It is hypothesized that the essence of instability of a zonally inhomogeneous basic flow is wave-packet resonance. This hypothesis is verified with an instability analysis of a generic localized barotropic westerly jet. The latter has a regional strength indicated by a parameter l , and a global characteristic indicated by a domain-average-basic-zonal-flow component, U . There is a critical easterly U for a given l with |U|critical being stronger for a larger l . If U were an easterly with a magnitude larger than |U|critical , the most unstable mode would be a stationary wave-packet located downstream of the localized jet. If U were a westerly or an easterly weaker than |U|critical , the instability would be oscillatory with an intra-seasonal period and a smaller mean growth rate than that of stationary wave-packet resonance. This unstable wave-packet is also most intense when it reaches downstream of the localized jet. The oscillation period would be shorter for a stronger westerly U . The nature of these features is interpreted from the perspective of vorticity dynamics. The local energetics of the instability is also diagnosed.
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