On the (In)Homogeneity of Reanalysis Products

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Abstract
The homogeneity of the ECMWF 40-yr Re-Analysis (ERA-40) is assessed. This is done by comparing ERA-40 data with results from the NCEP–NCAR reanalysis and also by investigating a known relationship between a modeled (latent heat flux) and an external (SST) quantity. The direct comparison between the two reanalyses reveals a lot of inhomogeneities. They occur mainly in the Southern Hemisphere and before 1980. While observational density was sufficient to effectively constrain the models in the Northern Hemisphere, it was not in the Southern Hemisphere. From the investigation of the relationship between latent heat flux and SST it is found that, because of an increasing amount of data, the reanalysis results become more reliable toward the end of the reanalysis period (approximately after 1980). When using the reanalysis data to investigate climate change issues care has to be taken not to confuse the inhomogeneities with real changes.