8.4.9 Quasi-Biennial Oscillation
The Quasi-Biennial Oscillation (QBO; see Chapter 3) is a quasi-periodic wave-driven zonal mean wind reversal that dominates the low-frequency variability of the lower equatorial stratosphere (3 to 100 hPa) and affects a variety of extratropical phenomena including the strength and stability of the winter polar vortex (e.g., Baldwin et al., 2001). Theory and observations indicate that a broad spectrum of vertically propagating waves in the equatorial atmosphere must be considered to explain the QBO. Realistic simulation of the QBO in GCMs therefore depends on three important conditions: (i) sufficient vertical resolution in the stratosphere to allow the representation of equatorial waves at the horizontally resolved scales of a GCM, (ii) a realistic excitation of resolved equatorial waves by simulated tropical weather and (iii) parametrization of the effects of unresolved gravity waves. Due to the computational cost associated with the requirement of a well-resolved stratosphere, the models employed for the current assessment do not generally include the QBO.
The inability of resolved wave driving to induce a spontaneous QBO in GCMs has been a long-standing issue (Boville and Randel, 1992). Only recently (Takahashi, 1996, 1999; Horinouchi and Yoden, 1998; Hamilton et al., 2001) have two necessary conditions been identified that allow resolved waves to induce a QBO: high vertical resolution in the lower stratosphere (roughly 0.5 km), and a parametrization of deep cumulus convection with sufficiently large temporal variability. However, recent analysis of satellite and radar observations of deep tropical convection (Horinouchi, 2002) indicates that the forcing of a QBO by resolved waves alone requires a parametrization of deep convection with an unrealistically large amount of temporal variability. Consequently, it is currently thought that a combination of resolved and parametrized waves is required to properly model the QBO. The utility of parametrized non-orographic gravity wave drag to force a QBO has now been demonstrated by a number of studies (Scaife et al., 2000; Giorgetta et al., 2002, 2006). Often an enhancement of input momentum flux in the tropics relative to that needed in the extratropics is required. Such an enhancement, however, depends implicitly on the amount of resolved waves and in turn, the spatial and temporal properties of parametrized deep convection employed in each model (Horinouchi et al., 2003; Scinocca and McFarlane, 2004).