Comparison of trends and low‐frequency variability in CRU, ERA‐40, and NCEP/NCAR analyses of surface air temperature

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Abstract
Anomalies in monthly mean surface air temperature from the 45‐Year European Centre for Medium‐Range Weather Forecasts (ECMWF) Re‐Analysis (ERA‐40) and the first National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis are compared with corresponding values from the Climatic Research Unit (CRU) CRUTEM2v data set derived directly from monthly station data. There is mostly very similar short‐term variability, especially between ERA‐40 and CRUTEM2v. Linear trends are significantly lower for the two reanalyses when computed over the full period studied, 1958–2001, but ERA‐40 trends are within 10% of CRUTEM2v values for the Northern Hemisphere when computed from 1979 onward. Gaps in the availability of synoptic surface data contribute to relatively poor performance of ERA‐40 prior to 1967. A few highly suspect values in each of the data sets have also been identified. ERA‐40's use of screen‐level observations contributes to the agreement between the ERA‐40 and CRUTEM2v analyses, but the quality of the overall observing system and general character of the ERA‐40 data assimilation system are also contributing factors. Temperatures from ERA‐40 vary coherently throughout the boundary layer from the late 1970s onward, in general, and earlier for some regions. There is a cold bias in early years at 500 hPa over the data‐sparse southern extratropics and at the surface over Antarctica. One indicator of this comes from comparing the ERA‐40 analyses with results from a simulation of the atmosphere for the ERA‐40 period produced using the same model and same distributions of sea surface temperature and sea ice as used in the ERA‐40 data assimilation. The simulation itself reproduces quite well the warming trend over land seen in CRUTEM2v and captures some of the low‐frequency variability.