Submesoscale frontal structure and mixing processes in the Kuroshio-Oyashio region

 Research Summary

   Vertical mixing in the ocean occurs through the energy transfer from large to small scales. The importance of mixing processes caused by bottom friction and internal gravity waves generated by the wind and tidal flows was recognized earlier and observational and numerical evidences have been accumulated. However, the wind does not only cause vertical mixing directly but also drives large-scale ocean circulation. The energy stored in the large-scale circulation is transferred to smaller scales through various instability processes and enhances vertical mixing. Satellite altimetry developed since 1990s revealed the ocean is full of mesoscale eddies that are emerged from large-scale circulations. However, structure and mixing processes have yet to be clarified by in situ observation at submesoscale, although high-resolution numerical simulations suggested the enhancement of submesoscale features near frontal areas.

  Recently, our group conducted high-resolution observations around the front between the Tsugaru Warm Current and the Oyashio using an underway CTD (Fig. 1) and a vertical microstructure profiler. We captured a very-sharp frontal structure formed between the Tsugaru Warm Current and the Oyashio above the continental shelf (Fig. 2), and suggested the enhancement of vertical mixing through capturing internal waves within the strong frontal band. In this study, we will make fine-scale observations of submesoscale fronts in the Kuroshio-Oyashio area and clarify physical processes related to vertical mixing and assess their impacts on nutrient transport and marine ecosystems.

Fig. 1. Underway CTD observation

Fig.2. Frontal structure between the Tsugaru Warm Current and the Oyashio off Sanriku (Itoh et al. 2016 doi: 10.1007/s10872-015-0320-6) al. 2016 doi: 10.1007/s10872-015-0320-6)

Principle investigator:

Sachihiko Itoh

AORI, UTokyo

Cooperative researcher: Shin-ichi Itoh (AORI, UTokyo)

Cooperative researcher: Daisuke Hasegawa (TNFRI)

Cooperative researcher: Hiroyasu Hasumi (AORI, UTokyo)

Cooperative researcher: Susumu Yoshizawa (AORI, UTokyo)