Submesoscale Frontal Instabilities Modulate Large‐Scale Distribution of the Winter Deep Mixed Layer in the Kuroshio‐Oyashio Extension. Issue 12 (9th December 2022)

• Record Type: Journal Article
• Authors: Ding, Yang; Xu, Lixiao; Xie, Shang‐Ping; Sasaki, Hideharu; Zhang, Zhengguang; Cao, Haijin; Zhang, Yang
• Is Part Of: Journal of geophysical research. Volume 127:Issue 12(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-09
• Subjects: submesoscale frontal instabilities -- mixed layer restratification -- baroclinic mixed layer instability -- Kuroshio‐Oyashio Extension -- the North Pacific -- deep mixed layer; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018915
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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°) 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°, 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°) 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°, 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. Key Points: The winter mixed layer is shallow in Kuroshio‐Oyashio Extension (KOE) frontal region despite the strong heat loss and wind stirring The mixed layer shoaling in the KOE frontal region is closely associated with submesoscale frontal dynamics The submesoscale ageostrophic motions slump the vertically oriented isopycnals due to buoyancy loss, and restratify the mixed layer