Nutrient supply to the euphotic layer by the advection and vertical mixing over the Kuroshio and Kuroshio Extension

Abstract

The Kuroshio carries a large amount of nutrients from the east of Luzon Island to the south of Japan. However, only nutrients transported into the euphotic layer can be utilized by phytoplankton. We use the results of an eddy-resolving coupled physical-biological model to investigate (1) the horizontal and vertical transport of nitrate into the euphotic layer (0–100 m) in the Kuroshio and Kuroshio Extension and (2) the contribution of different sources of nitrate (coastal, deep layer and open ocean) to the spatial variation of downstream transport within the euphotic layer along the Kuroshio and Kuroshio Extension. As a mean state, the downstream transport of nitrate in the euphotic layer varies as 2.8 kmol s⁻¹ east of Luzon Island, 7.9 kmol s⁻¹ east of Taiwan, 8.9 kmol s⁻¹ near the Tokara Strait, 21.5 kmol s⁻¹ near the Izu-Ogasawara Ridge and 19.6 kmol s⁻¹ around 160°E. Vertical transport from the bottom of the euphotic layer around the Luzon Strait due to the uplifting of the potential density layer is an important contribution to the increase in downstream transport of nitrate by 3.8 kmol s⁻¹. Horizontal transport from Japan coast area contributes 14.3 kmol s⁻¹ of nitrate to the Kuroshio mainstream. An interesting staggered upward and downward distribution of vertical velocity, which can be explained by conservation of the potential vorticity, was found in the Kuroshio Extension and contributed a total nitrate transport of 5.2 kmol s⁻¹ to the euphotic layer. We also calculated the vertical flux of nitrate resulting from vertical mixing based on observations and demonstrated that vertical mixing can provide a nitrate flux into the euphotic layer in the Kuroshio region comparable to that supplied by vertical velocity.