Beschreibung: European heat waves have increased during the two recent decades. Particularly 2015 and 2018 were characterized by a widespread area of cold North Atlantic sea surface temperatures (SSTs) in early summer as well as positive surface temperature anomalies across large parts of the European continent during later summer. The European heat wave of 2018 is further suggested to be induced by a quasi-stationary and high-amplified Rossby wave pattern associated with the so-called quasi-resonant amplification (QRA) mechanism. In this study, we evaluate the North Atlantic SST anomalies and the QRA theory as potential drivers for European heat waves for the first time in combination by using the ERA-5 reanalysis product. A composite and correlation study reveals that cold North Atlantic SST anomalies in early summer favour a more undulating jet stream and a preferred trough-ridge pattern in the North Atlantic–European sector. Further we found that cold North Atlantic SSTs promote a stronger double jet occurrence in this sector. Thus, favorite conditions for a QRA signature are evident together with a necessary preconditioning of a double jet. However, our wave analysis covering two-dimensional probability density distributions of phase speed and amplitude does not confirm a relationship between cold North Atlantic SSTs and the QRA theory, compositing cold SSTs, high double jet indices (DJIs) or both together. Instead, we can show that cold North Atlantic SST events enhance the dominance of transient waves. In the presence of a trough during cold North Atlantic events, we obtain a slow-down of the transient waves, but not necessarily an amplification or stationarity. The deceleration of the transient waves result in a longer duration of a trough over the North Atlantic accompanied by a ridge downstream over Europe, triggering European heat episodes. Although a given DJI preconditioning may also be subject to the onset of certain QRA events, our study found no general relation between cold North Atlantic SST events and the QRA diagnostics. Our study highlights the relevance of cold North Atlantic SSTs for the onset of high European temperatures by affecting travelling jet stream undulations (but without involving QRA in general). Further attention should be drawn not only to the influence of North Atlantic SST year-to-year variability, but also to the effect of the North Atlantic warming hole as a negative SST anomaly in the long term, which is projected to evolve through climate change.

Beschreibung: This study highlights the relevance of North Atlantic SSTs and certain jet stream properties for the onset of high European temperatures by using the ERA-5/ERA20c reanalysis product and a targeted experiment with the OpenIFS model. We found that certain European heat wave events could be related to the simultaneous appearance of cold North Atlantic SST events, specific jet stream wave numbers and further to transient and recurrent Rossby wave activity. The coexistence of cold North Atlantic sea surface temperature (SST) and positive European surface temperature anomalies during several summer seasons, like in 1994, 2015 and 2018 motivated us to evaluate whether and how widespread and significant North Atlantic SST anomalies could be associated with European heat waves.Therefore we investigated the role of the jet stream in serving as a medium for a downstream signal propagation. A composite study reveals that cold North Atlantic SST anomalies in summer are accompanied by a more undulating jet stream and a preferred trough-ridge pattern in the North Atlantic-European sector. A wave analysis covering two-dimensional probability density functions of phase speed and amplitude after compositing cold SSTs show that cold North Atlantic SST events reveal a preference for a dominance of transient waves. In the presence of a trough during cold North Atlantic events, we obtain a slow-down of the transient waves, but not necessarily an amplification or stationarity. The deceleration of the transient waves result in a longer duration of a trough over the North Atlantic accompanied by a ridge downstream over Europe, favouring the conditions for the onset of European heat episodes. A study of the jet stream energetics via a kinetic energy power spectrum of meridional wind anomalies reveals that generally a trend shows up towards wave numbers 4 to 6. This is supported by an enhanced activity of specific wave numbers within this increased range during summer seasons of European heat wave events happening in the last two decades. An arising question poses whether the increased energy for a certain wave number originates from an SST forcing or different drivers. We investigate this by performing targeted OpenIFS model runs forced by different SST conditions.