Climate Reanalysis

A climate reanalysis is a physically consistent reconstruction of past atmospheric or oceanic states made by assimilating historical observations into a fixed numerical model. Holding the model and assimilation scheme constant across the record avoids the spurious jumps that come from changing operational forecast systems, so the output is gridded, gap-free, and multi-decadal. ECMWF ERA5 covers 1940 to present at about 31 km resolution with hourly fields. Reanalyses supply the baseline for anomalies, mode indices, and reanalysis-driven hindcasts, though their early decades carry larger uncertainty where observations were sparse.