...
Fig. 1 shows the ERA5 zonal mean temperature (contours) and difference between the final product and the warm up (colours), for three months of the overlap, October to December 2009. In the troposphere and lower stratosphere, between 1000 hPa and 10 hPa, the differences are small (less than 0.2 K). From the mid-stratosphere to the stratopause (from 10 hPa to 1 hPa), the differences are larger, but generally below 2 K, except near the equator at about 1 hPa where the final product is more than 2K warmer than the warm up. In the mesosphere (from 1 hPa to 0.01 hPa), the final product is generally more than 2 K colder than the warm up near the equator, and its more than 5 K colder at about 0.05 hPa. In addition, this colder region spreads into the northern hemisphere near 0.5 hPa, and the southern hemisphere above 0.05 hPa. Differences of more than 2 K, but of different signs, are also apparent in both polar regions above about 0.3 hPa.
A similar, but not identical, comparison for the transition from 1988 to 1989 in ERA-Interim (Fig. 2) reveals that in the troposphere and stratosphere (from 1000 hPa to 1 hPa), the differences are, on the whole, of a similar magnitude to those in the 2009/2010 ERA5 transition, although ERA-Interim does exhibit transition differences lower down in the atmosphere. However, in the lower to mid-mesosphere (from 1 hPa to 0.1 hPa, with the latter being the top of the ERA-Interim domain), the differences in the ERA-Interim transition are generally smaller than in the ERA5 transition, having magnitudes less than 2 K, except near the equator at about 0.8 hPa where the magnitudes exceed 3 K. The spread in temperature, for September - November 2009, in the ERA5 10 member ensemble (Fig. 3), which gives some indication of the uncertainty, is generally larger than the ERA5 transition differences in the lower latitudes of the troposphere and stratosphere. The largest values of the spread are located in the mid to upper stratosphere, at lower latitudes, where values exceed 0.8 K. This region merges into a second area of relatively large uncertainty, in the northern mid-latitudes of the mid-mesosphere, where values exceed 0.7 K. In general, the spread in the mesosphere is smaller than the ERA5 transition differences.
...