Vertical Structure of Terdiurnal Tides in the Antarctic MLT Region: 15‐Year Observation Over Syowa (69°S, 39°E)

Abstract

The terdiurnal tide (TDT) in the Antarctic mesosphere and lower thermosphere region is poorly known. This study examines TDT using neutral wind observations at Syowa during years of 2004–2018. TDT is found to be a significant tidal component with distinct vertical structures and seasonal evolution. (1) It shows a prominent height-dependent seasonal variation with phase reversal at 94 km. (2) The vertical wavelength in summer is ∼40 km shorter than in winter. These features differ largely from those in the Arctic, indicating hemispheric asymmetry. The phase structure reveals a dominant upward propagating mode in local summer but superposition of more than one mode in other seasons. A downward propagating mode above 94 km in winter suggests Joule heating/ion drag as additional tidal sources to lower atmosphere ones. These results provide new constrains and benchmarks for model simulations that seek to understand terdiurnal tidal forcing mechanisms in polar regions. Plain Language Summary Terdiurnal tides in the Antarctica mesosphere and lower thermosphere region are poorly known. In this study, we examine its characteristics using long-term neutral wind observations at Syowa (69◦S, 39◦E) between January 2004 and July 2018. The analysis reveals terdiurnal tide being a significant tidal component in the Antarctica around solstices with distinct vertical structures. (1) Tides above 94 km has opposite climatological variation to that below 94 km. (2) The zonal and meridional wind components are 90◦ phase shifted, with similar amplitude in most seasons. (3) The vertical wavelength is shorter in summer than in winter. Most of these features differ from those reported in the Arctic, indicating hemispheric asymmetry. Furthermore, the phase structure reveals a dominant upward propagating mode in local summer but superposition of more than one mode in other seasons. A downward propagating mode in winter above 95 km suggests Joule heating/ion drag as likely tidal sources from above, in addition to those from lower atmosphere. The nearly 15-year Syowa observations provide new constrains and benchmark for models that seek to understand terdiurnal tidal forcing mechanisms in polar regions.