A regime view of future atmospheric circulation changes in northern mid-latitudes

Federico Fabiano,Paolo Ghinassi,Virna L Meccia,Paolo Davini,Susanna Corti

doi:10.5194/wcd-2-163-2021

Abstract

Abstract. Future wintertime atmospheric circulation changes in the Euro–Atlantic (EAT) and Pacific–North American (PAC) sectors are studied from a weather regimes perspective. The Coupled Model Intercomparison Project phases 5 and 6 (CMIP5 and CMIP6) historical simulation performance in reproducing the observed regimes is first evaluated, showing a general improvement in the CMIP6 models, which is more evident for EAT. The circulation changes projected by CMIP5 and CMIP6 scenario simulations are analysed in terms of the change in the frequency and persistence of the regimes. In the EAT sector, significant positive trends are found for the frequency and persistence of NAO+ (North Atlantic Oscillation) for SSP2–4.5, SSP3–7.0 and SSP5–8.5 scenarios with a concomitant decrease in the frequency of the Scandinavian blocking and Atlantic Ridge regimes. For PAC, the Pacific Trough regime shows a significant increase, while the Bering Ridge is predicted to decrease in all scenarios analysed. The spread among the model responses is linked to different levels of warming in the polar stratosphere, the tropical upper troposphere, the North Atlantic and the Arctic.

Highlights

A major challenge for the climate community is to understand how a warmer climate will affect the large-scale atmospheric circulation at mid-latitudes
We proposed here an alternative view of future changes in the atmospheric circulation at northern mid-latitudes, considering the future trends in weather regime frequency and persistence over the Euro–Atlantic and Pacific–North American sectors, as projected by the CMIP5 and CMIP6 models
The CMIP6 ensemble shows a non negligible improvement in the reproduction of the weather regimes when compared to CMIP5 (Sect. 3.2)

Summary

Introduction

A major challenge for the climate community is to understand how a warmer climate will affect the large-scale atmospheric circulation at mid-latitudes. The wintertime mid-latitude climate in the Northern Hemisphere is primarily influenced by the low-frequency variability (at timescales longer than 5 d) related to the strength and position of the eddy-driven jet stream (Woollings et al, 2010; Barnes and Polvani, 2013) This is true for the North Atlantic and North Pacific sectors, where the latitudinal shifts in the jet describe a significant fraction of the low-frequency variability (Athanasiadis et al, 2010) and determine specific impacts locally (Ma et al, 2020) and over downstream regions (i.e. Europe and North America) (Screen and Simmonds, 2014; Zappa et al, 2015a, b; Loikith and Broccoli, 2014). The emerging picture is that the fate of the eddydriven jet streams in a warmer climate is mainly controlled by the meridional temperature gradient at mid-latitudes, which in turn depends on three independent processes all of which are linked to the differential heating of different regions of the atmosphere

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Discussion

Conclusion