Origin and recycling processes of precipitating water over the Eurasian continent: Experiments using an atmospheric general circulation model

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Abstract
By using an atmospheric general circulation model, origin and transport processes of water in the atmosphere‐land system are examined. The water vapor and the land‐surface water in the model are tagged according to the geographical regions of water input (evaporation) and are separately treated. The results are examined focusing on the water cycle over the Eurasian continent. In the winter season, most of the precipitating water over the Eurasian continent is supplied by evaporation from the oceans. On the other hand, the precipitating water in the summer is mostly supplied by the evaporation from the continental surface, indicating active recycling of water between the atmosphere and the land in this season. Considering that the water in the continental surface should be supplied from the oceans sometime before, the history of the water from its origin (evaporation from the oceans) is examined, by separately treating the components of the soil water and the snow according to the geographical regions of the origin. Two additional types of tracers are included in order to determine the timescale of the transport and the frequency of the recycling between the atmosphere and the land. The results show that the main origin of water in the northern part of the Eurasian continent is the Northern Atlantic Ocean and that in the southern part is the Northern Indian Ocean. In the southern part the mean age of the precipitating water since its origin is 1 month or shorter, and the mean count of recycling is less than one, indicating that the water comming directly from the oceans by atmospheric transport is dominant. In the northern inland part in summer, however, the mean age is 3 months or longer, and the mean count of recycling is above two. These results suggest that a significant portion of the precipitating water in inland Eurasia in the summer originals in the Atlantic Ocean in the previous winter and is transported eastward with a few recycling cycles between the atmosphere and the continental surface.