[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-04] Submesoscale anticyclonic vorticity and subsurface eddy generations by the Kuroshio and cyclonic eddies along the coast

*Takeyoshi Nagai¹, Gloria Silvana Duran Gomez², Gandy Maria Rosales Quintana², Diego Pinglo³ (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.Universidad Nacional Agraria La Molina )

Keywords:Kuroshio, cyclonic eddy, subsurface anticyclonic eddy, submesoscale, turbulence, nutrients

While the dark subsurface layers of the Kuroshio carry a large amount of nutrients, it has been elusive whether and how these nutrients are supplied to sunlit layers along the southern coast of Japan before they reach the northern downstream regions, the Kuroshio Extension. The elevated nitrate concentrations found on the density surfaces along the subsurface Kuroshio may indicate that some fractions of nitrate are injected across the density surfaces through diapycnal mixing processes. A series of microstructure observations in the Kuroshio flowing over many seamounts in the regions south of Kyushu revealed an unprecedentedly large-scale hotspot of diapycnal mixing and nutrient supply with an average rate of O(1 mmol m^-2day^-1) over 200 km formed in the Tokara Strait. Besides turbulent diapycnal fluxes, submesoscale cyclonic eddies generated behind the islands in the same regions have also been found to induce net upwelling of nutrients at O(10 mmol m^-2day^-1). On the other hand, recent numerical studies in the Gulf Stream have shown that countercurrent associated with cyclonic eddies between the Gulf Stream and the coast produces opposite sign of potential vorticity (PV), that may induce submesoscale instability, such as inertial and symmetric instabilities. The previous studies on these cyclonic eddies along the oceanic currents, known as frontal eddies, have pointed out its importance in uplifting the subsurface nutrients to enrich the water and retaining fish juveniles within them, providing good nursery environments for pelagic fish. Despite this importance, the mixing and nutrient injection associated with the submesoscale processes have yet to be well clarified. In this study, a high-resolution numerical simulation coupled with a N₂P₂Z₂D₂ ecosystem model is used to reproduce anticyclonic vorticity generations by cyclonic eddies formed between the Kuroshio and the southern coast of Japan to elucidate whether negative PV can arise along the coast by cyclonic eddies, and how it affects the nutrient supply. The results show that negative PV can be generated near islands and seamounts in the Tokara Strait, subsurface layers off the east coast of Hyuganada, Cape Ashizuri southwest of Shikoku, and Cape Shiono. While the negative PV generations are caused directly by the Kuroshio in the Tokara Strait, they are seen when the cyclonic vorticity approaches the coast of Hyuganada, Cape Ashizuri, and Cape Shiono. The model KPP eddy diffusivity at 150 m depth shows elevated values with negative PV. The nitrate diffusive flux at 150 m depth in the model also shows large values with negative PV, but the maximum diffusive fluxes are found with cyclonic vorticity with larger vertical nitrate concentration gradient than that at negative relative vorticity. These negative or low PV waters are found to form submesoscale subsurface anticyclonic eddies that are advected by the Kuroshio to the downstream. Using the subsurface eddy tracking technique, 20% more subsurface anticyclones are found than subsurface cyclones within the model domain, and model subsurface anticyclones are found to have slightly longer lifetime than subsurface cyclones. In the presentation, we attempt to further quantify the role of subsurface anticyclones in transporting and fluxing the nutrients.