Frequency spectra and vertical profiles of wind fluctuations in the summer Antarctic mesosphere revealed by MST radar observations

Continuous observations of the polar mesosphere summer echoes in a height region from 81-93 km were performed by the first MST (Mesospehre-Stratosphere-Troposphere)/IS (Incoherent Scatter) radar in the Antarctic over three summer periods of 2013/14, 2014/15, and 2015/16. Power spectra of horizontal and vertical wind fluctuations, and momentum flux spectra in a wide frequency range from (8 min) -1 to (20 d) -1 were first estimated for the Antarctic mesosphere. The horizontal (vertical) wind power spectra obey a power law with an exponent of about -2 (-1) at higher frequencies than the inertial frequency (corresponding to a period of 13 h), and have isolated peaks around 1 day and a half day in the lower frequency region. In addition, an isolated peak around a 2 day period is observed in the horizontal wind spectra but absent in the vertical wind spectra, which is consistent with the characteristics of a normal-mode Rossby wave. Zonal (meridional) momentum flux spectra are mainly positive (negative), and large fluxes are observed in a relatively low frequency range from (1 day) -1 to (3 h) -1. Furthermore, a case study was performed for the change in vertical profiles of the time mean wind and momentum fluxes around January 3, 2015 when a minor sudden stratospheric warming (SSW) occurred in the northern hemisphere. Judged from the timing, it may not be a response to the SSW onset, but an interesting momentum flux convergence corresponding to an eastward acceleration of about 200 m s-1day-1 was observed before the SSW onset. This strong wave forcing is consistent with subsequent decrease in the mean westward wind and increase in the mean equatorward wind. This fact implies that the gravity wave forcing added positive angular momentum to the mean wind resulting in the excess of equatorward pressure force.