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November 06, 2002
OCEAN TEMPERATURES AFFECT INTENSITY OF THE SOUTH ASIAN MONSOON AND RAINFALL
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.
The summer monsoon blows northeast 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.
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.”
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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Contacts:
Robert Gutro
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-4044)
Stephanie Kenitzer
American Meteorological Society
(Phone: 425/432-2192)
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What is a Monsoon?
This movie gives a brief look and explanation of what a monsoon is and how it works. CREDIT: NASA Data Assimilation Office
Monsoon Floods of July 2002 in Southern Asia
In early July, monsoon rains have lashed the west and north east of India, particularly the Indian states of Assam and Bihar as well as northern Bangladesh. They caused extensive flooding, leaving hundreds of thousands homeless across both countries. CREDIT: International Federation of Red Cross and Red Crescent Societies
Monsoon Season Floodwaters Continue Rising in Bangladesh
In July of 2002, more than 200 villages in Bangladesh were inundated when an embankment was breached on the Jamuna river in the district of Sirajganj, 65 miles north east of the capital, Dhaka. Thousands of people were forced to leave their homes. CREDIT: International Federation of Red Cross and Red Crescent Societies
The Life Cycle of the Madden Julian Oscillation
This image is one of many from a continuously updated animation of the life cycle of the Madden-Julian Oscillation (MJO), an ebb and flow of surface and upper-level winds in the tropical Pacific Ocean. The MJO is also referred to as the 30-60 day or 40-50 day oscillation and is the main intra-annual fluctuation that explains weather variations in the tropics. It may also affect weather systems in the extratropics, such as the west coast of the U.S. In its simplest form, the MJO consists of coherent variations in wind, moisture, sea surface temperature (SST), cloudiness, and rainfall, that propagates from west to east along the equator. Because most tropical rainfall is caused by rising air (convection), and convective cloud tops are very cold (and emit little longwave radiation), the MJO is most obvious in the variation of outgoing longwave radiation (OLR), as measured by an infrared sensor on a satellite. The blue areas depict the least amount of radiation and indicate rainfall. The red areas indicate the lack of rain or high convective clouds. The images are spaced approximately 3 days apart and one whole cycle lasts approximately 48 days. CREDIT: NOAA
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