Have the Southern Westerlies changed in a zonally symmetric manner over the last 14,000 years? A hemisphere-wide take on a controversial problem

Abstract

The prevailing view in the palaeoclimate literature of the last 20 years is that the Southern Westerly Winds (SWW) were intensified over southern Australia and Tasmania during the warmer-than-present early Holocene (11e8 ka). At similar latitudes on the opposite side of the southern mid-latitudes, palaeoclimate studies have suggested a poleward shift of the northern edge of the westerlies and focusing at 49 S in southern South America. This zonal asymmetry contrasts with the modern day zonal symmetry displayed by the SWW and poses a formidable challenge to an understanding of the modes of climatic variability of the southern extra-tropics. This paper presents an updated synthesis of continuous, radiocarbon-dated palaeoenvironmental data from the westerlies zone of influence in all Southern Hemisphere continents. Synchronous multi-millennial trends in moisture, vegetation, fire, and hydrologic balance are remarkably consistent with the way the SWW changes impact upon the climate in Southern Hemisphere landmasses in the modern climate. Considering the modern relationships between local precipitation and zonal wind speeds, it is suggested that the SWW changed in a zonally symmetric manner at multi-millennial scale between 14 and 5 ka. Regional asymmetry develops after 5 ka across the Southern Hemisphere, with a pattern of precipitation anomalies akin to the modern functioning of El Niño e Southern Oscillation, which started ~6200 years ago.Institute of Ecology and Biodiversity, Chile, and Fondecyt 311018

Climatic records over the past 30 ka from temperate Australia – a synthesis from the Oz-INTIMATE workgroup

“…For example, it is likely that precipitation was not a limiting factor for the development of broad vegetation types (i.e. forest, grassland, alpine, etc) in the high rainfall mountainous zone of western Tasmania throughout the last 30 ka (Colhoun et al 1999), responding, rather, to temperature fluctuations (Colhoun et al, 1999;Fletcher and Thomas 2010a) and fire (Fletcher and Thomas, 2010b), with precipitation changes through this time implicated in compositional changes within vegetation types (Fletcher and Moreno, 2011;Fletcher and Moreno, 2012). Conversely, records on the mainland, where moisture is and has been a limiting factor for vegetation, pollen records are likely to have responded more strongly to changes in moisture availability (i.e.…”

Constant wind regimes during the Last Glacial Maximum and early Holocene: evidence from Little Llangothlin Lagoon, New England Tablelands, eastern Australia

“…It plays an important role in the delivery of winter rainfall to the southern half of Australia. Understanding the history of the westerlies in the Australasian region is therefore important for understanding the climate and environmental history of eastern Australia (Shulmeister et al, 2004;Fletcher and Moreno, 2012;Lorrey et al, 2012). In addition, changes in the Southern Hemisphere westerlies are inferred to modulate global atmospheric carbon dioxide concentrations and potentially trigger global climate changes (e.g.…”

Precipitation linked to Atlantic moisture transport: clues to interpret Patagonian palaeoclimate

“…Combined seismic and sedimentological investigations of the lake basin have provided a record of the lake-level changes for the Late Pleistocene and Holocene (Markgraf et al 2003, Gilli et al 2001, Haberzettl et al 2005, Anselmetti et al 2009). Additionally, the lake-level record coupled with other palaeoclimatic records located in Patagonia revealed substantial climate changes in the region (Ariztegui et al 2010, Fletcher & Moreno 2012, Kilian & Lamy 2012, Zolitschka et al 2013. The water level of Lago Cardiel relative to the modern depth (73 m) is shown in Fig.…”

“…It is an interpretative climatic tool that serves to help understand the time variations in climatic variables over Patagonia from the knowledge of changes in the position and intensity of the mid-latitude SH westerlies, and as such it has been widely used in the palaeoclimatic literature (Fletcher & Moreno 2012). The emphasis of this study was placed on Lago Cardiel and its surroundings, in central Eastern Patagonia, since it is an ideal place which combines key factors to analyse present and past climates.…”

“…Thus, from the combination of proxy-data and modelling, past environments are reconstructed using the westerlies mechanism. Furthermore, a recent update on the palaeoclimatic southern westerlies variability by Fletcher & Moreno (2012) concluded that the northerly (southerly) displacement has little, if any, effect on the mean latitudinal position of the westerlies relative to today, dealing with their functioning and impact on the mid-latitude climate 'is still a matter of discussion and consensus remains elusive ' (p. 33), and, thus, it 'poses a formidable challenge to an understanding of the modes of climatic variability of the southern extratropics' (p. 1, Abstract). The current palaeoclimatic literature virtually excludes the possible role of winds blowing westward from the South Atlantic.…”