East Asia and Western Pacific METEOROLOGY AND CLIMATE

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4.a Dependence of the instabiiity properties upon IL
The normal mode instability calculation confirms that in general there exist both
local and global unstable modes for a jet-streak with a background shear IL,. A local
mode consists of a group of dominant waves which jointly contribute to a maximum of
local energy in the downstream of the jet core. The growth rate of a local mode is
strongly dependent upon the vertical mean of the uniform part of the zonal wind, U 0 . A
global mode, on the other hand, mainly consists of a single wave and its growth rate is
only weakly dependent upon U 0 . Moreover, there exist only unstable local modes in the
absence of a background baroclinic shear (i.e., U T = 0.0).
Fig. 2 shows the variations of the eigenvalues of all unstable monopole modes
with respect to U 0 for the case of U T = 0.3. Panel (a) shows that the growth rates of the
local modes (modes #1, #2, #3, and S) decrease with U 0 after they reach their peak
values and eventually become zero for large U 0 . Mode #1 is again the most pronounced
local mode. Among the three travelling local modes, mode #1 is the most unstable
mode for U 0 < 0.38; mode #2 becomes most unstable in the range of 0.38 < U Q <
0.75; and mode #3 is most unstable when U 0 is sufficiently large. It is found that the
maximum energy center of a local mode gradually shifts to further downstream region of
the jet stream and the maximum value of the local energy also tends to decrease as U Q is
increased (not shown here). The detailed structures as well as the results of the various
diagnostic analyses of a representative travelling local mode are presented below. Mode
S also has a strongly localized structure but its growth rate is the smallest among the local
modes for this value of U T . The growth rate of this mode is also much smaller than that
for U T = 0.0. Hence, the most favorable condition for this mode to grow is when there
is no zonally uniform background baroclinic shear.
The growth rates of the global modes are shown in panel (b). In general, they
are much less sensitive to U Q and tend to gradually increase with U Q . Comparing panel
(b) with panel (a), we find that a local mode is the most unstable mode for U Q < 0.45
and the global mode #6 becomes the most unstable mode for U 0 > 0.45. Hence, a large
value U Q is favorable for a global mode but not for a local mode in the presence of a
background shear U T . This indicates that the constant zonal wind U Q not only changes
the instability properties of an individual mode but also alters the structure of the most
unstable mode (from a local mode to a global mode).
Fig. 2c shows the variations of the frequency of each branch of the unstable
monopole modes. The result reveals that it is possible to separate the travelling local
modes from the global modes according to their frequencies. The frequencies of the
travelling local modes are higher than those of the global modes. It is also found that the