Southern Ocean jet-topography interactions and their impact on eddy fluxes

• Main Author: Barthel, Alice
• Format: Doctoral or Postdoctoral Thesis
• Language: unknown
• Published: UNSW Sydney 2017
• Subjects: Eddies; Southern Ocean; ACC; MOC; Topography; Antarctic; The Antarctic; Antarc*
• Online Access: https://dx.doi.org/10.26190/unsworks/19949; http://hdl.handle.net/1959.4/58703

In the Southern Ocean, strong jets interact with large topographic features along the path of the Antarctic Circumpolar Current. These interactions generate eddies which in turn, impact the transport of passive tracers, the energy pathways through the ocean system and the meridional overturning circulation, all of which contribute to global ocean circulation and climate. This thesis investigates the dynamical processes underlying jet-eddy-topography interactions in the Southern Ocean. Firstly, topography impacts the eddy-induced lateral mixing of tracers. A two-layer quasigeostrophic ocean model is used to simulate an unstable jet impinging on an isolated seamount and quantify the resulting tracer mixing. In the absence of topography, the flow grows unstable and generates eddy-induced mixing as it evolves downstream. When a small seamount is present, eddies are enhanced in the lee of topography, increasing the mixing intensity relative to the flat-bottom case. When the topography is high, the spatial pattern of eddy activity and mixing is altered, with elevated eddy kinetic energy (EKE) and strong mixing occurring upstream, while mixing suppression occurs immediately downstream of the obstacle. Secondly, the topographic contribution to deep EKE is investigated using numerical simulations of idealised jet-topography interactions. The energy budget analysis performed identifies two energy sources for deep EKE, the relative magnitude of which depend on the topography and upstream flow characteristics. In particular, a jet impinging on a seamount generates EKE through the work of Reynolds stress, while an increase in the jet baroclinicity enhances the contribution from eddy form stress. The presence of a meridional ridge increases both energy sources, generating much larger values of EKE at depth compared to seamount or flat-bottom cases. Lastly, eddies around topography contribute to meridional overturning by transporting water polewards across the time-mean jet core. This eddy-driven poleward transport occurs only where EKE is growing through baroclinic instability, rather than in regions of elevated EKE. In addition, horizontal shear instability supports EKE growth without resulting in cross-jet transport by eddies. The results in this thesis highlight the key role of instability mechanisms in setting the magnitude and location of eddy fluxes near topography in the Southern Ocean.