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Idealized numerical simulations of the Kuroshio western boundary current flowing over the Hirase seamount were conducted to examine the mechanisms of phenomena observed by shipboard and mooring measurements. Along the Kuroshio, enhanced mixing [vertical diffusivity, Kρ = O(10−2) m2 s−1] was observed in a low-stratification layer between high-shear layers around low tide, and a V-shaped band of the negative vertical component of relative vorticity (ζz) was also observed. Those features were reproduced in simulations of the Kuroshio that included the D2 tide. In the simulation, a streak of negative ζz detached from the Hirase turned into vertically tilted 10-km-scale vortices. The buoyancy frequency squared (N2) budget at the mooring position showed that the low stratification was caused by vertical and horizontal advection and horizontal tilting. The Kρ tended to increase when the Ertel potential vorticity (PV) < 0, as expected given the inertial instability. However, the magnitude of Kρ also depended on the tidal phase near Hirase, and Kρ was increased in the high vertical shear zones at the periphery of vortices where a strain motion is large. These results indicate that not only inertial instability but also tidal and vertical shear effects are important for driving turbulent mixing. Significance Statement: A basin-scale distribution of wind stress drives a strong surface-intensified current in the western part of each ocean basin, such as the Gulf Stream and the Kuroshio. This western boundary current is regarded as a place where the kinetic energy and vorticity generated by winds are dissipated, allowing the basin-scale circulation to keep a steady state, but its dissipation mechanisms are not well understood. To understand the mechanisms, we conducted idealized numerical simulations that isolate the interactions between a seamount and the current as well as tidal currents, and compared results with observations. Our findings provide insights into how the current transfers kinetic energy to smaller scales when it flows over a seamount. [ABSTRACT FROM AUTHOR]

Two Argo floats equipped with oxygen, chlorophyll (Chl), backscatter, and nitrate sensors conducted daily vertical profiles of the water column from a depth of 2000 m to the sea surface in the western North Pacific from January to April of 2018. Data for calibrating each sensor were obtained via shipboard sampling that occurred when the floats were deployed and recovered. Float backscatter observations were converted to particulate organic carbon (POC) concentrations using an empirical relationship derived from contemporaneous float profiles of backscatter and shipboard observations of suspended organic carbon particles. During the float deployment periods, repeated meteorological disturbances (storms) passed over the study area and caused the mixed layer to deepen. During these events, nitrate was entrained from deeper layers into the surface mixed layer, while Chl and POC in the surface mixed layer were redistributed into deeper layers. After the storms, the upper layer gradually restratified, nitrate concentrations in the surface layer decreased, and Chl and POC concentrations increased. When the floats observed the same water mass, the net community production within the euphotic layer (0-70 m), determined from the increases in POC, was 126-664 mg C m-2 d-1 (10.5-55.3 mmol C m-2 d-1) close to the values reported from a nearby area. The C/N ratio of the increase in POC and the decrease in nitrate was closed to the Redfield ratio, which indicates that the sensors were able to observe the net biochemical processes in this area despite the relatively low concentrations of nitrate and POC. To determine the fate of particles transported from the surface ocean to the twilight layer, the ratio of oxygen consumption and nitrate regeneration rates were compared. This O2/N ratio approached the Redfield ratio when the floats followed the same water mass continuously, but the consumption rate of POC was significantly lower than what would be expected based on the oxygen consumption and nitrate release rates. This suggests that dissolved organic carbon was the main substrate for the respiration in the twilight layer. [ABSTRACT FROM AUTHOR]