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.