Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear
The transport by materially coherent surface-layer eddies was studied in a two-layer quasigeostrophic model driven by eastward mean shear. The coherent eddies were identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. At...
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MDPI AG
2019-12-01
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| Series: | Fluids |
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| Online Access: | https://www.mdpi.com/2311-5521/5/1/2 |
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| author | Wenda Zhang Christopher L. P. Wolfe Ryan Abernathey |
| author_facet | Wenda Zhang Christopher L. P. Wolfe Ryan Abernathey |
| author_sort | Wenda Zhang |
| collection | DOAJ |
| description | The transport by materially coherent surface-layer eddies was studied in a two-layer quasigeostrophic model driven by eastward mean shear. The coherent eddies were identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. Attention was restricted to eastward mean flows, but a wide range of flow regimes with different bottom friction strengths, layer thickness ratios, and background potential vorticity (PV) gradients were otherwise considered. It was found that coherent eddies become more prevalent and longer-lasting as the strength of bottom drag increases and the stratification becomes more surface-intensified. The number of coherent eddies is minimal when the shear-induced PV gradient is 10−20 times the planetary PV gradient and increases for both larger and smaller values of the planetary PV gradient. These coherent eddies, with an average core radius close to the deformation radius, propagate meridionally with a preference for cyclones to propagate poleward and anticyclones to propagate equatorward. The meridional propagation preference of the coherent eddies gives rise to a systematic upgradient PV transport, which is in the opposite direction as the background PV transport and not captured by standard Lagrangian diffusivity estimates. The upgradient PV transport by coherent eddy cores is less than 15% of the total PV transport, but the PV transport by the periphery flow induced by the PV inside coherent eddies is significant and downgradient. These results clarify the distinct roles of the trapping and stirring effect of coherent eddies in PV transport in geophysical turbulence. |
| first_indexed | 2024-12-16T16:52:39Z |
| format | Article |
| id | doaj.art-d0cdd036459d4e5f9c52ae3dfc77fbb5 |
| institution | Directory Open Access Journal |
| issn | 2311-5521 |
| language | English |
| last_indexed | 2024-12-16T16:52:39Z |
| publishDate | 2019-12-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Fluids |
| spelling | doaj.art-d0cdd036459d4e5f9c52ae3dfc77fbb52022-12-21T22:23:59ZengMDPI AGFluids2311-55212019-12-0151210.3390/fluids5010002fluids5010002Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward ShearWenda Zhang0Christopher L. P. Wolfe1Ryan Abernathey2School of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794, USASchool of Marine and Atmospheric Sciences, Stony Brook University, Stony Brook, NY 11794, USALamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USAThe transport by materially coherent surface-layer eddies was studied in a two-layer quasigeostrophic model driven by eastward mean shear. The coherent eddies were identified by closed contours of the Lagrangian-averaged vorticity deviation obtained from Lagrangian particles advected by the flow. Attention was restricted to eastward mean flows, but a wide range of flow regimes with different bottom friction strengths, layer thickness ratios, and background potential vorticity (PV) gradients were otherwise considered. It was found that coherent eddies become more prevalent and longer-lasting as the strength of bottom drag increases and the stratification becomes more surface-intensified. The number of coherent eddies is minimal when the shear-induced PV gradient is 10−20 times the planetary PV gradient and increases for both larger and smaller values of the planetary PV gradient. These coherent eddies, with an average core radius close to the deformation radius, propagate meridionally with a preference for cyclones to propagate poleward and anticyclones to propagate equatorward. The meridional propagation preference of the coherent eddies gives rise to a systematic upgradient PV transport, which is in the opposite direction as the background PV transport and not captured by standard Lagrangian diffusivity estimates. The upgradient PV transport by coherent eddy cores is less than 15% of the total PV transport, but the PV transport by the periphery flow induced by the PV inside coherent eddies is significant and downgradient. These results clarify the distinct roles of the trapping and stirring effect of coherent eddies in PV transport in geophysical turbulence.https://www.mdpi.com/2311-5521/5/1/2ocean mesoscale eddieslagrangian coherent structuregeostrophic turbulencepotential vorticity transporteddy diffusivity |
| spellingShingle | Wenda Zhang Christopher L. P. Wolfe Ryan Abernathey Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear Fluids ocean mesoscale eddies lagrangian coherent structure geostrophic turbulence potential vorticity transport eddy diffusivity |
| title | Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear |
| title_full | Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear |
| title_fullStr | Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear |
| title_full_unstemmed | Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear |
| title_short | Role of Surface-Layer Coherent Eddies in Potential Vorticity Transport in Quasigeostrophic Turbulence Driven by Eastward Shear |
| title_sort | role of surface layer coherent eddies in potential vorticity transport in quasigeostrophic turbulence driven by eastward shear |
| topic | ocean mesoscale eddies lagrangian coherent structure geostrophic turbulence potential vorticity transport eddy diffusivity |
| url | https://www.mdpi.com/2311-5521/5/1/2 |
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