Airborne Observations of Non-orographic Gravity Waves over the Southern Ocean: Tropospheric or Stratospheric Wave Excitation?

There are various different explanations of the belt of increased stratospheric temperature variances around 60° S surrounding Antarctica. Downstream propagation of mountain waves from the southern Andes and the Antarctic Peninsula, as well as from small islands, gravity wave emission at tropospheric fronts and jet streams, and secondary wave generation locally in regions of primary orographic waves have been addressed to as relevant sources of these increased temperature variances in literature. However, recent GPS radio occultation observations suggest that wave amplitudes are distributed likewise over orography and over the ocean. This indicates a possible non-orographic source like spontaneous adjustment around the stratospheric polar night jet.

Here, we report about unique observations from the research flight RF25 of the NSF/NCAR GV aircraft during DEEPWAVE. The in-situ and remote-sensing observations were conducted along two extended legs from New Zealand south to 63° S. They constitute the first observations in the stratospheric wave belt covering the troposphere and the middle atmosphere. Flight-level measurements, as well as dropsondes, MTP measurements and radiosoundings from Macquarie Island indicate that the strong polar front jet excited gravity waves propagating into the stratosphere. On the other hand, the largest stratospheric temperature fluctuations in the range of 30 to 60 km altitude were observed by the airborne Rayleigh lidar south of the polar front jet directly in the vicinity of the polar night jet. The calculated gravity wave potential density peaks close to 60° S.

Close agreements of the wave pattern simulated by the ECMWF IFS analyses and short-term forecasts with the observations encourage a closer examination of those gravity waves by means of a normal-mode decomposition of the high-resolution meteorological data. A normal-mode functions representation separates the flow into balanced and non-balanced circulation. These retrieved modes obey the dispersion relation of Rossby-Haurwitz and inertia-gravity (IG) waves, respectively. In addition, spectral filtering provides temperature and wind perturbations of IG waves only with a horizontal wavenumber larger than 30. IG wave patterns, locations, altitude ranges and temporal propagation are investigated and possible sources of their generation are discussed.