Relationship between precipitation isotopic compositions and synoptic atmospheric circulation patterns in the lower reach of the Yangtze River

In this study, the stable isotopic compositions (δ¹⁸O, δ²H and d-excess) in daily precipitation from October 2017 to May 2020 in the lower reach of the Yangtze River (LYR) were examined to understand the relationship between stable isotopic compositions in precipitation and synoptic atmospheric circulation patterns (SACPs). Atmospheric circulations over the LYR were classified into seven patterns according to the Kirchhofer method with three, one and three SACPs in the East Asian winter monsoon period (EAWM), the monsoon transition period and the East Asian summer monsoon period (EASM), respectively. Precipitation events between most SACPs are distinguishable for either δ¹⁸O or d-excess, except for two SACPs in the EAWM, and two SACPs in the EASM. Local meteorological variables are primary factors determining the variation of isotopic compositions in precipitation for three SACPs in the EAWM. Though the precipitation-producing air masses come from different seas under two different SACPs in the EASM, i.e., the South China Sea for one and the western Pacific for the other, neither δ¹⁸O nor d-excess is distinguishable. Nevertheless, the relationships between isotopic composition and meteorological variables appear to be distinguishable between these two SACPs. The SACP dominant in the monsoon transition tends to have higher δ¹⁸O (mean value) and δ²H (mean value) in precipitation than all other SACPs. The d-excess in precipitation of this SACP is more likely altered by subcloud evaporation, and the retrieval of moisture source d-excess information should be treated with cautions. Similarly, d-excess in Meiyu rain, classified in one of the SACPs in the EASM period, is very likely altered by moisture recycling along the atmospheric moisture transport pathway, evident from the significant negative relationship between d-excess in precipitation and mean specific humidity along the backward trajectories. Thus d-exess of Meiyu rain is very likely positively deviated from its oceanic moisture source. Overall, the findings can provide valuable insights for understanding the relationship between stable isotopes in precipitation and SACPs in the East Asian monsoon region.