Abstract
In this thesis, anomalous structures of oceanic mesoscale turbulence, as exemplified by coherent vortices and zonally-elongated transient flows (ZELTs), are considered. All simulations of mesoscale turbulence flows are performed in a two-layer, quasi-geostrophic model. The first chapter explores stability of and transport by baroclinic vortices on the β-plane. The study adapts a wave-mean flow formalism and examines interactions between the axisymmetric flow (“the vortex”) and residuals (“the waves”). Unlike baroclinically unstable vortices on the f-plane, such vortices on the β-plane can be also vulnerable to barotropic instability as revealed by the globally integrated energy balance analysis. The spatial structure of energy fluxes shows the energy leakage inside the vortex core when its breakdown occurs. Mixing by stable and unstable vortical flows is quantified through the computation of Finite-Time Lyapunov Exponent (FTLE) maps. Depending on the strength of wave radiation, the upper-layer FTLE maps of stable vortices show either an annulus or volute ring of vigorous mixing inside the vortex interior. This ring region is disrupted when the vortex becomes unstable. Both stable and unstable vortices show the wavy patterns of FTLE in the near- and far-fields. Despite the fact that the initial vortex resides in the top layer only, significant FTLE patterns are observed in the deep layer at later times. Lagrangian analysis of the vortex-induced change of large-scale tracer gradient demon- strates significant effects of vortex instability in the top layer and the importance of the wave-like anomalies in the bottom layer. The second chapter explores the phenomenology of zonally-elongated transients (ZELTs) in the ocean and the sensitivity of their properties to changes in several environmental factors. ZELTs explain a major part of anisotropy in mesoscale turbulent flow. Calculations are performed in a two-layer, quasi-geostrophic model. Empirical Orthogonal Functions (EOF) decomposition allows for the separation of ZELTs from the background turbulent flow as several leading EOF modes. The leading Extended EOF reveals that ZELTs propagate westward at the speed of ~ 1 cms−1. The decrease in the planetary vorticity gradient and increase in the bottom drag coefficient each leads to flattening of the variance spectrum, isotropization of the leading EOF and fast decay of the autocorrelation function of its corresponding Principal Component. The third chapter deals with the underpinning mechanisms of ZELTs formation. As evidenced by spatial Fourier spectrum, spatial structure of the leading EOF and the autocorrelation function of its corresponding Principal Component, simulations in reduced-dynamics models with completely removed eddy-eddy interactions show no presence of ZELTs, thereby suggesting that the physical mechanism based on energy cascade arguments is more plausible for the formation of ZELTs. The energy exchanges produced by baroclinic-baroclinic and mixed-mode interactions are the major cause of the emergence of ZELTs as revealed in simulations with both moderate and high values of bottom drag. Barotropic-barotropic interactions, which entail energy cascade in barotropic mode, play a secondary role in ZELTs development in the moderate-drag simulation, and these interactions have no impact on the dynamics of ZELTs in the high-drag simulation.