Transient Carbonate Chemistry in the Expanded Kuroshio Region

Abstract

The Kuroshio is the most significant current in the western North Pacific Ocean and affects a wide area. This work shows that the intrusion of the oligotrophic upper-layer West Philippine Sea seawater into the South China Sea (SCS) as the branch of Kuroshio reduced the productivity and hence the fluxes of sinking particles in the SCS between 2013 and 2017. Conversely, the productivity in the SCS increased during a large scale Kuroshio intrusion in 1998–2006, indicating that other factors also affected the productivity. Further, the western North Pacific Ocean is acidifying, with the surface seawaters to the west having lower acidification rates. This phenomenon is likely a consequence of enhanced productivity owing to more anthropogenic nutrient inputs from the continent in the west, but needs further investigation. In the East China Sea, the Kuroshio Intermediate Water has increased nutrient concentrations, but decreased in both dissolved oxygen (DO) concentration and pH, most likely owing to reduced ventilation in the North Pacific Intermediate Water. Further warming of the surface oceans would strengthen the stratification of the surface ocean, weakening ventilation. Consequently, DO and pH would continue to decline while nutrients level increases.