Tutorial 2.1
QBO: Introduction
The quasi-biennial oscillation (QBO) is a slow oscillation in the strength and direction of the zonal wind in the lower and middle stratosphere over the equator of the Earth's atmosphere. It has a period of roughly two years (hence the name ‘quasi-biennial’) and has been observed in climatological records since the 1950s. The diagram below shows a sequence of observations of the zonal wind profile above the equator in the form of contours of zonal wind in height and time.
These observations were compiled from station data at three locations in the tropics (Canton Island in Kiribati, Gan in the Maldives, and Singapore) by Barbara Naujokat of the Freie Universität Berlin (Naujokat 1986).
Basic mechanism
Our understanding of what causes this remarkable, almost quasi-periodic oscillation in the otherwise chaotic atmosphere is largely based on the simple theoretical arguments of Richard Lindzen and James Holton (Lindzen & Holton 1972), who showed how the critical-layer absorption of waves propagating both upwards, and either eastwards or westwards, could lead in turn to alternating acceleration of the zonal flow in either direction. They suggested that large-scale equatorial waves (with both easterly and westerly phase velocities) are generated in the troposphere and propagate upward through the tropical stratosphere. There they interact with the mean flow, mainly as a result of the radiative dissipation of their component temperature perturbations.
As the waves are dissipated, they deposit their zonal momentum (in the same sense as their phase velocity) into the mean zonal flow. This occurs most strongly where the mean zonal flow is similar in size to the phase velocity of the waves (the ‘critical layer’) – so the strongest wave acceleration tends to occur just below the peak velocity in the mean zonal flow. This leads to a wave-induced acceleration that tends to ‘pull’ the zonal wind peak downwards towards the tropopause. Once the zonal wind peak approaches the tropopause at the bottom of the stratosphere, it in turn tends to be dissipated. Meanwhile, waves propagating in the opposite zonal direction can propagate freely upwards until they are dissipated in the high stratosphere. This leads to an acceleration in the opposite direction at high altitudes, starting the cycle over again. This basic scenario is illustrated schematically in Fig. 2.1b above, after Plumb 1982.
In practice, the process is more complicated than this simple model would suggest. The waves participating in the QBO include a diverse collection of equatorially trapped planetary waves (Lindzen & Holton 1968 emphasised only the main large-scale Kelvin and mixed Rossby–gravity modes) and small-scale gravity waves. See, for instance, the review article by Mark Baldwin and colleagues (Baldwin et al. 2001) for an up-to-date summary.