NOAA/WDS Paleoclimatology - Andes Temperate-Mediterranean Transition 657 Year PDSI Reconstruction

The Andes Cordillera acts as regional "Water Towers" for several countries and encompasses a wide range of ecosystems and climates. Several hydroclimatic changes have been described for portions of the Andes during recent years, including glacier retreat, negative precipitation trends, an elevation rise in the 0° isotherm, and changes in regional streamflow regimes. The Temperate-Mediterranean transition (TMT) zone of the Andes (35.5°-39.5°S) is particularly at risk to climate change because it is a biodiversity hotspot with heavy human population pressure on water resources. In this paper we utilize a new tree-ring network of Austrocedrus chilensis to reconstruct past variations in regional moisture in the TMT of the Andes by means of the Palmer Drought Severity Index (PDSI). The reconstruction covers the past 657 years and captures interannual to decadal scales of variability in late spring-early summer PDSI. These changes are related to the north-south oscillations in moisture conditions between the Mediterranean and Temperate climates of the Andes as a consequence of the latitudinal position of the storm tracks forced by large-scale circulation modes. Kernel estimation of occurrence rates reveals an unprecedented increment of severe and extreme drought events during the last century in the context of the previous six centuries. Moisture conditions in our study region are linked to tropical and high-latitude ocean-atmospheric forcing, with PDSI positively related to Nino-3.4 SST during spring and strongly negatively correlated with the Antarctic Oscillation (AAO) during summer. Geopotential anomaly maps at 500-hPa show that extreme dry years are tightly associated with negative height anomalies in the Ross-Amundsen Seas, in concordance with the strong negative relationship between PDSI and AAO. The twentieth century increase in extreme drought events in the TMT may not be related to ENSO but to the positive AAO trend during late-spring and summer resulting from a gradual poleward shift of the mid-latitude storm tracks. This first PDSI reconstruction for South America demonstrates the highly significant hindcast skill of A. chilensis as an aridity proxy.

安第斯山脉（Andes Cordillera）是多个国家的区域性“水塔”，涵盖了极为丰富的生态系统与气候类型。近年来，安第斯山脉部分区域已被观测到多种水文气候变化，包括冰川退缩、降水呈负趋势、0℃等温线抬升，以及区域径流格局变化。安第斯山脉的温带-地中海过渡带（Temperate-Mediterranean transition, TMT，南纬35.5°-39.5°）尤其易受气候变化影响，因其既是生物多样性热点区域，又面临着水资源承载的沉重人口压力。本文利用新建立的智利雪松（Austrocedrus chilensis）树轮网络，借助帕默尔干旱指数（Palmer Drought Severity Index, PDSI）重建了安第斯山脉温带-地中海过渡带区域过去的湿度变化。本次重建覆盖了过去657年的时段，能够捕捉春末至初夏时节帕默尔干旱指数的年际至年代际变率特征。上述湿度变化与安第斯山脉地中海气候区与温带气候区之间的经向湿度振荡存在关联，该振荡由大尺度环流模态驱动的风暴路径的纬度位置变化所引发。对事件发生频次的核密度估计结果显示，在过去六个世纪的背景下，上个世纪的严重与极端干旱事件出现了前所未有的增长。本研究区域的湿度状况与热带及高纬度海气强迫过程紧密相关：春季帕默尔干旱指数与尼诺3.4区海表温度（Nino-3.4 SST）呈正相关关系，而夏季则与南极涛动（Antarctic Oscillation, AAO）呈显著负相关。500百帕位势高度距平图表明，极端干旱年份与罗斯-阿蒙森海区域的负高度距平存在紧密关联，这与帕默尔干旱指数与南极涛动之间的强负相关关系相一致。温带-地中海过渡带地区在二十世纪出现的极端干旱事件增多现象，可能并非与厄尔尼诺-南方涛动（ENSO）相关，而是与春末至夏季的南极涛动正趋势有关，该正趋势源于中纬度风暴路径的逐渐向极偏移。这是南美地区首次开展的帕默尔干旱指数重建研究，证实了智利雪松作为干旱代用指标具有极高的后报技能。