Subsurface nutrient maximum and submesoscale structures in the southwestern Japan Sea

Abstract

From summer to autumn, in the southwestern Japan Sea, the maximum subsurface nutrient (DIN: nitrate + nitrite) concentration and minimum dissolved oxygen (DO) concentration layers lie at depths of approximately 75–125 dbar. The subsurface nutrient-rich water originates from the East China Sea through the lower layer (50–100-dbar depths) of the Tsushima Straits. On a smaller scale, in the subsurface nutrient-rich layer, submesoscale structures (~10 km) with high DO and chlorophyll-a (Chl-a) concentrations were observed below the surface euphotic zone and the minimum Chl-a layer in late October 2008. Considering that photosynthesis cannot occur in the minimum Chl-a layer, the submesoscale Chl-a maxima did not coincide with the normal maximum subsurface chlorophyll layer, where phytoplankton can mature, in the seasonal pycnocline in summer. The subsurface submesoscale water masses were zonally distributed from west to east downstream of the Tsushima Warm Current. In late November 2008, high DO and Chl-a water subducted from the surface density front to the subsurface layers along the isopycnal surface. In the large-scale frontal region of the Tsushima Warm Current, the subsurface isopycnals were convex upward both in October and November 2008, suggesting meso- and submesoscale upwellings.