The 68-year monthly resolved time series of δ18O from ice cores retrieved from the glaciated col of Nevado Huascarán, Peru (9°S, 77°W, 6050 m) reflects climate variability over Amazonia and the western tropical Atlantic. Over the 25-year period (1968–1993) of midtroposphere observations, the interannual variations in Huascarán δ18O relate closely with the zonal wind variations over tropical South America at the 500 hPa level. Additionally, there is some evidence that the spatial distribution of temperature anomalies in the western tropical Atlantic has influence on the 500 hPa circulation and hence the isotopic fractionation of moisture advected across Amazonia. During El Niño warming, the moisture convergence axis over the Atlantic Ocean is commonly diverted northward, leading to unusual warm and dry conditions over northeast Brazil, and 18O-enriched snowfall at Huascarán. This enrichment phase is enhanced when the peak Pacific warming occurs during the first half of the calendar year, coincident with the wet season over Amazonia. Approximately 12 months later, the El Niño demise is affiliated with a reprisal of strong trade wind circulation, and the resultant cool, pluvial environment over Amazonia triggers a reversal to strongly depleted isotope anomalies.