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The basic concept of monsoon, revisited (discussion paper)

This is a discussion paper 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 .

My abstract has been put on this blog on 2021-06-13 [in English], and [in Japanese].

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Needs for re-thinking about the concept
The word “monsoon” is often used as a key word to describe the natural environment in some parts of the world (in particular, South, Southeast and East Asia) in the discussion of the relationship between it and the human society. The connotation of the term is not the same between one user of the term and another, however. The broadness of the meaning may be a good thing as a key word of a movement (either academic or social) for participants with various backgrounds join together. On the other hand, when we make serious discussions about, for example, the effect of changes in monsoon on the human society of a certain region, we need an agreement between the discussants about the meaning of the term. Perhaps we cannot reach overall agreement. We will probably need context-dependent definitions of the term.

Actually, situation in the Japanese Language is a little more complicated. In Japanese, there are two words, “monsûn” and “kisetsufû”. The former is obviously a word borrowed from some foreign language, perhaps English. The latter consists of two elements which came to Japanese from Classical Chinese, “kisetsu” meaning season, “fû” meaning wind. Some Japanese speakers (both professional and non-professonal) use the two words as synonyms. Others make distinctions, and think that “monsûn” is something tropical and “kisetsufû” is something they experience in Japan. I belong to the group who take them as synonyms. I occasionally remark about the usage of the other group.

Within a broad stretch of the meaning of the term, I can depict several clusters.
1. Seasonal alternation of wind direction;
2. Rainy (or snowy) season;
3. Warm and humid climate; climate which enables cultivation of rice.
Understanding of climate based on principles of physics has advanced in the 20-th Century. There are two basic causes of the seasonal alternation of wind direction (to be discussed below), and the second one, land-sea contrast, is important for monsoon to be outstanding feature of certain regions of the world. Seasonal alternation of winds usually accompanies seasonal alternation of precipitation, but the manner is not always the same between one region and another.

Seasonal alternation of wind direction
It would be endless to discuss the concept of season. I start from the basic physical cause of the seasonal cycle. The axis of the rotation of the earth is tilted from the axis of the orbital motion of the earth around the sun. To an observer living at a place on the earth, both the elevation angle of the sun and the duration of daytime is largest around the time of summer solstice (around 22 June in the northern hemisphere) and smallest around the time of winter solstice (around 22 December in the northern hemisphere). Therefore, income of energy from the sun per unit area of the earth’s surface is largest around the summer solstice. I tentatively call a half-year time period which contains the summer solstice “summer”, and the other half year “winter”. Note that summer in the northern hemisphere is winter in the southern hemisphere. Also note that “summer” in this tentative definition does not always mean the time when the surface temperature (or air temperature near the surface) is highest of the year. (See the typical situation of the tropical monsoon region to be discussed below.)

Prevailing wind direction is the same all over the year in some parts of the world. But it is almost opposite between summer and winter in some parts of the world. Notably, in the tropical Indian Ocean to the north of the equator, southwesteries prevail in summer and northeasterlies in winter. This is surely the original type of the concept of monsoon.

The global distribution of alternation of prevailing wind direction was shown by Khromov, a climatologist in the USSR then, well known by the German version (Chromov, 1957). I tried similar analysis with modern data and made a presentation (Masuda, 2002) but not yet finished an article. The distribution has both zonally symmetric (dependent only on latitude, not on longitude) and zonally asymmetric (dependent on longitude) features.

To explain the distribution, it is better to discuss the causes together.

Seasonal shift of zones of general circulation
As for the first approximation, the atmospheric circulation of the troposphere (from the surface up to 10-15 km) can be summarized in a zonally symmetric and annual average state.

In low latitudes, prevailing winds near the surface is easteries, called “trade winds”. There is a zone of upward motion near the equator (which can be equated with “the Intertropical Convergence Zone”), and zones of downward motion around 30 degree latitude north and south. The lower branch of the meridional-vertical circulation is towards the equator, and it becomes easterlies because of the rotation of the earth (Coriolis effect). George Hadley, in 1735, envisaged such a pattern of motion in order to explain the cause of trade winds. Hence we call the meridional-vertical circulation “the Hadley Circulation”.

In middle latitudes, prevailing winds near the surface is westerlies, and that in the upper troposphere is even stronger westerlies. The westerlies are not actually zonally symmetric, but undulated. The waves in the westerlies accompany extratropical cyclones.

Up to here we have not considered the seasonal cycle. Actually, the general circulation is caused by the energy balance, and the maximum of energy income migrateds between the Tropic of Cancer and the Tropic of Capricorn. Hence, the whole structure consisting of the Hadley Circulation and the mid-latitude westerly zone shifts towards the summer hemisphere of the time of the year.

This shift can explain some part of the seasonal alternation of prevailing wind direction. In particular, around 30 degree latitudes, where trade winds prevails in summer and mid-latitude westerlies in winter.

In 1950s-1970s, Atsushi Kurashima, among others, extended this kind of idea to explain monsoons of the world (Figure 2.4 of Nemoto et al. 1959; Figure 42 of Kurashima 1968, Figure 10 of Kurashima 1972). It was a remarkable achievement. But it cannot explain zonally asymmetric features of the monsoon.

Circulation caused by the land-sea thermal contrast
The seasonal alternation of prevailing winds is prominent around Eurasia (and also in tropical Africa north of the equator), but not so in North and South America. (American people do speak of monsoon there. In my understanding, they talk about the sudden start of rainfall occurring after warming of the land surface, which is another typical feature of monsoon but is not necessarily caused of seasonal alternation of prevailing winds.) This fact suggests that existence of large continental land mass has something to do.

We know about the phenomenon called land-sea breeze. The time period is one day, and the horizontal scale is tens of kilometres. In daytime, the air over the land becomes warmer than that over the sea, hence its density becomes lower. The difference of density causes upward motion over the land and downward over the sea, flow near the surface from the sea to the land (sea breeze), and return flow somewhere above (typically at the height around 2 km). At night the temperature difference is opposite and land breeze occurs.

It is caused by the difference of thermal characteristics, namely heat capacity, between the land and the sea. Heat capacity is the amount of energy needed to cause unit (typically 1 degree C) rise of temperature of a certain material body. In this context, heat capacity per unit area of the earth’s surface matters. The land has smaller heat capacity than the sea (per unit area), so daytime-nighttime difference of temperature becomes larger there, given similar amount of diurnally changing energy input.

To explain this cause-and-effect relationship, people often talk about “specific heat”. It is true that specific heat of rock or soil is smaller than that of water. But it is not the main cause of the land-sea thermal contrast. “Specific heat” is abbreviation of “specific heat capacity”, that is, heat capacity per mass. Heat capacity is specific heat capacity times density times volume. Heat capacity per unit area is specific heat capacity times density times the depth of the layer to be considered. Specific heat capacity of rock and soil is typically 1/2 to 1/4 of that of water. Density of rock and soil is typically 2 times that of water. There can be orders-of-magnitude difference in depths of the layer that contributes to diurnal variation, however. It is because the land is solid so that the way of vertical heat transfer is just conduction, but the sea is fluid so that convection is possible.

The land-sea thermal contrast can cause not only diurnal cycle phenomena but also annual cycle phenomena. The effective depth of the layer that contributes to the annual cycle is typically 1 m for the land and 100 m for the sea. Because of the different time scale, the Coriolis effect is crucial to annual cycle phenomena, though it is marginal to diurnal cycle phenomena.

Tropical summer monsoon and pre-monsoon
Webster (1987) starts his explanation of the mechanism of tropical monsoon from this kind of land-sea contrast. The explanation is cited by the new textbook by Yasunari (2018, Sections 2-3 and 2-6). I do not find it in the new textbook by Webster (2020), however.

Because of difference in the way of vertical heat transfer, temperature near the surface in summer becomes higher over the land than over the sea. Thus, there will be flow from the sea to the land near the surface, and the return flow somewhere above.

Up to here, condensation of water vapor is not considered. This is not yet the typical summer monsoon situation.

Near-surface winds from the ocean brings air rich in water vapor to the land. By the upward motion over the land, the temperature of the moving air parcel will become lower, and part of the water vapor it contains will become rainfall. So, summer monsoon brings rain to this part of the land!

But then, the land surface is cooled by rain. The contrast of near-surface air density between the land and the sea is no longer the main cause of the monsoon circulation. On the other hand, condensation in rain clouds converts energy associated with water vapor into internal energy of the air proportional to temperature. So the rain clouds themselves create low density air parcels and drives the monsoon circulation. The center of upward motion associated with clusters of rain clouds may move, and it may not remain where the surface temperature was high in the beginning.

We should distinguish “pre-monsoon” and “mature monsoon” situations. In pre-monsoon, land temperature will be highest of the year, and circulation caused by land-sea thermal contrast will prevail. In mature monsoon, the effect of land-sea contrast is indirect.

This kind of description can be applied not only to the South and Southeast Asia north of the equator, but also to the tropical Africa north of the equator, and to Indonesia and northern Australia south of the equator. I am not sure whether it can also be applied to tropical Americas where seasonal alternation of wind direction is not evident.

East Asian winter monsoon and its tropical extension
The prevailing wind in winter in the central zone of Japan (30 to 40 degrees north) is northwesterlies. In summer, the average wind is not strong there, but it is likely to be southwesterliies, as located to the northwest edge of the North Pacific Subtropical High. So this zone can be said to have seasonal alternation of prevailing wind, though less typical than the tropical Indian Ocean.

The difference of the situation here from those of the other regions of the same latitude can be explained by the continental-scale land-sea thermal contrast.

In winter, because of smaller heat capacity, temperature of air just above the land surface over the Eurasian continent will become lower than that over the Pacific Ocean. So there will be downward motion over the continent and upward motion over the ocean, flow near the surface from the continent to the ocean and return flow somewhere above.

The center of low high pressure at the surface resides in Siberia or Mongolia, and, because of the Coriolis effect, there is a clockwise (seen from above) circulation around the center.

To the south of 30 degrees north, the wind direction changes to northeasterly, adding to the trade winds. The flow sometimes crosses the equator and seamlessly merges into the northwesterly summer monsoon of the southern hemisphere.

The near-surface wind, as blowing out of the continent, is cold, and has little water vapor. But as it runs through the sea surface, it gains much water vapor from below. And if it hits the land again, it can bring much precipitation there.

The central zone of Japan has mountain ranges. When northwesterly wind prevails, the windward (the side facing the Sea of Japan) gets much snow, and the leeward (the side facing the Pacific Ocean) tends to have clear sky. (Even in winter, the weather is not always dominated by the monsoon. The part of Japan facing the Pacific Ocean can have much snow when the weather is dominated by migrating extratropical cyclones.)

Some parts of the Southeast Asia (to the north of Equator) also get much precipitation (rain) by the northeasterly winter monsoon: e.g. areas near the east coasts of the Philippines, Viet Nam, southern Thailand, peninsular Malaysia, and near the north coast of Borneo Island.

In either case, the area where winter monsoon precipitation prevails is probably 100 or 200 km from the coastline, and surely is minority within the tropical monsoon areas. But it should be noted that the condition relevant to human life there may not be the same as in the majority.

In the zonally symmetric view, the zones around 30 degree latitude is dominated by the Subtropical High, i.e. downward branch of the Hadley circulation, and low cloudiness and little precipitation.

The actual situation in East Asia is somewhat complicated. The condition of “mature summer”, typical in August, is really dominated by the Subtropical High (except occasional arrival of tropical storms). On the other hand, the early summer (June to July) is a rainy season, called Baiu in Japan and Meiyu in the Chang Jiang river basin in China.

Kodama (1992) explained Meiyu-Baiu as one of subtropical precipitation zones.

I am reluctant to include Meiyu-Baiu in monsoons, because I do not have a cause-and-effect explanation of Meiyu-Baiu either by seasonal shifts of general circulation zones or by land-sea thermal contrast. I will not object if someone includes it, however.

【[Addendum 2021-09-09] Kurashima (contribution to Nemoto et al. 1959; 1968; 1972) was eager to include Baiu in Asian monsoon. I understand that he was driven by the following two pieces of knowledge that was novel in 1950s.

  • Coincidence (in multiple-year average sense) of the onset of summer monsoon rain in India (west coast or Deccan Plateau) and the onset of Meiyu (Chang Jiang basin) - Baiu (central Japan) rain.
  • Jump of upper-tropospheric (ca. 10 km height) westerly “jet stream” from the south to the north of the Tibetan Plateau in early summer.】

‘Am’ type in Köppen’s classification of climates
The term “monsoon(-al) climate” is often associated with the type “Am” in Köppen’s classification of climates. It is a subtype of the tropical climate “A”, and something intermediate between “f” (humid year-around) and “w” (dry in winter). I have not examined Köppen’s original intention yet, but since I know that he tried to explain climatic condition of natural vegetation, I guess that Am is a situation favorable for dry-deciduous forests, unlike Af for tropical evergreen forests or Aw for savannas.

If we accept this definition of “monsoon climate”, places of climate type ‘Af’ (e.g. Singapore) will be excluded.

【[Addendum 2021-09-09] Several group make maps of climate classification based on Köppen’s formulation and more recent meteorological data. One of those achievements is by Beck et al. (2018). Wikipedia (English) article Köppen climate classification adopts it. A map showing distribution of 'Am' is found in the article Tropical monsoon climate. Beck et al.'s area of Am does include the west coast of India, the west coasts of Myanmar and Cambodia and some mountain ranges of Laos, but most of the Indochina Peninsula are categorized as 'Aw'. 'Am' is not a good proxy of the area dominated by tropical monsoon circulation as discussed above.】

Warm and humid climate
On the other hand, the word “monsoon” is often associated, mainly by non-experts of climate science, to warm and humid climates in general, including Köppen’s “Af” type.

One of the influential books in the Japanese-speaking world is that of a philosopher Watsuji, originally published in 1935. Watsuji discussed three types of landscapes, “monsoon”, “desert” and “meadow”. He mainly think about India when discussing the monsoon climate. He occasionally mentions that there is a dry season in India, but, as far as I understand, he does not distinguish monsoonal climate from always humid climate.

Also, the term “monsoon Asia” is often equated with the part of the world where rice is the main crop. Section 6.1 of Nemoto et al. (1959), written by Yoshino, may be one example of writing in this attitude.

Too little water and too much water
Musiake (2002) wanted to promote studies of hydrology and water resources in “Monsoon Asia”. He emphasized that both “too little water” and “too much water” cause problems (typically droughts and floods) to the human society there. Though it is not the same concept as Köppen’s “Am”, it is relevant that it is neither always wet nor always dry.

Tentative conclusion
The definition of “monsoon” may be different from one context to another, and it is unlikely to unify it. We have to make tentative agreement in each context in order to make fruitful discussions.

When I am able to make a choice, I use “monsoon” to denote large-scale atmospheric circulation somehow associated to land-sea thermal contrast, including both summer monsoon and winter monsoon.


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