Middle Atmosphere Dynamics
• Gravity waves (GWs) transfer momentum and energy of the atmosphere from the troposphere to the stratosphere and mesosphere. The GWs generated by tropospheric convection, for example, play a crucial role in driving the tropical quasi-biennial oscillation (QBO) in the stratosphere and the semi-annual oscillation in the lower mesosphere.
• In most climate models, however, the GW processes have been parameterized without any connection with their convective sources. Recently, we implemented our parameterization of convective GWs (CGWs), in which the GW spectra are obtained using convection properties, into the Met Office Unified Model (UM).
< Global distribution of the westward- and eastward-CGW momentum flux into the stratosphere in January and July >
• Impacts of the CGWs on the tropical stratospheric variability are investigated in AMIP-type simulations using the UM (Kim et al., 2013). Overall, the CGWs increase the tropical variability in the stratosphere, especially for the periods of ~28 and ~12 months, compared to the simulation without the CGW parameterization. Regarding the QBO, including the CGW parameterization results in a greater variability in the duration and amplitude of the easterly and westerly phases.
< (left) Frequency spectra of the tropical zonal wind >
< (right) Deseasonalized tropical zonal wind >
• This was the first successful simulation of the QBO with the parameterization of CGWs. Extending this study, recently we have performed a 54-year AMIP simulation. The contribution of the gravity waves as well as the planetary waves to the evolution of the QBO is being investigated using this simulation.
Kim, Y.-H., A. C. Bushell, D. R. Jackson, and H.-Y. Chun, 2013: Impacts of introducing a convective gravity-wave parameterization upon the QBO in the Met Office Unified Model. Geophys. Res. Lett., 40, 1873–1877.