• Abstract and Figures
• Public Full-text

Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Clim. Past Discuss., 11, 2159–2180, 2015 www.clim-past-discuss.net/11/2159/2015/ doi:10.5194/cpd-11-2159-2015 CPD © Author(s) 2015. CC Attribution 3.0 License. 11, 2159–2180, 2015 This discussion paper is/has been under review for the journal Climate of the Past (CP). A 250 year periodicity Please refer to the corresponding final paper in CP if available. in Southern Hemisphere westerly A 250 year periodicity in Southern winds over the last 2600 years Hemisphere westerly winds over the last C. Turney et al. 2600 years Title Page C. Turney1, R. Jones2, C. Fogwill1, J. Hatton2, A. N. Williams3,4, A. Hogg5, 1 1 1 Z. Thomas , J. Palmer , and S. Mooney Abstract Introduction 1School of Biological, Earth and Environmental Sciences, University of New South Wales, Conclusions References Sydney, Australia Tables Figures 2Department of Geography, Exeter University, Devon, EX4 4RJ, UK 3Fenner School of Environment and Society, The Australian National University, Canberra, ACT 0200, Australia J I 4Archaeological and Heritage Management Solutions Pty Ltd, 2/729 Elizabeth Street, J I Waterloo, NSW 2017, Australia 5Waikato Radiocarbon Laboratory, University of Waikato, Private Bag 3105, Back Close Hamilton, New Zealand Full Screen / Esc Received: 24 April 2015 – Accepted: 8 May 2015 – Published: 3 June 2015 Correspondence to: C. Turney ([email protected]) Printer-friendly Version Published by Copernicus Publications on behalf of the European Geosciences Union. Interactive Discussion 2159 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Abstract CPD Southern Hemisphere westerly airflow has a significant influence on the ocean– atmosphere system of the mid- to high-latitudes with potentially global climate impli- 11, 2159–2180, 2015 cations. Unfortunately historic observations only extend back to the late nineteenth 5 century, limiting our understanding of multi-decadal to centennial change. Here we A 250 year periodicity present a highly resolved (30 yr) record of past westerly air strength from a Falkland in Southern Islands peat sequence spanning the last 2600 years. Situated under the core latitude Hemisphere westerly of Southern Hemisphere westerly airflow, we identify highly variable changes in exotic winds over the last pollen derived from South America which can be used to inform on past westerly air 2600 years 10 strength and location. The results indicate enhanced airflow over the Falklands be- tween 2000 and 1000 cal. yr BP, and associated with increased burning, most probably C. Turney et al. as a result of higher temperatures and/or reduced precipitation, comparable to records in South America. Spectral analysis of the charcoal record identifies a 250 year period- icity within the data, suggesting solar variability has a modulating influence on Southern Title Page 15 Hemisphere westerly airflow with potentially important implications for understanding Abstract Introduction global climate change through the late Holocene. Conclusions References 1 Introduction Tables Figures A major limitation for quantifying the magnitude and impact of change across the J I Southern Ocean is the relatively short duration or low resolution of ocean–atmosphere J I 20 records. This is particularly significant with regards the Southern Hemisphere west- erly storm belt, which since the mid-1970s, has undergone a significant intensification Back Close and southward shift (Gillett et al., 2008; Messié and Chavez, 2011). One measure of Full Screen / Esc this change in atmospheric circulation is the Southern Annular Mode (SAM), described ◦ as the pressure difference between Antarctica (65 S) and the latitude band at around Printer-friendly Version ◦ 25 40 S (Karpechko et al., 2009; Marshall, 2003). Since the mid-1970s SAM appears to have undergone a positive shift in the troposphere, which has been associated with Interactive Discussion 2160 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | hemispheric-wide changes in the atmosphere–ocean-ice domains, including the deliv- ery of precipitation and significant surface and subsurface warming (Cook et al., 2010; CPD Delworth and Zeng, 2014; Domack et al., 2005; Gille, 2008, 2014; Thompson et al., 11, 2159–2180, 2015 2011). This trend is projected to continue during the 21st century as a result of both 5 ongoing greenhouse gas emissions and a persistence of the Antarctic ozone hole (Liu and Curry, 2010; Thompson et al., 2011; Yin, 2005), potentially resulting in reduced A 250 year periodicity Southern Ocean uptake of anthropogenic CO2 (Ito et al., 2010; Le Quére et al., 2009; in Southern Lenton et al., 2013; Marshall, 2003; Marshall and Speer, 2012). Hemisphere westerly While no observational records for SAM extend beyond the late nineteenth century winds over the last 10 (Fogt et al., 2009; Marshall, 2003; Visbeck, 2009), proxy records of past westerly air- 2600 years flow have been generated on annual to centennial timescales through the Holocene (Abram et al., 2014; Björck et al., 2012; Lamy et al., 2010; Strother et al., 2015; Villalba C. Turney et al. et al., 2012). Crucially the association between proxies and changes in westerly wind strength and/or latitude is often implied but few provide a direct measure of past airflow Title Page 15 or directly test their interpretation through time. One possibility is the identification of exotic airborne pollen preserved in sedimentary sequences. Ideally, the peat or lake Abstract Introduction record should be close enough to the source vegetation to have a relatively high in- Conclusions References put of pollen but not so close that the influx is constant over time. Whilst numerous studies have been undertaken in the Arctic (Fredskild, 1984; Jessen et al., 2011) and Tables Figures 20 the high-latitudes of the Indian and Pacific oceans (McGlone et al., 2000; Scott and van Zinderen Barker, 1985), few have been reported from the south Atlantic. Recent J I work on a lake core taken from Annekov Island, South Georgia (Strother et al., 2015) demonstrates the considerable potential of this approach but the relatively large dis- J I tance from the nearest source in South America (Fig. 1) (approximately 2100 km) limits Back Close 25 the delivery of pollen. Full Screen / Esc Here we report a new high-resolution record of westerly airflow over the past 2600 years from the Falkland Islands. The Falkland Islands (52◦ S) lie west of Annekov Printer-friendly Version Island and 500 to 730 km east of Argentina within the main latitudinal belt of Southern Hemisphere westerly airflow. The close proximity to South America means that these Interactive Discussion 2161 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | islands receive a relatively high input of exotic pollen (Barrow, 1978), making them an ideal location to investigate past changes in westerly airflow. CPD 11, 2159–2180, 2015 2 Methods A 250 year periodicity The Falkland Islands are a low-lying archipelago in the South Atlantic Ocean, situated in Southern 5 in the furious fifties wind belt on the southeast South American continental shelf at ◦ ◦ Hemisphere westerly 51–52 S, 58–61 W (Fig. 1). The Falkland Islands experience a cool temperate but rel- winds over the last atively dry oceanic climate, dominated by westerly winds (Otley et al., 2008). Across ◦ ◦ 2600 years the year, the temperature ranges from 2.2 C (July) to 9 C (February), with the islands experiencing a relatively low but variable precipitation (typically ranging between 500 C. Turney et al. −1 10 and 800 mmyear ) lying in the lee of the Andes to the west (Lister and Jones, 2014). In the modern climate the prevailing wind direction across the Falkland Islands is predom- inantly from the west with strong winds throughout the year and no significant seasonal Title Page variation (Upton and Shaw, 2002). To investigate past westerly airflow an exposed Ericaceous-grass peatland was Abstract Introduction ◦ ◦ 15 cored above Port Stanley Airport (51.691 S, 57.785 W, approximately 30 ma.s.l.) Conclusions References (Fig. 1). The one metre sequence reported here comprises a uniform dark-brown peat sequence from which the uppermost 90 cm was contiguously sampled for pollen, char- Tables Figures coal and comprehensive dating. Pollen samples were prepared using standard palynological techniques (Faegri and J I 20 Iverson, 1975). Volumetric samples were taken every 1 cm along the core and Ly- J I copodium spores were added as a “spike”. The samples were deflocculated with hot 10 % NaOH and then sieved through a 150 µm mesh. The samples then underwent Back Close acetolysis, to remove extraneous organic matter before the samples were mounted Full Screen / Esc in silicon oil. Pollen types/palynomorphs were counted at 400 × magnification until 25 a minimum of 300 target grains were identified. The pollen counts were expressed Printer-friendly Version as percentages, with only terrestrial land pollen (TLP) contributing to the final pollen Interactive Discussion sum. Pollen/Palynomorphs were identified using standard pollen keys (Barrow, 1978; 2162 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Macphail and Cantrill, 2006) and the pollen type slide collection at Exeter University. Past fire activity was assessed using micro-charcoal counts of fragments identified on CPD the pollen slides e.g. (Whitlock and Larsen, 2001). Counts were undertaken at each 11, 2159–2180, 2015 level until a fixed total of 50 lycopodium spores was counted and the total expressed 3 5 as a concentration (fragments per cm ). The results were sub-divided into two size classes: < 50 and 50–150 µm. A 250 year periodicity Terrestrial plant macrofossils (fruits and leaves) were extracted from the peat se- in Southern quence and given an acid-base-acid (ABA) pretreatment and then combusted and Hemisphere westerly graphitized in the University of Waikato AMS laboratory, with 14C=12C measurement by winds over the last 10 the University of California at Irvine (UCI) on a NEC compact (1.5SDH) AMS system.

Load more

  22

Copy Link