9 The Tropical Ice Core Record of ENSO

Ice core records from tropical and subtropical ice caps provide unique information about the chemical and physical character of the atmosphere. Seasonal variations in the chemical composition of the snowfall and amount of precipitation accumulating on these ice caps produce annual laminations that allow these stratigraphic sequences to be dated. The thickness of an annual lamination reflects the net accumulation, while the physical and chemical constituents (e.g., dust, 8180, various ions) record atmospheric conditions during deposition. The information presented in this chapter builds upon an earlier investigation of the preservation of an El Nino/Southern Oscillation (ENSO) history in the 1,500-year record from ice cores recovered from the Quelccaya ice cap, Peru (Thompson et al. 1992). Recent ice cores from Nevado Huascaran, Peru (90°7'S, 77°37'W, 6,048 m), which provided the first tropical ice core history extending back to the Late Glacial Stage (Thompson et al. 1995), also contain an annually resolvable record for the past 270 years. This study is based upon the most recent 68-year period from the Huascaran ice cores, from which a methodology for isolation of ENSO events is developed. The Quelccaya ENSO study (Thompson et al. 1992) revealed that in the Peruvian Andes the ice core constituent most highly correlated with sea surface temperatures (SSTs) is 8180 (r = 0.36, significant at the 95% level) . The current investigation is based upon a 68-year monthly resolved 80 time series from the col of Huascaran. 8180, as preserved on Huascaran, has been shown to reflect large-scale climatological variability over Amazonia and the western tropical Atlantic on timescales of decades to centuries (Thompson et al. 1995). Over the 25-year period (1968-93) of available climatological observations, the interannual variations of 8180 are closely related to zonal 326 Long-Term Changes in ENSO: Historical, Paleoclimatic, and Theoretical Aspects wind variations over tropical South America at the 500 hPa level. Limited evidence suggests that the spatial distribution of SST anomalies in the western tropical Atlantic influences the 500 hPa circulation, which affects the isotopic fractionation of moisture advected across Amazonia and subsequently the 8180 of Andean precipitation. A composite response to ENSO developed by using superposed epoch analysis suggests that often during ENSO warming, the moisture convergence axis over the Atlantic Ocean is diverted northward, resulting in unusually warm and dry conditions over northeast Brazil and 180 enrichment of snowfall on Huascaran. Roughly one year later, as ENSO declines, the Atlantic trade wind circulation strengthens and the associated cooler, moister conditions over the Amazon result in Andean moisture that is more depleted in 180. The Huascaran results illustrate how the ENSO signal is recorded in Andean snowfall and provide a methodology for application to other annually resolved ice core records. These include recently recovered tropical ice cores (Sajama, Bolivia) and subtropical ice cores (Dasuopu Glacier, Himalayas of China), which are introduced briefly in the final section of the chapter. The Dasuopu record is most promising in its potential to link ENSO histories from the Andes with monsoonal variability over southern Asia.