The height-time structure of the equatorial winds between 70 hPa and 10 hPa is analyzed using a periodic nonlinear principal component fit. The result provides an objective measure of the phase of the quasi-biennial oscillation (QBO) based on the winds at all levels. It also shows that the QBO variability from cycle to cycle is almost entirely in the rate of phase progression, and there is very little variability in the sequence of vertical structures realized through each cycle.

The QBO phase is then used in a statistical correction made to multimodel ensemble seasonal tropospheric forecasts for Northern winter. It is shown that including the QBO phase as a predictor can improve practical forecasts in the extratropics, particularly over the North Atlantic sector.