Evidence for increased expression of the Amundsen Sea Low over the South Atlantic during the late Holocene

Zoë A Thomas,Scott Mooney,Jackie Hatton,Paul Mayewski,Alan N Williams,Richard T Jones,Alan Hogg,Philip Jones,Chris J Fogwill,David Lister,Chris S M Turney

doi:10.5194/cp-14-1727-2018

Abstract

Abstract. The Amundsen Sea Low (ASL) plays a major role in the climate and environment of Antarctica and the Southern Ocean, including surface air temperature and sea ice concentration changes. Unfortunately, a relative dearth of observational data across the Amundsen and Bellingshausen seas prior to the satellite era (post-1979) limits our understanding of the past behaviour and impact of the ASL. The limited proxy evidence for changes in the ASL are primarily restricted to the Antarctic where ice core evidence suggests a deepening of the atmospheric pressure system during the late Holocene. However, no data have previously been reported from the northern side of the ASL. Here we report a high-resolution, multi-proxy study of a 5000-year-long peat record from the Falkland Islands, a location sensitive to contemporary ASL dynamics which modulates northerly and westerly airflow across the southwestern South Atlantic sector of the Southern Ocean. In combination with climate reanalysis, we find a marked period of wetter, colder conditions most likely the result of enhanced southerly airflow between 5000 and 2500 years ago, suggesting limited ASL influence over the region. After 2500 years ago, drier and warmer conditions were established, implying more westerly airflow and the increased projection of the ASL onto the South Atlantic. The possible role of the equatorial Pacific via atmospheric teleconnections in driving this change is discussed. Our results are in agreement with Antarctic ice core records and fjord sediments from the southern South American coast, and suggest that the Falkland Islands provide a valuable location for reconstructing high southern latitude atmospheric circulation changes on multi-decadal to millennial timescales.

Highlights

The leading mode of variability in atmospheric circulation across the southern mid–high latitudes is the Southern Annular Mode (SAM), manifested as the pressure difference between 65 and 40◦ S (Marshall, 2003; Thompson et al, 2011).Published by Copernicus Publications on behalf of the European Geosciences Union.Z
Wetter conditions are associated with a weakening of the Amundsen Sea Low (ASL) and the delivery of more southerly and easterly airflow across the South Atlantic
A similar picture emerges with variations in temperature (Fig. 2a and d) with a deeper ASL associated with warmer temperatures over the Falkland Islands, and a weaker ASL with cooler conditions

Summary

Introduction

The leading mode of variability in atmospheric circulation across the southern mid–high latitudes is the Southern Annular Mode (SAM), manifested as the pressure difference between 65 and 40◦ S (Marshall, 2003; Thompson et al, 2011).Z. Thomas et al.: Evidence for increased expression of the Amundsen Sea Low. The multi-decadal trend to a more positive SAM since the mid-20th century (Fogt et al, 2012; Hosking et al, 2013) is expressed by a strengthening and poleward shift of midlatitude westerly airflow and storm tracks over the Southern Ocean (Marshall, 2003; Thompson et al, 2011; Visbeck, 2009; Fig. 1), and has been linked to changes in climate, ocean ventilation, air–sea carbon flux, sea ice trends, and ice sheet dynamics on interannual to multi-decadal timescales Proxy reconstructions of SAM and/or associated westerly winds have been generated for the Holocene (Sime et al, 2010; Dixon et al, 2012; Fletcher and Moreno, 2012; Moreno et al, 2012; Abram et al, 2014; Mayewski et al, 2017; Turney et al, 2017b), but there is a relative dearth of records regarding the past behaviour of the ASL (Mayewski et al, 2013)

Methods

Results

Conclusion