## Doctoral Dissertations

Spring 1996

Dissertation

#### Program or Major

Earth Sciences: Oceanography

#### Degree Name

Doctor of Philosophy

Wendel Brown

#### Abstract

The dynamics of meandering and eddy detachment in separating western boundary currents are explored by considering inviscid, quasi-geostrophic current systems with both data analysis and modeling.

In chapter two, hydrographic and velocity observations of the North Brazil Current (NBC) retroflection region during the 1990-1991 Western Tropical Atlantic Experiment are examined. A three-layer quasi-geostrophic approximation to the NBC region is built. Stream function and potential vorticity (PV) fields are presented. The upper and middle layer flows simulate the retroflection of the NBC waters at surface and subthermocline levels, respectively. The deepest layer is represented by a meandering weak southward flow that resembles the Deep Western Boundary Current. The participation of baroclinic instability in the NBC eddy-shedding is verified by isolating the effect of PV anomalies in each of the layers on each of the layers.

In chapter three, a 1$1\over2$ layer "contour dynamical" model of two converging western boundary currents that form a zonal jet is built. It is intended to investigate the role of the coastline tilt and transport asymmetry between the converging currents in meander formation. The respective effects of coastline tilt and coastal current asymmetry can reinforce or cancel each other. In the former case, a retroflection type of boundary current separation, as observed in the Brazil Current separation, is obtained. In the latter case, a much smoother separation results, as observed in the Gulf Stream.

In chapter four, a 2$1\over2$-layer "contour dynamical" model is built to investigate the role of baroclinic instability in forming eddies. The upper active layer flow structure is similar to that of the 1$1\over2$-layer model (chapter three). The lower active layer flow structure ranges from converging to diverging coastal current configurations. Unstable waves must either propagate westward or slowly in the eastward direction to allow coast/retroflection eddy formation. When retroflection eddies pinch off, both upper and lower layers present opposite-signed, closed, PV contours. This characterizes a dipolar vortex which can self-propagate away from the jet axis.

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