Southern Ocean Mesoscale Eddy-Mean Flow Interaction, Mixed Layer Dynamics, and Their Relationships with the Southern Annular Mode
- Restrictions on Access:
- Open Access.
- The Antarctic Circumpolar Current (ACC) extends unbroken around the Southern Ocean and is important to the global ocean circulation and Earths climate. The ACC dynamics remains elusive in part because the role of turbulent mesoscale eddies on setting the state of the Southern Ocean remains less certain. In this dissertation, the relationship between the ACC jets and mesoscale eddy fluxes is investigated in the Indo-western Pacific Southern Ocean using an eddy-resolving model simulation. In this region, where the jets are relatively well-defined, the analysis shows that transient eddy momentum fluxes drive the ACC jets. Associated with these ACC jets, there are jet-scale overturning circulations (JSOCs). Analogous to the eddy momentum flux-driven portion of the atmospheric Ferrel Cell, these JSOCs, which are thermally indirect with sinking motions on the equatorward flank of the jet and rising motions on the poleward flank of the jet, are also discernible in transformed Eulerian mean framework and potential density coordinates. Therefore, these JSOCs describe Lagrangian motion. The JSOCs cannot be attributed to Ekman downwelling because the Ekman vertical velocities are much weaker than those of the JSOCs and Ekman meridional structure shares little resemblance to the rapidly varying JSOCs pattern that we observe in the model simulation. Furthermore, for the first time, observational evidence of the existence of JSOCs is demonstrated using Argo float data. The significantly enhanced negative cross-stream motion of the JSOCs across the jet cores is revealed by Argo float trajectories, and the perturbation vertical motion is inferred from Argo salinity fields.The eddy-driven JSOCs have a pronounced impact on the formation of a narrow band of the deep mixed layer (hereinafter mixed layer wedge) in the Indo-western Pacific Southern Ocean. The mixed layer wedge starts to deepen in June, centered at 47.5S, with a meridional scale of only ~2. Its center is located ~1 north of the Subantarctic Front (SAF), the northernmost front of the ACC. This structure is obtained from both the eddy-resolving model simulation and Argo float data. The formation of the mixed layer wedge is found to coincide with destratification underneath the mixed layer. This destratification can be attributed primarily to the descending branch of the JSOC on the warmer, equatorward flank of the SAF, promoting destratification during the warm season. Ekman advection contributes to the formation of the mixed layer, but it occurs farther north of the region where the mixed layer initially deepens. The winter net air-sea heat flux is only a response to the earlier mixed layer. These findings suggest that the eddy-driven JSOC associated with the SAF plays an important role in initiating the narrow and deep mixed layer wedge that forms north of the SAF.The Southern Ocean mixed layer depth (MLD) shows a significant non-zonal variability in response to the Southern Annular Mode (SAM) on seasonal-to-interannual timescales. As the leading mode of atmospheric variability in the Southern Hemisphere extratropics, the SAM is characterized by a zonally symmetric pattern with its positive phase of anomalously low pressure over the polar cap and anomalously high pressure over the mid-latitudes. Following the prominent SAM events that occur in austral summer, MLD anomalies appear in the subsequent austral winters, from June to August. These winter MLD anomalies show two significantly developed regions of Indo-western Pacific and eastern Pacific Southern Oceans, which peak in August in the former and in June in the latter. The complex spatial and temporal MLD anomalies are attributed to mixed-layer potential density anomalies, which are dependent on both potential temperature and salinity anomalies. The analysis indicates that wave-like, rather than zonally symmetric, atmospheric circulation anomalies lead to the potential temperature and salinity anomalies through air-sea fluxes of heat and fresh water, respectively.
- Dissertation Note:
- Ph.D. Pennsylvania State University 2018.
- Technical Details:
- The full text of the dissertation is available as an Adobe Acrobat .pdf file ; Adobe Acrobat Reader required to view the file.
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