14:45 〜 15:00
[AOS11-17] The Kuroshio nutrient stream, where the diapycnal mixing matters
キーワード：黒潮、栄養塩ストリーム、密度面を横切る混合
While the Kuroshio and the Gulf Stream carry a large amount of nutrients in dark subsurface layers and supply them to the downstream subpolar regions, it has remained 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 the relatively less dense water along the nutrient streams can be injected more easily to the euphotic zone. In the North Pacific, the regions of anomalously high nitrate concentrations along the Kuroshio Extension coincide with one of the major net CO2 sinks of the world oceans. Although the elevated nutrient concentrations on the density surface were attributed to diapycnal nutrient flux along the nutrient stream in the earlier studies, recent studies concluded that the horizontal advection carries this anomaly from the upstream tropical regions and that diapycnal mixing can be negligible in the Gulf Stream. However, because the Kuroshio flows over rough topography more frequently than the Gulf Stream, it still remains elusive whether the diapycnal mixing is really not playing a role at all to form the high nutrient concentration layer along the Kuroshio nutrient stream.
In this study, using a numerical model of the Kuroshio nutrient stream, and direct microscale turbulence measurements, the importance of the diapycnal mixing in the Kuroshio is examined.
The eddy flux analyses on the density surface in the simulation suggest that the eddies stir and dilute the elevated nitrate concentration, resulting in the decreasing trend of nitrate along the nutrient stream with the increasing trends on the both sides of the stream.
A series of the tow-yo microstructure observations in the Kuroshio flowing over the rough topography shows that the large nitrate diffusive diapycnal flux of 1-10 mmol m-2day-1 are frequently observed in the Tokara Strait, the Hyuganada Sea, and over the Izu Ridge. The time scale required for the diapycnal nitrate flux at 1 mmol m-2day-1 to generate the observed positive anomaly of nitrate concentrations, which is estimated to be 26 Gmol over 5000 km, is found to be as short as 50 days, assuming the area of nitrate injection of 1000 km2. The observations suggest that the nitrate injection at 1 mmol m-2day-1 over the area of 1000 km2 is plausible if the all the contributions in these mixing hot-spots along the Kuroshio are integrated.
This time scale of 50 days is comparable to or slightly less than the advection time scale, and therefore, unlike the Gulf Stream, the diapycnal turbulent diffusive flux is unlikely negligible in the Kuroshio nutrient stream. Furthermore, in the Kuroshio Extension, rich high vertical wavenumber thermohaline interleaving structures are found to induce a large double-diffusive nitrate flux at 1-10 mmol mmol m-2day-1 in the pycnostad of sigma_theta=26-27 kgm-3. Because these relatively dense layers in this region are not outcropping 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. In addition, although the detailed mechanisms still remain unclear, the Argo float data analyses show that these double-diffusive flux in the dense layers modulates interannually.
In this study, using a numerical model of the Kuroshio nutrient stream, and direct microscale turbulence measurements, the importance of the diapycnal mixing in the Kuroshio is examined.
The eddy flux analyses on the density surface in the simulation suggest that the eddies stir and dilute the elevated nitrate concentration, resulting in the decreasing trend of nitrate along the nutrient stream with the increasing trends on the both sides of the stream.
A series of the tow-yo microstructure observations in the Kuroshio flowing over the rough topography shows that the large nitrate diffusive diapycnal flux of 1-10 mmol m-2day-1 are frequently observed in the Tokara Strait, the Hyuganada Sea, and over the Izu Ridge. The time scale required for the diapycnal nitrate flux at 1 mmol m-2day-1 to generate the observed positive anomaly of nitrate concentrations, which is estimated to be 26 Gmol over 5000 km, is found to be as short as 50 days, assuming the area of nitrate injection of 1000 km2. The observations suggest that the nitrate injection at 1 mmol m-2day-1 over the area of 1000 km2 is plausible if the all the contributions in these mixing hot-spots along the Kuroshio are integrated.
This time scale of 50 days is comparable to or slightly less than the advection time scale, and therefore, unlike the Gulf Stream, the diapycnal turbulent diffusive flux is unlikely negligible in the Kuroshio nutrient stream. Furthermore, in the Kuroshio Extension, rich high vertical wavenumber thermohaline interleaving structures are found to induce a large double-diffusive nitrate flux at 1-10 mmol mmol m-2day-1 in the pycnostad of sigma_theta=26-27 kgm-3. Because these relatively dense layers in this region are not outcropping 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. In addition, although the detailed mechanisms still remain unclear, the Argo float data analyses show that these double-diffusive flux in the dense layers modulates interannually.