Mixing characteristics of the subarctic front in the Kuroshio-Oyashio confluence region

Abstract

Summary: This paper analyzes the mixing characteristics of the Subarctic Front (SAF) in the Kuroshio-Oyashio Confluence Region based on temperature, salinity, and current data obtained from surveys and remote sensing in June 2016. The frontal zone of the observed area is at 145°–151°E, 38°–41°N. The front is distributed between 25.5–26.7 σθ in a band pattern inclined from north to south and is deeper in the south. The region shallower than 200 m and distributed along the isopycnal of 25.9–26.1 σθ has the strongest horizontal temperature and salinity gradients, and the largest of the former can reach over 0.7°C/km. Diapycnal mixing of the SAF is mainly turbulent; it is stronger in the north than in the south. The region with stronger turbulence (Kρ > 10−3.5 m2/s) is distributed mainly in water layers within and under the front (26.1–26.7 σθ), showing that the SAF is shallower in the north and deeper in the south along the front. Symmetric instability may be the main factor causing strong turbulent mixing in the frontal zone. Double diffusion mixing is stronger in the south than in the north; the region with stronger double diffusion (Kθ > 10−4.5 m2/s) is distributed mainly in water layers within and above the front (25–26.5 σθ) on the southern side of the SAF. These water layers are dominated mainly by ``salt-fingering’’ double diffusion, with only a few water layers dominated by ``diffusive layering’’ double diffusion mixing in middle and lower waters deeper than 300 m. Keywords: Kuroshio–Oyashio confluence region, Subarctic front, Mixing, Turbulent eddy diffusivity, Thermal diffusivity