南海北部次中尺度过程数值研究
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摘要
近年来的观测与理论研究发现,海洋上混合层存在一类水平尺度为0.1~10km、时间尺度为~O(1天)的重要物理过程,称之为次中尺度过程。该过程具有较大的罗斯贝数(Ro)和较小的理查森数(Ri),它能有效地通过次级不稳定从中尺度地转过程中汲取能量,并向小尺度湍流混合串级,从而对上层海洋物质能量输运、中尺度过程变异、海气相互作用,以及混合层再分层等产生重要影响。利用区域海洋模式系统ROMS(Regional Ocean Modeling System)进行水平分辨率约为1km的高分辨率数值实验,对南海北部的次中尺度过程进行了初步探讨。分析结果表明,南海北部海域有着丰富的中尺度涡旋与海洋锋面活动,且涡旋与锋区边缘存在显著的次中尺度现象。通过对次中尺度涡旋个例的稳定性和能量分析发现,锋面海域强烈的水平浮力梯度导致了涡丝边缘的Ertel位涡小于0,并引起对称不稳定,锋生作用是该次中尺涡旋南侧发生对称不稳定的主要动力机制。同时,对称不稳定能有效地从地转剪切中汲取能量并向小尺度湍流混合串级,其能量汲取的最大值出现在20m深度,约为4×10~(–7)W×kg~(–1)。
Recent field observations and theoretical analysis revealed the processes with the lateral scale of 0.1-10 km and time scale of O(1 day) in the mixed layer, which are termed sub-mesoscale processes. Physics at these scales are distinguished by big Rossby number(Ro) and small Richardson number(Ri). They are able to effectively extract energy from geostrophic shear through sub-mesoscale instabilities, and then forward cascade the energy to dissipation at smaller scales. Moreover, sub-mesoscale processes can play a crucial role in the transport of upper-ocean mass and energy, mesoscale variability, air-sea interaction, and upper-ocean restratification, among others. Base on the Regional Ocean Modeling System(ROMS) high-resolution(1-km) numerical experiment, we preliminarily discuss sub-mesoscale processes in the northern South China Sea(NSCS) in this paper. The results suggest that mesoscale eddies and fronts were ubiquitous in the NSCS with significant sub-mesoscale process at their edges. Analytical investigation of stability and energy in one sub-mesoscale eddy as a case study indicates that the generation of negative potential vorticity(q) resulting in symmetric instability(SI) was primarily attributable to the strong lateral buoyancy gradients adjacent to frontal vortex filament. This instability is mainly produced by the frontogenesis at the southern edge of the sub-mesoscale eddy. Moreover, the extracted energy by the SI from geostrophic shear tends to forward cascade to the turbulence at small scale, with a maximum energy extraction of 4×10~(–7)W×kg~(–1) at about 20-m depth.
Recent field observations and theoretical analysis revealed the processes with the lateral scale of 0.1-10 km and time scale of O(1 day) in the mixed layer, which are termed sub-mesoscale processes. Physics at these scales are distinguished by big Rossby number(Ro) and small Richardson number(Ri). They are able to effectively extract energy from geostrophic shear through sub-mesoscale instabilities, and then forward cascade the energy to dissipation at smaller scales. Moreover, sub-mesoscale processes can play a crucial role in the transport of upper-ocean mass and energy, mesoscale variability, air-sea interaction, and upper-ocean restratification, among others. Base on the Regional Ocean Modeling System(ROMS) high-resolution(1-km) numerical experiment, we preliminarily discuss sub-mesoscale processes in the northern South China Sea(NSCS) in this paper. The results suggest that mesoscale eddies and fronts were ubiquitous in the NSCS with significant sub-mesoscale process at their edges. Analytical investigation of stability and energy in one sub-mesoscale eddy as a case study indicates that the generation of negative potential vorticity(q) resulting in symmetric instability(SI) was primarily attributable to the strong lateral buoyancy gradients adjacent to frontal vortex filament. This instability is mainly produced by the frontogenesis at the southern edge of the sub-mesoscale eddy. Moreover, the extracted energy by the SI from geostrophic shear tends to forward cascade to the turbulence at small scale, with a maximum energy extraction of 4×10~(–7)W×kg~(–1) at about 20-m depth.
引文
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闫桐,齐义泉,经志友,2015.南海上层环流对不同气候态风场响应的数值研究[J].热带海洋学报,34(4):1-11.YANTONG,QI YIQUAN,JING ZHIYOU,2015.A numerical study on the responses of the South China Sea upper circulation to different climatological wind products[J].Journal of Tropical Oceanography,34(4):1-11(in Chinese).
BRUCE J G,1995.Eddies southwest of the Denmark Strait[J].Deep Sea Research Part I:Oceanographic Research Papers,42(1):13-29.
CAPET X,MCWILLIAMS J C,MOLEMAKER M J,et al,2008a.Mesoscale to submesoscale transition in the California Current System.Part I:Flow structure,eddy flux,and observational tests[J].Journal of Physical Oceanography,38(1):29-43.
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CHEN J,1983.Some explanations for the real-time distribution of sea surface temperature in the northern South China Sea winter[J].Acta Oceanologica Sinica,5(3):391-395.
CHU P C,FAN CHENWU,LOZANO C J,et al,1998.An airborne expendable bathythermograph survey of the South China Sea,May 1995[J].Journal of Geophysical Research:Oceans(1978-2012),103(C10):21637-21652.
D’ASARO E,LEE C,RAINVILLE L,et al,2011.Enhanced turbulence and energy dissipation at ocean fronts[J].Science,332(6027):318-322.
DONG CHANGMING,MCWILLIAMS J C,SHCHEPETKIN A F,2007.Island wakes in deep water[J].Journal of Physical Oceanography,37(4):962-981.
FLAMENT P,ARMI L,WASHBURN L,1985.The evolving structure of an upwelling filament[J].Journal of Geophysical Research:Oceans,90(C6):11765-11778.
HAINE T W N,MARSHALL J,1998.Gravitational,symmetric,and baroclinic instability of the ocean mixed layer[J].Journal of Physical Oceanography,28(4):634-658.
HOLMES R M,THOMAS L N,THOMPSON L,et al,2014.Potential vorticity dynamics of tropical instability vortices[J].Journal of Physical Oceanography,44(3):995-1011.
HOSKINS B J,1974.The role of potential vorticity in symmetric stability and instability[J].Quarterly Journal of the Royal Meteorological Society,100(425):480-482.
HOSKINS B J,1982.The mathematical theory of frontogenesis[J].Annual Review of Fluid Mechanics,14(1):131-151.
HU JIANGYU,KAWAMURA H,HONG HUASHENG,et al,2000A review on the currents in the South China Sea:Seasonal circulation,South China Sea Warm Current and Kuroshio intrusion[J].Journal of Oceanography,56(6):607-624.
HU J Y,KAWAMURA H,TANG D L,2003.Tidal front around the Hainan Island,northwest of the South China Sea[J].Journal of Geophysical Research Oceans,108(C11):3342.
JING ZHIYOU,QI YIQUAN,DU YAN,et al,2015.Summer upwelling and thermal fronts in the northwestern South China Sea:Observational analysis of two mesoscale mapping surveys[J].Journal of Geophysical Research:Oceans,120(3):1993-2006.
KLEIN P,LAPEYRE G,2009.The oceanic vertical pump induced by mesoscale and submesoscale turbulence[J].Annual Review of Marine Science,1:351-375.
LEMARIéF,KURIAN J,SHCHEPETKIN A F,et al,2012.Are there inescapable issues prohibiting the use of terrainfollowing coordinates in climate models?[J].Ocean Modelling,42:57-79.
LéVY M,KLEIN P,TREGUIER A M,2001.Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime[J].Journal of Marine Research,59(4):535-565.
LI LI,GUO XIAOGANG,WU RISHENG,2000.Oceanic fronts in southern Taiwan Strait[J].Journal of Oceanography in Taiwan Strait,19(2):145-156.
LIU SUMEI,GUO XINYU,CHEN QI,et al,2010.Nutrient dynamics in the winter thermohaline frontal zone of the northern shelf region of the South China Sea[J].Journal of Geophysical Research,115(C11):C11020.
LIU F,TANG S,CHEN C,2015.Satellite observations of the small-scale cyclonic eddies in the western South China Sea[J].Biogeosciences,12(2):299-305.
MARCHESIELLO P,CAPET X,MENKES C,et al,2011.Submesoscale dynamics in tropical instability waves[J].Ocean Modelling,39(1/2):31-46.
MCGILLICUDDY D J,ANDERSON L A,DONEY S C,et al,2003.Eddy-driven sources and sinks of nutrients in the upper ocean:Results from a 0.1°resolution model of the North Atlantic[J].Global Biogeochemical Cycles,17(2):1035.
MCGILLICUDDY D J JR,ANDERSON L A,BATES N R,et al,2007.Eddy/wind interactions stimulate extraordinary mid-ocean plankton blooms[J].Science,316(5827):1021-1026.
MCWILLIAMS J C,2010.A perspective on submesoscale geophysical turbulence[C]//IUTAM Symposium on Turbulence in the Atmosphere and Oceans.Dordrecht,Netherlands:Springer,28:131-141.
MUNK W,ARMI L,FISCHER K,et al,2000.Spirals on the sea[J]Proceedings of the Royal Society A:Mathematical,Physical and Engineering Sciences,456(1997):1217-1280.
PI QINGLING,HU JIANYU,2010.Analysis of sea surface temperature fronts in the Taiwan Strait and its adjacent area using an advanced edge detection method[J].Science China Earth Science,53(7):1008-1016.
QU TANGDONG,MITSUDERA H,YAMAGATA T,2000Intrusion of the North Pacific waters into the South China Sea[J].Journal of Geophysical Research:Oceans(1978-2012),105(C3):6415-6424.
SHAW P T,CHAO S Y,FU L L,1999.Sea surface height variations in the South China Sea from satellite altimetry[J].Oceanologica Acta,22(1):1-17.
SHCHEPETKIN A F,MCWILLIAMS J C,2005.The regional oceanic modeling system(ROMS):A split-explicit,freesurface,topography-following-coordinate oceanic model[J]Ocean Modelling,9(4):347-404.
STONE P H,1966.On non-geostrophic baroclinic stability[J]Journal of the Atmospheric Sciences,23(4):390-400.
SU JILAN,2004.Overview of the South China Sea circulation and its influence on the coastal physical oceanography outside the Pearl River Estuary[J].Continental Shelf Research,24(16):1745-1760.
TANDON A,GARRETT C,1994.Mixed layer restratification due to a horizontal density gradient[J].Journal of Physical Oceanography,24(6):1419-1424.
TAYLOR J R,FERRARI R,2011.Ocean fronts trigger high latitude phytoplankton blooms[J].Geophysical Research Letters,38:L23601.
THOMAS L N,RHINES P B,2002.Nonlinear stratified spin-up[J]Journal of Fluid Mechanics,473:211-244.
THOMAS L N,TANDON A,MAHADEVAN A,2008.Submesoscale processes and dynamics[M]//HECHT M W,HASUMI H.Ocean modeling in an eddying regime,geophysical monograph series,177.Washington:American Geophysical Union:17-38.
THOMAS L N,TAYLOR J R,2010.Reduction of the usable wind-work on the general circulation by forced symmetric instability[J].Geophysical Research Letters,37(18):L18606.
THOMAS L N,TAYLOR J R,FERRARI R,et al,2013.Symmetric instability in the Gulf Stream[J].Deep Sea Research Part II:Topical Studies in Oceanography,91:96-110.
WANG DONGXIAO,LIU YUN,QI YIQUAN,et al,2001.Seasonal variability of thermal fronts in the northern South China Sea from satellite data[J].Geophysical Research Letters28(20):3963-3966.
WANG GUIHUA,SU JILAN,CHU P C,2003.Mesoscale eddies in the South China Sea observed with altimeter data[J].Geophysical Research Letters,30(21):OCE 6-1.
WANG GUIHUA,LI JIAXUN,WANG CHUNZAI,et al,2012.Interactions among the winter monsoon,ocean eddy and ocean thermal front in the South China Sea[J].Journal of Geophysical Research Oceans,117(C8):C08002.
刘国强,2011.南海北部以及吕宋海峡次级中尺度动力过程数值模拟研究[D].青岛:中国科学院研究生院(海洋研究所):131.LIU GUOQIANG,2011.Numerical study of submesoscale processes in northern South China Sea and Luzon Strait[D]Qingdao:Graduate University of Chinese Academy of Sciences(Institute of Oceanology,Chinese Academy of Sciences):131(in Chinese).
闫桐,齐义泉,经志友,2015.南海上层环流对不同气候态风场响应的数值研究[J].热带海洋学报,34(4):1-11.YANTONG,QI YIQUAN,JING ZHIYOU,2015.A numerical study on the responses of the South China Sea upper circulation to different climatological wind products[J].Journal of Tropical Oceanography,34(4):1-11(in Chinese).
BRUCE J G,1995.Eddies southwest of the Denmark Strait[J].Deep Sea Research Part I:Oceanographic Research Papers,42(1):13-29.
CAPET X,MCWILLIAMS J C,MOLEMAKER M J,et al,2008a.Mesoscale to submesoscale transition in the California Current System.Part I:Flow structure,eddy flux,and observational tests[J].Journal of Physical Oceanography,38(1):29-43.
CAPET X,MCWILLIAMS J C,MOLEMAKER M J,et al,2008b.Mesoscale to submesoscale transition in the California Current System.Part II:Frontal processes[J].Journal of Physical Oceanography,38(1):44-64.
CAPET X,MCWILLIAMS J C,MOLEMAKER M J,et al,2008c.Mesoscale to submesoscale transition in the California current system.Part III:Energy balance and flux[J].Journal of Physical Oceanography,38(10):2256-2269.
CHEN J,1983.Some explanations for the real-time distribution of sea surface temperature in the northern South China Sea winter[J].Acta Oceanologica Sinica,5(3):391-395.
CHU P C,FAN CHENWU,LOZANO C J,et al,1998.An airborne expendable bathythermograph survey of the South China Sea,May 1995[J].Journal of Geophysical Research:Oceans(1978-2012),103(C10):21637-21652.
D’ASARO E,LEE C,RAINVILLE L,et al,2011.Enhanced turbulence and energy dissipation at ocean fronts[J].Science,332(6027):318-322.
DONG CHANGMING,MCWILLIAMS J C,SHCHEPETKIN A F,2007.Island wakes in deep water[J].Journal of Physical Oceanography,37(4):962-981.
FLAMENT P,ARMI L,WASHBURN L,1985.The evolving structure of an upwelling filament[J].Journal of Geophysical Research:Oceans,90(C6):11765-11778.
HAINE T W N,MARSHALL J,1998.Gravitational,symmetric,and baroclinic instability of the ocean mixed layer[J].Journal of Physical Oceanography,28(4):634-658.
HOLMES R M,THOMAS L N,THOMPSON L,et al,2014.Potential vorticity dynamics of tropical instability vortices[J].Journal of Physical Oceanography,44(3):995-1011.
HOSKINS B J,1974.The role of potential vorticity in symmetric stability and instability[J].Quarterly Journal of the Royal Meteorological Society,100(425):480-482.
HOSKINS B J,1982.The mathematical theory of frontogenesis[J].Annual Review of Fluid Mechanics,14(1):131-151.
HU JIANGYU,KAWAMURA H,HONG HUASHENG,et al,2000A review on the currents in the South China Sea:Seasonal circulation,South China Sea Warm Current and Kuroshio intrusion[J].Journal of Oceanography,56(6):607-624.
HU J Y,KAWAMURA H,TANG D L,2003.Tidal front around the Hainan Island,northwest of the South China Sea[J].Journal of Geophysical Research Oceans,108(C11):3342.
JING ZHIYOU,QI YIQUAN,DU YAN,et al,2015.Summer upwelling and thermal fronts in the northwestern South China Sea:Observational analysis of two mesoscale mapping surveys[J].Journal of Geophysical Research:Oceans,120(3):1993-2006.
KLEIN P,LAPEYRE G,2009.The oceanic vertical pump induced by mesoscale and submesoscale turbulence[J].Annual Review of Marine Science,1:351-375.
LEMARIéF,KURIAN J,SHCHEPETKIN A F,et al,2012.Are there inescapable issues prohibiting the use of terrainfollowing coordinates in climate models?[J].Ocean Modelling,42:57-79.
LéVY M,KLEIN P,TREGUIER A M,2001.Impact of sub-mesoscale physics on production and subduction of phytoplankton in an oligotrophic regime[J].Journal of Marine Research,59(4):535-565.
LI LI,GUO XIAOGANG,WU RISHENG,2000.Oceanic fronts in southern Taiwan Strait[J].Journal of Oceanography in Taiwan Strait,19(2):145-156.
LIU SUMEI,GUO XINYU,CHEN QI,et al,2010.Nutrient dynamics in the winter thermohaline frontal zone of the northern shelf region of the South China Sea[J].Journal of Geophysical Research,115(C11):C11020.
LIU F,TANG S,CHEN C,2015.Satellite observations of the small-scale cyclonic eddies in the western South China Sea[J].Biogeosciences,12(2):299-305.
MARCHESIELLO P,CAPET X,MENKES C,et al,2011.Submesoscale dynamics in tropical instability waves[J].Ocean Modelling,39(1/2):31-46.
MCGILLICUDDY D J,ANDERSON L A,DONEY S C,et al,2003.Eddy-driven sources and sinks of nutrients in the upper ocean:Results from a 0.1°resolution model of the North Atlantic[J].Global Biogeochemical Cycles,17(2):1035.
MCGILLICUDDY D J JR,ANDERSON L A,BATES N R,et al,2007.Eddy/wind interactions stimulate extraordinary mid-ocean plankton blooms[J].Science,316(5827):1021-1026.
MCWILLIAMS J C,2010.A perspective on submesoscale geophysical turbulence[C]//IUTAM Symposium on Turbulence in the Atmosphere and Oceans.Dordrecht,Netherlands:Springer,28:131-141.
MUNK W,ARMI L,FISCHER K,et al,2000.Spirals on the sea[J]Proceedings of the Royal Society A:Mathematical,Physical and Engineering Sciences,456(1997):1217-1280.
PI QINGLING,HU JIANYU,2010.Analysis of sea surface temperature fronts in the Taiwan Strait and its adjacent area using an advanced edge detection method[J].Science China Earth Science,53(7):1008-1016.
QU TANGDONG,MITSUDERA H,YAMAGATA T,2000Intrusion of the North Pacific waters into the South China Sea[J].Journal of Geophysical Research:Oceans(1978-2012),105(C3):6415-6424.
SHAW P T,CHAO S Y,FU L L,1999.Sea surface height variations in the South China Sea from satellite altimetry[J].Oceanologica Acta,22(1):1-17.
SHCHEPETKIN A F,MCWILLIAMS J C,2005.The regional oceanic modeling system(ROMS):A split-explicit,freesurface,topography-following-coordinate oceanic model[J]Ocean Modelling,9(4):347-404.
STONE P H,1966.On non-geostrophic baroclinic stability[J]Journal of the Atmospheric Sciences,23(4):390-400.
SU JILAN,2004.Overview of the South China Sea circulation and its influence on the coastal physical oceanography outside the Pearl River Estuary[J].Continental Shelf Research,24(16):1745-1760.
TANDON A,GARRETT C,1994.Mixed layer restratification due to a horizontal density gradient[J].Journal of Physical Oceanography,24(6):1419-1424.
TAYLOR J R,FERRARI R,2011.Ocean fronts trigger high latitude phytoplankton blooms[J].Geophysical Research Letters,38:L23601.
THOMAS L N,RHINES P B,2002.Nonlinear stratified spin-up[J]Journal of Fluid Mechanics,473:211-244.
THOMAS L N,TANDON A,MAHADEVAN A,2008.Submesoscale processes and dynamics[M]//HECHT M W,HASUMI H.Ocean modeling in an eddying regime,geophysical monograph series,177.Washington:American Geophysical Union:17-38.
THOMAS L N,TAYLOR J R,2010.Reduction of the usable wind-work on the general circulation by forced symmetric instability[J].Geophysical Research Letters,37(18):L18606.
THOMAS L N,TAYLOR J R,FERRARI R,et al,2013.Symmetric instability in the Gulf Stream[J].Deep Sea Research Part II:Topical Studies in Oceanography,91:96-110.
WANG DONGXIAO,LIU YUN,QI YIQUAN,et al,2001.Seasonal variability of thermal fronts in the northern South China Sea from satellite data[J].Geophysical Research Letters28(20):3963-3966.
WANG GUIHUA,SU JILAN,CHU P C,2003.Mesoscale eddies in the South China Sea observed with altimeter data[J].Geophysical Research Letters,30(21):OCE 6-1.
WANG GUIHUA,LI JIAXUN,WANG CHUNZAI,et al,2012.Interactions among the winter monsoon,ocean eddy and ocean thermal front in the South China Sea[J].Journal of Geophysical Research Oceans,117(C8):C08002.