Re-examining Southern Hemisphere westerly wind behavior: insights from a late Holocene precipitation reconstruction using New Zealand fjord sediments

Abstract

The behavior of the Southern Hemisphere westerly winds (SWW) is inherently linked with climate variability. Understanding past SWW-climate feedbacks is key to predicting future climate change; however we are limited by a lack of SWW reconstructions, particularly from the southwest Pacific. Here we examine fjord sediments from Fiordland, New Zealand (44.5–46.5°S), to determine how the SWW have fluctuated in position and strength in response to regional late Holocene climate change, and we compare our record to Patagonia climate reconstructions in order to evaluate large-scale SWW behavior. SWW flow over the Southern Alps of New Zealand drives orographic rainfall that is recorded within fjord sediments as changes in the relative contributions of terrestrial to marine organic matter, which can be measured by bulk sediment δ 13C and organic C/N ratios. Using these proxies, cores from three New Zealand fjords show that precipitation decreased between 1400–1100 and 750–150 cal yr BP with short-duration shifts toward drier conditions at 1900 and 900 cal yr BP. Precipitation increased 2100–1400 and 1100–700 cal yr BP with a short interval of wetter conditions around 400 cal yr BP. These results indicate that westerly airflow over New Zealand was stronger (weaker) during intervals of relatively warm (cool) climate. Our comparison of precipitation reconstructions from Fiordland and Patagonia shows dissimilarities in SWW flow between regions. While to date, most mechanisms for changing the SWW have invoked purely zonal shifts, our results suggest that meridional changes in SWW flow occurred in addition to zonal shifts. In Fiordland, the meridional airflow associated with −SAM and/or El Niño-like climate phenomena may be responsible for wet intervals, while +SAM and/or La Niña-like phenomena may promote dry conditions.