Large-scale distribution and variations of active salt-finger double-diffusion in the western North Pacific

Large-scale distribution and variations in active salt-finger (SF) in the western North Pacific were examined by detecting the active SF with a vertical density ratio Rρ = 1 − 2 at depths of 10-300m using a monthly gridded hydrographic dataset from 2001 to 2016. The active SF is distributed mostly in March along 40°N around the Subarctic Boundary (SAB), where the mixed layer deepens northward and corresponds to the Central Mode Water formation site with a density of +0.02σθ to +0.2σθ of surface density and mainly in 26.1-26.4σθ. This active SF along 40°N underwent seasonal variation and decayed rapidly from March to August from the shallower and less dense parts of the active SF with increasing mean density. The features of the active SF in March are consistent with the hypothesis that surface water with a horizontal density ratio RL = 1 − 2 is subducted and vertically superposed, resulting in an active SF. The mean density of the active SF in March is well correlated with the surface density with RL = 1 − 2, and both mean densities showed a decreasing trend from 2001 to 2016, following the surface warming trend (~0.057°C/yr) in the surface water with RL = 1 − 2. Large year-to-year variations in the active SF in March are explained by both horizontal and vertical extensions, and can be reproduced by four conditions: 1) from 1°N to 3°S of SAB, 2) RL=1-2, and 3) northward deepening of the mixed layer depth, and 4) the part with a density of +0.02σθ to +0.2σθ of surface density.