80 Historical Drying in the Southeast Pacific and Central-South Chile: Characterization and Attribution of Driving Causes

Monday, 23 January 2017
Juan Pablo Boisier, Universidad de Chile, Santiago, Chile; and R. Rondanelli, R. D. Garreaud, and D. Christie

Model simulations including the historical evolution and future scenarios of atmospheric greenhouse gases, stratospheric ozone concentration and other drivers of climate change show a clear rainfall decline across the subtropical southeast Pacific region (SEP). This anthropogenic effect is particularly consistent within state-of-the-art models and relates to hemispheric-scale changes in circulation, notably to the Hadley cell widening and the strengthening of the mid-high latitudes pressure gradient (a positive Southern Annular Mode-like pattern of change).

Notwithstanding the robustness of the SEP drying across multiple model data, this signal has not been detected nor attributed properly because of the lack of reliable long-term data over the South Pacific ocean. To move forward in this query, we assess the rainfall variability along the adjacent southwestern coast of South America, based on a revised dataset from about 300 rain gauge stations in central and southern Chile. Within a large interannual variability, this region has indeed experienced a precipitation decline during the last half century, in line with the model results. A persistent deficit of precipitation during the recent years (2000-2015), with no precedents in records, has further highlighted the need for a deeper understanding of internal climate variability and externally forced rainfall change in the SEP region and particularly in Chile.

The analysis of observational data, contrasted to a large ensemble of both sea surface temperature-forced and fully coupled simulations, evidences the leading but non exclusive influence of natural low frequency phenomena — the Pacific Decadal Oscillation, specifically— on short-term (~30 years) SEP rainfall trends. The same analysis indicates a very likely contribution of the anthropogenic forcing to these trends. Further, another set of simulations disentangling the climate response to increasing greenhouse gases and ozone depletion indicate a significant contribution of both drivers to the SEP precipitation trends, with a regional and seasonal distribution fairly consistent with the observations. Our results confirm then an emerging anthropogenic effect on the precipitation regime in Chile, with major implications for the country’s water resources. These results are also shown in the context of multi-century rainfall variability; based on fewer rain gauge stations with records going back to the 19th century and on dendro-cronological rainfall reconstructions, we discuss the presence and origin of long-term precipitation trends in Chile.

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