The validity of long-term prediction of quasi-biennial oscillation (QBO) as a proof of the exact seasonal synchronization of the equatorial stratospheric QBO cycle

• All easterly wind regime descents in 1953–2014 in the equatorial stratosphere are examined.
• Stage of stagnation of easterly downward propagation is observed during each QBO cycle.
• Seasonal regularities of stagnation allow reliable long-term forecast of QBO.
• Method of QBO forecasting is updated considering sequences of cycles of different periods.
• Two subsequent QBO-cycles are predicted for the time interval until the end of 2018.

Sixty years of radiosonde data of the equatorial stratospheric zonal wind have been used to develop the method of long-term wind variations forecast based on the seasonal regularities of quasi-biennial oscillation (QBO) evolution. The wind changes during all easterly wind regime descents in 1953–2014 in the middle stratosphere (from 10 to 70 hPa) have been analyzed elaborately by the experimental data. Arguments are presented which show that the delay of easterly downward propagation is observed during each QBO cycle, not just in some cycles, as is commonly believed. Moreover it is shown that the start and the end of all easterly delays (the stages of stagnation) are strongly linked to seasons of a year. It causes the discretely varying length of the stagnation stages and the clearly defined quantized QBO cycle period which can only be equal to 24, 30 or 36 months. These regularities allow developing the method of long-term QBO-cycle prediction. The verification of previous forecasts by comparing with actually observed zonal wind changes in 2005–2014 is presented. Also the QBO development for the following period as far as December 2018 is predicted. We show that the predictability of the wind QBO in the equatorial stratosphere for a long time interval (up to five years) is the result of the existence of a strong seasonal dependence of the QBO evolution. On the other hand, the correctness of these QBO predictions is a strong proof of the exact seasonal synchronization of the QBO cycle in the equatorial stratosphere.