A volcanically triggered regime shift in the subpolar North Atlantic Ocean as a possible origin of the Little Ice Age

C F Schleussner,G Feulner

doi:10.5194/cp-9-1321-2013

C F Schleussner, G Feulner

Open Access

https://doi.org/10.5194/cp-9-1321-2013

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Abstract

Abstract. Among the climatological events of the last millennium, the Northern Hemisphere Medieval Climate Anomaly succeeded by the Little Ice Age are of exceptional importance. The origin of these regional climate anomalies remains a subject of debate and besides external influences like solar and volcanic activity, internal dynamics of the climate system might have also played a dominant role. Here, we present transient last millennium simulations of the fully coupled model of intermediate complexity Climber 3α forced with stochastically reconstructed wind-stress fields. Our results indicate that short-lived volcanic eruptions might have triggered a cascade of sea ice–ocean feedbacks in the North Atlantic, ultimately leading to a persistent regime shift in the ocean circulation. We find that an increase in the Nordic Sea sea-ice extent on decadal timescales as a consequence of major volcanic eruptions in our model leads to a spin-up of the subpolar gyre and a weakened Atlantic meridional overturning circulation, eventually causing a persistent, basin-wide cooling. These results highlight the importance of regional climate feedbacks such as a regime shift in the subpolar gyre circulation for understanding the dynamics of past and future climate.

Highlights

The so-called Little Ice Age (LIA), a period of cool temperatures over the Northern Hemisphere, is among the most debated climatological events of the Holocene
Since the LIA coincides with a series of minima in sunspot activity, it has been interpreted as an example of the importance of multi-decadal variations in the total solar irradiance (TSI) to the earth’s climate (Eddy, 1976), a hypothesis investigated in a range of model simulations of the last millennium (e.g., CroSwloeyl,id20E00a; Zrothrita et al, 2004; Swingedouw et al, 2012)
The Atlantic meridional overturning circulation (AMOC) at 30N is found to be about 6.5 % weaker in the strong compared to the SPG weak regime in man Suosucremptoibdiellit(yM, Secnigeenlceet, 3al3.1,2507182–).582, doi:10.1126/science. 1197175,In20th1e1.simulations presented here, the SPG regime shift is Cook, E.a,cDco’mArpraignoie,dRb.y, aantdraMnsaiennnt, AMM.:OAC rweedlul-cvtieornifioefd,abmouultti5- % proxy breectwonesetnruMctiCoAn oafntdheLwIAintaenrdNaonrtihncArtelaasneticinOssecaililcaetioonveInr -the dex sinNcoeraddic1s4e0a0s*(,NJoorutrhnAaltloafnCticlimSeaate-I,c1e5E, x1t7e5n4t–(1N7A6S4I,E2)0,0420.◦ W– Crowley,2T0.:◦ CEa, u6s4e◦sNof–c8l0im◦ Nat;esceheanFgige.o2vae)rtthhaetpiasscto1n0s0i0styeneat rws,iSthcis-eaence, 2ic8e9,re2c7o0n–s2t7ru7c,tdiooni:s1b0y.1M12a6c/isacsieFnacuer.i2a8e9t.5a4l.7(72.207009,)

Summary

Introduction

The so-called Little Ice Age (LIA), a period of cool temperatures over the Northern Hemisphere, is among the most debated climatological events of the Holocene. Since the LIA coincides with a series of minima in sunspot activity, it has been interpreted as an example of the importance of multi-decadal variations in the total solar irradiance (TSI) to the earth’s climate (Eddy, 1976), a hypothesis investigated in a range of model simulations of the last millennium (e.g., CroSwloeyl,id20E00a; Zrothrita et al, 2004; Swingedouw et al, 2012). In a modeling study,Tthhesee lCowryeostismpathesehraeve been shown to yield results that are consistent with Northern Hemisphere temperature reconstructions (Feulner, 2011). In these simulations, LIA cooling is dominated by the effect of volcanic eruptions, an alternative hypothesis that was suggested earlier (Robock, 1979; Crowley, 2000). Internal variability of the climate system can lead to pronounced climate anomalies, though model simulations suggest that long-term anomalies like the LIA cannot be sustained without external forcing (Hunt, 2006)

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