Abstract
This study investigates the submesoscale fronts and their dynamic effects on the mean flow due to frontal instabilities in the wind-driven summer offshore jet of the western South China Sea (WSCS), using satellite observations, a 500 m-resolution numerical simulation, and diagnostic analysis. Both satellite measurements and simulation results show that the submesoscale fronts occupying a typical lateral scale of O(∼10) km are characterized with one order of Rossby (Ro) and Richardson (Ri) numbers in the WSCS. This result implies that both geostrophic and ageostrophic motions feature in these submesoscale fronts. The diagnostic results indicate that a net cross-frontal Ekman transport driven by down-front wind forcing effectively advects cold water over warm water. By this way, the weakened local stratification and strong lateral buoyancy gradients are conducive to a negative Ertel potential vorticity (PV) and triggering frontal symmetric instability (SI) at the submesoscale density front. The cross-front ageostrophic secondary circulation caused by frontal instabilities is found to drive an enhanced vertical velocity reaching O(100) m/d. Additionally, the estimate of the down-front wind forcing the Ekman buoyancy flux (EBF) is found to be scaled with the geostrophic shear production (GSP) and buoyancy flux (BFLUX), which are the two primary energy sources for submesoscale turbulence. The large values of GSP and BFLUX at the fronts suggest an efficient downscale energy transfer from larger-scale geostrophic flows to the submesoscale turbulence owing to down-front wind forcing and frontal instabilities. In this content, submesoscale fronts and their instabilities substantially enhance the local vertical exchanges and geostrophic energy cascade towards smaller-scale. These active submesoscale processes associated density fronts and filaments likely provide new physical interpretations for the filamentary high chlorophyll concentration and frontal downscale energy transfer in the WSCS.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
Barkan R, Winters K B, Smith S G L. 2015. Energy cascades and loss of balance in a reentrant channel forced by wind stress and buoyancy fluxes. Journal of Physical Oceanography, 45(1): 272–293, doi: https://doi.org/10.1175/JPO-D-14-0068.1
Boccaletti G, Ferrari R, Fox-Kemper B. 2007. Mixed layer instabilities and restratification. Journal of Physical Oceanography, 37(9): 2228–2250, doi: https://doi.org/10.1175/JPO3101.1
Capet X, McWilliams J C, Molemaker M J, et al. 2008. Mesoscale to submesoscale transition in the California current system. Part I: Flow structure, eddy flux, and observational tests. Journal of Physical Oceanography, 38(1): 29–43, doi: https://doi.org/10.1175/2007JPO3671.1
Carton J A, Giese B S. 2008. A reanalysis of ocean climate using Simple Ocean Data Assimilation (SODA). Monthly Weather Review, 136(8): 2999–3017, doi: https://doi.org/10.1175/2007MWR1978.1
Charney J G. 1971. Geostrophic turbulence. Journal of the Atmospheric Sciences, 28(6): 1087–1095, doi: https://doi.org/10.1175/1520-0469(1971)028<1087:GT>2.0.CO;2
Chen Gengxin, Gan Jianping, Xie Qiang, et al. 2012. Eddy heat and salt transports in the South China Sea and their seasonal modulations. Journal of Geophysical Research: Oceans, 117(C5): C05021
Chen Gengxin, Hou Yijun, Chu Xiaoqing. 2011. Mesoscale eddies in the South China Sea: Mean properties, spatiotemporal variability, and impact on thermohaline structure. Journal of Geophysical Research: Oceans, 116(C6): C06018
Chen Changlin, Wang Guihua. 2014. Interannual variability of the eastward current in the western South China Sea associated with the summer Asian monsoon. Journal of Geophysical Research: Oceans, 119(9): 5745–5754, doi: https://doi.org/10.1002/2014JC010309
Chu P C, Fan C W, Lozano C J, et al. 1998. An airborne expendable bathythermograph survey of the South China Sea, May 1995. Journal of Geophysical Research: Oceans, 103(C10): 21637–21652, doi: https://doi.org/10.1029/98JC02096
D’Asaro E, Lee C, Rainville L, et al. 2011. Enhanced turbulence and energy dissipation at ocean fronts. Science, 332(6027): 318–322, doi: https://doi.org/10.1126/science.1201515
Dong Jihai, Zhong Yisen. 2018. The spatiotemporal features of submesoscale processes in the northeastern South China Sea. Acta Oceanologica Sinica, 37(11): 8–18, doi: https://doi.org/10.1007/s13131-018-1277-2
Donlon C J, Martin M, Stark J, et al. 2012. The operational sea surface temperature and sea ice analysis (OSTIA) system. Remote Sensing of Environment, 116: 140–158, doi: https://doi.org/10.1016/j.rse.2010.10.017
Fang Wendong, Fang Guohong, Shi Ping, et al. 2002. Seasonal structures of upper layer circulation in the southern South China Sea from in situ observations. Journal of Geophysical Research: Oceans, 107(C11): 3202, doi: https://doi.org/10.1029/2002JC001343
Fox-Kemper B, Ferrari R, Hallberg R. 2008. Parameterization of mixed layer eddies. Part I: Theory and diagnosis. Journal of Physical Oceanography, 38(6): 1145–1165, doi: https://doi.org/10.1175/2007JPO3792.1
Gan Jianping, Li H, Curchitser E N, et al. 2006. Modeling South China sea circulation: Response to seasonal forcing regimes. Journal of Geophysical Research: Oceans, 111(C6): C06034
Gan Jianping, Qu Tangdong. 2008. Coastal jet separation and associated flow variability in the southwest South China Sea. Deep Sea Research Part I: Oceanographic Research Papers, 55(1): 1–19, doi: https://doi.org/10.1016/j.dsr.2007.09.008
Gula J, Molemaker M J, McWilliams J C. 2014. Submesoscale cold filaments in the gulf stream. Journal of Physical Oceanography, 44(10): 2617–2643, doi: https://doi.org/10.1175/JPO-D-14-0029.1
Gula J, Molemaker M J, McWilliams J C. 2015. Gulf stream dynamics along the southeastern U.S. seaboard. Journal of Physical Oceanography, 45(3): 690–715, doi: https://doi.org/10.1175/JPO-D-14-0154.1
Gula J, Molemaker M J, McWilliams J C. 2016. Submesoscale dynamics of a gulf stream frontal eddy in the south atlantic bight. Journal of Physical Oceanography, 46(1): 305–325, doi: https://doi.org/10.1175/JPO-D-14-0258.1
He Qingyou, Zhan Haigang, Cai Shuqun, et al. 2018. A new assessment of mesoscale eddies in the South China Sea: Surface features, three-dimensional structures, and thermohaline transports. Journal of Geophysical Research: Oceans, 123(7): 4906–4929, doi: https://doi.org/10.1029/2018JC014054
Hoskins B J. 1974. The role of potential vorticity in symmetric stability and instability. Quarterly Journal of the Royal Meteorological Society, 100(425): 480–482, doi: https://doi.org/10.1002/qj.49710042520
Hu Jianyu, Gan Jianping, Sun Zhenyu, et al. 2011. Observed three-dimensional structure of a cold eddy in the southwestern South China Sea. Journal of Geophysical Research: Oceans, 116(C5): C05016
Kuo N J, Zheng Quanan, Ho C R. 2000. Satellite observation of upwelling along the western coast of the South China Sea. Remote Sensing of Environment, 74(3): 463–470, doi: https://doi.org/10.1016/S0034-4257(00)00138-3
Large W G, McWilliams J C, Doney S C. 1994. Oceanic vertical mixing: a review and a model with a nonlocal boundary layer parameterization. Reviews of Geophysics, 32(4): 363–403, doi: https://doi.org/10.1029/94RG01872
Lemarié F, Kurian J, Shchepetkin A F, et al. 2012. Are there inescapable issues prohibiting the use of terrain-following coordinates in climate models?. Ocean Modelling, 42: 57–79, doi: https://doi.org/10.1016/j.ocemod.2011.11.007
Li Yuanlong, Han Weiqing, Wilkin J L, et al. 2014. Interannual variability of the surface summertime eastward jet in the South China Sea. Journal of Geophysical Research: Oceans, 119(10): 7205–7228, doi: https://doi.org/10.1002/2014JC010206
Liu Xiao, Levine N M. 2016. Enhancement of phytoplankton chlorophyll by submesoscale frontal dynamics in the North Pacific Subtropical Gyre. Geophysical Research Letters, 43(4): 1651–1659, doi: https://doi.org/10.1002/2015GL066996
Liu Fenfen, Tang Shilin, Chen Chuqun. 2015. Satellite observations of the small-scale cyclonic eddies in the western South China Sea. Biogeosciences, 12(2): 299–305, doi: https://doi.org/10.5194/bg-12-299-2015
Mahadevan A, Tandon A. 2006. An analysis of mechanisms for submesoscale vertical motion at ocean fronts. Ocean Modelling, 14(3–4): 241–256, doi: https://doi.org/10.1016/j.ocemod.2006.05.006
McWilliams J C. 2016. Submesoscale currents in the ocean. Proceedings of the Royal Society A-Mathematical Physical and Engineering Sciences, 472(2189): 20160117, doi: https://doi.org/10.1098/rspa.2016.0117
McWilliams J C. 2017. Submesoscale surface fronts and filaments: secondary circulation, buoyancy flux, and frontogenesis. Journal of Fluid Mechanics, 823: 391–432, doi: https://doi.org/10.1017/jfm.2017.294
McWilliams J C. 2019. A survey of submesoscale currents. Geoscience Letters, 6: 3, doi: https://doi.org/10.1186/s40562-019-0133-3
McWilliams J C, Molemaker M J, Yavneh I. 2001. From stirring to mixing of momentum: Cascades from balanced flows to dissipation in the oceanic interior. In: Proceedings of the 12th’ Aha Huliko’a Hawaiian Winter Workshop. Hawaii: University of Hawaii, 59–66
Metzger E J. 2003. Upper ocean sensitivity to wind forcing in the South China Sea. Journal of Oceanography, 59(6): 783–798, doi: https://doi.org/10.1023/B:JOCE.0000009570.41358.c5
Nagai T, Tandon A, Yamazaki H, et al. 2012. Direct observations of microscale turbulence and thermohaline structure in the Kuroshio Front. Journal of Geophysical Research: Oceans, 117(C8): C08013
Omand M M, D’Asaro E A, Lee C M, et al. 2015. Eddy-driven subduction exports particulate organic carbon from the spring bloom. Science, 348(6231): 222–225, doi: https://doi.org/10.1126/science.1260062
Penven P, Debreu L, Marchesiello P, et al. 2006. Evaluation and application of the ROMS 1-way embedding procedure to the central california upwelling system. Ocean Modelling, 12(1–2): 157–187, doi: https://doi.org/10.1016/j.ocemod.2005.05.002
Risien C M, Chelton D B. 2008. A global climatology of surface wind and wind stress fields from eight years of QuikSCAT scatterometer data. Journal of Physical Oceanography, 38(11): 2379–2413, doi: https://doi.org/10.1175/2008JPO3881.1
Shchepetkin A F, McWilliams J C. 2005. The regional oceanic modeling system (ROMS): a split-explicit, free-surface, topography-following-coordinate oceanic model. Ocean Modelling, 9(4): 347–404, doi: https://doi.org/10.1016/j.ocemod.2004.08.002
Su Zhan, Ingersoll A P, Stewart A L, et al. 2016. Ocean convective available potential energy. Part I: Concept and calculation. Journal of Physical Oceanography, 46(4): 1081–1096, doi: https://doi.org/10.1175/JPO-D-14-0155.1
Su Zhan, Wang Jinbo, Klein P, et al. 2018. Ocean submesoscales as a key component of the global heat budget. Nature Communications, 9: 775, doi: https://doi.org/10.1038/s41467-018-02983-w
Taylor J R, Ferrari R. 2009. On the equilibration of a symmetrically unstable front via a secondary shear instability. Journal of Fluid Mechanics, 622: 103–113, doi: https://doi.org/10.1017/S0022112008005272
Thomas L N. 2005. Destruction of potential vorticity by winds. Journal of Physical Oceanography, 35(12): 2457–2466, doi: https://doi.org/10.1175/JPO2830.1
Thomas L N. 2012. On the effects of frontogenetic strain on symmetric instability and inertia-gravity waves. Journal of Fluid Mechanics, 711: 620–640, doi: https://doi.org/10.1017/jfm.2012.416
Thomas L N, Lee C M. 2005. Intensification of ocean fronts by down-front winds. Journal of Physical Oceanography, 35(6): 1086–1102, doi: https://doi.org/10.1175/JPO2737.1
Thomas L N, Tandon A, Mahadevan A. 2008. Submesoscale processes and dynamics. In: Hecht M W, Hecht H, eds. Ocean Modeling in an Eddying Regime. Washington DC: Geophysical Monograph Series, 17–38
Thomas L N, Taylor J R. 2010. Reduction of the usable wind-work on the general circulation by forced symmetric instability. Geophysical Research Letters, 37(18): L18606
Thomas L N, Taylor J R, D’Asaro E A, et al. 2016. Symmetric instability, inertial oscillations, and turbulence at the gulf stream front. Journal of Physical Oceanography, 46(1): 197–217, doi: https://doi.org/10.1175/JPO-D-15-0008.1
Thomas L N, Taylor J R, Ferrari R, et al. 2013. Symmetric instability in the Gulf Stream. Deep Sea Research Part II: Topical Studies in Oceanography, 91: 96–110, doi: https://doi.org/10.1016/j.dsr2.2013.02.025
Wang Guihua, Chen Dake, Su Jilan. 2006. Generation and life cycle of the dipole in the South China Sea summer circulation. Journal of Geophysical Research: Oceans, 111(C6): C06002
Wang Guihua, Chen Dake, Su Jilan. 2008. Winter eddy genesis in the eastern South China Sea due to orographic wind jets. Journal of Physical Oceanography, 38(3): 726–732, doi: https://doi.org/10.1175/2007JPO3868.1
Wang Guihua, Su Jilan, Chu P C. 2003. Mesoscale eddies in the South China Sea observed with altimeter data. Geophysical Research Letters, 30(21): 2121, doi: https://doi.org/10.1029/2003GL018532
Wang Guihua, Wang Chunzai, Huang Ruixin. 2010. Interdecadal variability of the eastward current in the South China Sea associated with the summer Asian Monsoon. Journal of Climate, 23(22): 6115–6123, doi: https://doi.org/10.1175/2010JCLI3607.1
Woodruff S D, Worley S J, Lubker S J, et al. 2011. ICOADS Release 2.5: extensions and enhancements to the surface marine meteorological archive. International Journal of Climatology, 31(7): 951–967, doi: https://doi.org/10.1002/joc.2103
Wunsch C, Ferrari R. 2004. Vertical mixing, energy, and the general circulation of the oceans. Annual Review of Fluid Mechanics, 36: 281–314, doi: https://doi.org/10.1146/annurev.fluid.36.050802.122121
Wyrtki K. 1961. Physical oceanography of the southeast Asian waters. NAGA Report Vol. 2, Scientific Results of Marine Investigations of the South China Sea and the Gulf of Thailand. La Jolla, California: Scripps Institution of Oceanography, 195
Xie Shangping, Chang C H, Xie Qiang, et al. 2007. Intraseasonal variability in the summer South China Sea: Wind jet, cold filament, and recirculations. Journal of Geophysical Research: Oceans, 112(C10): C10008, doi: https://doi.org/10.1029/2007JC004238
Xie Shangping, Xie Qiang, Wang Dongxiao, et al. 2003. Summer upwelling in the South China Sea and its role in regional climate variations. Journal of Geophysical Research: Oceans, 108(C8): 3261, doi: https://doi.org/10.1029/2003JC001867
Xie Lingling, Zheng Quanan. 2017. New insight into the South China Sea: Rossby normal modes. Acta Oceanologica Sinica, 36(7): 1–3, doi: https://doi.org/10.1007/s13131-017-1077-0
Xie Lingling, Zheng Quanan, Zhang Shuwen, et al. 2018. The Rossby normal modes in the South China Sea deep basin evidenced by satellite altimetry. International Journal of Remote Sensing, 39(2): 399–417, doi: https://doi.org/10.1080/01431161.2017.1384591
Xiu Peng, Chai Fei. 2011. Modeled biogeochemical responses to mesoscale eddies in the South China Sea. Journal of Geophysical Research: Oceans, 116(C10): C10006, doi: https://doi.org/10.1029/2010JC006800
Yang Qingxuan, Nikurashin M, Sasaki H, et al. 2019. Dissipation of mesoscale eddies and its contribution to mixing in the northern South China Sea. Scientific Reports, 9: 556
Yu Jie, Zheng Quanan, Jing Zhiyou, et al. 2018. Satellite observations of sub-mesoscale vortex trains in the western boundary of the South China Sea. Journal of Marine Systems, 183: 56–62, doi: https://doi.org/10.1016/j.jmarsys.2018.03.010
Zhang Huaimin, Bates J J, Reynolds R W. 2006. Assessment of composite global sampling: Sea surface wind speed. Geophysical Research Letters, 33(17): L17714, doi: https://doi.org/10.1029/2006GL027086
Zhang Zhengguang, Qiu Bo, Klein P, et al. 2019. The influence of geostrophic strain on oceanic ageostrophic motion and surface chlorophyll. Nature Communications, 10: 2838, doi: https://doi.org/10.1038/s41467-019-10883-w
Zhang Zhiwei, Tian Jiwei, Qiu Bo, et al. 2016. Observed 3D structure, generation, and dissipation of oceanic mesoscale eddies in the South China Sea. Scientific Reports, 6: 24349, doi: https://doi.org/10.1038/srep24349
Zheng Quanan, Lin Hui, Meng Junmin, et al. 2008. Sub-mesoscale ocean vortex trains in the Luzon Strait. Journal of Geophysical Research: Oceans, 113(C4): C04032
Zheng Quanan, Xie Lingling, Xiong Xuejun, et al. 2020. Progress in research of submesoscale processes in the South China Sea. Acta Oceanologica Sinica, 39(1): 1–13, doi: https://doi.org/10.1007/s13131-019-1521-4
Zheng Quanan, Xie Lingling, Zheng Zhewen, et al. 2017. Progress in research of mesoscale eddies in the South China Sea. Advances in Marine Science (in Chinese), 35(2): 131–158
Acknowledgements
We thank GHRSST-PP (http://ghrsst-pp.metoffice.com), AVISO+ (http://www.aviso.altimetry.fr), NODC/NOAA (http://data.nodc.noaa.gov), and ESA (ftp://merisfrs-ftp-ds.eo.esa.int) for providing a suite of high-resolution satellite data and reanalysis products.
Author information
Authors and Affiliations
Corresponding author
Additional information
Foundation item
This work is supported by the Chinese Academy of Sciences under contract Nos ZDBS-LY-DQC011, ZDRW-XH-2019-2 and ISEE2018PY05; the Southern Marine Science and Engineering Guangdong Laboratory (Guangzhou) under contract No. GML2019ZD0303; the National Natural Science Foundation of China under contract Nos 41776040 and 92058201; the Pilot National Laboratory for Marine Science and Technology (Qingdao) under contract No. OCFL-201804; the State Key Laboratory of Tropical Oceanography under contract No. LTO1907; the Guangzhou Science and Technology Project under contract No. 201904010420.
Rights and permissions
About this article
Cite this article
Huang, X., Jing, Z., Zheng, R. et al. Dynamical analysis of submesoscale fronts associated with wind-forced offshore jet in the western South China Sea. Acta Oceanol. Sin. 39, 1–12 (2020). https://doi.org/10.1007/s13131-020-1671-4
Received:
Accepted:
Published:
Issue Date:
DOI: https://doi.org/10.1007/s13131-020-1671-4