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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-32
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  07 Aug 2020

07 Aug 2020

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This preprint is currently under review for the journal WCD.

Impact of North Atlantic SST and Jet Stream anomalies on European Heat Waves

Julian Krüger1, Robin Pilch Kedzierski1, Karl Bumke1, and Katja Matthes1,2 Julian Krüger et al.
  • 1Marine Meteorology Department, GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
  • 2Faculty of Mathematics and Natural Sciences, Christian-Albrechts-Universität zu Kiel, Kiel, Germany

Abstract. 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.

Julian Krüger et al.

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Julian Krüger et al.

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Latest update: 24 Sep 2020
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Short summary
Motivated by the European heat wave occurrences of 2015 and 2018, this study evaluates the influence of cold North Atlantic SST anomalies on European heat waves by using the ERA-5 reanalysis product. Our findings show that widespread cold North Atlantic SST anomalies may be a precursor for a persistent jet stream pattern and are thus important for the onset of high European temperatures.
Motivated by the European heat wave occurrences of 2015 and 2018, this study evaluates the...
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