MIXED BAROTROPIC-BAROCLINIC EDDIES GROWING ON AN EASTWARD MIDLATITUDEJET

The instability of a swift and narrow current flowing eastward in the mid-latitude open ocean is studied with a multi-mode quasi-geostrophic model. Linear analysis shows that there exist two groups of unstable waves: short and long. The phase velocity of the former is much larger ; the growth rate of both is almost proportional to the maximum speed of the baroclinic current. As the width of the jet increases, the grow th rate of the long waves increases and that of the short waves decrea ses. The evolution of the waves has Four stages: (i) linear growth; (i i) formation of large meanders, cyclonic to the south and anticyclonic to the north of the jet; (iii) meander necking and detachment; and (i v) westward propagation of the detached eddies. Available potential en ergy is transferred to kinetic energy and into the barotropic mode mos tly during the second and third stages, when strong cells develop in t his mode. The main Feature of the meander and eddy evolution is the in teraction of baroclinic monopoles, dominating the surface flow, with b arotropic dipoles that become more important at depth. There is qualit ative and quantitative similarity between the simulated eddies and obs erved Gulf-Stream rings: their diameter is 150-250 km, their maximum s urface speed 150 cms(-1) and propagation is predominantly westward.