Mixed barotropic-baroclinic eddies growing on an eastward mid-latitude jet

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Abstract
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 growth rate of the long waves increases and that of the short waves decreases. The evolution of the waves has four stages: (i) linear growth; (ii) formation of large meanders, cyclonic to the south and anticyclonic to the north of the jet; (iii) meander necking and detachment; and (iv) westward propagation of the detached eddies Available potential energy is transferred to kinetic energy and into the barotropic mode mostly during the second and third stages, when strong cells develop in this mode. The main feature of the meander and eddy evolution is the interaction of baroclinic monopoles, dominating the surface flow, with barotropic dipoles that become more important at depth. There is qualitative and quantitative similarity between the simulated eddies and observed Gulf-Stream rings: their diameter is 150–250 km, their maximum surface speed 150 cms−1, and propagation is predominantly westward.