Reanalysis

A reanalysis is a consistent reconstruction of the atmospheric or oceanic state produced by assimilating historical observations into a single frozen model and data-assimilation system. Freezing the system across the whole record removes the artificial discontinuities of evolving operational forecasts, yielding gridded, gap-free, multi-decadal fields. ECMWF ERA5 spans 1940 to present at hourly output and about 31 km; ocean reanalyses such as ORAS5 and GLORYS do the equivalent for the ocean. Reanalyses define the climatologies behind anomalies and supply forcing and verification for hindcasts.