Six hundred years of South American tree rings reveal an increase in severe hydroclimatic events since mid-20th century

South American (SA) societies are highly vulnerable to droughts and pluvials, but lack of long-term climate observations severely limits our understanding of the global processes driving climatic variability in the region. The number and quality of SA climate-sensitive tree ring chronologies have significantly increased in recent decades, now providing a robust network of 286 records for characterizing hydroclimate variability since 1400 CE. We combine this network with a self-calibrated Palmer Drought Severity Index (scPDSI) dataset to derive the South American Drought Atlas (SADA) over the continent south of 12°S. The gridded annual reconstruction of austral summer scPDSI is the most spatially complete estimate of SA hydroclimate to date, and well matches past historical dry/wet events. Relating the SADA to the Australia–New Zealand Drought Atlas, sea surface temperatures and atmospheric pressure fields, we determine that the El Niño–Southern Oscillation (ENSO) and the Southern Annular Mode (SAM) are strongly associated with spatially extended droughts and pluvials over the SADA domain during the past several centuries. SADA also exhibits more extended severe droughts and extreme pluvials since the mid-20th century. Extensive droughts are consistent with the observed 20th-century trend toward positive SAM anomalies concomitant with the weakening of midlatitude Westerlies, while low-level moisture transport intensified by global warming has favored extreme rainfall across the subtropics. The SADA thus provides a long-term context for observed hydroclimatic changes and for 21st-century Intergovernmental Panel on Climate Change (IPCC) projections that suggest SA will experience more frequent/severe droughts and rainfall events as a consequence of increasing greenhouse gas emissions

The College News, 1923-01-24, Vol. 09, No. 13

Bryn Mawr College student newspaper. Merged with The Haverford News in 1968 to form the Bi-college News (with various titles from 1968 on). Published weekly (except holidays) during the academic year

Environmental correlates of stem radius change in the endangered Fitzroya cupressoides forests of southern Chile

Relationships between environmental factors and stem radius variation at short temporal scales can provide useful information regarding the sensitivity of tree species' productivity to climate change. This study used automatic point dendrometers to assess the relationship between environmental variables and stem radius contraction and increment in ten Fitzroya cupressoides trees growing in two sites, the Coastal Range (Alerce Costero National Park) and the Andean Cordillera (Alerce Andino National Park) of southern Chile. The growing season in each site, determined using stem daily cycle patterns for each month, was longer in the Coastal Range site than in the Andes. Warmer and sunnier conditions were positively related with daytime tree radius contraction in both areas, and relationships were stronger in the Coastal Range site where more pronounced shrinking events were associated with prolonged warm and dry conditions compared to the Andes. Stem increment was positively related with precipitation and humidity in both sites, reflecting the positive effect of water on cell turgidity and consequent enlargement. Relationships between stem radius change and environmental variables considering longer temporal scales (7 to 31 days), confirmed a stronger association with humidity/vapor pressure deficit and precipitation, rather than with temperature. Although Fitzroya grows in particularly wet and cool areas, current and projected drier and warmer summer conditions in southern Chile may have a negative effect on Fitzroya stem increment and carbon accumulation in both sites. This effect would be more critical in the Coastal Range compared with the Andes though, due in part to more limiting soil conditions and less summer precipitation in this area. Long-term research is needed to monitor different aspects of the response of these endangered ecosystems to this additional threat imposed by climate change

Streamflow variability in the Chilean Temperate-Mediterranean climate transition (35\ub0S\u201342\ub0S) during the last 400\ua0years inferred from tree-ring records

As rainfall in South-Central Chile has decreased in recent decades, local communities and industries have developed an understandable concern about their threatened water supply. Reconstructing streamflows from tree-ring data has been recognized as a useful paleoclimatic tool in providing long-term perspectives on the temporal characteristics of hydroclimate systems. Multi-century long streamflow reconstructions can be compared to relatively short instrumental observations in order to analyze the frequency of low and high water availability through time. In this work, we have developed a Biobío River streamflow reconstruction to explore the long-term hydroclimate variability at the confluence of the Mediterranean-subtropical and the Temperate-humid climate zones, two regions represented by previous reconstructions of the Maule and Puelo Rivers, respectively. In a suite of analyses, the Biobío River reconstruction proves to be more similar to the Puelo River than the Maule River, despite its closer geographic proximity to the latter. This finding corroborates other studies with instrumental data that identify 37.5°S as a latitudinal confluence of two climate zones. The analyzed rivers are affected by climate forcings on interannual and interdecadal time-scales, Tropical (El Niño Southern Oscillation) and Antarctic (Southern Annular Mode; SAM). Longer cycles found, around 80-years, are well correlated only with SAM variation, which explains most of the variance in the Biobío and Puelo rivers. This cycle also has been attributed to orbital forcing by other authors. All three rivers showed an increase in the frequency of extreme high and low flow events in the twentieth century. The most extreme dry and wet years in the instrumental record (1943–2000) were not the most extreme of the past 400-years reconstructed for the three rivers (1600–2000), yet both instrumental record years did rank in the five most extreme of the streamflow reconstructions as a whole. These findings suggest a high level of natural variability in the hydro-climatic conditions of the region, where extremes characterized the twentieth century. This information is particularly useful when evaluating and improving a wide variety of water management models that apply to water resources that are sensitive to agricultural and hydropower industries.Fil: Muñoz, Ariel A.. Pontificia Universidad Católica de Valparaíso; ChileFil: González Reyes, Alvaro. Universidad de Chile; ChileFil: Lara, Antonio. Universidad Austral de Chile; ChileFil: Sauchyn, David. University of Regina; CanadáFil: Christie, Duncan. Universidad Austral de Chile; ChileFil: Puchi, Paulina. Pontificia Universidad Católica de Valparaíso; ChileFil: Urrutia Jalabert, Rocío. Universidad Austral de Chile; ChileFil: Toledo Guerrero, Isadora. Pontificia Universidad Católica de Valparaíso; ChileFil: Aguilera Betti, Isabella. Pontificia Universidad Católica de Valparaíso; ChileFil: Mundo, Ignacio Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Sheppard, Paul R.. University of Arizona; Estados UnidosFil: Stahle, Daniel. University of Arkansas for Medical Sciences; Estados UnidosFil: Villalba, Ricardo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Provincia de Mendoza. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales. Universidad Nacional de Cuyo. Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales; ArgentinaFil: Szejner, P.. University of Arizona; Estados UnidosFil: LeQuesne, Carlos. Universidad Austral de Chile; ChileFil: Vanstone, Jessica. University of Regina; Canad