Error in ERA5 2‐m temperature identified using GraphCast

Abstract Reanalyses such as the European Centre for Medium‐Range Weather Forecasts (ECMWF) Reanalysis Version 5 (ERA5) have long been foundational for weather and climate science. They have also found a new use case, as training and verification data for machine‐learnt weather prediction (MLWP) models. Here we compare short‐lead‐time (6 h) forecasts from the MLWP model GraphCast against ERA5. In doing so, we identify a recurrent, spatially coherent error in 2‐m temperature centred on the Ethiopian Highlands, which occurs predominantly at 0600 UTC. We find that the same error feature is present in other MLWP models trained on ERA5. We show that these error events arise from errors in ERA5 that are also present in the ECMWF operational analysis. They arise from the two‐dimensional (2D) optimal interpolation procedure, when surface reports that are temporally displaced compared with the background forecast are assimilated. This produces spuriously warm analysis increments over Ethiopia on approximately 7% of dates at 0600 UTC across the reanalysis record. The spread from the ensemble of data assimilation partially flags these cases but is underdispersive. We assess the impact on a MLWP system trained on ERA5. While the MLWP model can largely ignore these unphysical error events, a small systematic degradation in forecast skill over the region is observed. We discuss the implications for using reanalysis as truth in machine‐learning training and verification, and recommend simple changes to reduce such artefacts in future analyses.