Abstract
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.
Highlights
The quasi-biennial oscillation (QBO) is the predominant variability of the tropical stratosphere with periods of about 20–35 months (Baldwin et al, 2001)
The eastward forcing by the Kelvin waves appears in the QBO phase of strong westerly shear
We have examined four reanalyses with the aim of estimating the momentum forcing of the QBO due to equatorial waves over the period 1979–2010
Summary
The quasi-biennial oscillation (QBO) is the predominant variability of the tropical stratosphere with periods of about 20–35 months (Baldwin et al, 2001). The alternating jets modulate interannual extratropical wave activities and impact on the strength of the polar stratospheric vortex (Holton and Tan, 1980; Watson and Gray, 2014). The QBO induces the secondary meridional circulation (Plumb and Bell, 1982), which modulates the distribution of chemical species in the tropics and extratropics (Hilsenrath and Schlesinger, 1981; Li and Tung, 2014). For these reasons, it is important to understand and model the QBO. Such modulations of the polar vortex and chemical species distributions cannot be reproduced by global models in which the QBO is not simulated
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