Seasonal variations of precipitation δ18O in eastern Asia

Abstract

Climate change often occurs through changes in seasonal influences, but paleoclimate records rarely have intra-annual resolution. The precipitation δ18O, either directly measured or indirectly inferred from other datable materials, is one of the most widely used proxies for paleoclimate studies. It is important, therefore, that we understand the seasonal distribution of meteoric δ18O in relation to climate dynamics. In most midlatitude and high-latitude land areas, the precipitation is 18O enriched in the summer and depleted in the winter. However, this isotopic seasonality is reversed in the coastal regions of east Asia, while farther inland it returns to a summer-enriched /winter-depleted seasonal distribution. This spatial distribution of the isotopic seasonality in east Asia has been previously observed, but the mechanisms causing it have not been adequately explained. In this paper, we discuss nine climatic factors that influence isotopic seasonality, either summer-enriched or the reverse. Using a simple two-dimensional δ18O model, we show that one of the important mechanisms for the observed isotopic pattern in Asia may be interseasonal variations in vertical atmospheric stability. These variations are reflected in the large-scale vertical advection of the Walker circulation and in small-scale diffusional mixing, both of which are modulated by the greater stability of air above land relative to that above the ocean in the winter and the opposite in the summer. Our results indicate that the complex causes of interseasonal, interannual, and interepochal variations of meteoric δ18O require considerable caution in the attribution of records of past δ 18O variations to changes in specific climate variables. Copyright 2004 by the American Geophysical Union.