We identified the interannual variations in δ13CPOM and δ15NPOM during summer using repetitive measurements along the SI-line. In 2019, low δ15NPOM with high δ13CPOM was observed compared to that in 2018, despite the fact that the environmental conditions were not significantly different among these four years (2016, 2018–2020). These interannual variations may be influenced by horizontal advection. We were unable to gather data on the contribution of the horizontal advection processes, however, interannual variations in salinity and phosphate concentration in the ECS are linked to those of SOJ (Kodama et al., 2016; Kosugi et al., 2021). In the ECS, the interannual difference in δ15NPOM in the Changjiang diluted water was reported by Sukigara et al. (2017) and Ho et al. (2021) revealed an interannual difference in δ15NPOM in the Changjiang diluted water in the ECS, which may influence δ13CPOM and δ15NPOM in the SOJ. The causes of interannual variations are not described by Sukigara et al. (2017) or Ho et al. (2021), and we were unable to determine them in our study.