Vortex life cycles in two-and three-layer quasi-geostrophic models

Abstract

Coherent vortices and their properties were studied in two- and three-layer quasi-geostrophic beta-plane turbulence. Much research has discussed vortex characteristics in a number of applications, but no significant study of vortices in turbulent regimes with jets has occurred. This research attempted to first determine the typical lifetime of a vortex, with considerations of its birth, evolution, and cessation. A vortex census was also performed in an attempt to describe the life cycle of vortices in terms of characteristic properties. Parameters such as a vortex's amplitude, diameter, life span, elliptical area, and birth distance from the mean jet location were investigated in the two-layer model. Also, the vertical structure and alignment of vortices between layers was investigated using a three-layer, two-jet model. Studying vortex life cycles resulted in the formulation of a typical life cycle. Most vortices arose as the result of filament roll-up. Vortices then were found to possess one of three characteristic radial profiles, one of which was similar to generalizations made by McWilliams (1984) about the average radial profile. Performing a census on vortices formed during two-layer, one- and two-jet simulations provided statistical data on the life span, amplitude, and diameter of coherent vortices. Also, the percentage of vortices classified as cyclones and anti-cyclones were determined and suggested to be significantly different than the expected 50%/50% of cyclones and anti-cyclones for quasi-geostrophic turbulence. Sample sizes of greater than 140 and 160 vortices were investigated for the two-layer, two-jet and one-jet cases, respectively. Linear regression and multiple linear regression suggested no correlation between vortex amplitude or diameter with vortex life span. Considerations of the distance at which a vortex's birth occurred suggested no correlation with vortex life span. Multiple linear regression suggested the parameter most influential in the prediction of vortex life span was amplitude. The three-layer model investigated potential vorticity fields in all three layers, allowing research into the alignment of vortices between the layers. Results of a vortex census for this simulation suggested that a majority of vortices aligned between the top two layers. Vortices in the bottom-layer did not correlate with the behavior in the top two layers.