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ABSTRACT
The phase speed spectrum of ocean mesoscale eddies is fundamental to understanding turbulent baroclinic flows. Since eddy phase propagation has been shown to modulate eddy fluxes, an understanding of eddy phase speeds is also of practical importance for the development of improved eddy parameterizations for coarse resolution ocean models. However, it is not totally clear whether and how linear Rossby wave theory can be used to explain the phase speed spectra in various weakly turbulent flow regimes. Using linear analysis, theoretical constraints are identified that control the eddy phase speed in a two-layer quasigeostrophic (QG) model. These constraints are then verified in a series of nonlinear two-layer QG simulations, spanning a range of parameters with potential relevance to the ocean. In the two-layer QG model, the strength of the inverse cascade exerts an important control on the eddy phase speed. If the inverse cascade is weak, the phase speed spectrum is reasonably well approximated by the phase speed of the linearly most unstable mode. A significant inverse cascade instead leads to barotropization, which in turn leads to mean phase speeds closer to those of barotropic-mode Rossby waves. The two-layer QG results are qualitatively consistent with the observed eddy phase speed spectra in the Antarctic Circumpolar Current and may also shed light on the interpretation of phase speed spectra observed in other regions.
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1.Introduction
Mesoscale eddies (on scales of tens to hundreds of kilometers) are ubiquitous in the ocean, and are believed to be crucial in the transport of tracers and the rectification of the mean flow (e.g., Gill et al. 1974; Johnson and Bryden 1989; Hallberg and Gnanadesikan 2006; McWilliams 2008; Waterman et al. 2011). Yet many of their fundamental properties are still poorly understood. This contribution focuses on the question of what controls the zonal propagation (i.e., phase speed) of mesoscale eddies in a two-layer model of quasigeostrophic baroclinic turbulence. Eddy phase speed has recently been highlighted as an important factor in modulating the magnitude of eddy fluxes (e.g., Marshall et al. 2006; Ferrari and Nikurashin 2010; Abernathey et al. 2010; Klocker et al. 2012a; Klocker and Abernathey 2014; Bates et al. 2014). As described by the theory of Ferrari and Nikurashin (2010), eddy propagation relative to...