Assessing the impact of riverine water on the Northwest Pacific using normalized Total Alkalinity

Abstract. The impact of riverine water was assessed using salinity-normalized Total Alkalinity observations of the Northwest Pacific, including the coastal areas of Japan (20–50° N, 120–160° E). The observational data included surface carbonate parameters obtained from decades of surveys conducted by volunteer cargo ships and research vessels in this area. This study uses data and statistical methods (e.g., re-gridding and Fourier regression) like those in a previous study that analysed air-sea CO2 flux but focuses instead on the diffusion of normalized Total Alkalinity from land. First, the seawater area affected by riverine water was identified using an Empirical Orthogonal Function analysis of normalized Total Alkalinity. The differences in normalized Total Alkalinity and Dissolved Inorganic Carbon from the surrounding area were then analysed to evaluate the potential drivers, such as riverine water supply, advection effects, and biological activities. In addition, the impact of riverine water on oceanic CO2 uptake and acidification in the study area was assessed. The analysis showed that riverine water was the main cause of the higher total Alkalinity compared to the surrounding area, whereas its contribution to the increase in Dissolved Inorganic Carbon was relatively minor. The supply of riverine water had little effect on oceanic CO2 uptake throughout the year. The supply of riverine water had a minor effect on pH but contributed to coastal acidification, as indicated by a decrease in the calcification index (Ωarg, the aragonite saturation state) by 0.09±0.01 over the past 20 years, even after accounting for the buffering effect of riverine Total Alkalinity, which reduced the overall decrease by approximately 71 %. The results of this study are expected to be further improved by enhancing observations, such as the vertical profiles of carbonate parameters, and are expected to expand to other sea areas and be applied to global budgets.