[A-CG31] Nutrient footprint of primary production in the coastal and marginal seas of East and Southeast Asia

[ACG31-01] Is the subsurface Kuroshio just a nutrient conduit or a spreader?

★Invited Papers

*Takeyoshi Nagai¹, Gloria Silvana Duran Gomez², Hiroaki Saito³, Hiroshi Ogawa³, Toru Kobari⁴, Naoki Yoshie⁵, Hirohiko Nakamura⁴ (1.Department of Ocean Sciences, Tokyo University of Marine Science and Technology, 2.Graduate School of Marine Science and Technology, Tokyo University of Marine Science and Technology, 3.Atmosphere and Ocean Research Institute, The University of Tokyo, 4.Faculty of Fisheries, Kagoshima University, 5.Center for Marine Environmental Studies, Ehime University)

Keywords:Kuroshio, Nutrient stream, Turbulent Mixing

While the Kuroshio and the Gulf Stream transport a large amount of nutrients in their dark subsurface layers, it has been unclear whether these nutrients are supplied to the sunlit surface layers before they reach to their northern induction sites, such as north of the Gulf Stream, and the Kuroshio Extension regions. If these nutrient streams merely send subsurface nutrients to these downstream regions, they are just passing through the southern coast all the way along the western boundaries as if they are limited express trains without stopping. On the other hand, the nutrient concentrations on the density surface along these nutrient streams shows elevated values suggesting diapycnal nutrient flux, implying the opposite story. This is particularly important as these positive nutrient anomalies on the relatively less dense water along the nutrient streams can be more susceptive to surface mixing processes and likely injected 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. However, 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. In contrast, the Kuroshio flows over rough topography more frequently. Thus, 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 reexamined. 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, Osumi 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 day 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.