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.