Stable isotopes in precipitation in the Asian monsoon region

Abstract

The influence of the Asian monsoon on the δ 18 O composition of precipitation is investigated on the basis of the ECHAM-4 Atmospheric General Circulation Model (AGCM), fitted with stable isotopic tracers. The model is forced with prescribed sea surface temperatures (SST) over the last few decades of the 20th century. The simulated climate and climate-stable isotope relationships are validated with observational data from the International Atomic Energy Agency-Global Network of Isotopes in Precipitation (IAEA-GNIP) and reanalysis data. The model shows deficiencies when simulating interannual variations of monsoon precipitation, but the associated monsoon circulation is quite accurately reproduced, in particular when run in a high-resolution (T106) version. The modeled stable isotope distribution is quite similar to observations, but the local climatic controls on δ 18 O are overestimated. The influence of the Asian monsoon on δ 18 O is analyzed on the basis of a vertical wind shear index M, indicative of variations in large-scale monsoon strength. The ECHAM model simulates a significant negative relationship between δ 18 O composition of precipitation and M over most monsoon-affected areas, consistent with the IAEA-GNIP data. Variations in the amount of precipitation provide a first-order explanation for this relationship. Distillation processes during transport and hence increased rainout and depletion of heavy isotopes upstream may also lead to a significant monsoon-δ 18 O relationship in areas where local precipitation is not affected by monsoon variability. The modern δ 18 O record from the Dasuopu ice core in the Himalayas is a good indicator of the large-scale monsoon circulation, a relationship that is correctly simulated by the T106 version of the ECHAM model. Our results suggest that δ 18 O variations in this region are sensitive to fluctuations in Asian monsoon intensity. Copyright 2005 by the American Geophysical Union.