NOAA/WDS Paleoclimatology - Southern Annular Mode (SAM) Index 600 Year Tree Ring Reconstruction

STUDY NOTES: Southern Annular Mode (SAM) Index reconstruction for the past 600 years based on 3000 trees from 102 sites across South America, Tasmania and New Zealand. We identified regional tree growth patterns for the temperate forests in these three regions during the past 400-600 years. Using time-varying subsets of regional chronologies as predictors, we develop two reconstructions of mean summer (December-February) SAM variations for the period 1409-2006. Two instrumental target series were selected for analysis: the SAM-Marshall (based on selected station pressure records) and the SAM-NCEP (based on mean sea-level pressure south of 20S). The first principal component (PC1) of the three regional chronologies best correlated with the predictands explains 44 and 47% of the total variance of the SAM-Marshall and SAM-NCEP instrumental records, respectively. Data set updated 28 July 2014 with addition of underlying tree-ring chronology and raw ring width data. Added files include a). villalba2012sam.xls, an Excel format file containing the SAM reconstruction and all tree-ring chronologies; b.) text data file villalba2012sam-crns.txt containing the 3 chronology predictor series; c.) 6 raw ring width tree ring data files used to develop the chronologies listed in SI_Table 2 (Regional tree-ring chronologies): villalba2012sam-araucaria.txt, villalba2012sam-austrocedrus.txt, villalba2012sam-halocarpus.txt, villalba2012sam-lagarostrobos.txt, villalba2012sam-nothofagus.txt, and villalba2012sam-phyllocadus.txt. These tree ring data make it possible to validate the SAM reconstructions starting from the regional chronologies (included on the Excel file) or from the raw data contained in the raw measurement files. This is all the data used to support the conclusions of the Nature Geoscience paper. Regional chronologies were developed by combining some of the individual chronologies listed in SI_Table 1 (Tree-ring chronologies). Those chronologies not included in the Regional chronologies can be obtained by contacting the producers listed in Source of SA_Table 1. Most of these chronologies are presently included in the International Tree-Ring Data Bank.
ABSTRACT SUPPLIED BY ORIGINATOR: Recent changes in the summer climate of the Southern Hemisphere extra-tropics are primarily related to the dominance of the positive phase of the Southern Annular Mode. This shift in the behaviour of the Southern Annular Mode - essentially a measure of the pressure gradient between Southern Hemisphere mid and high latitudes - has been predominantly induced by polar stratospheric ozone depletion. The concomitant southward expansion of the dry subtropical belts could have consequences for forest growth. Here, we use tree-ring records from over 3,000 trees in South America, Tasmania and New Zealand to identify dominant patterns of tree growth in recent centuries. We show that the foremost patterns of growth between 1950 and 2000 differed significantly from those in the previous 250 years. Specifically, growth was higher than the long-term average in the subalpine forests of Tasmania and New Zealand, but lower in the dry-mesic forests of Patagonia. We further demonstrate that variations in the Southern Annular Mode can explain 12-48% of the tree growth anomalies in the latter half of the twentieth century. Tree-ring-based reconstructions of summer Southern Annular Mode indices suggest that the high frequency of the positive phase since the 1950s is unprecedented in the past 600 years. We propose that changes in the Southern Annular Mode have significantly altered tree growth patterns in the Southern Hemisphere.