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OCEAN
TEMPERATURES AFFECT INTENSITY OF THE SOUTH ASIAN MONSOON AND
RAINFALL
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Warmer
or colder sea surface temperatures (SST) may affect one of
the world's key large-scale atmospheric circulations that
regulate the intensity and breaking of rainfall associated
with the South Asian and Australian monsoons, according to
new research from NASA.
A
monsoon is a wind that changes direction with the seasons.
Monsoons develop from changing patterns of atmospheric circulation
which are caused by changes in heating and cooling of land
and oceans. One of the strongest and most well-known monsoons
is the one which affects India and Southeast Asia in June
through September.
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The
summer monsoon blows southwesterly across the Indian Ocean
and is extremely wet. During the summer monsoon, particularly
in July, there is usually a break period in the monsoon, when
the rains stop and re-start. A phenomenon known as the Madden
Julian Oscillation (MJO) has been found to affect that break
and the variation of the MJO is affected by the variation
of the SSTs.
The
MJO is the main fluctuation of atmospheric circulation that
explains variations of weather in the tropics and that regulates
south Asian monsoons. The variation of the MJO involves variations
in wind, SST, cloudiness, and rainfall. The MJO can be characterized
by a large-scale eastward movement of air in the upper troposphere
with a period of about 20-70 days, over the tropical eastern
Indian and western Pacific Oceans at approximately 7.5 miles
(200 millibars) high in the atmosphere.
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Man
Li Wu, a researcher from NASA's Goddard Space Flight Center
in Greenbelt, Md., and lead author of the study, used computer
models that simulate the atmosphere in the region. One of
the purposes of this study is to examine to what extent intra-seasonal
SST variations affect the MJO.
"Changes
in SSTs will affect a large scale atmospheric circulation
known as the MJO in the area of southern Asia, which regulates
the variability of the monsoon," Wu said. The variation
of the MJO coincides with the variation of tropical precipitation
over the Indian Ocean and western Pacific Ocean. She noted
that from observations the warmer sea surface temperatures
are usually found 5 to 10 days before the strengthening of
the precipitation on the MJO time scale.
Siegfried
Schubert, a co-author on the paper from the Goddard Space
Flight Center said that "the MJO plays an important role
in the variability of the South Asian monsoons. Changes in
SST may be responsible for between 15% and 30% of the fluctuations
that occur in the strength of the MJO."
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The
ability of current computer models to simulate fluctuations
of MJO is still in question. Wu said, "Improvements in
predicting the MJO are an important step in making further
progress in weather and climate prediction for areas affected
by the monsoon. For weather forecasts, improved modeling of
the MJO offers hope for extending the range of useful forecasts."
For seasonal and climate prediction, the MJO is a key component.
These
findings appear in the American Meteorological Society's October
15th issue of the Journal of Climate, Volume 15, Number 20.
This
research was funded under NASA's Earth Science Enterprises
Global Modeling and Analysis Program, dedicated to better
understanding and protecting our home planet, and is a contribution
to the CLIVAR/Monsoon Global Climate Model Intercomparison
project. Support was also provided by the Atmospheric Sciences
Division of the National Science Foundation.
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