Articles | Volume 13, issue 15
Atmos. Chem. Phys., 13, 7813–7824, 2013
Atmos. Chem. Phys., 13, 7813–7824, 2013

Research article 14 Aug 2013

Research article | 14 Aug 2013

The roles of vertical advection and eddy diffusion in the equatorial mesospheric semi-annual oscillation (MSAO)

R. L. Gattinger1, E. Kyrölä2, C. D. Boone3, W. F. J. Evans4,5, K. A. Walker3,6, I. C. McDade7, P. F. Bernath3,8, and E. J. Llewellyn1 R. L. Gattinger et al.
  • 1ISAS, Department of Physics and Engineering Physics, 116 Science Place, University of Saskatchewan, Saskatoon SK, S7N 5E2, Canada
  • 2Finnish Meteorological Institute, Earth Observation, P.O. Box 503, 00101, Helsinki, Finland
  • 3Department of Chemistry, University of Waterloo, Waterloo, ON, N2L 3G1, Canada
  • 4NorthWest Research Associates Inc., 4118 148 Avenue N.E., Redmond, WA 98052, USA
  • 5Centre for Research in Earth and Space Science (CRESS), York University, 4700 Keele Street, Toronto, ON, M3J 1P3, Canada
  • 6Department of Physics, University of Toronto, 60 St. George Street, Toronto, ON, M5S 1A7, Canada
  • 7Department of Earth and Space Science and Engineering (ESSE), York University, 4700 Keele Street, Toronto, ON, M3J 1P3, Canada
  • 8Department of Chemistry and Biochemistry, Old Dominion University, 4541 Hampton Boulevard, Norfolk, Virginia 23529-0126, USA

Abstract. Observations of the mesospheric semi-annual oscillation (MSAO) in the equatorial region have been reported dating back several decades. Seasonal variations in both species densities and airglow emissions are well documented. The extensive observations available offer an excellent case study for comparison with model simulations. A broad range of MSAO measurements is summarised with emphasis on the 80–100 km region. The objective here is not to address directly the complicated driving forces of the MSAO, but rather to employ a combination of observations and model simulations to estimate the limits of some of the underlying dynamical processes. Photochemical model simulations are included for near-equinox and near-solstice conditions, the two times with notable differences in the observed MSAO parameters. Diurnal tides are incorporated in the model to facilitate comparisons of observations made at different local times. The roles of water vapour as the "driver" species and ozone as the "response" species are examined to test for consistency between the model results and observations. The simulations suggest the interactions between vertical eddy diffusion and background vertical advection play a significant role in the MSAO phenomenon. Further, the simulations imply there are rigid limits on vertical advection rates and eddy diffusion rates. For August at the Equator, 90 km altitude, the derived eddy diffusion rate is approximately 1 × 106 cm2 s−1 and the vertical advection is upwards at 0.8 cm s−1. For April the corresponding values are 4 × 105 cm2 s−1 and 0.1 cm s−1. These results from the current 1-D model simulations will need to be verified by a full 3-D simulation. Exactly how vertical advection and eddy diffusion are related to gravity wave momentum as discussed by Dunkerton (1982) three decades ago remains to be addressed.

Final-revised paper