Topographically driven vigorous vertical mixing supports mesoscale biological production in the Tsugaru Gyre

The Tsugaru Strait is a critical conduit linking the Sea of Japan and the Pacific Ocean and is characterized by a vigorous eastward current. Here, we use high-resolution surface current data acquired from a high-frequency radar system in conjunction with direct turbulence measurements to show that the large increase in chlorophyll-a within a hundred kilometre-scale anticyclonic gyre is driven by the intensification of turbulence, extraordinarily high diffusive nitrate fluxes and divergent flow fields limited to a very narrow area. Enhanced mixing and nutrient fluxes are caused by the presence of a distinct topographic feature-an underwater seamount-within the path of the Tsugaru Strait water flow. Our long-term datasets reveal a positive correlation between recurring submesoscale divergence related to the topographic features and the downstream distribution of chlorophyll-a. This finding reinforces the significant role of the underwater seamount in enhancing the productivity of the downstream anticyclonic gyre. The results of this study emphasize the importance of re-evaluating nutrient supply processes with respect to submesoscale interactions with underwater seamounts for large-scale enhanced biological productivity.