Millenial water availability changes and teleconnections in southern South America / Variations millénaires de la disponibilité en eau et analyse des téléconnections dans le Sud de l’Amérique du Sud Étienne Boucher 1 , Joel Guiot 2 , Myriam Khodri 3 and Emmanuel Chapron 4 1 Université du Québec à Montréal, Dept. of Geography, Montréal, Québec, Canada 2 Centre Européen de Recherche et d’Enseignement des Géosciences de l’Environnement (CEREGE), UMR 6635, Aix-en-Provence, France. 3 LOCEAN, Université Pierre et Marie Curie, Paris, France. 4 Université d’Orléans, UMR-6113, Orléans, France. ABSTRACT We present the first highly resolved and spatially explicit millennial reconstruction of the summer PDSI (Palmer Drought Severity Index) in the Southern Hemisphere. Our work focuses on southern South America (hereafter named SSA, south of 20°S) and reveals strong links between the summer PDSI and the major ocean-atmospheric forcings in the area: AAO (Antarctic Oscillation), ENSO (El Niño Southern Oscillation) and PDO (Pacific Decadal Oscillation). Regime shift detection and trends in extreme wet spells / droughts are presented, discussed and compared to climate runs (IPSLCM4_V4). Both the proxy and the climate reconstruction highlight the strong teleconnections that prevail in that area. Given these results, data assimilation studies become not only possible, but appear like the next logical step. RÉSUMÉ Nous présentons la première reconstitution spatialement explicite et à haute résolution du PDSI estival (Palmer Drought Severity Index) de l’Hémisphère Sud. Notre travail cible sur le Sud de l’Amérique du Sud (SSA, Sud de 20°S) et révèle d’importantes téléconnections entre le PDSI estival et les principaux mécanismes de forcage océano-atmosphériques : AAO (Oscillation Arctique), ENSO (Oscillation Australe El-Niño) et PDO (Pacific Decadal Oscillation). L’analyse statistique des changements de régime et des tendances dans les extrêmes humides / secs est présentée, discutée, et confrontée à des sorties de modèles climatiques (IPSLCM4_V4). À la fois la reconstitution basée sur des proxies et celle basée sur un modèle climatique font ressortir les importantes téléconnections qui existent dans cette région du monde, ce qui pave la voie pour de futures études d’assimilation de données. 1 INTRODUCTION Until now, most PDSI reconstructions focused on the northern Hemisphere (Cook et al., 1999, 2004; Woodhouse and Overpeck, 1998; Woodhouse and Brown, 2001, Nicault et al., 2008, Esper et al., 2007) owing to the quality and quantity of both proxy and climate datasets in this part of the world. Over the last decades, a substantial research effort was invested in the Southern Hemisphere to develop new high-quality proxies (see Villalba et al., 2009 for a review) and produce the first climate field reconstructions (Neukom et al., 2010a and 2010b). Using this new network of proxies, we reconstructed the summer (DJF) PDSI in SSA. Our aim was to explore millennial changes in water-availability in an area where extremely dry and wet spells have among the costliest impact on the economy, ecology and society (Magrin et al., 2007). In this work, we focus on millennial regime changes and trends in extremely dry / wet spells. We discuss the potential response of the PDSI to the most important ocean-atmospheric forcings such as AAO, ENSO and PDO and compare our results with climate model runs (IPSLCM4_V4) over the common era (1000-1850). 2 DATA AND METHODS 2.1 Proxy and PDSI data The proxies used in the present study are presented on figure 1 but were listed elsewhere (see Boucher et al. 2011). High resolution proxies were mostly tree-ring series. Low frequency proxies were typically marine sediments, lake sediments and ice cores from the Andes and Antarctica. We used Dai et al. (2004) gridded (2.5°x 2.5°) PDSI data and extended it to the last millennium in SSA Figure 1. Proxy and PDSI data used in this analysis. Position of Dai et al (2004)’s 2.5 x 2.5 degrees PDSI in SSA (a). Location and type of proxies used in this reconstruction (b).