East Asia and Western Pacific METEOROLOGY AND CLIMATE

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5.625 N and 15 N (region Bl) and 61.875 E between 0 TV and 18.75 N (region
B2), and plot each in time sequence of five day average (Fig.8). It is very obvious
from Fig.8 that the temporal variation of the pressure gradient is in close
resemblance to that of KE. This result impies that the adjustment between the
wind field and the mass field is very important in determining the variation of the
monsoon circulation,
Although the increase of pressure gradient during the onset period can be
explained by differential heating due to insolation and condensation heating as
the season progress, the reason for the decrease of pressure gradient prior to the
break of the monsoon is still unresolved.lt may be due to a reduced differential
heating by the shading of insolation caused by the development of cloud convection
as suggested by Krishnamurti and Bhalme 9^ or due to the distribution of ground
hydrology in response to the northward migration of the rainfall system ( Webster
and Chou 10^). However, the Indian monsoon region in summer is perhaps one of
the few regions around the world that both baroclinic and barotropic instabilities
(as well as inertia! instability) are easy to occur. It is possible that the release of the
combined baroclinic and barotropic instability is the reason for this low-frequency
oscillation. This problem deserves further investigation.
3.3 Temporal Variation of the Rossby Wave Sources
According to Sardeshmukh and Hoskins 11^,the response of the large scale rotational
flow to diabatic heating can be written as:
(|+V-V)C = S'+f (2)
where V$ is the rotational wind associated with ( ,and ( is the absolute vorticity.
The source of Rossby waves is defined as:
V x is the divergent wind and D = V • V x while F is frictional dissipation. Fig.9
depicts the temporal variation of the Rossby wave source in a region bounded by
45°JEJ and 106.875° E and 5.625° N and 26. 25° AT . The source of the Rossby wave
shows a clear low-frequency cycle.
In- comparison to the pressure gradient variation of the Somali jet core region
(Fig.9a), we can see that the peaks of Rossby waves occur within five days immediately
after the peaks of pressure gradient and KE of the jet. Similar low-frequency