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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.
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This document has been produced in the context of the Copernicus Climate Change Service (C3S). The activities leading to these results have been contracted by the European Centre for Medium-Range Weather Forecasts, operator of C3S on behalf of the European Union (Delegation agreement Agreement signed on 11/11/2014 and Contribution Agreement signed on 22/07/2021). All information in this document is provided "as is" and no guarantee or warranty is given that the information is fit for any particular purpose. The users thereof use the information at their sole risk and liability. For the avoidance of all doubt , the European Commission and the European Centre for Medium - Range Weather Forecasts have no liability in respect of this document, which is merely representing the author's view. |
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