3.6 Simulation of the Quasi-biennial Oscillation (QBO)

The Quasi Biennial Oscillation (QBO) of the tropical zonal mean wind is a prominent feature of the tropical stratosphere, and the main source of interannual variability in the Tropics between the tropopause and 40 km. The QBO has been observed for over 50 years, and its influence on stratospheric dynamics has been well documented; however, very few General Circulation Models (GCMs) can model this phenomenon.

Giorgetta et al. (2002) and Richter et al. (2014) have shown that high vertical resolution is needed in order to simulate the QBO in a GCM. To address this problem, a high-resolution version of WACCM has been developed by J. Richter (CGD) and R. Garcia (ACOM). This version is based upon the standard version of WACCM, but with a vertical grid of 110 levels (110L) that provides very high vertical resolution (500 m) between the boundary layer and the middle stratosphere. The 110L model is now being used to simulate the QBO for present-day climate and for high-CO2 conditions. Figure 1 shows a comparison of the simulated QBO winds at the Equator with ERA-Interim reanalysis data and with a standard WACCM run (70 vertical levels; 70L). The structure, period and amplitude of the QBO in the 110L model compares very well with the reanalysis. By contrast, the 70L model produces a much weaker QBO that fails to propagate below 50 hPa. The difference is directly attributable to the ability to represent short-vertical wavelength equatorially trapped waves (Kelvin, Rossby-gravity). These waves, whose vertical wavelength is as small as 4-6 km are properly represented in the 110L, but not in the 70L model, whose vertical resolution in the critical upper troposphere and lower stratosphere region is no better than 1.25 km.

Sample template image
Figure 1: Tropical (2°S to 2°N average) zonal-mean zonal wind from ERA-Interim (top panel), 70L WACCM (middle panel) and 110L WACCM (bottom panel).

 Simulations from the 110L model are part of the NCAR contribution to the QBO Inter-comparisonproject (QBOi; http://www.sparc-climate.org/activities/quasi-biennial-oscillation/)

References:

Giorgetta, M. A., E. Manzini, and E. Roeckner, 2002: Forcing of the quasi-biennial oscillation from a broad spectrum of atmospheric waves, Geophys. Res. Lett., 29(8), 1245, doi:10.1029/2002GL014756.

Richter, J. H., A. Solomon, and J. T. Bacmeister, 2014: On the simulation of the quasi-biennial oscillation in the Community Atmosphere Model, version 5, J. Geophys. Res. Atmos., 119, 3045–3062, doi:10.1002/2013JD021122.