The A.D. 1500-1880 Little Ice Age (LIA) in the central Andean highlands of South America was the expression of sustained climatic anomalies that yielded significant environmental and ecological responses. The linkages between climatic change and glacial and ecological response over the past several centuries are being studied in the Cordillera Vilcanota of southern Peru (14°S, 71°W) to better understand the observed changes accompanying ongoing climatic warming in the region. The Vilcanota is renowned for the Quelccaya Ice Cap (5,670 m MSL), from which a 1500-year proxy record of annually resolvable climatic variation was obtained from two ice cores extracted in 1983. The ice strata provide a detailed record of climatic behavior across a range of timescales. When paired with the corresponding environmental history of the Vilcanota this chronology offers interesting inference on regional climatic variability, especially with regard to ENSO and the South American monsoon system.

Analysis reveals that Central Andean climate is characterized by distinct centennial-scale modes that switch abruptly and completely within 1-2 decades. Two highly dissimilar modes jointly yielded the LIA glacial advance: first, an excessively moist epoch lasting 150 years which caused a significant increase in snow-ice thickness and areal extent; and second, an arid and cold period of 200 years duration, during which glacial advance continued to the maximum extent reached since the Last Glacial Maximum. A third, post-LIA climatic mode has been evident since ~1880, a variable climatic regime intermediate to the two LIA modes. This contemporary pattern encompasses the instrumental climatic record, and as such, has shaped perceptions of climatic behavior and norms in the Andes.

These modal shifts must be accompanied by concomitant changes in tropospheric circulation, particularly during the wet monsoon of the austral summer. The multi-century environmental history of the Vilcanota thus suggests that present-day character and variability of the South American monsoonal system exists in just one of several modes of behavior, and that the system undergoes abrupt modal shifts even without external influences such as anthropogenic warming.