East Asia and Western Pacific METEOROLOGY AND CLIMATE

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to the pre-onset level by the end of August. Thus,the kinetic energy undergoes
a low-frequency oscillation of a period arround 40 to 45 days. Murakami 8^ had
qualitatively related this oscillation to the alternate active and break periods of the
summer monsoon . Cross-isobaric generation of KE dominate the energy budget
terms. The inertia! terms are relatively small and cross-isobaric generation is
balanced by frictional dissipation throughout the summer monsoon season. Thus
a quasi-Ekman low-level monsoon flow persists during the Indian summer monsoon
as is pointed out by Stout and Young 6 }. Temporal variations of the cross-isobaric
generation of KE resemble that of KE and have a similar low- frequency cycle.
To examine the Somali jet further, we did similar calculations for regions Bl
and B2 which represent areas with wind speeds greater than 15 m/s and 10 m/s
respectively (Fig.4 and 5). Despite more intense kinetic energy and larger crossisobaric
generation near the jet core , the pattern of temporal variation is similar
to that of region Al. Contribution from horizontal flux convergence is significant
during the onset period while signifcant horizontal flux divergence is observed
after the peaks of maximum KE. The contribution of horizontal flux convergence
by the divergent wind is in general small as compared to that of the rotational
parts (figure not shown).
3.1.2 Region A2
Region A2 covers the low-level southeasterly summer monsoon flow of the
southern hemisphere. Temporal variations of KE and energy bueget terms are
shown in Fig.6. It is of interest. to note that, two KE peaks correspond to two
active periods of summer monsoon are again evident in this region. Prior to the
onset of northern hemisphere monsoon ,the intensity of the flow in region A2 is
larger than that in region Al.
During the onset of the India monsoon, a large amoumt of cross-isobaric
generation of KE was present but the intensity of the flow in this region did not
increase as did its northern hemisphere counterpart until around July 5 due to a
strong horizontal flux divergence of the rotational winds. This imply a strong crossequatorial
flux to the northern hemisphere during this stage. This corresponds to
the strong inflow which was observed in region A1 during the onset period.
3.1.3 Regions Dl and D2
For comparison purposes, we have also computed the energy budget for low
level flows in the western North Pacific region ( Fig.7a). In region Dl, that is,