Climatic Signals in the Stable Isotope Composition of Precipitation in Northeast Asia

The IAEA/WHO stable isotopic data from various locations in Northeast Asia were evaluated to examine the climatic factors controlling the temporal and spatial variations in stable isotopic compositions of precipitation. To further understand these factors, stable isotopic data were obtained for individual rainfall events over a two-year period at Jeju Island, Korea. The temperature appears to be the main factor controlling the stable isotope composition of precipitation in the northwestern (inner continental) region of the study area, whereas the amount effect for summer rains is overshadowing the temperature effect in the southeastern (coastal) region. The deuterium excess values, which range from 3.0 to 40.6 per mil at Jeju Island, show a distinct seasonal variation with higher d-values in winter (>~15 per mil) and lower values in summer (~10 per mil). Such a seasonal variation appears to be closely related to two air masses with different moistures affecting the Jeju Island during different seasons. To assess the physical mechanism for this, air parcel trajectories were back calculated for precipitation events with the highest five d-values. In all five cases, the modeled trajectories show that air masses originate over northeast Asia and pass over the Yellow Sea before reaching Jeju Island. As the dry air passes over the relatively warmer water, large amounts of net evaporation occur from the sea-surface to the atmosphere that serves to increase the moisture content of the air mass. Thus, the isotopically enriched winter precipitation is a function of oceanic moisture close to Korea and Japan. In summer, the lower deuterium excess values reflect a very different climatic regime in which the air mass reflects the hot and humid characteristics of North Pacific. The trajectories of rainfall events with the lowest five d-values illustrate that the source regions for these air masses occur in the South China Sea or the tropical North Pacific Ocean. The trajectory for 5 July 1995 is representative of the movement of these air masses and shows how the air originated in the South China Sea, migrated over eastern China and the Yellow Sea before arriving at Jeju Island.