Articles | Volume 15, issue 12
Atmos. Chem. Phys., 15, 6577–6587, 2015
https://doi.org/10.5194/acp-15-6577-2015

Special issue: The SPARC Reanalysis Intercomparison Project (S-RIP) (ACP/ESSD...

Atmos. Chem. Phys., 15, 6577–6587, 2015
https://doi.org/10.5194/acp-15-6577-2015

Research article 16 Jun 2015

Research article | 16 Jun 2015

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

Y.-H. Kim and H.-Y. Chun Y.-H. Kim and H.-Y. Chun
  • Department of Atmospheric Sciences, Yonsei University, Seoul, South Korea

Abstract. The momentum forcing of the QBO (quasi-biennial oscillation) by equatorial waves is estimated using recent reanalyses. Based on the estimation using the conventional pressure-level data sets, the forcing by the Kelvin waves (3–9 m s−1 month−1) dominates the net forcing 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 results with those from the native model-level data set of the ERA-Interim reanalysis, it is suggested that the use of conventional-level data causes the Kelvin wave forcing to be underestimated by 2–4 m s−1 month−1. The momentum forcing by mesoscale gravity waves, which are unresolved in the reanalyses, is deduced from the residual of the zonal wind tendency equation. In the easterly-to-westerly transition phase at 30 hPa, the mesoscale gravity wave forcing is found to be smaller than the resolved wave forcing, whereas the gravity wave forcing dominates over the resolved wave forcing in the opposite phase. Finally, we discuss the uncertainties in the wave forcing estimates using the reanalyses.

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Short summary
The momentum forcing of the stratospheric quasi-biennial oscillation (QBO) by equatorial waves is estimated using recent reanalysis data sets. The Kelvin wave produces primary forcing of the QBO in the easterly-to-westerly transition phase below 30hPa, whereas mesoscale gravity waves produce larger forcing than the Kelvin wave above 30hPa. Still, the recent reanalyses underestimate the forcing by the large-scale equatorial waves.
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