Offshore Spreading of River Discharge in the Japan Sea: Glider Surveys and Numerical Experiments

The region offshore of Sado Island, Japan (i.e., along the southern margin of the central Japan Sea) is characterized primarily by a surface-intensified jet with a lateral scale of O(10 km) and mesoscale eddies whose speeds exceeding 0.5 m s⁻¹. We have investigated those mesoscale structures and their temporal variabilities using gliders and mooring system having ADCP for several years. Upper-layer, low-salinity water (ULSW) was first found within an anticyclonic eddy in our glider surveys during April–June 2016. The ULSW is characterized by salinity less than 34.0 and potential density (σ_θ) 25.5 kg m^-3< σ_θ< 26.1 kg m^-3with horizontal scale <60 km and vertical thickness >100 m. To detect the ULSW, we need to collect offshore and high-resolution measurements under severe oceanic conditions in springtime. Because of the difficulties associated with such measurements, ULSW has never been observed by earlier studies. The ULSW was found in our recent glider surveys during January–April 2019. This result suggests that the ULSW was not incidentally observed in 2016 but it substantially spreads over this area during winter and spring every year. The formation mechanisms of the ULSW were investigated by numerical experiments using the newly developed 2-km resolution model which well reproduced the spatiotemporal changes in multiscale phenomena observed by gliders. The model results showed that the ULSW was formed by offshore spreading of river discharge from the Japanese coast. The spreading processes were affected by mesoscale and submesoscale features such as fronts and eddies. Therefore, the ULSW that originated in the bay and supplied freshwater and nutrients from land runoff into offshore regions would have impacted on the biochemical environment.