms-1 at 67 km altitude occurs in mid December (62 ms-1 at 37 km in mid July). The 0 ms-1 wind contour reaches a maximum altitude of 90 km in mid November and a minimum altitude of 18 km in January extending into mid March at 75 km and early October at 76 km

Seasonal variations of the gravity wave momentum flux in the Antarctic mesosphere and lower thermosphere

Airglow imager and dynasonde/imaging Doppler interferometer (IDI) radar wind measurements at Halley Station, Antarctica (75.6degreesS, 26.6degreesW) have been used to estimate the seasonal variation of the vertical fluxes of horizontal momentum carried by high-frequency atmospheric gravity waves. The cross-correlation coefficients between the vertical and horizontal wind perturbations were calculated from sodium (Na) airglow imager data collected during the austral winter seasons of 2000 and 2001. These were combined with wind velocity variances from coincident radar measurements to estimate the daily averaged upper limit of the vertical flux of horizontal momentum due to gravity waves. The resulting momentum flux at the Na airglow altitudes, while displaying a large day-to-day variability, showed a marked rotation from the northwest to the southeast throughout the winter season. Calculations show that this rotation is consistent with seasonal changes in the wind field filtering of gravity waves below the Na airglow region. The calculations also indicate that while the magnitude of the meridional wind is small, this filtering leads to the observed seasonal changes in the meridional momentum flux

A climatology of tides and gravity wave variance in the MLT above Rothera, Antarctica obtained by MF radar

A cumulative total of over 5 years of data from an MF radar situated at Rothera (67°S, 68°W) on the Antarctic Peninsula have been used to derive climatologies of periodic motions in the wind field in the mesosphere and lower thermosphere with periods less than or equal to 1 day. Strong tidal motions are observed at 24, 12 and 8 h and monthly mean climatologies are presented between 74 and 94 km altitude for comparison with the HWM-93 horizontal wind model. The 24 h tide shows a strong seasonal dependence in both the zonal and meridional components with a summertime maximum and wintertime minimum over all altitudes. The monthly mean maximum amplitude is 12(±2) ms−1 at 94 km in January and the minimum is <1 ms−1 around 86 km in early winter. The 12 h wave shows large short-term amplitude variability with a peak in amplitude around late autumn. It reaches a minimum at high altitudes in winter and below 80 km during summer, characteristic of a mixture of migrating and non-migrating modes. The phase of the 12 h wave is relatively constant throughout winter with a minimum mean vertical wavelength of 75 km around equinox. The 8 h wave is predominantly a summertime high altitude phenomenon. It is seen most strongly in the winds above 85 km and reaches monthly mean amplitudes of 6(±2) ms−1 in the zonal winds at 94 km altitude. Finally, a seasonal climatology of gravity wave variances is generated by calculating the daily mean variance in the raw winds after subtracting the fitted tidal components. This index shows a strong seasonal and height dependence in both components with a wintertime peak of 2000 m2s−2 in the zonal component at the highest altitudes. This peak occurs when the stratospheric zonal jets are strongest and therefore the filtering of upward-propagating waves in the stratosphere should be greatest; implying that either a significant part of this wintertime wave activity is generated from a region above the peak stratospheric wind or that there is a strong annual variability in the source or propagation of the gravity wave activity at Rothera