Northward shift of the southern westerlies during the Antarctic Cold Reversal

doi:10.1016/j.quascirev.2021.107189

Quaternary Science Reviews

Volume 271, 1 November 2021, 107189

https://doi.org/10.1016/j.quascirev.2021.107189 Get rights and content

Highlights

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Empirical data demonstrate a northward shift of the southern westerlies during the Antarctic Cold Reversal (ACR).
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The northward shift of the southern westerlies during the ACR drove antiphase west-east environmental responses across Tasmania.
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Wetter conditions, lower fire activity and cold tolerant vegetation prevailed on western (windward) slopes during the ACR.
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Stronger westerly winds drive increased evaporation on the eastern (leeward) side of the Tasmanian mountains during the ACR.
Consistent with models depicting global reorganisation of ocean and atmospheric circulation and heat transport during the ACR.

Abstract

Inter-hemispheric asynchrony of climate change through the last deglaciation has been theoretically linked to latitudinal shifts in the southern westerlies via their influence over CO₂ out-gassing from the Southern Ocean. Proxy-based reconstructions disagree on the behaviour of the westerlies through this interval. The last deglaciation was interrupted in the Southern Hemisphere by the Antarctic Cold Reversal (ACR; 14.7 to 13.0 ka BP (thousand years Before Present)), a millennial-scale cooling event that coincided with the Bølling–Allerød warm phase in the North Atlantic (BA; 14.7 to 12.7 ka BP). We present terrestrial proxy palaeoclimate data that demonstrate a migration of the westerlies during the last deglaciation. We support the hypothesis that wind-driven out-gassing of old CO₂ from the Southern Ocean drove the deglacial rise in atmospheric CO₂.

Graphical abstract

Introduction

The southern westerlies are part of a zonally-symmetric system that dominates the climate of the mid-to high-latitudes of the Southern Hemisphere (Garreaud, 2007). Changes in the strength and latitudinal position of the southern westerlies are believed to modulate global atmospheric CO₂ concentration via changes in wind stress over the Southern Ocean. Wind stress influences the upwelling of CO₂ saturated deep waters and the capacity of the surface ocean to absorb, or release, CO₂ (Siani et al., 2013). In the ocean, the latitudinal position of the southern westerlies is linked to the position of the Subpolar Oceanic Front and the Antarctic Circumpolar Current (Toggweiler et al., 2006), while on land changes in the southern westerlies govern mid-to high-latitude terrestrial climate, principally hydroclimate (Garreaud, 2007), which has a profound influence over a range of terrestrial processes (Fletcher and Moreno, 2012; Mariani and Fletcher, 2017). Parallel latitudinal shifts of the southern westerlies and Intertropical Convergence Zone (ITCZ) have also been proposed for the last deglaciation as part of the atmospheric response to changes in ocean heat transport by the Atlantic Meridional Overturning Circulation (AMOC) (Buizert et al., 2018; Denton et al., 2010; Markle et al., 2017; Pedro et al., 2016; Sigman et al., 2020). However, while paleoclimate proxy data confirm a cooling in the mid-to high latitudes of the Southern Hemisphere (poleward of ca. 40⁰S) during the ACR, the available data provide no clear constraint on the latitudinal behaviour of the southern westerlies through this interval.

We reconstruct westerly wind behaviour in the mid-latitudes of the Australian sector of the Southern Hemisphere from multiproxy data from six radiocarbon-dated lake sediment sequences from Tasmania (40–44⁰S; Fig. 1a). Tasmania is uniquely situated to investigate the behaviour of the southern westerlies during the ACR due to its location at the northern margins of the westerly wind belt and an exceptionally strong correlation between southern westerly wind speed and its rainfall anomalies (Fig. 1b) (Gillett et al., 2006). This means that rainfall proxies can be applied to reconstruct past changes in wind regimes. Tasmania's mountainous west coast and contrasting lowland east coast create an orographic effect that splits the island into clear zones of positive (western sites) and negative (eastern sites) correlation between southern westerly wind speed and rainfall (Fig. 1b). This allows us to target sites where there is an unambiguous southern westerly influence over rainfall.

We compile new lake sediment rainfall proxies including charcoal, pollen and geochemistry from six lakes located in zones of both significant positive and significant negative correlation between southern westerly wind speed and rainfall (Fig. 1b). While the charcoal proxy is influenced by human-caused fire ignitions (Bowman and Brown, 1986), the occurrence and spread of fires is moisture-limited in the high-rainfall west of Tasmania (McWethy et al., 2013; Styger and Kirkpatrick, 2015). Sedimentary charcoal analyses have previously revealed a coherence between changes in regional charcoal (biomass burning) and changes in moisture delivery by the southern westerlies through the Holocene: increased southern westerly flow over Tasmania results in reduced sedimentary charcoal content (and vice-versa), reflecting the primacy of southern westerly-derived orographic rainfall over regional fire regimes (Mariani and Fletcher, 2016, 2017).

Section snippets

Construction of age models

¹⁴C analysis using accelerator mass spectrometry (AMS) was used to date each sediment record utilised in this analysis. Results for each sample submitted for analysis, along with their calibrated age ranges, are provided in Table S1. All radiocarbon ages were calibrated using SHCal20 (Hogg et al., 2020) and age-depth models for each core (Fig. 3) were constructed using the rbacon v2.3.9.1 (Blaauw and Christen, 2011) package in R. Modelling was restricted to ∼9 to 18 ka. Modelled age outputs

Results

All graphed results are in the Supplementary Information (Figs. S1-5) and this section only briefly describes the results here. All selected cores span the ACR interval with sufficient dating to resolve environmental changes through this interval (Figure S1). Synthesis of the charcoal records from the western Tasmanian sites (n = 4) demonstrates a sharp increase in CHAR across the west commencing at ca. 17.8 ka BP and continuing until ca. 15 ka BP, before declining between ca. 13.5 ka BP. CHAR

Discussion

Charcoal influx to the western Tasmanian lakes decreases during the ACR (Fig. 2d, S2-3), consistent with enhanced westerly winds and rainfall, reducing biomass burning. This change is synchronous with a marked increase in carbonate precipitation and subsequent deposition (sedimentary Ca/Ti ratio) (Kylander et al., 2011) at two sites in eastern Tasmania (Fig. 2a,b) and a concomitant decrease in carbonate deposition in the west (Fig. 2c). Carbonate precipitation occurs under increased evaporative

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

MSF was funded by Australian Research Council grants (DI110100019, IN140100050, IN170100062, IN170100063). JBP received support from the Australian Government as part of the Antarctic Science Collaboration Initiative program. We thank two anonymous reviewers for their positive and constructive reviews of an initial draft of this manuscript.

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View full text

Northward shift of the southern westerlies during the Antarctic Cold Reversal

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Construction of age models

Results

Discussion

Declaration of competing interest

Acknowledgements

Quat. Sci. Rev.

Comput. Geosci.

Earth Planet Sci. Lett.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Int.

Rev. Palaeobot. Palynol.

Quat. Sci. Rev.

Palaeogeogr. Palaeoclimatol. Palaeoecol.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Quat. Sci. Rev.

Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO2

Science

Rapid loss of CO2 from the south pacific ocean during the last glacial termination

Paleoceanography and Paleoclimatology

Rapid interhemispheric climate links via the Australasian monsoon during the last deglaciation

Nat. Commun.

Flexible paleoclimate age-depth models using an autoregressive gamma process

Bayesian Analysis

Bushfires in Tasmania: a botanical approach to anthropological questions

Archaeol. Ocean.

An analysis of transformations

J. Roy. Stat. Soc. B

Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north

Nature

Multiple expressions in the anatomy of past abrupt warmings recorded in Greenland ice

Nat. Commun.

The relationship between the ITCZ and the Southern Hemispheric eddy-driven jet

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Aspects of the mean wintertime circulation along Australia's southern shelves: numerical studies

J. Phys. Oceanogr.

Robust locally weighted regression and smoothing scatterplots

J. Am. Stat. Assoc.

Paleolimnology: the History and Evolution of Lake Systems

Synchronous timing of abrupt climate changes during the last glacial period

Science

Micro-XRF Studies of Sediment Cores: Applications of a Non-destructive Tool for the Environmental Sciences

Predictability of biomass burning in response to climate changes

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Influence of the tropics and southern westerlies on glacial interhemispheric asymmetry

Nat. Geosci.

The last glacial termination

Science

Wind-driven upwelling in the Southern Ocean and the deglacial rise in atmospheric CO₂