The Kuroshio Nutrient Stream: Where Diapycnal Mixing Matters

Abstract

AbstractWhile the Kuroshio and Gulf Stream carry a large amount of nutrients in the dark subsurface layers and supply them to the downstream regions, it remains unclear how the elevated nutrient concentrations along these nutrient streams are formed and/or maintained. This is particularly important as these positive nutrient anomalies on relatively less dense water along the nutrient streams can be injected more easily into the euphotic zone. The regions of anomalously high nitrate concentrations along the Kuroshio Extension coincide with one of the major net CO2 sinks for the Earth’s atmosphere. Although, in the earlier studies, the elevated nutrient concentrations on the density surfaces were attributed to diapycnal nutrient flux along the nutrient stream, recently it has been concluded that horizontal advection carries the preexisting anomaly from the upstream tropical regions, and that diapycnal mixing can be negligible in the Gulf Stream. Nonetheless, since the Kuroshio flows over more rough topography than the Gulf Stream, it remains elusive whether diapycnal mixing plays any role in the formation of a high nutrient concentration layer along the Kuroshio nutrient stream. In this chapter, by using a numerical model of the Kuroshio and direct microscale turbulence measurements, the importance of diapycnal mixing in the Kuroshio is examined. The time scale required for the diapycnal nitrate flux of 1 mmol m−2 day−1 to generate the observed positive anomaly of nitrate concentrations, which is estimated to be 26 Gmol over 5,000 km, is found to be as short as 50 days, assuming the nitrate injection area of 1,000 km2. However, the question is whether there are such large-scale nutrient injection hot-spots, and where they may be formed. The tow-yo microstructure observations of the Kuroshio flowing over rough topography show that the large nitrate diffusive diapycnal flux of 1–10 mmol m−2 day−1 is frequently observed in the Tokara Strait, the Hyuganada Sea, and over the Izu Ridge. These observations suggest that the nitrate injection at 1 mmol m−2 day−1 over the 1,000 km2 area is plausible if all the contributions in these mixing hot-spots along the Kuroshio are integrated. This time scale of 50 days is estimated to be comparable to the advection time scale over a distance of 5,000 km along the Kuroshio. Thus, unlike the Gulf Stream, the diapycnal turbulent diffusive flux can be important in the Kuroshio nutrient stream. From simulations, the eddy flux analyses on the density layer suggest that eddies can stir and dilute elevated nitrate concentrations, resulting in the decreasing trend of nitrate along the nutrient stream, but with increasing trends on both sides across the stream. Additionally, in the Kuroshio Extension region, rich high vertical wavenumber thermohaline interleaving structures are found to induce a large double-diffusive nitrate flux of 1–10 mmol m−2 day−1 in the density range of σθ = 26–27 kgm−3. Because these relatively dense layers in this region do not outcrop even during winter, whether this nitrate diffused upward by double-diffusion can be supplied to the euphotic zone depends on further stirring processes, such as the northward and upward eddy fluxes.KeywordsBaroclinicBarotropicDiapycnal mixingEuphotic zoneThermohaline