Uncertainties within Weather Regime definitions for the Euro-Atlantic sector in ERA5 and CMIP6

Abstract

Certain Weather Regimes (WR) are associated with a higher risk of en- ergy shortages, i.e. Blocking regimes for European winters. However, there are many uncertainties tied to the implementation of WRs and associated risks in the energy sector. Especially the impact of climate change is unknown. We investigate these uncertainties by looking at three methodologically diverse Euro-Atlantic WR definitions. We carry out a thorough validation of these methods and analyse their methodological and spatio-temporal sensitivity using ERA5 data. Furthermore, we look into the suitability of CMIP6 models for WR based impact as- sessments. Our sensitivity assessment showed that the persistence and occurrence of regimes are sensitive to small changes in the methodology. We show that the training period used has a very significant impact on the persistence and occurrence of the regimes found. For both WR4 and WR7, this results in instability of the regime patterns. All CMIP6 models investigated show instability of the regimes. Meaning that the normalised distance between the CMIP6 model regimes and our baseline regimes exceeds 0.4 or are visually extremely dissimilar. Only the WR4 regimes clustered on historical CMIP6 model data consistently have a normalised distance to our baseline regimes smaller than 0.4 and are visually identifiable. The WR6 definition exceeds the normalised distance threshold for all investigated CMIP6 experiments. Though all CMIP6 model experiments clustered with the WR7 definition have a normalised distance to the baseline regimes below 0.4, visual inspection of the regimes indicates instability. Great caution should be taken when applying WR’s in impact models for the en- ergy sector, due to this large instability and uncertainties associated with WR defini- tions