There is a new paper which is critical of the ERA-40 Reanalysis. This is an important issue as this data set has been used in long-term climate studies; e.g. see which has over 2000 citations in the peer-reviewed literature according to google scholar. The new paper is
The abstract reads [highlight added]
“Atmospheric reanalyses can be useful tools for examining climate variability and change; however, they must be used cautiously because of time-varying biases that can induce artificial trends. This study explicitly documents a discontinuity in the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) that leads to significantly exaggerated warming in the Arctic mid- to lower troposphere, and demonstrates that the continuing use of ERA-40 to study Arctic temperature trends is problematic. The discontinuity occurs in 1997 in response to refined processing of satellite radiances prior to their assimilation into the reanalysis model. It is clearly apparent in comparisons of ERA-40 output against satellite-derived air temperatures, in situ observations, and alternative reanalyses. Decadal or multidecadal Arctic temperature trends calculated over periods that include 1997 are highly inaccurate, particularly below 600 hPa. It is shown that ERA-40 is poorly suited to studying Arctic temperature trends and their vertical profile, and conclusions based upon them must be viewed with extreme caution. Consequently, its future use for this purpose is discouraged. In the context of the wider scientific debate on the suitability of reanalyses for trend analyses, the results show that a series of alternative reanalyses are in broad-scale agreement with observations. Thus, the authors encourage their discerning use instead of ERA-40 for examining Arctic climate change while also reaffirming the importance of verifying reanalyses with observations whenever possible.”
Text in the paper includes
“ERA-40 has been recently used to assess Arctic temperature trends and their vertical structure. Most notably, ERA-40 formed the basis of a now-controversial examination of central Arctic temperature trends by Graversen et al. (2008). The results of that study have been strongly contested, mainly because of concerns about the accuracy of trends calculated from ERA-40 temperatures (Bitz and Fu 2008; Grant et al. 2008; Thorne 2008; Screen and Simmonds 2010b). Yet, ERA-40 continues to be used for Arctic temperature trend analysis (e.g., Yang et al. 2010). In light of this, we show here—explicitly and more thoroughly than previous studies—that inhomogeneities in ERA-40 lead to a poor representation of Arctic temperature trends, particularly in the mid- to lower troposphere, and we demonstrate that its continued use for this purpose is problematic.”
Such an error not only affects the Arctic troposphere, but necessarily must effect the entire northern hemisphere jet stream. It is the poleward to equatorward layer average temperature gradient which causes this wind, as we discuss, for example, in
Pielke, R.A. Sr., T.N. Chase, T.G.F. Kittel, J. Knaff, and J. Eastman, 2001: Analysis of 200 mbar zonal wind for the period 1958-1997. J. Geophys. Res., 106, D21, 27287-27290 [we used the NCEP Reanalysis in our study]
Christy, J.R., B. Herman, R. Pielke, Sr., P. Klotzbach, R.T. McNider, J.J. Hnilo, R.W. Spencer, T. Chase and D. Douglass, 2010: What do observational datasets say about modeled tropospheric temperature trends since 1979? Remote Sensing, 2(9), 2148-2169.
The authors of the Journal of Climate paper [Screen and Simmonds] are commended for alerting everyone to this serious error.