FORSGC Network for isotopes in precipitation over the Monsoon Asia

Full text: The GNIP project provides the monthly data of stable isotopes in precipitation all over the world. For the validation of atmospheric water cycle by reanalysis datasets or GCM simulations, however, there are a few stations in Asia. Also, spatial and temporal variability of the stable isotope composition of precipitation in the Asia Pacific region was discussed based on the GNIP monthly dataset, and regional climatology and atmospheric circulation patterns. However, the relationship between isotope signature of precipitation and climate in tropics is not well understood, because the daily isotopic data in precipitation is limited. Since 2000, the isotope group of the Frontier Observational Research System for Global Change (FORSGC) had been collecting the daily precipitation samples for stable isotopes over the Monsoon Asia, such as Siberia (3 stations), Tibetan Plateau (10 stations), Nepal (5 stations), Thailand (3 stations), Indonesia (5 stations), and Palau Island (1 station). In this study, we will introduce the FORSGC network for stable isotopes in precipitation database and show the short-term (monthly and daily) variability of precipitation isotopes observed in Thailand and Indonesia and so on. The daily rainfall samples were collected during August to December 2001 at ChiangMai, Bangkok and Phuket stations in Thailand. These stations showed similar monthly variability in Oxygen-18, and most ranged from -15 per mille to 0 per mille. There were increasing trends in August and November, while decreasing trend in September. Daily variability, however, showed a different trend. A depletion of early-August at ChiangMai and a sudden enrichment around 10 October at Phuket were appeared. Considering relationships between Oxygen-18 and precipitation amount, there were positive correlations in three stations by the monthly basis. In general, isotopic compositions are heavy in low precipitation and light in high precipitation caused by the amount effect. The precipitation was small from August to early September and large from late September to early November. There was some correlations between δ18O and precipitation if daily precipitation was less than 30mm, however, there was not significant if that was more than 30mm. Also, the daily rainfall samples were collected during April to December in 2001 at Bukittinggi and Jambi stations which located western and eastern part of Sumatra Island, Indonesia. Most Oxygen-18 ranged from -15 per mille to 0 per mille in both stations. Only Jambi station showed seasonal variability trend, that is, enrichment and depletion trends from April to August and from August to October, respectively. From November to December, there was depletion trend in both stations. The d-excess was ranged 10 to 15 permil in Bukittinggi, whereas sometimes ranged below 5 in Jumbi. This fact indicates the origin of rain water was much different between these two places. The daily rainfall variability was much different between them. Furthermore, the time series in δ18O and the origin of rain water was simulated using Isotope Circulation Model (ICM). The temporal and spatial resolution is daily time-scale and 2.5 degrees in latitude and longitude, respectively. Notice that this model devises to integrate all vertical atmospheric- and isotopic-physics in each grid. When water transit its phase among solid, liquid and gas, the evaluation of the isotopic composition is described by the Rayleigh distillation process. The atmospheric water budgets were calculated by using the variables from NCEP/NCAR renalysis. From the preliminary result of the ICM simulation, the time series of δ18O in two places were almost same pattern during May to December in 2001. However, the origin of rain water was much difference between these two places. Most of rain water in Bukittinggi was originated from the Indian Ocean through whole year. From a half to two third of rain water in Jumbi was originated from the Indian Ocean, and the remain was from Java Island and Java Sea. The mixing ratio of dif ferent original water was changed weekly or smaller time-scale. This results coincides with the observed d-excess variations. (author)