Abstract
Kelvin–Helmholtz (KH) billows can facilitate microscale turbulent mixing around seamounts in the Kuroshio. This study sought to describe the influence of billow intensity (i.e., “intermittent and small” and “steady and large” billows) on vertical nitrate fluxes. KH billows led to turbulent kinetic energy dissipation rates [ε = O (10–7 to 10–6) W kg–1] and eddy diffusivities [Kρ = O (10–4 to 10–3) m2 s–1] that were significantly stronger than those outside the billow depths. The mean nitrate flux estimated using Kρ in the billow depths had a maximal value of 10.0 mmol m–2 day–1, which was much higher than estimates for the open ocean. The nitrate flux associated with the shallow KH billows at two vertical levels contributed to enrich the subsurface phytoplankton maximum, while the deeper billows closer to the summit were found to induce a large amount of nitrate flux from the deeper to the subsurface water. This study showed that KH billows make important contributions to seamount ecosystems, particularly in the cycling and vertical mixing of nutrients to make them available for potential downstream transport.
Highlights
The Kuroshio originates at the bifurcation of the North Equatorial Current, which carries oligotrophic waters low in nutrient concentrations and plankton biomass (Barkley, 1970; Kobari et al, 2020)
Because of the complexity of the various interacting mechanisms in the upstream Kuroshio, it is difficult to determine whether nutrient enrichment downstream, over seamounts, is caused exclusively by horizontal advection
The spatial flow patterns of the Kuroshio characterized by the satellite-derived velocities were used (Figure 2)
Summary
The Kuroshio originates at the bifurcation of the North Equatorial Current, which carries oligotrophic waters low in nutrient concentrations and plankton biomass (Barkley, 1970; Kobari et al, 2020). Submesoscale eddies generated from the interaction of the Kuroshio with the southern tip of Taiwan have been observed to entrain high concentrations of chlorophyll a (Chl a) (Cheng et al, 2020). The Kuroshio is affected by small islands (e.g., Green Island), that produce island wakes and enhance chlorophyll concentration (Chang et al, 2013). These reports identified a general leeward advection of chlorophyll in the Kuroshio. Because of the complexity of the various interacting mechanisms in the upstream Kuroshio, it is difficult to determine whether nutrient enrichment downstream, over seamounts, is caused exclusively by horizontal advection
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