TREE-RING AND GLACIAL EVIDENCE FOR THE MEDIEVAL WARM EPOCH AND THE LITTLE ICE-AGE IN SOUTHERN SOUTH-AMERICA

A tree-ring reconstruction of summer temperatures from northern Patagonia shows distinct episodes of higher and lower temperature during the last 1000 yr. The first cold interval was from A.D. 900 to 1070, which was followed by a warm period A.D. 1080 to 1250 (approximately coincident with the Medieval Warm Epoch). Afterwards a long, cold-moist interval followed from A.D. 1270 to 1660, peaking around 1340 and 1640 (contemporaneously with early Little ke Age events in the Northern Hemisphere). In central Chile, winter rainfall variations were reconstructed using tree rings back to the year A.D. 1220. From A.D. 1220 to 1280, and from A.D. 1450 to 1550, rainfall was above the long-term mean. Droughts apparently occurred between A.D. 1280 and 1450, from 1570 to 1650, and from 1770 to 1820. In northern Patagonia, radiocarbon dates and tree-ring dates record two major glacial advances in the A.D. 1270-1380 and 1520-1670 intervals. In southern Patagonia, the initiation of the Little ke Age appears to have been around A.D. 1300, and the culmination of glacial advances between the late 17th to the early 19th centuries. Most of the reconstructed winter-dry periods in central Chile are synchronous with cold summers in northern Patagonia, resembling the present regional patterns associated with the El Nino-Southern Oscillation (ENSO). The years A.D. 1468-69 represent, in both temperature and precipitation reconstructions from tree-rings, the largest departures during the last 1000 yr. A very strong ENSO event was probably responsible for these extreme deviations. Tree-ring analysis also indicates that the association between a weaker southeastern Pacific subtropical anticyclone and the occurence of El Nino events has been stable over the last four centuries, although some anomalous cases are recognized.