[A-OS16] Coastal ocean circulation and material cycle

[AOS16-08] Effect of river water on surface seawater around Japan derived from Absolute Salinity anomaly

*Hiroshi Uchida¹, Masahide Wakita¹, Mitsuho Oe² (1.Japan Agency for Marine-Earth Science and Technology, 2.Japan Meteorological Agency)

Keywords:Absolute Salinity anomaly, river water

The spatial distribution of Absolute Salinity anomaly (dSA) obtained from shipboard observations was investigated to understand the influence of river water on surface seawater around Japan. dSA is defined as the difference between Absolute Salinity and Reference Composition Salinity (Practical Salinity converted to SI units) based on electrical conductivity measurements. Absolute salinity can be obtained from direct measurement of seawater density. On the other hand, dSA can also be estimated from seawater composition analysis. In the open ocean, dSA estimation equations have been proposed mainly as a function of total alkalinity (TA), dissolved inorganic carbon, silicate, nitrate, and dissolved organic carbon (DOC). In this study, the spatial distribution of dSA in surface seawater around Japan was determined based on seawater composition data obtained from shipboard CTD/water sampling observations; CTD/water sampling data in the Pacific Ocean, East China Sea, and Japan Sea obtained by Japan Meteorological Agency from 2010 to 2022; WOCE and GO-SHIP data in the Okhotsk Sea, Bering Sea, and Pacific Ocean. The influence of DOC on dSA is relatively small and there are very few DOC data, so the influence of DOC was ignored here. The influence of river water (mainly the influence of TA) was investigated based on the relationship between dSA and Practical Salinity estimated from the water sampling data, and the following characteristics were obtained. The Practical Salinity in the subtropical surface water varies greatly due to water evaporation and precipitation, but dSA is almost zero. On the other hand, in the East China Sea, low salinity and high dSA seawater were observed, which may be due to the influence of the Yangtze River. After entering the Japan Sea, such characteristic seawater mixes with the Japan Sea Proper Water, and is widely distributed in the central part of the Japan Sea. In the Sea of Okhotsk, low-salinity, high dSA seawater is observed, possibly due to the influence of the Amur River. Such characteristic seawater is directly discharged into the Pacific Ocean, or is mixed with high dSA seawater in the deep layer as it passes through the Kuril Straits, and is distributed in the subarctic region. In the Tsugaru Strait, the seawater modified by the Japan Sea Proper Water and the lower salinity/higher dSA water are found, but they are distributed in the area where the influence of the Yangtze River and the Amur River have similar characteristics (intersection of Practical Salinity-dSA relationship), and it is difficult to determine which influence is responsible for the change in dSA alone. Although the water sampling data are discrete in time and space, the spatially continuous distribution of dSA is evaluated by measuring the density by using the refractive index density sensor in addition to the thermosalinograph based on continuous observations of surface seawater by research vessels.