In this study, we attempt to divide
global divergent field at the upper troposphere
in contributions from the Hadley,
Walker, and monsoon circulations, using monthly
mean velocity potential field at 200 hPa level.
First, the zonal mean of the velocity potential is analyzed to
represent the Hadley circulation.
The deviation from the zonal mean is then divided in
its annual mean and the seasonal cycle parts, which are
considered to represent the Walker and monsoon circulations, respectively.
The intensities of each circulation are measured by their peaks in the
velocity potential field separated in each component.
According to this separation, the mean intensities of the Walker,
monsoon, and Hadley circulations appear to be
120: 60: 40 (${\times}$10$^{5}$m$^2$ s$^{-1}$) in January
and 120: 90: 45 (${\times}$10$^{5}$m$^2$ s$^{-1}$) in July, respectively.

Based on this simple definition, interannual variabilities of each circulation
are then examined quantitatively using the NCEP/NCAR reanalysis.
The time series of the intensity of the Walker circulation coincides
with the Southern Oscillation index (SOI), and the intensity has weakened
in recent decades.
That of the Hadley circulation indicates intensifying trend in boreal winter.

Finally, the same analysis is applied for the model atmosphere
by the MRI CGCM1 with a gradual increase in CO$_2$ at a compound
rate of 1 \%/yr for 150 years.
It is shown that the Hadley circulation intensifies by 40\%
and the monsoon circulation decays by 20\% in boreal summer
when the global warming has occurred in a century later.
The result demonstrates that the
proposed simple separation of the tropical circulation in the Walker,
monsoon, and Hadley components is useful, though it is not rigorous,
for the initial assessment of the
model response to the global warming.

Full Article: http://air.geo.tsukuba.ac.jp/~tanaka/papers/paper133.pdf

Last Updated: 2007-02-27