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In summary, the ERA5 transition differences for 2009/2010 are small in the troposphere and stratosphere, but much larger in the mesosphere. There, the transition differences exceed those in ERA-Interim and are much larger than the ensemble spread. The large differences in the mesosphere are particularly apparent in the polar and equatorial and polar regions. For the former latter region, the differences in eastward wind (and temperature) are related to the spurious mesospheric jet that this cycle of the IFS (CY41R2) suffers from, particularly in the transition seasons.
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