University of Crete Island of Crete

Dynamics of the QBO and SAO revealed by
a gravity-wave resolving GCM simulation

K. Sato1, Y. Kawatani2, S. Watanabe2, Y. Tomikawa3,
S. Miyahara4 and M. Takahashi1,5
  1. Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo, Tokyo, Japan
  2. Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Yokohama, Japan
  3. National Institute of Polar Research, Tokyo, Japan
  4. Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, Fukuoka, Japan
  5. Center for Climate System Research, University of Tokyo, Kashiwa, Japan

A high-resolution atmospheric general circulation model (AGCM) has been developed to study various aspects of small-scale phenomena including gravity waves and their role on the large-scale fields in the middle atmosphere (Watanabe et al. 2008). Our spectral model has a T213 truncation in the horizontal (horizontal resolution of about 60 km) and 256 layers (L256) in the vertical from the surface to about 85 km with an interval of 300 m in the upper troposphere and above. No gravity wave parameterizations are included in our model and hence all gravity waves are spontaneously generated. The model simulates large-scale oscillations with realistic amplitudes in the equatorial atmosphere such as the quasi-biennial oscillation (QBO) and semi-annual oscillation (SAO), although the period of the QBO-like oscillation is shorter (about 1.5 years) than the real one.

Model outputs with a time interval of 1 hour are analyzed to elucidate relative importance of the internal gravity waves (IGWs) and equatorially-trapped waves (EQWs) to drive the QBO-like oscillation. It is shown that the zonal wavenumber versus frequency spectra of simulated precipitation and outgoing long wave radiation (OLR) represent realistic signals of convectively-coupled EQWs. The horizontal wind spectra reveals clear signals of equatorial Kelvin waves, Rossby-gravity waves, and n=0 eastward, n=1 and n=2 eastward/westward propagating gravity waves in the stratosphere. These wave signals are separately extracted for further examination following Wheeler and Kiladis (1999). The horizontal distribution of each EQW amplitude generally corresponds well to that of the activity of cumulus convection. On the other hand, it is seen that IGWs are strongly influenced by the vertical wind shear associated with the Walker circulation in the troposphere, which results in different distribution of IGW amplitudes between the eastern and western hemisphere. In the westerly shear phase of the QBO-like oscillation, IGWs contribute to 50-70% of the total eastward acceleration. The equatorial Kelvin waves contribute to the largest forcing among EQWs especially around the altitude with a zonal wind of 0 m/s, while the forcing due to n=0, 1, 2 eastward-propagating EQWs becomes comparable to that of Kelvin waves at higher altitudes. It is interesting that the distribution of the wave forcing is not zonally uniform and is different depending on the wave types.

The simulated SAO has larger amplitudes of the zonal-mean zonal wind in the first cycle than in the second cycle. The easterly and westerly maxima are about -70 (m/s) and 35 (m/s) for the first cycle, respectively, while those are about -50 (m/s) and 35 (m/s) for the second cycle. Fast Kelvin waves are obvious in the easterly phase of the SAO, probably because slow Kelvin waves are effectively filtered by the QBO westerly below. The candidates of driving mechanism of the SAO are the momentum deposition by internal waves with large phase velocities and the momentum transport by the meridional circulation. Tomikawa et al. (2008) focused on a temperature maximum observed in the winter subtropical region around the stratopause and showed the importance of the meridional circulation appearing in the easterly wind of the SAO, using the same model outputs. It is likely that this meridional circulation is driven by the E-P flux convergence associated with planetary waves. This circulation may be important to maintain the SAO easterly phase.

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