Abstract
Submesoscale activity in the upper ocean has received intense studies through simulations and observations in the last decade, but in the eddy-active South China Sea (SCS) the fine-scale dynamical processes of submesoscale behaviors and their potential impacts have not been well understood. This study focuses on the elongated filaments of an eddy field in the northern SCS and investigates submesoscale-enhanced vertical motions and the underlying mechanism using satellite-derived observations and a high-resolution (∼500 m) simulation. The satellite images show that the elongated highly productive stripes with a typical lateral scale of ∼25 km and associated filaments are frequently observed at the periphery of mesoscale eddies. The diagnostic results based on the 500 m-resolution realistic simulation indicate that these submesoscale filaments are characterized by cross-filament vertical secondary circulations with an increased vertical velocity reaching O(100 m/d) due to submesoscale instabilities. The vertical advections of secondary circulations drive a restratified vertical buoyancy flux along filament zones and induce a vertical heat flux up to 110 W/m2. This result implies a significant submesoscale-enhanced vertical exchange between the ocean surface and interior in the filaments. Frontogenesis that acts to sharpen the lateral buoyancy gradients is detected to be conducive to driving submesoscale instabilities and enhancing secondary circulations through increasing the filament baroclinicity. The further analysis indicates that the filament frontogenesis detected in this study is not only derived from mesoscale straining of the eddy, but also effectively induced by the subsequent submesoscale straining due to ageostrophic convergence. In this context, these submesoscale filaments and associated frontogenetic processes can provide a potential interpretation for the vertical nutrient supply for phytoplankton growth in the high-productive stripes within the mesoscale eddy, as well as enhanced vertical heat transport.
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Acknowledgements
We thank Haijin Cao of Hohai University and Xiaolong Huang, who improved this manuscript with helpful comments and fruitful discussions. The numerical simulation is supported by the High Performance Computing Division in the South China Sea Institute of Oceanology. The authors would like to thank GSFC of NASA (http://oceancolor.gsfc.nasa.gov), AVISO+ (https://www.aviso.altimetry.fr), BODC (https://www.bodc.ac.uk), APDRC (http://apdrc.soest.hawaii.edu) for providing a suite of high-resolution satellite data and reanalysis products.
Funding
The National Natural Science Foundation of China under contract Nos 92058201, 41776040, 41830538 and 41949907; the Talents Team Project of Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) under contract No. GML2019ZD0303; the Program of Chinese Academy of Sciences under contract Nos ZDBS-LY-DQC011, ZDRW-XH-2019-2, XDA15020901 and ISEE2021PY01.
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Zheng, R., Jing, Z. Submesoscale-enhanced filaments and frontogenetic mechanism within mesoscale eddies of the South China Sea. Acta Oceanol. Sin. 41, 42–53 (2022). https://doi.org/10.1007/s13131-021-1971-3
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DOI: https://doi.org/10.1007/s13131-021-1971-3