Abstract
The Southern Hemisphere westerly (SWW) winds are a strong, zonally symmetric wind belt that directly affect hydrology and carbon cycling in the middle to high latitudes. In order to determine the response of the wind belt to anthropogenic climate change, a better understanding of the past variability in SWW circulation, including strength and location, is critical. The strength and location of the SWW is influenced by large-scale climate variability, such as the Southern Annular Mode (SAM), which is the dominant mode of atmospheric variability in the Southern Hemisphere and influences atmosphere, ocean, and sea ice dynamics across the southern latitudes.
In the southwest Pacific Ocean, there is a strong correlation between hydrology and SWW strength and location, and there are a variety of sedimentary environments where climate records can be developed over a variety of temporal scale, including lakes, peatlands, fjords, and deep marine basins. Here, I present three records of paleoenvironmental change collected along a transect through the modern wind belt that provide well-constrained high-resolution reconstructions of past climate from the northern margin, core, and southern margin of the modern wind belt. In addition, this compilation of climate records provides an opportunity to reconstruct latitudinal shifts in the wind belt through time.
At the northern edge of the wind belt in New Zealand, the water level at Lake Von (45°S), which is topographically closed, is affected by variability in westerly wind derived precipitation over the Southern Alps. The continuous well-constrained record developed from the lake depocenter provides the first evidence of centennial-scale hydrological changes in the South Island of New Zealand during the Late Quaternary. Low lake levels caused by weaker westerlies and reduced precipitation occur between 3.2 - 6.4 and 8.4 - 11.6 ka cal BP, while high organic matter input between 6.4 - 8.4 and 11.6 - 12.6 ka cal BP suggests higher lake levels as a result of stronger westerly flow and increased delivery of precipitation to the lake.
The subantarctic Auckland Islands (50.5°S) are located in the core of the modern wind belt and a sediment core collected from a fjord ingression basin has evidence for four different depositional environments over the last ~19 ka cal BP: deglacial, lacustrine, transgressive, and marine. Multiple geochemical proxies in the lacustrine sediment deposited during the Late Glacial and early Holocene provide evidence for physical changes in lake circulation and precipitation amount as a result of changes in SWW strength. Water column mixing and influx of terrestrial matter from the catchment are indicative of strong winds between 11 and 15.7 ka cal BP, while reduced wind strength during the early Holocene (8.7 to 11 ka cal BP), drives centennial scale intervals of water column stratification and lake bottom-water anoxia.
Located south of the southern margin of the SWW, the ocean-atmosphere-sea ice system during the early Holocene in Moubray Bay, northwest Ross Sea (72°S) is influenced by the phase of the SAM, as opposed to the SWW. A high-resolution well-constrained record provides evidence for changes in high latitude atmosphere and ocean circulation between 9.4 and 11.6 ka cal BP, which drove upwelling of Circumpolar Deepwater (CDW) onto the Ross Sea continental shelf that contributed to rapid retreat of the Ross Ice Shelf after ~11 ka cal BP. CDW was upwelling under a stabilised or stratified water column that formed due to low surface winds and high meltwater input between 10.2 and 11.6 ka cal BP, but after 10.2 ka cal BP increased flow of cold meridional winds off the continent cooled the sea surface and drove sea ice expansion across the Ross Sea.
Synchronous hemisphere-wide change in climate during the early Holocene is observed in the three paleoclimate records I present here and in multiple records developed from New Zealand, subantarctic, Antarctica, and southern South America. I interpret that this is due to contraction and southward shift in the westerly wind belt to a narrow zone south of 54°S, with the zone of maximum wind stress located around 60°S. Poleward migration of the SWW is due to +SAM-like circulation during the early Holocene that affected ocean upwelling, atmospheric and sea surface temperatures, and the efficiency of the Southern Ocean carbon cycle. The correspondence between records that span multiple latitudes and longitudes during the early Holocene suggests that centennial to millennial scale changes in large-scale climate variability (SAM) simultaneously affects climate across the Southern Hemisphere. The past link between SWW and SAM is still poorly reconstructed and additional paleoclimate records at centennial-scale resolution are required to determine if the mode of SAM had as significant an influence on SWW in the past at it does today.