Evolution Of The Westerly Winds Belt In The Middle Latitudes Of The Southern Hemisphere Since The Last Glacial Maximum
The Southern Westerly Winds (SWW) are a symmetric component of the global climate system that govern the modern climate of all Southern Hemisphere landmasses south of ~30°S. Changes in the strength and latitudinal position of the SWW influence the precipitation patterns in the southern mid-latitudes, and have been postulated as fundamental drivers of ocean-atmospheric CO2 exchange since the Last Glacial Maximum (LGM: ~34.0-18.0 ka). Despite their role in modern and past climatic dynamics, the evolution of the SWW at locations within their zone of influence is still uncertain; this is largely because of the paucity of paleoclimate records with well constrained chronology, adequate sampling resolution and an appropriate depositional setting. Resolving these issues will help understand the behaviour of the SWW in the past at different spatial (regional and hemisphere) and temporal (centennial to multi-millennial) scales. Here I present new paleoclimate data based on the examination of detailed chronologies of fossil pollen, charcoal and chironomids preserved in lake sediments from western Patagonia: Lago Emerenciana (43°S) and Lago Pintito (52°S) and New Zealand’s southwestern South Island: Lake Von (45°S). These data, spanning a broad range of the SWW zone of influence, provide insights into the role of shifting SWW in environmental and climate dynamics of the middle latitudes of the Southern Hemisphere spanning the last ~24,000 years.
In the first study site, I performed detailed fossil pollen and charcoal analyses from sediment cores collected from Lago Emerenciana, a relatively small closed-basin lake located in northwestern Patagonian (43°S), to examine past vegetation, fire regime and climate change during the last ~24,000 years. I detect very low temperature and increased precipitation between ~24.0 and ~17.0 ka, followed by a warming trend and reduced precipitation between ~17.0 and ~14.3 ka. A cold reversal and increased precipitation regime occurred between ~14.3 and ~12.4 ka, followed by a return to warming and a slight decline in precipitation between ~12.4 and ~11.0 ka. I identify warmer temperatures and a major decline in precipitation at the beginning of the Holocene between ~11.0 and ~9.0 ka, conditions that persisted until ~6.2 ka. Centennial to millennial precipitation variability occurred during the last ~6200 years.
In the second study site, I developed high resolution fossil pollen and charcoal records, along with an exploratory chironomid record from sediment cores obtained from Lake Von, a small closed-basin lake located in the southwestern sector of the South Island of New Zealand (45°S), to examine vegetation, fire and climate trends spanning the last ~18,000 years. I observe a trend toward warming and relatively dry conditions between ~18.0 and ~14.8 ka with relatively wet conditions between ~18.0 and ~16.7 ka, increased precipitation between ~16.7 and ~14.8 ka, and cooling conditions and enhanced precipitation between ~14.8 and ~12.8 ka, followed by a marked drop in precipitation between ~12.6 and ~11.2 ka. I detect warmer and diminished precipitation between ~10.8 and ~7.2 ka, followed by lower temperature and enhanced precipitation between ~7.2 and ~3.7 ka. The mid-late Holocene is also characterised by alternating dry and wet oscillations of millennial- and centennial-scale phases with low precipitation between ~6.0 and ~5.2, ~4.4 and ~4.1, ~3.7 and ~2.9, and ~1.9 and ~0.56 ka, and increased precipitation in the intervening intervals. In the third study site, I produced high resolution fossil pollen and charcoal records from sediment cores I collected from Lago Pintito, a small and shallow closed-basin lake located in southwestern Patagonia (52°S). This record allows the detection of past vegetation, fire and hydroclimatic shifts at millennial and centennial scales over the last ~17,000 years. From these data, I identify cold and dry conditions between ~17.0 and ~16.4 ka, increased precipitation between ~16.4 and ~14.2 ka and ~12.5 and ~11.4 ka, and intense precipitation but lower in magnitude than the neighbouring intervals between ~14.2 and~12.5 ka. I detect a major decline in precipitation at the beginning of the Holocene between ~11.4 and ~6.8 ka, followed by centennial-scale changes in precipitation until the present.
The comparison between precipitation variability reconstructed from the records from western Patagonia (Lago Emerenciana and Lago Pintito) and New Zealand’s southwestern South Island (Lake Von) allows the inference of SWW changes at a hemispheric scale during and since the LGM, based on the premise that there is a strong and positive correlation between zonal wind speeds and local precipitation in these regions. The results of this thesis suggest: i) strong SWW influence at 43°S between ~24.0 and ~17.5 ka, ii) a southward shift of the SWW between ~17.5 and ~16.5 ka and reduced SWW influence north of 52°S, iii) strengthening and/or a northward shift of the SWW between ~16.5 and ~ 14.5 ka, with strong SWW influence between 52°S and 43°S, iv) a northward shift of the SWW between ~14.5 and ~12.6 ka which resulted in stronger SWW influence between 43°S and 46° S and weaker SWW influence at 52°S, v) a southward shift of the SWW between ~12.6 and ~11.2 ka leading to weaker SWW influence between 43°S and 46°S and stronger SWW influence at 52°S, vi) a generalized multi-millennial decline in the strength of the SWW between ~11.2 and ~7.2 ka, and vii) high variability in the SWW in Western Patagonian and New Zealand’s southwestern South Island during the last ~7200 years. Based on these findings, I postulate that hemisphere-wide changes in the position and/or strength of the SWW have modulated the atmospheric CO2 concentration through wind-driven upwelling of CO2-rich deep waters in the high southern latitudes during and since the LGM.