The deuterium excess (d-excess) of precipitation aids in identifying vapor source regions because it reflects humidity conditions in those regions. For Japan, studies have assumed that the Sea of Japan was the dominant source of winter precipitation when the d-excess value in winter is >20‰ or higher than the average value in summer. Because this assumption is based on an interpretation that the high d-excess value is due to an interaction between the continental winter monsoon (WM) and warm Sea of Japan, it may not be appropriate for winter precipitation due to extratropical cyclones (ECs). Here we clarify local patterns of isotopic composition and the origins of precipitation in WM and EC types over Japan using a regional isotope circulation model. The results indicate that the Sea of Japan made the highest contribution to precipitation on the Sea of Japan side in the WM type, whereas the Pacific Ocean is the dominant source of precipitation over Japan in the EC type. Because d-excess values are higher in the WM type than the EC type, we can assume that the Sea of Japan is the dominant source of precipitation on the Sea of Japan side when the d-excess value is high. In comparison, we cannot identify the source regions from d-excess values alone for Honshu Island bordering the Pacific Ocean, because the difference in the d-excess value between the WM and EC types is unclear. Moreover, WM variability can be estimated from observed d-excess values due to their clear positive correlation.
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