Submesoscale Ageostrophic Motions Within and Below the Mixed Layer of the Northwestern Pacific Ocean. Issue 2 (22nd February 2022)

• Record Type: Journal Article
• Authors: Cao, Haijin; Jing, Zhiyou
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-22
• Subjects: submesoscale -- vertical velocity -- Nothern Pacific Ocean -- spectral analysis; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2021JC017812
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Submesoscale dynamics below the mixed layer (ML) and their mechanisms are still unclear. By a series of nested simulations in the Pacific Northwest with high horizontal resolution of ∼500 m, this study reveals that there exist strong submesoscale ageostrophic motions in the upper pycnocline of the Kuroshio Extension region. These motions exhibit enhanced lateral buoyancy gradient and vigorous vertical velocity but with weak vertical vorticity distinct from the ML submesoscale activities. The vertical velocity in the high‐resolution simulation reaches tens of meters per day, consistent with recent observations (e.g., SubMESI and OSMOSIS). Our analysis shows that the enhanced vertical velocity is mostly attributed to the along‐isopycnal motions at the Kuroshio front, but in the region nearby the large vertical velocity mostly arises from the wave‐like vertical movement of isopycnals. To understand the mechanisms for the large vertical velocity, this paper further examined the instability of the flow and the frequency‐wavenumber spectra of vertical vorticity, lateral divergence, lateral buoyancy gradient, and vertical velocity. A criterion based on the ratio between divergence and vorticity variance in spectral space is used to roughly identify the upper bound of unbalanced submesoscales. The results suggest that the high‐frequency, high‐wavenumber processes dominate the vertical motions within and below the ML and significantly enhance the net vertical heat transport between the ML and the ocean interior. This study seeks to provide comprehension of the submesoscale ageostrophic motions below the ML and their impacts on the upper ocean. Plain Language Summary: The ocean is usually known as a dynamical system of geostrophic turbulence. This study uses a high‐resolution (∼500 m) numerical simulation to investigate the geostrophically unbalanced motions with horizontal length scales of a few kilometers (submesoscales) in the upper ocean of the northwestern Pacific. The results show active submesoscale processes (e.g., submesoscale fronts and eddies) at the surface and even below the mixed layer (ML), which have distinct dynamical features from large‐scale or mesoscale flows. Although these motions are not the primary energy reservoir, they play a significant role in the energy transfer between scales and can drive much stronger vertical motions than large‐scale and mesoscale processes within and below the ML. This is consistent with the recent observations in the other regional oceans. Importantly, our results suggest that these motions can drive a significant net heat transport between the ML and the ocean interior at horizontal length scales much smaller than the baroclinic Rossby deformation radius. Key Points: The submesoscale motions below the mixed layer (ML) show different features from the mixed‐layer submesoscale processes The mechanisms for the enhanced vertical velocity rely on along‐isopycnal motions and internal waves The high‐frequency, high‐wavenumber processes dominate the net vertical heat transport between the ML and the ocean interior