1. INTRODUCTION
The isotopes of hydrogen (H, 2H or D,3H) and oxygen (16O,17O, 18O) are distributed in different phases by different ratios during the water cycle due to isotope fractionation (Sun et al., 2020). Atmospheric precipitation is one of the most crucial parts of the global water cycle. When the water vapor condenses, the heavier isotopes (D and 18O) preferentially enter into the liquid water, resulting in a positive hydrogen and oxygen isotopic composition (δD and δ18O) of rainwater at the beginning of the rainfall, and then the δD and δ18O value of precipitation gradually decreases with the condensation of D and 18O (Gedzelman et al., 2001). As climate changes, the δD and δ18O in atmospheric precipitation also show spatial and temporal variations (Cai et al., 2018; Ansari et al., 2020). Therefore, the δD and δ18O of atmospheric precipitation can either be used as tracers of moisture sources or to invert atmospheric processes (Lawrence et al., 1982; Lawrence et al., 2004; Liu et al., 2009; Wolf et al., 2020), which reflect seasonal and synoptic-related climatic features to some extent (Gedzelman et al., 2001).
Seasonal variations in the isotopic composition of precipitation are mostly controlled by the regional climate, including the source of moisture controlled by the monsoon (Cai et al., 2018; Guo et al., 2021), whereas variations in synoptic scales are mainly influenced by meteorological factors, including local temperature, humidity, etc. (Xie et al., 2011; Salamalikis et al., 2016). In addition, extreme weather events such as typhoons are also known to have significant effects on precipitation isotope values (Ohsawa and Yusa, 2000). Although typhoons have a wide range of effects, their short lifetime and complex impact processes have led to a lack of understanding of typhoon-related precipitation isotopic composition compared to normal precipitation. Previous studies have shown that the δ18O of typhoon-related precipitation is significantly depleted than that of other tropical and subtropical summer precipitation (Lawrence and Gedzelman, 1996; Lawrence et al., 2002), with the δ18O decreasing gradually from the periphery of the typhoon toward the center (Gedzelman et al., 2001). The depleted isotopic composition of typhoon-related precipitation may be attributed to the high and deep clouds, the large precipitation amount, the high stratiform precipitation fractions, and the deep convection system (Lawrence et al., 1998; Fudeyasu et al., 2008; Han et al., 2021). In addition, the inward decrease of isotopic composition was proposed to be due to the isotopic exchange between precipitation and lossy water vapor in the atmospheric boundary layer (Lawrence et al., 1998; Xu et al., 2019). The isotopes of typhoon-related precipitation are also affected by the physical processes at different stages of the typhoon, with significantly higher isotope values in the early and late stages of precipitation than in the middle stage (Xu et al., 2019). There are both similarities and differences in the effects of different typhoons on precipitation isotopes, which may be related to the intensity, lifetime, and migration route of the typhoons as well as the source of moisture for typhoon-related precipitation (Gedzelman et al., 2001; Lawrence et al., 2004; Xu et al., 2019). Therefore, it is important to explore the differences between typhoon and normal precipitation processes and their mechanisms, to further clarify the role of moisture sources in the typhoon precipitation process, and to analyze the differences in the effects of various typhoon processes on precipitation isotopes as an important basis to understand and investigate the isotopic fractionation effects of typhoon-related precipitation.
The southeastern coast of China is in the prevalent East Asian monsoon region, and thus the seasonal variation of precipitation isotopes is significantly controlled by the monsoon climate. In addition, it has been experiencing a large number of typhoons every year that have caused risks for the safety of people’s lives and properties, as well as social and economic impacts on the region. In recent years, under the influence of global climate change, the frequency of typhoon events in the western Pacific region has decreased, whereas their intensity has increased significantly (Zhang et al., 2015a; Walsh et al., 2019), and thus their destructive power has intensified too. Because of these enhanced risks, it is even more essential to further understand the past and present typhoon dynamics and their evolutionary trends under the effect of persistent global climate change. Heavy rainfall caused by typhoons is one of the most dramatic processes that can affect the natural environment, thus the study of typhoon-related precipitation is helpful to understand the typhoon meteorological processes (Zhang et al., 2018). Studying the hydrogen and oxygen isotopes of typhoon-related precipitation can provide a reference for typhoon meteorological processes, typhoon geological records, and paleotempestology studies, which is crucial for quantifying global changes and for assessing natural disaster risks in typhoon-affected regions.
In this study, we analyzed the δD and δ18O values of atmospheric precipitation in Xiamen, Southeast China Coast, based on the measured data of 162 atmospheric precipitation samples (including 35 typhoon-related precipitation samples) collected from June 2018 to August 2019. The objectives of this study were to investigate the seasonal variations, synoptic processes, and typhoon impact on isotopic composition of atmospheric precipitation. We analyzed the influential factors responsible of the seasonal variation of precipitation isotopic compositions, explored the differences between typhoon and normal precipitation processes, and clarified the role of a typhoon on the isotopic composition of atmospheric precipitation. This study is important for understanding and investigation of the effects of typhoon processes on the isotopic fractionation of precipitation.