Recording of El Nino in ice core delta O-18 records from Nevado Huascaran,Peru

The 68-year monthly resolved time series of delta(18)O from ice cores retri eved from the glaciated col of Nevado Huascaran; Peru (9 degrees S, 77 degr ees W, 6050 m) reflects climate variability over Amazonia and the western t ropical Atlantic. Over the 25-year period (1968-1993) of midtroposphere obs ervations: the interannual variations in Huascaran delta(18)O relate closel y 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 ad vected across Amazonia. During El Nino warming, the moisture convergence ax is over the Atlantic Ocean is commonly diverted northward, leading to unusu al warm and dry conditions over northeast Brazil, and O-18-enriched snowfal l at Huascaran. This enrichment phase is enhanced when the peak Pacific war ming occurs during the first half of the calendar year, coincident with the wet season over Amazonia, Approximately 12 months later, the El Nino demis e is affiliated with a reprisal of strong trade wind circulation; and the r esultant cool, pluvial environment over Amazonia triggers a reversal to str ongly depleted isotope anomalies.