Submesoscale Frontal Instabilities Modulate Large-Scale Distribution of the Winter Deep Mixed Layer in the Kuroshio-Oyashio Extension

In the mid-latitude North Pacific, the wintertime ocean heat loss reaches the maximum in the Kuroshio-Oyashio Extension (KOE) region. However, the mixed layer depth (MLD) there is shallow (<50 m), flanked by a zonal band of deep MLD (>100 m) on either side. Such an observed mixed layer pattern is not reproduced by coarse-resolution or mesoscale eddy-resolving (1/10 degrees) models. What causes the observed MLD shoaling along the KOE frontal region? Here we show that the mixed layer shoaling in the front is well reproduced by a 1/30 degrees, submesoscale-permitting ocean general circulation model, indicating that it is closely related to the submesoscale frontal dynamics. The geostrophic strain rate is strong along the sharp KOE front, favoring submesoscale frontal instabilities. The surface buoyancy loss during winter reduces stratification and enhances the fronts (i.e., with steeper, northward rising isopycnals). The available potential energy stored in the narrow fronts can be released in the form of eddy kinetic energy by the mixed layer baroclinic instabilities (MLI). The associated vigorous ageostrophic motions associated with MLI efficiently slump the vertically oriented isopycnals and restratify the mixed layer. Thus, the MLD is shallow in the KOE frontal region despite the strong surface heat loss and wind stirring during winter. The submesoscale processes revealed here have important implications for improving climate models. Plain Language Summary Although the ocean heat loss reaches its maximum in the Kuroshio-Oyashio Extension (KOE) region, the mixed layer depth (MLD) there is shallow (<50 m), flanked by a zonal band of deep MLD (>100 m) either to the north or south of the front. Such observed mixed layer pattern cannot be resolved by neither climate models nor eddy-resolving (1/10 degrees) models, and the detailed physical processes are still unclear. Here we show that the mixed layer shoaling in the KOE is well reproduced by a 1/30 degrees, submesoscale-permitting ocean general circulation model, indicating that it is closely related to the submesoscale frontal dynamics. We show that the strong geostrophic strain in the KOE region acts to break the front to submesoscale, while the large surface buoyancy loss during winter enhances the submesoscale front and catalyzes the frontal instabilities. The associated vigorous ageostrophic motions associated with mixed layer baroclinic instabilities (MLI) efficiently slump the vertically oriented isopycnals and restratify the mixed layer in winter. Our results indicate that explicitly resolving submesoscale is vital to accurately simulate the MLD in the KOE region. Alternatively, appropriate parameterization of frontal instabilities is needed for climate models.

Automated summary

In the mid-latitude North Pacific, the winter ocean heat loss reaches its maximum in the Kuroshio-Oyashio Extension (KOE) region. However, the observed shallow mixed layer depth (MLD) there, flanked by deep MLD on either side, is not reproduced by coarse-resolution or mesoscale eddy-resolving models. This study shows that the shallow MLD in the KOE region is closely related to submesoscale frontal dynamics, where the geostrophic strain rate and surface buoyancy loss during winter play important roles.