Quasi-biennial oscillation

The QBO (quasi-biennial oscillation) is a quasi-periodic oscillation of easterlies and westerlies in the tropical stratosphere with a mean period of 28 months. It was discovered in the 1950s, but its origin remained unclear for some time. Observations showed that its phase was not related to the annual cycle, as is the case for all other stratospheric circulation patterns. In the 1970s it was recognized that the periodic wind change was driven by atmospheric waves emanating from the tropical troposphere that travel upwards and are dissipated in the stratosphere by radiative cooling. The precise nature of the waves responsible for this effect was heavily debated, in recent years however gravity waves have come to be seen as a major contributor.

Effects of the QBO include mixing of stratospheric ozone by its secondary circulation, modification of monsoon precipitation, and an influence on stratospheric circulation in Northern hemisphere winter (the sudden stratospheric warmings).