East Asia and Western Pacific METEOROLOGY AND CLIMATE

the jet core. The maximum value is 1.63 and is found at the longitude "Xj = 1.0" (the
first grid point to the right of the center in the plot) (Fig.6a). The spatial distribution of
the generation rate is fairly symmetric about its maximum. The domain integrated
potential energy generation rate is 2.1. The local kinetic energy generation rate {E*
D}is found to have a localized negative center at the longitude "xj = 2.0" with a peak
value of - 0.26 (Fig. 4b). Therefore, the instability of this mode is attributable to the
baroclinic process. Advection (Fig. 4c) removes energy from the immediate-downstream
region to a region further downstream of the jet core. The same is true for the work done
by the ageostrophic geopotential (Fig. 4e). On the other hand, the mechanical work
done by the geostrophic geopotential on the ageostrophic wind tends to redistribute
energy from the exit region to the entrance region of the jet stream (Fig.4 d). The
contribution to the local energetics from the baroclinic ageostrophic component is much
stronger than that from the barotropic ageostrophic component (Fig. 4d vs 4e). It has
been verified that the time-mean local rate of change of the energy is exactly twice the
product of the local energy and the growth rate of this mode (Fig. 4f). The effective
local energy generation rate {G} is equal to the sum of panels (a), (b), (d), and (e) plus
the dissipation term. Its maximum value is centered at longitude "x { = 0.5".
Therefore, the maximum local energy growth rate is located east of the maximum
effective local energy generation rate (longitude "xj = 1.0").
(iii) Feedback effects on the basic flow
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The feedback tendencies of the perturbation normal mode on the basic flow are
shown in Fig. 5. The feedback effects are quite localized as one might expect from the
structure of the normal mode. The disturbance at the upper level (Fig. 5a) tends to
reinforce the basic jet stream by having a positive tendency of *F on the south side of the
jet and negative tendency on the nortli side. The disturbance at the lower level (Fig. 5b)
Kg, B Feedback effects of the unstable mode shown in Fig.3 on the basic flow.
(a) Geopotential tendency at the upper level (b) Geopotential tendency at the lower
• • ' • • •• level' • • • ' " • • ' ' • . ' • • ' • • ' : • • . • . ' .-•• ' • • - • • • • - • • . ". .