The Asian Monsoon from the GEOS-1 Multiyear Assimilation

The Asian monsoon system, often characterized by heavy rains, provides abundant water resources to most of the Asian countries during the summer. The Asian summer monsoon, largely divided into the Indian monsoon and the East Asian monsoon, has been studied for decades by researchers around the world. However, our understanding of the monsoon has been hampered by the limited physical and dynamical information over the vast regions around the high mountains in Asia. The GEOS Multiyear Assimilation provides useful information for understanding the monsoon circulation and the global hydrologic cycle. The following animation, created by averaging the daily data from the GEOS assimilation for five years (1985-1989), illustrates the seasonal evolution of the Asian monsoon system.

This is a still image from the animation showing the average rainfall and winds on July 12 over the 5 year period from 1985 to 1989.

2D Monsoon Animation (Quicktime using JPEG compression - 2.8MB)

The monsoon starts over Southeast Asia early in March and April. Heavy monsoon rainfall is also found over South China before a monsoon sets up over the Indian monsoon region. In the middle of May, cross-equatorial winds become pronounced in association with heavy convective rainfall in the southern tip of the Indian subcontinent. The enire Indian monsoon region experiences the most rapid transition of circulations in May.

Strong low-level winds, the so-called Somali Jet, are continuously intensified throughout June, while progressing northward steadily. The heavy rainfall near the west coast of the Indian subcontinent is associated with high mountain terrain and abundant moisture supply transported by the Somali Jet. The northern part of the Bay of Bengal is known to have about the heaviest rainfall in the world during the summer monsoon period. The low-level cyclonic circulation east of the mountains may provide a favorable condition for the development of tropical depressions, where the westward propagating disturbances from the western Pacific are often intensified. The source of moisture for the heavy rains is not clear yet. However, the moisture evaporated from the Arabian Sea and the South Indian Ocean appears to be an important source.

In June, the monsoon rainbands continuously march northward over the East Asian region as well. While the total precipitation amount is not comparable to the tropical rainfall, the concentrated rainfall during the monsoon period, usually no longer than two weeks, is crucial for the agriculture over that region. The East Asian monsoon has different names as it progress northward; Mai-Yu in China, Baiu in Japan, and Changma in Korea. The failure of the monsoon often results in devastating drought over these countries.

3D Animation of Indian Monsoon:

How much rain?

These are yearly precipitation estimates for some selected cities extracted from the GEOS-1 Multiyear Assimilation. Calcutta and NW Burma are in the heart of the Indian monsoon region.

[Users with graphics would see a graph here showing yearly rainfall totals
     in a selection of cities around the world.  It would show much more rain
     falling (up to 125 inches) in Calcutta and NW Burma than in London, L.A.,
     Washington D.C., or Seattle.]

The information on this page is also to be included in an interactive CD-ROM currently under development. This project is led by Sara Tweedie of Tweedie & Associates.

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Data Assimilation Office / Code 910.3 / NASA - Goddard Space Flight Center

Page Authors: Rob Lucchesi & Chung-Kyu Park
Office Head: Ricky Rood