Tree-ring evidence for long-term precipitation changes in subtropical South America

In recent years there has been a notable increase in the number of tree-ring chronologies for the temperate and cold regions of the Americas. In comparison, few advances have been reported for the American tropics and subtropics, where the absence of seasonality appears to be the main reason for the lack of well-defined growth bands in most species. Distinct, annually formed tree-rings have recently been reported for subtropical montane trees on the eastern slope of the Andes (22-28 degrees S). Six absolute-dated chronologies from Juglans australis and Cedrela lilloi at the upper treeline (between 1700 and 2000 m) in the montane forest of north-western Argentina were selected to explore the potential of these records to infer decade- to century-scale climatic variations in the subtropics. These tree-rings capture a significant percentage of the variances in regional temperature and precipitation records and appear to be suitable to reconstruct decade-long changes in large-scale circulation over the South American subtropics. In particular, tree-growth at xeric sites has been strongly influenced by precipitation changes, which in turn respond to alternating patterns of zonal versus meridional flows over subtropical South America. The upper treeline records indicate that the increase in precipitation during the past three decades, caused by an enhanced transport of humid air masses from the Brazilian-Bolivian lowland tropics to the semiarid subtropics, has been unprecedented for the past 200 years. Although this precipitation increase may reflect natural variability in the subtropics, it is also consistent with 2 x CO2 climatic simulations from five general circulation models. There is a general agreement among model results about a noticeable increase in precipitation in north-western Argentina due to an intensification of the water transport across subtropical South America in response to a southward displacement of the continental low and an increasing warming at these latitudes. A larger network of tree-ring chronologies will aid efforts to understand long-term climatic interactions between tropical and subtropical regions in South America, and shed light on the pole of natural versus anthropogenic forcings on regional climatic changes. (C) 1998 Royal Meteorological Society.