Changes in mean flow and atmospheric wave activity in the North Atlantic sector

Heiko Paeth,Felix Pollinger

doi:10.1002/qj.3660

Abstract

AbstractIn recent years, the midlatitudes are characterized by more intense heatwaves in summer and sometimes severe cold spells in winter that might emanate from changes in atmospheric circulation, including synoptic‐scale and planetary wave activity in the midlatitudes. In this study, we investigate the heat and momentum exchange between the mean flow and atmospheric waves in the North Atlantic sector and adjacent continents by means of the physically consistent Eliassen–Palm flux diagnostics applied to reanalysis and forced climate model data. In the long‐term mean, momentum is transferred from the mean flow to atmospheric waves in the northwest Atlantic region, where cyclogenesis prevails. Further downstream over Europe, eddy fluxes return momentum to the mean flow, sustaining the jet stream against friction. A global climate model is able to reproduce this pattern with high accuracy. Atmospheric variability related to atmospheric wave activity is much more expressed at the intraseasonal rather than the interannual time‐scale. Over the last 40 years, reanalyses reveal a northward shift of the jet stream and a weakening of intraseasonal weather variability related to synoptic‐scale and planetary wave activity. This pertains to the winter and summer seasons, especially over central Europe, and correlates with changes in the North Atlantic Oscillation as well as regional temperature and precipitation. A very similar phenomenon is found in a climate model simulation with business‐as‐usual scenario, suggesting an anthropogenic trigger in the weakening of intraseasonal weather variability in the midlatitudes.

Highlights

Human activity is supposed to have manifold implications for the Earth's climate system (IPCC, 2013)
It is distinguished between synoptic-scale and planetary waves in order to improve our process understanding of the implications of changing wave activity for regional climate and weather extremes
Observed trends are compared with the output of a climate model simulation with strong radiative forcing to support the hypothesis of an anthropogenic trigger

Summary

Introduction

Human activity is supposed to have manifold implications for the Earth's climate system (IPCC, 2013). While heatwaves may be intuitively associated with global warming, cold spells often lead to some uncertainty about the evidence of anthropogenic climate change in the public and require plausible scientific explanations. This implies a thorough look at how atmospheric dynamics may respond to radiative forcing. These mechanisms are much less understood because atmospheric circulation is a paradigm for the chaotic nonlinear nature and internal variability of the climate system (Shepherd, 2014; Barnes and Screen, 2015; Overland et al, 2016)

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Results

Conclusion