This is a longer version (than the official one) of the abstract for my presentation at The Sixth Biennial Conference of East Asian Environmental History (EAEH 2021) to be held on-line (based on Kyoto) on 7-10 September 2021 http://www.aeaeh.org/eaeh2021.htm .
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The basic concept of monsoon, revisited
Kooiti Masuda
We can agree that monsoon is crucial in human activities in the large part of Asia and Oceania. But the term "monsoon" has a broad stretch of meaning. We must take care that what people want to mean by the word may not always be the same. The two centers of its usage is "seasonally changing winds" and "the rainy season".
The atmosphere has a basic zonal configuration of winds: tropical easterlies (trade winds) and mid-latitude westerlies. The zonal pattern shifts according to seasons, to the north in the northern summer, to the south in the southern summer. This shift, which occurs irrespective of longitude, may be included in monsoon, but not the essential part of monsoon.
The basic cause of monsoon is the difference of thermal properties between the land and the ocean. The land is solid, so conduction is the only way to transmit energy below the surface. The ocean is liquid, so convection is possible, which is more efficient then conduction. Therefore, the depth of layer that can contribute to the seasonal change is larger over the ocean than over land. Given similar amount of seasonal change of incoming solar radiation, it causes larger seasonal change of surface temperature over land. The seasonally changing wind system is strongest around the largest land mass of the world, Eurasia.
The basic mechanism is like this. In summer, the air contacting the land surface has lower density than its counterpart over the ocean. So there will be a circulation with upward motion over land and downward motion over the ocean. The wind near the surface will be from the ocean to the land, and there will be the opposite motion several kilometers above.
Actually, in the tropical monsoon climate such as in the central India, the typical situation happens just before the main rainy season, and the situation is called "pre-monsoon". After the onset of the rainy season, rainfall itself, rather than the hot land surface, becomes the main cause of upward motion, so its position may be movable. The term "monsoon" is often used to mean this situation.
In winter, the basic machanism results in a high pressure zone around Siberia and Mongolia, and surface northwesterlies around Japan, and northeasterlies around the South China Sea. As the wind crosses the equator, it becomes northwesterly, and confluences into the summer monsoon of Indonesia and northern Australia.
The winter monsoon causes much precipitation (mostly snow) in the part of Japan which is faced to the Sea of Japan, and generaly clear sky in the leeward of the central mountains. It also brings precipitation in some tropical and subtropical areas, such as the east-coast zones of Taiwan, Philippines, Vietnam, southern Thailand and peninsular Malaysia, and the north-coast zones of Borneo and Jawa.
Variability of monsoon affects human life in various ways. The typical one is via precipitation. In the area of monsoon climate, both "too little water" and "too much water" causes problems (typically droughts and floods), as Katumi Musiake often said. Note that the system that brings water can be summer monsoon or winter monsoon, according to the location. In addition, the seasonal winds themselves can be important, especially in the days of sail.
References
- Katumi Musiake, 2002: Hydrology and water resources in Monsoon Asia: A consideration of necessity to organize “Asian Association of Hydrology and Water Resources”. Journal of Japan Society of Hydrology and Water Resources (水文・水資源学会誌), 15: 428-434. https://doi.org/10.3178/jjshwr.15.428 (This article is written in English though most of the contents of the journal are in Japanese.)
- Peter J. Webster, 1987: The elementary monsoon. in: Monsoons (edited by J. S. Fein and P. L. Stephens, Wiley), 3-32.
