The representation of the Quasi-Biennial Oscillation (QBO) and Semi-Annual Oscillation (SAO) in the ECMWF Integrated Forecast System (IFS) depends on the model formulation in the tropical region, particularly the parameterisation of the effects of gravity waves. Therefore, the representation of the QBO and SAO vary with model cycle and resolution. Observations do provide constraints for the analyses, but poorly observed regions, such as the upper stratosphere and the mesosphere, will exhibit similar characteristics to those of the forecast model.
a | b |
c  | d |
Figure 1. The zonal mean, eastward wind for 2008-2017, averaged over 5S-5N, for the monthly mean in a) ERA5, and b) ERA-Interim and for the climatological monthly mean (mean annual cycle) in c) ERA5, and d) ERA-Interim. The units are ms-1. The vertical coordinate is the reference pressure of the model levels.
The depiction of the QBO in the lower to mid stratosphere (from about 50 to 5 hPa) in ERA5 is very similar to that in ERA-Interim (Figure 1 a and b), at least for the period considered here, 2008 to 2017. The similarity is present in both the strength of the descending easterlies and westerlies and in their phase and periodicity. The disruption of the westerlies just above 50 hPa in early 2016, followed by prolongation of the westerlies, is clearly visible in both reanalyses.
Although the broad representation of the SAO between 5 and 0.5 hPa is reasonably similar in the two reanalyses, with descending easterlies and westerlies at similar times of the year, the magnitude of the winds and the exact pattern of descent are not the same (Figure 1 c and d). However, between 0.5 and 0.1 hPa the agreement between the reanalyses is poor, with the two representations of the SAO being very different. In this region, ERA5 has westerlies that are 30 ms-1, or more, larger than those in ERA-Interim. At these levels, ERA5 also has a complete absence of descending easterlies around the time of the solstices, though, due to a dearth of observations, it is not certain what happens in reality. This predominance of westerlies in ERA5 is related to the spurious equatorial mesospheric jet that occurs in CY41R2 of the IFS and which peaks in the transition seasons. For more information on this spurious equatorial mesospheric jet, see Section 7 in the ECMWF Technical Memorandum No. 824 Report on Stratosphere Task, Force by T.G. Shepherd, I. Polichtchouk, R.J. Hogan and A.J. Simmons.
