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Abstract
The Southern Ocean’s Antarctic Circumpolar Current (ACC) naturally lends itself to interpretations using a zonally averaged framework. Yet, navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models. In this study, both observational and numerical results indicate that zonal asymmetries, in the form of topography, impact global flow structure and transport properties.
The conclusions are based on a suite of more than 1.5 million virtual drifter trajectories advected using a satellite altimetry–derived surface velocity field spanning 17 years. The focus is on sites of “cross front” transport as defined by movement across selected sea surface height contours that correspond to jets along most of the ACC. Cross-front exchange is localized in the lee of bathymetric features with more than 75% of crossing events occurring in regions corresponding to only 20% of the ACC’s zonal extent.
These observations motivate a series of numerical experiments using a two-layer quasigeostrophic model with simple, zonally asymmetric topography, which often produces transitions in the front structure along the channel. Significantly, regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic. Jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets. Eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering. The combination of high eddy kinetic energy and recirculation features enhances particle exchange. These results stress the complications in developing consistent circumpolar definitions of the ACC fronts.
Abstract
The Southern Ocean’s Antarctic Circumpolar Current (ACC) naturally lends itself to interpretations using a zonally averaged framework. Yet, navigation around steep and complicated bathymetric obstacles suggests that local dynamics may be far removed from those described by zonally symmetric models. In this study, both observational and numerical results indicate that zonal asymmetries, in the form of topography, impact global flow structure and transport properties.
The conclusions are based on a suite of more than 1.5 million virtual drifter trajectories advected using a satellite altimetry–derived surface velocity field spanning 17 years. The focus is on sites of “cross front” transport as defined by movement across selected sea surface height contours that correspond to jets along most of the ACC. Cross-front exchange is localized in the lee of bathymetric features with more than 75% of crossing events occurring in regions corresponding to only 20% of the ACC’s zonal extent.
These observations motivate a series of numerical experiments using a two-layer quasigeostrophic model with simple, zonally asymmetric topography, which often produces transitions in the front structure along the channel. Significantly, regimes occur where the equilibrated number of coherent jets is a function of longitude and transport barriers are not periodic. Jet reorganization is carried out by eddy flux divergences acting to both accelerate and decelerate the mean flow of the jets. Eddy kinetic energy is amplified downstream of topography due to increased baroclinicity related to topographic steering. The combination of high eddy kinetic energy and recirculation features enhances particle exchange. These results stress the complications in developing consistent circumpolar definitions of the ACC fronts.
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.1029/2008JC005248 . Speer , K. , S. R. Rintoul , and B. Sloyan , 2000 : The diabatic Deacon cell . J. Phys. Oceanogr. , 30 , 3212 – 3222 , doi: 10.1175/1520-0485(2000)030<3212:TDDC>2.0.CO;2 . Sun , C. , and D. R. Watts , 2002 : Heat flux carried by the Antarctic Circumpolar Current mean flow . J. Geophys. Res. , 107 ( C9 ), 3119 , doi: 10.1029/2001JC001187 . Thompson , A. F. , and J.-B. Sallée , 2012 : Jets and topography: Jet transitions and the impact on transport in the
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. J. Richards , 2011 : Low-frequency variability of Southern Ocean jets . J. Geophys. Res. , 116 , C09022 , https://doi.org/10.1029/2010JC006749 . 10.1029/2010JC006749 Thompson , A. F. , and J. B. Sallée , 2012 : Jets and topography: Jet transitions and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , https://doi.org/10.1175/JPO-D-11-0135.1 . 10.1175/JPO-D-11-0135.1 Thompson , A. F. , and A. C. Naveira Garabato , 2014
and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . Treguier , A. M. , and R. L. Panetta , 1994 : Multiple zonal jets in a quasigeostrophic model of the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 24 , 2263 – 2277 , doi: 10.1175/1520-0485(1994)024<2263:MZJIAQ>2.0.CO;2 . Vallis , G. K. , and M. E. Maltrud , 1993 : Generation of mean flows and jets on a beta plane and over topography . J
and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . Treguier , A. M. , and R. L. Panetta , 1994 : Multiple zonal jets in a quasigeostrophic model of the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 24 , 2263 – 2277 , doi: 10.1175/1520-0485(1994)024<2263:MZJIAQ>2.0.CO;2 . Vallis , G. K. , and M. E. Maltrud , 1993 : Generation of mean flows and jets on a beta plane and over topography . J
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.1 Thompson , A. F. , and J.-B. Sallée , 2012 : Jets and topography: Jet transitions and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . 10.1175/JPO-D-11-0135.1 Thompson , L. , 1995 : The effect of continental rises on the wind-driven ocean circulation . J. Phys. Oceanogr. , 25 , 1296 – 1316 , doi: 10.1175/1520-0485(1995)025<1296:TEOCRO>2.0.CO;2 . 10.1175/1520-0485(1995)025<1296:TEOCRO>2.0.CO;2 Treguier , A
.1 Thompson , A. F. , and J.-B. Sallée , 2012 : Jets and topography: Jet transitions and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . 10.1175/JPO-D-11-0135.1 Thompson , L. , 1995 : The effect of continental rises on the wind-driven ocean circulation . J. Phys. Oceanogr. , 25 , 1296 – 1316 , doi: 10.1175/1520-0485(1995)025<1296:TEOCRO>2.0.CO;2 . 10.1175/1520-0485(1995)025<1296:TEOCRO>2.0.CO;2 Treguier , A
: Jet transitions and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . Thompson , A. F. , and A. C. Naveira Garabato , 2014 : Equilibration of the Antarctic Circumpolar Current by standing meanders . J. Phys. Oceanogr. , 44 , 1811 – 1828 , doi: 10.1175/JPO-D-13-0163.1 . Tseng , Y. , and J. H. Ferziger , 2001 : Mixing and available potential energy in stratified flows . Phys. Fluids , 13 , 1281
: Jet transitions and the impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 , doi: 10.1175/JPO-D-11-0135.1 . Thompson , A. F. , and A. C. Naveira Garabato , 2014 : Equilibration of the Antarctic Circumpolar Current by standing meanders . J. Phys. Oceanogr. , 44 , 1811 – 1828 , doi: 10.1175/JPO-D-13-0163.1 . Tseng , Y. , and J. H. Ferziger , 2001 : Mixing and available potential energy in stratified flows . Phys. Fluids , 13 , 1281
impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 . Thompson , D. , S. Solomon , P. Kushner , M. England , K. Grise , and D. Karoly , 2011 : Signatures of the Antarctic ozone hole in Southern Hemisphere surface climates change . Nat. Geosci. , 4 , 741–749 , doi:10.1038/NGEO1296 . van Sebille , E. , W. Johns , and L. Beal , 2012 : Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South
impact on transport in the Antarctic Circumpolar Current . J. Phys. Oceanogr. , 42 , 956 – 972 . Thompson , D. , S. Solomon , P. Kushner , M. England , K. Grise , and D. Karoly , 2011 : Signatures of the Antarctic ozone hole in Southern Hemisphere surface climates change . Nat. Geosci. , 4 , 741–749 , doi:10.1038/NGEO1296 . van Sebille , E. , W. Johns , and L. Beal , 2012 : Does the vorticity flux from Agulhas rings control the zonal pathway of NADW across the South
