The quasi-biennial oscillation

The quasi-biennial oscillation (QBO) dominates the variability of the equat orial stratosphere (similar to 16-50 km) and is easily seen as downward pro pagating easterly and westerly wind regimes, with a variable period averagi ng approximately 28 months. From a fluid dynamical perspective, the QBO is a fascinating example of a coherent, oscillating mean flow that is driven b y propagating waves with periods unrelated to that of the resulting oscilla tion, Although the QBO is a tropical phenomenon, it affects the stratospher ic flow from pole to pole by modulating the effects of extratropical waves. Indeed, study of the QBO is inseparable from the study of atmospheric wave motions that drive it and are modulated by it. The QBO affects variability in the mesosphere near 85 km by selectively filtering waves that propagate upward through the equatorial stratosphere, and may also affect the streng th of Atlantic hurricanes. The effects of the QBO are not confined to atmos pheric dynamics. Chemical constituents, such as ozone, water vapor, and met hane, are affected by circulation changes induced by the QBO. There are als o substantial QBO signals in many of the shorter-lived chemical constituent s. Through modulation of extratropical wave propagation, the QBO has an eff ect on the breakdown of the wintertime stratospheric polar vortices and the severity of high-latitude ozone depletion. The polar vortex in the stratos phere affects surface weather patterns, providing a mechanism for the QBO t o have an effect at the Earth's surface. As more data sources (e.g., wind a nd temperature measurements from both ground-based systems and satellites) become available, the effects of the QBO can be more precisely assessed. Th is review covers the current state of knowledge of the tropical QBO, its ex tratropical dynamical effects, chemical constituent transport, and effects of the QBO in the troposphere (similar to0-16 km) and mesosphere (similar t o 50-100 km). It is intended to provide a broad overview of the QBO and its effects to researchers outside the held, as well as a source of informatio n and references for specialists. The history of research on the QBO is dis cussed only briefly, and the reader is referred to several historical revie w papers. The basic theory of the QBO is summarized, and tutorial reference s are provided.