Intense Isopycnal Stirring and Double‐Diffusion Contributing to Microscale Variance in the Kuroshio Extension Front

In oceanic frontal regions, isopycnal stirring induces lateral thermohaline intrusion. Previous studies have observed interleaving layers in the Izu Ridge and Kuroshio Extension attributed to isopycnal stirring driven by mesoscale eddies and submesoscale flows. To quantify the strength of isopycnal stirring and multiscale temperature variance generation and dissipation, we applied a triple decomposition framework incorporating double diffusion, which had been ignored in previous studies using the triple decomposition framework. In the Izu Ridge, strong diapycnal diffusivity () near the topographic features was observed in the Kuroshio. In this region, turbulence generates microscale temperature variance by acting directly on large‐scale temperature gradients rather than on those of the mesoscale. In contrast, the section‐averaged isopycnal stirring coefficient, , in the Kuroshio Extension front was , which is an order of magnitude greater than that estimated over the Izu Ridge. The Turner angle and enhanced effective thermal diffusivity , which is parameterized fairly well with the double diffusion model diffusivity , indicate the presence of active double diffusion leading to the generation of microscale thermal variance through the terms of double diffusion acting on the vertical gradient of the mesoscale temperature. These results suggest that isopycnal stirring generates mesoscale temperature variance, which is acted upon by double diffusion, thus producing significant microscale variance and its dissipation. One to two orders of magnitude larger than in the Kuroshio Extension implies non‐negligible contributions from thermal variance generation through lateral temperature gradients at submesoscale fronts.