Observed Subdecadal Variations of European Summer Temperatures

W.A Müller,L Borchert,R Ghosh

doi:10.1029/2019gl086043

W.A Müller, L Borchert + Show 1 more

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https://doi.org/10.1029/2019gl086043

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Abstract

AbstractWe identify subdecadal variations in European summer temperatures in coupled and uncoupled century‐long reanalyses. Spectral analyses reveal significant peaks at 5–10 years in the midtwentieth century. The subdecadal variations show substantial amplitudes of ~1–1.5 °C, associated with extremely warm summers during their positive phases. We use forced ocean model experiments and show that the European summer temperature variations are associated with the subdecadal coupled North Atlantic climate system. A positive winter NAO‐like forcing is associated with changes in the ocean circulation and mass and heat convergence occurring 1–2 years prior to European summer temperature rise. Ocean heat content and sea surface temperature increase in the subtropical North Atlantic. The atmospheric response is barotropic and induces wave activity fluxes toward the European continent, modulation of the jet positions, and blocking frequency. The atmospheric response establishes a pathway connecting the subdecadal coupled North Atlantic climate system to European summer temperature.

Highlights

The observed European summer temperatures have increased by ~1.5–2 °C since the 1990s and are expected to increase further due to the global mean temperature rise associated with increasing greenhouse gas (GHG) emissions (Christidis et al, 2015; Schär et al, 2004; Suárez‐Gutiérrez et al, 2018)
We identify subdecadal variations in European summer temperatures in coupled and uncoupled century‐long reanalyses
We use the century‐long reanalyses of the NOAA 20th century (Compo et al, 2011) and the ECMWF ERA‐20C (Poli et al, 2016) and CERA‐20C (Laloyaux et al, 2018), in addition to forced ocean model experiments with the Max‐Planck‐Institute Ocean model (MPIOM), and identify new subdecadal variations of European summer temperature linked to the internal variations of the coupled North Atlantic climate system

Summary

Introduction

The observed European summer temperatures have increased by ~1.5–2 °C since the 1990s and are expected to increase further due to the global mean temperature rise associated with increasing greenhouse gas (GHG) emissions (Christidis et al, 2015; Schär et al, 2004; Suárez‐Gutiérrez et al, 2018). Observations and modeling results indicate an active role of the atmospheric heat and momentum forcing from the winter NAO, together with a delayed effect of the redistribution of North Atlantic water masses on a subdecadal to decadal time scale (Czaja & Marshall, 2001; Eden & Greatbatch, 2003; Martin et al, 2019; Reintges et al, 2017). Three questions are addressed: (I) Do European summer temperatures show significant variations on a subdecadal time scale, (II) are the variations of European summer temperatures coupled with the subdecadal North Atlantic climate system, and (III) how would an associated atmospheric pathway look like linking the North Atlantic with European summer temperatures

Century‐Long Reanalyses and Forced Ocean Model Experiments

Analysis Methods

Subdecadal Variations of European Summer Temperatures

A Subdecadal Coupled North Atlantic Climate System

A Subdecadal Atmospheric Pathway

Summary and Conclusion