Dynamics of ocean jets over topography
File(s)
Author(s)
Khatri, Hemant
Type
Thesis
Abstract
Alternating jets seen in the oceans are often transient in nature as they possess spatio-temporal variability. Ocean bathymetry is believed to be one of the primary causes for the jet variability. In this thesis, the dynamics of alternating jets and mesoscale eddies is studied in the presence of a zonally sloped topography in a baroclinic quasi-geostrophic model, which is forced with an imposed vertical velocity shear. It is observed that the jets tilt from the zonal direction and drift meridionally. The jets tend to align with the barotropic potential vorticity isolines and drift speeds match well with the phase speeds of linear Rossby waves. Thus, the linear dynamics controls the jet drift. Also, the tilted jets are coupled to the imposed shear and are able to gain energy directly from the imposed shear. On the other hand, eddies remove energy from the jets; hence, eddies act against the jets. The results are further confirmed by analysing the mean-flow and eddy energy budgets.
These results are limited to continuously forced dissipative systems. In weak dissipation regimes, in addition to the tilted jets, purely zonal large-scale modes are observed. The zonal modes gain energy from eddies and the tilted jets through nonlinear interactions. The results suggest that alternating jet patterns in the oceans can also form due to interactions among eddies and various large-scale modes. The mechanism is different from the classical arguments, in which only mesoscale eddies force the jets. Also, a direct energy transfer from the imposed shear to jets is not possible in the model of stationary zonally symmetric jets. Further, it is found that energy transfer to the jets via Reynolds stress work is higher in the layer having a positive meridional gradient in the background potential vorticity. This is qualitatively explained by simple reasoning based on Rossby wave group velocity.
These results are limited to continuously forced dissipative systems. In weak dissipation regimes, in addition to the tilted jets, purely zonal large-scale modes are observed. The zonal modes gain energy from eddies and the tilted jets through nonlinear interactions. The results suggest that alternating jet patterns in the oceans can also form due to interactions among eddies and various large-scale modes. The mechanism is different from the classical arguments, in which only mesoscale eddies force the jets. Also, a direct energy transfer from the imposed shear to jets is not possible in the model of stationary zonally symmetric jets. Further, it is found that energy transfer to the jets via Reynolds stress work is higher in the layer having a positive meridional gradient in the background potential vorticity. This is qualitatively explained by simple reasoning based on Rossby wave group velocity.
Version
Open Access
Date Issued
2019-05
Date Awarded
2019-09
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Berloff, Pavel
Sponsor
Imperial College London
Publisher Department
Mathematics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)