Collaborative Research: P2C2--2000 Years of Variability in the Southern Annular Mode from Tree Rings and Ice

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This project aims to use new data from King Billy pine trees (Athrotaxis selaginella) located on subalpine ridges in southern Tasmania, Australia, in combination with two high-resolution East Antarctic ice cores, to reconstruct the behavior of the Southern Annular Mode (SAM) over the last 2,000 years. This is important because the SAM influences not only Southern Hemisphere climate but also carbon and heat uptake and Antarctic ice melt.The SAM is the leading mode of climate variability in the middle to high latitudes of the Southern Hemisphere, where it affects the strength and position of the westerly winds and, therefore, precipitation and temperature anomalies across oceans and continents. The SAM controls the strength and position of the Southern Hemisphere westerlies, and therefore links large-scale climate anomalies across multiple continents and oceans. These subpolar winds also modulate the uptake of heat and carbon by the Southern Ocean and affect the cryosphere.While investigations of Northern Hemisphere climate dynamics have benefitted from a robust network of annually resolved climate proxies, the Southern Hemisphere has far more limited paleoclimate data during the Common Era, particularly in the Indian Ocean sector, the most climate data-sparse region globally. The recently discovered sub-fossil wood from King Billy pine trees provides a new opportunity for generating multi-millennial reconstructions of regional temperature and sea level pressure - key indicators of the SAM at mid-latitudes. This network of tree sites builds on and complements another emerging network of high-resolution climate records from east Antarctic ice. A new environmental reconstruction, centered on the Indian Ocean sector of the Southern Hemisphere, will help address significant uncertainties in existing SAM reconstructions from the Pacific sector and allow an assessment of the role of internal dynamics and natural forcing of Southern Hemisphere climate prior to human modifications of the atmosphere. Further, new data and reconstructions help make possible identification and quantification of uncertainty related to proxy type, location, and seasonality, and the influence of these on inferences about the timing and emergence of trends in the SAM. New environmental and climatological reconstructions also help explore what drives variability in this dynamical feature. These new reconstructions may extend existing reconstructions of SAM from the Antarctic Peninsula by at least 1,000 years. Greater understanding of the long-term variability of SAM and its links to Indian Ocean and ENSO dynamics, as well as to external forcing mechanisms (e.g ozone depletion, greenhouse gases) using data-model comparison techniques is possible. This project strengthens international collaborations between American and Australian scientists as well as across paleoclimate disciplines (tree rings and ice cores).This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.