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The momentum forcing of the QBO (quasi-biennial oscillation) by equatorial waves is estimatedusing recent reanalyses. Based on the estimation using theconventional pressure-level data sets, the forcing by the Kelvinwaves (3–9 m s−1 month−1) dominates the netforcing by all equatorial wave modes (3–11 m s−1 month−1)in the easterly-to-westerly transition phase at 30 hPa.In the opposite phase,the net forcing by equatorial wave modes is small(1–5 m s−1 month−1). By comparing the resultswith those from the native model-level data set of theERA-Interim reanalysis, it is suggested that the use ofconventional-level data causes the Kelvin wave forcing to beunderestimated by 2–4 m s−1 month−1. Themomentum forcing by mesoscale gravity waves, which areunresolved in the reanalyses, is deduced from the residual ofthe zonal wind tendency equation. In the easterly-to-westerlytransition phase at 30 hPa, the mesoscale gravity wave forcing isfound to be smaller than the resolved wave forcing, whereas thegravity wave forcing dominates over the resolved wave forcing inthe opposite phase. Finally, we discuss the uncertainties inthe wave forcing estimates using the reanalyses

Momentum forcing of the quasi-biennial oscillation by equatorial waves in recent reanalyses