TRANSITIONS BETWEEN BLOCKED AND ZONAL FLOWS IN A ROTATING ANNULUS WITH TOPOGRAPHY

The mid-latitude atmosphere is dominated by westerly, nearly zonal flo w. Occasionally, this flow is deflected poleward by blocking anticyclo nes that persist for 10 days or longer. Experiments in a rotating annu lus used radial pumping to generate a zonal jet under the action of th e Coriolis force. In the presence of two sym metric ridges at the bott om of the annulus, the resulting flaws were nearly zonal at high forci ng or blocked ai lewd forcing Intermittent switching between blocked a nd zonal patterns occurs because of the jet's interaction with the top ography. These results shed further light on previous atmospheric obse rvations and numerical simulations.