Assessment of the Kuroshio Large Meander’s Impact on the Dispersion Pathways of Fukushima Tritium-treated Water

The discharge of ALPS treated water from the Fukushima Daiichi Nuclear Power Station into the North Pacific Ocean has necessitated a detailed assessment of long-term dispersion pathways. The transport of this ALPS treated water is primarily governed by the Kuroshio Extension (KE) system. However, the upstream Kuroshio has been experiencing a persistent “Kuroshio Large Meander (KLM)” event since August 2017. Since the variability of the KE is dynamically linked to the path of the Kuroshio south of Japan, understanding how the KLM modulates the downstream flow field is critical for evaluating environmental impacts.

In this study, we investigate the influence of the KLM on the dispersion of tritium-treated water by Lagrangian particle tracking model with a continuous release scheme. The model was forced by ocean current data from the Hybrid Coordinate Ocean Model (HYCOM) to capture the spatiotemporal variability of the Kuroshio Current. We specifically examined the differences in transport patterns during the KLM period (2017–2022) versus non-meander period (2011–2016).

Preliminary results indicate that the presence of the upstream Large Meander induces specific downstream responses in the Kuroshio Extension that distinctively deviate from the reference non-meander period. We focus on how the KLM modulates the stability and position of the KE jet, thereby altering the initial advection pathways of the ALPS treated water. The analysis aims to clarify whether these KLM-induced changes in the KE system act to retard zonal transport or enhance regional retention, creating significant discrepancies in tracer arrival times and concentration fields between the two periods.

This study quantifies these deviations and discusses the implications of the “Kuroshio-Kuroshio Extension coupling” mechanism in determining the dispersion patterns and concentration distributions of passive tracers. The findings highlight the necessity of incorporating low-frequency climate variability into environmental risk assessments for oceanic discharges.