Flow regimes in a wide 'sliced-cylinder' model of homogeneous beta-plane circulation

We report new experiments with the 'sliced-cylinder' beta-plane model of Pe dlosky & Greenspan (1967) and Beardsley (1969), but with a much wider basin such that the western boundary current and its eddies occupy a small fract ion of the basin width. These experiments provide new insights into nonline ar aspects of the flow: the critical conditions for boundary current separa tion and the transition from stable to unstable flow are redefined, and a f urther transition from periodic to chaotic eddy shedding under strong antic yclonic forcing is also found. In the nonlinear regimes the western boundar y current separates from the western wall and shoots into the interior as a narrow jet that undergoes a rapid adjustment to join with the broad slow i nterior flow. In the unstable regimes this adjustment involves eddy sheddin g. Each transition occurs at a fixed critical value of a Reynolds number Re -gamma based on the velocity and width scales for a purely viscous boundary current: the flow is unstable for Re-gamma > 123 +/- 4 and aperiodic for R e-gamma > 231 +/- 5. The results provide evidence that the mechanism causin g instability is shear in the separated jet rather than the breaking of a l arge-amplitude Rossby wave. A quasi-geostrophic numerical model applied to the laboratory conditions yields a stability boundary and detailed characte ristics of the flow largely consistent with those determined from the exper iments. It also reveals a strong dependence of the circulation pattern on b asin aspect ratio, and shows that an adverse higher-order pressure gradient is responsible for western boundary current separation in this model. Eddy -eddy interactions and feedback of fluctuations from the eddy formation reg ion to upstream parts of the boundary current contribute to aperiodic behav iour. As a result of eddy shedding, passive tracer from each streamline in the boundary current can be stirred across much of the width of the basin.