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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 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 equatorial and polar regions. At low latitudesFor the former region, the differences in zonal eastward wind (and temperature) are related to the spurious mesospheric jet that this cycle of the IFS (CY41R2) suffers from in the transition seasons. The resolution of this problem is still the subject of ongoing research at ECMWF.
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