Warm summers during the Younger Dryas cold reversal

Frederik Schenk,Francesco Muschitiello,Maija Heikkilä,Svante Björck,Jens-Ove Näslund,Jenny Brandefelt,Arne V Johansson,Minna Väliranta,Barbara Wohlfarth,Lev Tarasov

doi:10.1038/s41467-018-04071-5

Abstract

The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. The sudden cooling is typically linked to an abrupt slowdown of the Atlantic Meridional Overturning Circulation (AMOC) in response to meltwater discharges from ice sheets. However, inconsistencies regarding the YD-response of European summer temperatures have cast doubt whether the concept provides a sufficient explanation. Here we present results from a high-resolution global climate simulation together with a new July temperature compilation based on plant indicator species and show that European summers remain warm during the YD. Our climate simulation provides robust physical evidence that atmospheric blocking of cold westerly winds over Fennoscandia is a key mechanism counteracting the cooling impact of an AMOC-slowdown during summer. Despite the persistence of short warm summers, the YD is dominated by a shift to a continental climate with extreme winter to spring cooling and short growing seasons.

Highlights

The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts
Consistent with new proxy evidence where we reconstruct July temperatures based on the climate indicator-plant species approach[41], we find clear evidence for warm European summers during the YD as a result of persistent atmospheric blocking over the NE-Atlantic and Fennoscandian Ice Sheet
The model is forced by the strong oceanic cooling and expansion of sea-ice during the YD reflecting global sea-surface temperatures (SSTs) in response to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC) of ~36% (−5.27 Sv) during the YD compared to the preceding BA25,26, consistent with geological evidence[22]

Summary

Introduction

The Younger Dryas (YD) cold reversal interrupts the warming climate of the deglaciation with global climatic impacts. It was suggested that forcings other than the AMOC slowdown and the associated cooling of the North Atlantic Ocean are required to explain the YD such as a reduction in solar activity[28], a change in wind patterns[29] or ice sheets[30,31], or possibly a combination of an AMOC slowdown with lower solar activity[11] As evidence for such a ~1100long negative forcing is lacking, the mechanisms and/or regional climatic impacts of the YD remain elusive. Consistent with new proxy evidence where we reconstruct July temperatures based on the climate indicator-plant species approach[41], we find clear evidence for warm European summers during the YD as a result of persistent atmospheric blocking over the NE-Atlantic and Fennoscandian Ice Sheet

Methods

Results

Conclusion