Experimental and numerical studies of an eastward jet over topography

Motivated by the phenomena of blocked and zonal flows in Earth's atmosphere , we conducted laboratory experiments and numerical simulations to study th e dynamics of an eastward jet flowing over wavenumber-two topography. The l aboratory experiments studied the dynamical behaviour of the flow in a baro tropic rotating annulus as a function of the experimental Rossby and Ekman numbers. Two distinct flow patterns, resembling blocked and zonal flows in the atmosphere, were observed to persist for long time intervals. Earlier model studies had suggested that the atmosphere's normally upstream -propagating Rossby waves can resonantly lock to the underlying topography, and that this topographic resonance separates zonal from blocked flows. In the annulus, the zonal flows did indeed have super-resonant mean zonal vel ocities, while the blocked flows appear subresonant. Low-frequency variabil ity, periodic or irregular, was present in the measured time series of azim uthal velocity in the blocked regime, with dominant periodicities in the ra nge of 6-25 annulus rotations. Oscillations have also been detected in zona l states, with smaller amplitude and similar frequency. In addition, over a large region of parameter space the two flow states exhibited spontaneous, intermittent transitions from the one to the other. We numerically simulated the laboratory flow geometry in a quasi-geostrophi c barotropic model over a similar range of parameters. Both flow regimes, b locked and zonal, were reproduced in the simulations, with similar spatial and temporal characteristics, including the low-frequency oscillations asso ciated with the blocked flow. The blocked and zonal flow patterns are prese nt over wide ranges of forcing, topographic height, and bottom friction. Fo r a significant portion of parameter space, both model flows are stable. De pending on the initial state, either the blocked or the zonal flow is obtai ned and persists indefinitely, showing the existence of multiple equilibria .