Investigating European heatwaves and their medium-range predictability in relation to weather regimes

The successful prediction of heatwave onsets on the medium-range forecast time scale (here, 5-12 days) mainly relies on the adequate forecasting of large-scale Rossby wave patterns and their dynamics. In the mid-latitude regions of Europe, lasting heatwaves are often associated with a substantial blocking of the large-scale atmospheric flow due to amplified and/or breaking Rossby waves. To characterize such anomalous flow configurations, which may come in different patterns, we adopt the concept of Euro-Atlantic weather regimes. Based on Empirical Orthogonal Function analysis and subsequent k-means clustering, this widely-used metric reduces the complexity of the atmospheric flow field by projecting it onto the seven main modes of synoptic-scale variability in this domain.  In this study, we therefore examine heatwave characteristics in different European regions in relation to Euro-Atlantic weather regimes. A focus is set on the question to which extent the medium-range predictability of heatwave onsets depends on the current or preceding weather regime as well as to flow anomalies further upstream or other potential precursors not directly related to Rossby wave dynamics such as abnormally dry soils. Heatwaves are objectively diagnosed as a 90th percentile exceedance in 2m maximum temperatures for a minimum of 3 days in both a local and regional context.  Using ERA-5 data for the period 1979-present, we find that British and Scandinavian heatwaves are mainly associated with classic blocking regimes (Scandinavian and European blocking), whereas the picture is more diverse for Central Europe where the „no regime“ case is also frequently observed. Remarkably, over the last 20 years, European heatwaves associated with a European blocking seem to be significantly related to pre-existing anomalously dry soils over large parts of Northern America which is, however, not the case for heatwaves related to any other weather regime. The medium-range predictability of heatwaves is investigated for the period 2001-2018, using hindcast ensembles of two state-of-the art weather forecast models ECMWF-IFS and GEFS-v12, by means of usual metrics such as 500hPa geopotential anomaly correlation coefficients (ACC) and 850hPa temperature mean absolute errors. Preliminary results with a focus on Central Europe suggest that heatwaves in this region seem to be slightly more predictable (roughly one more day until ACC drops below 0.8) when they occur in conjunction with a Scandinavian or European blocking compared to the case with no apparent regime. This may be explained by the overall more transient and phase-error prone nature of the „no regime“-type heatwaves. Interestingly, heatwaves with the worst predictability at 10 days lead time show an intensified jet stream over the Atlantic one week prior and a slight tendency toward wetter than normal soils over North America and Central Europe.Finally, we also investigate to which extent medium-range forecasts of local maximum temperatures further depend on more local, diabatic processes (soil moisture, cloud cover forecast) and whether there are systematic differences between lead times and weather regimes.