471 / 2024-09-17 18:49:42

Submesoscale ageostrophic processes generated by Kuroshio under seamount forcing

Kuroshio, submesoscale ageostrophic processes, flow-topography interactions, seamounts, Tokara Strait.

Abundant submesoscale ageostrophic processes (SAPs) are induced by the flow-topography interactions as the Kuroshio flows through the Tokara Strait. The interaction between complex seamount and geostrophic flow is an important mechanism for generating SAPs. The SAPs in the Tokara Strait and Hiki-sone Seamount were studied in this study. The results show that upstream of the seamount, the Kuroshio turns sharply northward due to the forcing of the seamount, resulting in a rapid increase of 10-100 times in the lateral shear of the northward velocity component higher than the background field. This intensified shear induces a strong stretching of the flow field, leading to an Ertel’s potential vorticity of less than zero and symmetric instability. Downstream of the seamount, the combined effects of flow field stretching and vortex flow causes the north velocity's transverse shear to rapidly drop and turn negative. Consequently, this prompts a rapid increase in the horizontal buoyancy gradient, inducing an Ertel’s potential vorticity of less than zero and inertial/centrifugal instability. The fluid instability upstream and downstream of the seamount creates conditions for the emergence of a submesoscale front; at a depth of 122-300 m from the seamount, a submesoscale front with vertical scale of approximately 200 m was observed. SAPs upstream of the seamount are more active than those downstream, and the conversion rate of ageostrophic kinetic energy is higher upstream.