# Freshwater spreading far offshore the Japanese coast

## TL;DR

This study shows how freshwater from rivers spreads far offshore along the Japanese coast, affecting ocean circulation and biological production.

## Contribution

The first observation of thick, low-salinity coastal water spreading offshore in the Sea of Japan and its link to biological production.

## Key findings

- Low-salinity water from the Japanese coast spreads offshore due to interaction with mesoscale eddies.
- River discharge influences offshore biological production, as shown by a relationship between low-salinity transport and chlorophyll-a levels.

## Abstract

River discharge to the ocean influences the transport of salts and nutrients and is a source of variability in water mass distribution and the elemental cycle. Recently, using an underwater glider, we detected thick, low-salinity water offshore for the first time, probably derived from coastal waters, in the central-eastern Sea of Japan, whose primary productivity is comparable to that of the western North Pacific. Thereafter, we aimed to investigate the offshore advection and diffusion of coastal water and its variability and assess their impact. We examined the effects of river water discharge on the flow field and biological production. Numerical experiments demonstrated that low-salinity water observed by the glider in spring was discharged from the Japanese coast to offshore regions. The water is discharged offshore because of its interaction with mesoscale eddies. A relationship between the modeled low-salinity water transport to the offshore region and the observed chlorophyll-a in the offshore region was also observed, indicating the influence of river water on offshore biological production. This study contributes to understanding coastal-offshore water exchange, ocean circulation, elemental cycles, and biological production, which are frontiers in the Sea of Japan and throughout the world.