5. CONCLUSION
In summary, there are significant seasonal in the δD and δ18O of precipitation samples in Xiamen, which are mainly controlled by the East Asia Monsoon with significant influence of typhoon events in summer. The influence of moisture sources on δ18O values overrides the influence of precipitation fractionation process on δ18O values which leads to an inverse temperature effect in the study area. Although summer temperatures are higher than winter and high temperatures promote re-evaporation, high humidity and abundant precipitation in summer inhibited re-evaporation during the precipitation process, leading to a higher slope of the meteoric water line in summer monsoon period.
In comparison to the seasonal scale, the synoptic time-series variation of precipitation isotopes is much more complicated. There are three types of isotopic variations in the normal precipitation processes. The δ18O value of type Ⅰ might be mainly controlled by re-evaporation, whereas that of type II was less affected by re-evaporation and mainly controlled by the continuing equilibrium fractionation during condensation. The δ18O values of type III show fluctuating variations, indicating that there might be a combination of influencing factors. The local meteorological parameters during normal precipitation, which mainly control the re-evaporation process, are the dominant factors for the variation patterns of δD and δ18O, whereas moisture sources control the overall isotope values of precipitation.
The differences between the time-series of normal and typhoon-related precipitation are mainly controlled by the changes of physical processes and meteorologic parameters during the precipitation process. However, due to the unique structural and dynamic characteristics of typhoons, the impact of typhoons changes gradually as the distance between the typhoon and the study area changes, leading to three stage changes in the isotope values of typhoon-related precipitation (Xu et al., 2019). Compared with normal precipitation, although there are differences in the route and intensity of typhoons, which have significant effects on the intensity and duration of typhoon-related precipitation, the uniformity of typhoon structure leads to a similar stage change in different typhoon-related precipitations. The moisture source trajectory of typhoon-related precipitation shows a clear spiral structure (except for typhoon Yutu), and the moisture sources at different heights control the δD and δ18O values of typhoon-related precipitation. The results of correlation and linear regression analyses suggest that the temperature during typhoon-related precipitation was independent of δD and δ18O, which may be due to the small temperature variation during the same precipitation event as well as between different typhoon-related precipitation events. However, the relative humidity and pressure show significant correlation with δD and δ18O, which may be related to the significant re-evaporation effect of the first and third stages of typhoon-related precipitation. This study is important for quantifying the global changes of typhoon processes and paleotempestology reconstruction of typhoon activities.