Impact of cold North Atlantic SST and jet stream anomalies on European heat waves

Abstract:

This study highlights the relevance of cold North Atlantic SSTs and jet stream undulations for the onset of high European temperatures by using the ERA-5 reanalysis product. The quasi-resonant amplification (QRA) mechanism, which is associated with quasi-stationary and high amplified waves, is not identified here to be a contributor to European heat waves in general.

Motivated by a coexistence of cold North Atlantic sea surface temperature (SST) and positive European surface temperature anomalies during the summer seasons of 2015 and 2018, we evaluate the North Atlantic SST anomalies and the QRA theory as potential drivers for European heat waves for the first time in combination.

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 find that cold North Atlantic SSTs promote a stronger double jet occurrence in this sector. The presence of a double jet as a necessary preconditioning for a QRA signature is thus evident. In order to verify a QRA imprint, we make a wave analysis covering two-dimensional probability density functions of phase speed and amplitude, compositing cold SSTs, high double jet indices (DJIs) or both together. However, our analysis does not confirm a relationship between cold North Atlantic SSTs, the double jet and the QRA theory. 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.

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.