Towards understanding potential atmospheric contributions to abrupt climate changes: characterizing changes to the North Atlantic eddy-driven jet over the last deglaciation

Lev Tarasov,Heather J Andres

doi:10.5194/cp-15-1621-2019

Abstract

Abstract. Abrupt climate shifts of large amplitudes were common features of the Earth's climate as it transitioned into and out of the last full glacial state approximately 20 000 years ago, but their causes are not yet established. Midlatitude atmospheric dynamics may have played an important role in these climate variations through their effects on heat and precipitation distributions, sea ice extent, and wind-driven ocean circulation patterns. This study characterizes deglacial winter wind changes over the North Atlantic (NAtl) in a suite of transient deglacial simulations using the PlaSim Earth system model (run at T42 resolution) and the TraCE-21ka (T31) simulation. Though driven with yearly updates in surface elevation, we detect multiple instances of NAtl jet transitions in the PlaSim simulations that occur within 10 simulation years and a sensitivity of the jet to background climate conditions. Thus, we suggest that changes to the NAtl jet may play an important role in abrupt glacial climate changes. We identify two types of simulated wind changes over the last deglaciation. Firstly, the latitude of the NAtl eddy-driven jet shifts northward over the deglaciation in a sequence of distinct steps. Secondly, the variability in the NAtl jet gradually shifts from a Last Glacial Maximum (LGM) state with a strongly preferred jet latitude and a restricted latitudinal range to one with no single preferred latitude and a range that is at least 11∘ broader. These changes can significantly affect ocean circulation. Changes to the position of the NAtl jet alter the location of the wind forcing driving oceanic surface gyres and the limits of sea ice extent, whereas a shift to a more variable jet reduces the effectiveness of the wind forcing at driving surface ocean transports. The processes controlling these two types of changes differ on the upstream and downstream ends of the NAtl eddy-driven jet. On the upstream side over eastern North America, the elevated ice sheet margin acts as a barrier to the winds in both the PlaSim simulations and the TraCE-21ka experiment. This constrains both the position and the latitudinal variability in the jet at LGM, so the jet shifts in sync with ice sheet margin changes. In contrast, the downstream side over the eastern NAtl is more sensitive to the thermal state of the background climate. Our results suggest that the presence of an elevated ice sheet margin in the south-eastern sector of the North American ice complex strongly constrains the deglacial position of the jet over eastern North America and the western North Atlantic as well as its variability.

Highlights

The last deglaciation encompassed a period of large-scale global warming of the Earth’s surface climate, with regional patterns of millennial-timescale variability (Blunier et al, 1998; Shakun and Carlson, 2010; Clark et al, 2012)
As in Woollings et al (2010), the algorithm used to detect the latitude of the eddy-driven jet for the remainder of this analysis finds the location of maximum zonal winds averaged over the North Atlantic (NAtl) basin within 15 to 75◦ N and over atmospheric levels 700 to 925 hPa
This paper explores the question of whether winter eddydriven-jet changes over the North Atlantic could have contributed to the abrupt climate changes detected in Greenland ice cores over the last deglaciation

Summary

Introduction

The last deglaciation encompassed a period of large-scale global warming of the Earth’s surface climate, with regional patterns of millennial-timescale variability (Blunier et al, 1998; Shakun and Carlson, 2010; Clark et al, 2012). The Oldest Dryas (OD, 19–14.7 kyr BP, which includes Heinrich Stadial 1, 17.5–14.7 kyr BP), the Bølling-Allerød (BA, 14.7–12.8 kyr BP), and the Younger Dryas (YD, 12.8– 11.7 kyr BP) denote consecutive periods of stadial and interstadial conditions with rapid adjustments between them (Hammer et al, 1986; Grachev and Severinghaus, 2005; Clark et al, 2012) Signatures of these climate variations are present in the middle to high latitudes of both hemispheres with increasing amplitude toward the poles, the signs tend to be anti-phased between the two hemispheres. Abrupt shifts have been detected in reconstructed lake levels and speleothem data in the subtropics and Indian and African monsoon regions, suggesting latitudinal shifts of the Intertropical Convergence Zone (ITCZ) (e.g. Jacob et al, 2007; Mohtadi et al, 2016)

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Conclusion