3.4.1.3 Reanalyses

A comprehensive global reanalysis completed since the TAR, ERA-40 (Uppala et al., 2005), extends from September 1957 to August 2002. Reanalysis is designed to prevent changes in the analysis system from contaminating the climate record, as occurs with global analyses from operational numerical weather prediction, and it compensates for some but not all of the effects of changes in the observing system (see Appendix 3.B.5.4). Unlike the earlier NRA that assimilated satellite retrievals, ERA-40 assimilated bias-adjusted radiances including MSU data (Harris and Kelly, 2001; Uppala et al., 2005), and the assimilation procedure itself accounts for orbital drift and change in satellite height – factors that have to be addressed in direct processing of MSU radiances for climate studies (e.g., Christy et al., 2003; Mears et al., 2003; Mears and Wentz, 2005). Onboard calibration biases are treated indirectly via the influence of other data sets. Nonetheless, the veracity of low-frequency variability in atmospheric temperatures is compromised in ERA-40 by residual problems in bias corrections.

Trends and low-frequency variability in large-scale surface air temperature from ERA-40 and from the monthly climate station data analysed by Jones and Moberg (2003) are in generally good agreement from the late 1970s onwards (see also Section 3.2.2.1). Temperatures from ERA-40 vary quite coherently throughout the planetary boundary layer over this period, and earlier for regions with consistently good coverage from both surface and upper-air observations (Simmons et al., 2004).

Processed MSU records of layer temperature have been compared with equivalents derived from the ERA-40 analyses (Santer et al., 2004). The use of deep layers conceals disparate trends at adjacent tropospheric levels in ERA-40. Relatively cold tropospheric values before the satellite era arose from a combination of scarcity of radiosonde data over the extratropical SH and a cold bias of the assimilating model, giving a tropospheric warming trend that is clearly too large when taken over the full period of the reanalysis (Bengtsson et al., 2004; Simmons et al., 2004; Karl et al., 2006). ERA-40 also exhibits a middle-tropospheric cooling over most of the tropics and subtropics since the 1970s that is certainly too strong owing to a warm bias in the analyses for the early satellite years.

Tropospheric patterns of trends from ERA-40 are similar to Figure 3.19, with coherent warming over the NH, although with discrepancies south of 45°S. These differences are not fully understood, although the treatment of surface emissivity anomalies over snow- and ice-covered surfaces may contribute (Swanson, 2003). At high southern latitudes, ERA-40 shows strong positive temperature trends in JJA in the period 1979 to 2001, in good accord with antarctic radiosonde data (Turner et al., 2006). The large-scale patterns of stratospheric cooling are similar in ERA-40 and the MSU data sets (Santer et al., 2004). However, the ERA-40 analyses in the lower stratosphere are biased cold relative to radiosonde data in the early satellite years, reducing downward trends. Section 3.5 relates the trends to atmospheric circulation changes.