An Internally-Generated Quasi-biennial Oscillation in WACCM

The quasi-biennial oscillation (QBO) has been difficult to simulate in comprehensive, global models of the atmosphere. Such models are normally run at moderate horizontal resolution (~1° in latitude and longitude) and relatively coarse vertical resolution (1.25-2 km in the stratosphere). It has now been clear for some time that, while current horizontal resolution is adequate for simulating the QBO, vertical resolution is not. In addition, even at horizontal resolution of 1°, models cannot simulate the mesoscale gravity wave spectrum that is evidently a major contributor to the momentum budget of the QBO. Both these deficiencies are addressed in new simulations with a high-vertical resolution version of WACCM.

The QBO in Equatorial (±2.5° average) zonal-mean zonal wind
Figure 1. The QBO in Equatorial (±2.5° average) zonal-mean zonal wind from the ERA-Interim reanalysis (top) and in a WACCM simulation with high vertical resolution (bottom). Click for larger image.

Figure 1 shows the QBO as simulated by a 110-level version of WACCM in which vertical resolution has been increased to 500 m from the top of the boundary layer through 10 hPa, decreasing slowly above that level. The model also implements a parameterization of mesoscale gravity waves excited by tropical convection (Richter et al., 2010). The QBO simulated by this model compares favorably to that seen in the ERA-Interim reanalysis (Figure 1). In particular, the period matches the observed period almost exactly, and the oscillation reaches the tropical tropopause, near 100 hPa. The momentum budget of the parameterized and explicitly-resolved waves that drive the QBO is discussed in detail by Garcia and Richter (2019). While it is possible to obtain a “QBO-like” oscillation in WACCM using coarser vertical resolution in the stratosphere, that oscillation is deficient in that its amplitude decreases rapidly below 50 hPa and  does not reach the tropical tropopause (Gettelman et al., 2019).

References

Garcia, R. R. and J. H. Richter, 2019: On the momentum budget of the quasi-biennial oscillation in the Whole Atmosphere Community Climate Model. J. Atmos. Sci, 76, 69-87, DOI: 10.1175/JAS-D-18-0088.1.

Gettelman, A. et al., 2019:  The Whole Atmosphere Community Climate Model, Version 6 (WACCM6). J. Geophys. Res., in press.

Richter, J. H., F. Sassi, and R. R. Garcia, 2010: Toward a physically based gravity wave source parameterization in a General Circulation Model. J. Atmos. Sci, 67 (1), 136–156, doi:10.1175/2009JAS3112.1.