Abstract

On interannual timescales, winter and summer precipitation along the eastern and southern fringes of the Atacama Desert is usually in antiphase and associated with tropical Pacific circulation anomalies. Strong positive anomalies in the equatorial Pacific displace the ITCZ northwards, shifting the subtropical high and intensifying the subtropical jet, all of which increase the frequency of winter frontal storms in the southern Atacama. Negative anomalies weaken the subtropical jet and enhance the advection of tropical moisture across the Altiplano and onto the eastern fringes of the Atacama during austral summer. Consequently, mean annual precipitation shifts from 90% summer at 22°S to 90% winter at 26°S. Paleoenvironmental evidence is surprisingly ubiquitous along this hyper-arid gradient and offers an opportunity to evaluate phasing of seasonal precipitation and large-scale circulation anomalies on millennial timescales. Over 260 14C-dated rodent middens from the fringes of the hyper-arid Atacama Desert record vegetation responses to precipitation changes during the past 50,000 yrs. Steppe grasslands expanded into absolute desert at our low elevation southern sites at 25°30'S during the LGM, between 25 – 15 kyr, most likely due to increased winter precipitation. Plant macrofossils present in LGM-age middens from the central Atacama (22-25°S), however, indicate overall aridity. A widespread summer pluvial ensued throughout the region between 17.5 and 16 and 13.8 and 9.5 kyr. As the summer pluvial waned during the early Holocene, abrupt desiccation followed, occurring earlier in the south and later in the north over a period of ca. 1000 years. These results lead us to conclude that the winter (Westerlies) and summer rainfall (Easterlies) belts were in antiphase in northern Chile during the last termination, possibly forced by equatorial Pacific SST anomalies on multimillenial timescales during major Southern Hemisphere deglaciation.