The monsoon (from Arabic موسم mausim 'season') is a large-scale air circulation of the lower troposphere in the area of the tropics and subtropics in the area of influence of the trade winds . A characteristic of the monsoon is its very stable seasonal monsoon winds in connection with a reversal of the most frequent wind direction twice in the course of a year.
The monsoon winds are caused both by the migration of the zenith of the sun between the tropics and by the different warming and cooling properties of different earth surfaces. Since the air over large land masses warms up more than the air over the oceans due to the increased solar radiation in the summer half-year, humid air masses move from the sea to the land during this season. The Coriolis force causes a large wind deflection.
The term monsoon has its strongest expression and at the same time its word origin in the area of the Indian Ocean , especially in relation to the Indian , but also to the North Australian and East African monsoons.
The monsoon has a strong influence on the climate of the affected regions due to the high humidity carried by the monsoon wind in summer , which is therefore known as the monsoon climate . The long, pronounced summer monsoon rains are characteristic . The typical vegetation of these regions with a humid summer climate is the monsoon forest . The great economic and cultural-historical importance, especially of the Indian monsoons, is derived from this very strong influence on the natural environment.
Meteorology and Climatology
In the monsoon climate, the seasons differ mainly in the amount of precipitation, a dry and a rainy season. The monsoons have a decisive impact on the climate in tropical and subtropical regions - for example in India , Central Africa and Southeast Asia .
Origin of the monsoons
Main article: Formation of a monsoon
Seasonal wind direction changes arise initially because of the shift in the intertropical convergence zone (intertropical convergence, ITC), a low-pressure gully that is created by the warming and rising of the air near the equator . The comparatively low pressure of the ITC attracts air and creates winds, the trade winds. The intra-tropical convergence zone follows with a slight delay the migration of the zenith of the sun between the tropics caused by the inclination of the earth's axis . The ITC, in the case of a monsoon phenomenon influenced by a continental soil depth, which are also called monsoon low designated and by the strong heating of the over the continents located air masses caused. The reason for the increased warming of the air over the continents is the different thermal properties of the land and sea surfaces . The warming, but also the cooling of the land surface occurs about two to three times as fast as that of the water surface.
The Indus plain and the Tibetan plateau , for example, form the core areas of ITC influence by a monsoon low ( Lit .: Weischet 2002). Due to this influence on the ITC, however, the Passats are also shifting. The winds in the northern hemisphere receive an eastern component through the Coriolis force deflecting to the right in the direction of movement , as long as the air masses move towards the equator, so that a northeast trade wind is created. In the southern hemisphere , the trade wind is deflected by the Coriolis force, in contrast to the northern hemisphere, in the direction of movement to the left, i.e. to the west, to a southeast trade wind. If the sun is in the vicinity of the Tropic of Capricorn, the ITC is also there. All air masses now flow into this low pressure channel. However, air from the south-east trade wind must cross the equator, the Coriolis force now causes a clockwise rotation, the wind turns from south-east to south-west: The south-west monsoon (actually south-west monsoon wind) ( Lit .: Borchert 1993). If the sun is south of the equator, the northwest monsoon (actually northwest monsoon wind) occurs; However, due to the secondary conditions (monsoon low) and the few existing land masses, this effect is weaker in the southern hemisphere.
On its way from the ocean to the continent, the monsoon wind absorbs moisture over the surface of the water and rains it down to a large extent on the windward side of weather divisions such as the Himalayas . The summer monsoon is therefore characterized by very humid conditions in these regions , which take on the character of a rainy season and, especially in recent times, have led to above-average flood conditions.
In the respective winter, however, high pressure areas develop over the continents. The ITC then shifts again towards the equator or crosses it towards the other hemisphere. As a result, the northeast trade wind in the northern hemisphere and the southeast trade wind in the southern hemisphere are the dominant winds. These are also known as the winter monsoons and carry dry, continental air masses with them. They are therefore usually expressed in a pronounced dry season .
Regional monsoon phenomena
There are many different regions with full-fledged monsoons, monsoons with a low persistence or those that only have a monsoon tendency. It is a matter of monsoon phenomena of various degrees and characteristics with numerous regional peculiarities. These have always played an enormous role in the entire history of the people living there, which also led to the fact that most of the recurring winds were given different regional names . Due to the abundance of different monsoon phenomena and their strongly regionally resolved effects, it is only possible to give a very large-scale overview below.
The most important factor of regional differentiation is the continent's orography and the effect primarily of very high mountains as weather divisions with damming rain and foehn effects. As a result, topographically separate areas, for example on the windward and lee side of a mountain range, can sometimes have considerable differences in annual precipitation and the main wind direction, such as the annual course of the monsoons, can also vary greatly. In addition, influences from other climatic factors or neighboring climatic zones also play a role. Ocean currents and changes in the local sea surface temperature can also influence the precipitation distribution and the monsoon as a whole.
It should be noted that this diversity and regional differentiation contradicts any generalizing statement and that many regional monsoon phenomena in their form and especially their genesis represent a field of research, i.e. cannot yet be considered conclusively understood and thus represented in this context. A correct representation of the current state of research cannot be realized in this article either.
- Indian monsoon
- North Australian monsoons
- East Asian monsoon trend
- West African monsoon system
- American monsoon system
- Monsoon trend in the Mediterranean
A monsoon can also occur outside of the cases outlined above, for example in Southeast Asia or North Australia or, to a lesser extent, in South Japan or East Asia, South Africa and Central America. A strong north-south distribution of ocean and land masses has a positive effect on the monsoons, as this best takes into account the migration movement of the zenith of the sun. In general, all coastal areas between 5 ° and 25 ° from the equator towards the poles can be specified as areas of occurrence for monsoon phenomena, although seasonal precipitation cycles can also occur here, but these are not or hardly associated with a dominant wind direction ( Lit .: Goudie 2002). Because of the superimposition of the westerly wind zone , one can only rarely see monsoon-related characteristics in the areas north and south of it. Using the example of the Meltemi (Etesia), summer north winds in Greece , monsoon influences can also be discovered in the Mediterranean .
Influence on vegetation
A strong monsoon climate transforms landscapes that resemble a semi-desert during the dry season and into fertile green land during the rainy season.
Plants that grow in the monsoon climate do not necessarily have to be adapted to frost. However, they must be adapted to both long periods of drought and heavy rain in order to survive. That means: They must not dry out during a long period of drought. During periods of heavy rain, they should be able to grow quickly to take advantage of the rain, and they should be deeply rooted to avoid being washed away.
Accordingly, farmers in the monsoon climate must grow plants that can withstand these conditions. Plants that need a lot of water to grow (for example rice ) have to be grown during the rainy season - or have to be artificially watered.
The monsoon forest therefore consists of plants that are adapted to these conditions.
The monsoons are of particular importance with regard to the dynamics of climate development. It is a very unstable climatic element, yet it has a very high impact on the climate of large parts of the world. This means that even small changes and developments, even at a regional level, can trigger or weaken a monsoon or significantly change its appearance, even and especially in comparatively short periods of time. Orogeny , tectonic plate movements , changes in large wind and ocean currents , as well as changes in the thermal behavior of continental surfaces, for example through a reduction in the albedo in the course of global warming , are examples of this. The latter in particular shows that short-term, anthropogenic causes are also present Disturbances in the climate system can change entire climate zones, even if only comparatively small changes occur at a global level, statistically adjusted.
Climate history of the monsoons
Monsoon phenomena have existed on earth since oceans and land masses exist. Since climate history is necessary in order to understand today's climate and to correctly forecast its future development, the monsoon phenomena of past geological ages (paleomonsoons) are of great importance. The precipitation caused by them in the past was also stored in low-lying rock layers in many places on earth and is now available as fossil drinking water sources in these areas , despite possible climate change and continental drift . Since these are often desert climates , these underground deposits are of the greatest economic importance for the people who live there. The occurrence of earlier monsoon phenomena is researched through paleoclimatology . It is assumed here that monsoon phenomena played an enormous role, especially in periods of the earth's history in which there was a supercontinent . These very large land masses could cause monsoon phenomena due to their strong continental heat depression , which went far beyond the strength of today's monsoons. Due to the very long periods of time, however, it is hardly possible to provide reliable evidence for this.
Influence on people
The importance of monsoons for the World Food that supply with drinking water and the irrigation of agricultural soils is of fundamental nature. More than 60% of the world population is directly or indirectly affected by monsoon phenomena, especially in India and southern China. This shows the dual character of the monsoon as a guarantee of precipitation on the one hand and, due to its variability (see monsoon rain ), also as a cause of droughts and floods on the other.
All cultures that developed in the regions affected by monsoon phenomena were and are dependent on the climate. A change in the monsoons is always associated with a change in the way of life of the people affected by it. This is especially true for agrarian societies in places where monsoon phenomena develop to their full extent, for example in the Indian region.
The variability of the monsoons, which has always existed and has determined people's lives for thousands of years, is not only of purely economic importance. The interrelationships between monsoons and humans - especially their dependence on monsoons - entered culture, art, religion, and even thought and philosophy, again especially in India. This can already be seen in the Indus culture , whose dependence on monsoon rains is shown in the article Indian monsoons . In addition, the monsoon winds have been a vehicle for cultural exchange in the Indian Ocean for many centuries , which is discussed in more detail in the following section.
Influence on shipping
In the years 120–117 BC Chr. Took Eudoxus of Cyzicus an exploratory trip to India and it recognized the importance of the monsoon winds for sailing ships in the Indian Ocean. Eudoxus then probably passed on his knowledge of the monsoon winds to Hippalus , to whom this discovery in the Periplus of the Erythrean Sea is ascribed. Hippalus became a legendary sailor and for a long time he was considered the first to take advantage of the monsoon wind. The monsoon was therefore formerly referred to as Hippalus in the greater area of the Indic . However, it is more likely that both Greeks were not the first to take advantage of the monsoons, as Yemeni sailors traded in the area long before that.
As seafarers, the southern Arabs use their knowledge of the monsoon winds in the Indian Ocean and have been cruising with their dhows between the Arab, Indian and African coasts for over two thousand years , taking in the changing winds of the monsoon, the Kaskasi and take advantage of the Kusi in one-year trading trips.
In addition to the incense trade , Yemen owned in the 8th century BC Chr. Already close trade contacts with India and East Africa. Especially those to Africa were so close that colonies of South Arabian settlers emerged in Eritrea . Due to the extensive trade, cultural influences from the Middle East could also have an effect in Yemen. So the South Arabic script was used in the 8th century BC. Developed from the Phoenician alphabet . Further influence gained from the 3rd century BC. Chr. The Hellenistic culture in Yemen, which is here about indirectly joined with residents of the Indian Ocean in trade contacts. This is shown by the cultural assets of Hellenistic origin still preserved in this room.
In the cultural exchange of goods and traditions, the mixed culture of Swahili arose on the East African coast, characterized by trade and Islam . The Arabs brought Islam with them to Africa, mixed with the local Bantu peoples and founded cities such as Lamu , Sofala and Mombasa .
It should therefore be noted that sailing ships using the monsoon wind together with the large caravans ( Silk Road , Frankincense Road ) often represented the only economic and cultural connection between the Orient and thus the Occident with the Indian and, above all, Southeast Asian region for centuries . The monsoon served as a mediator between these cultures, promoted their exchange and is therefore of central importance in the area of the Indic, in addition to its economic importance for maritime trade, in the history of culture and civilization.
Arab sailors described with the word mausim /موسم / ' Season ' is the phenomenon of a wind in the Arabian Sea that changes with the season. Basically, the term monsoon still expresses a change in wind direction between the seasons, even if it has changed in modern times and with an understanding of the causes of these winds. In the course of this cognitive process, the scientific understanding of the term has developed from a purely phenomenological wind or a season to the entirety of the causes, dynamics and effects that determine this phenomenon. To clarify this understanding of the term, one often speaks of a monsoon system or a monsoon circulation. The monsoon is therefore also a climate factor.
Monsoon research and scientific monsoon definition
History of Monsoon Research
The exploration of the monsoons and thus its definition of terms has a long history, which is usually closely linked to the effects on humans. This is also the reason why the term monsoon is often used as a synonym for the Indian summer monsoon and its rainfall. In the last century, too, the simple connection between the complex dependence on the monsoon and the interest in researching it became evident.
Meteorological records of the monsoon rains in the Indian area have been kept for more than 2000 years, even if these have only been handed down in fragments and do not represent a continuous series of measurements. In modern times Edmond Halley (1668) did pioneering work in monsoon research and recognized its thermal conditioning. This was followed by the research of Blanford (1860), Supan (1881) and Todd (1888), which were particularly influenced by the exceptionally weak monsoons in 1877/78. A very important role played in the result, the research by Gilbert Walker (1909, 1924), which the interactions between bears his name Walker circulation explored and later Jacob Bjerknes enabled the monsoon in relation to other climate phenomena such as El Niño put .
More precise forecasts can be made as part of a weather forecast .
Scientific monsoon definition
The most widespread definition of the term today is that of SP Chromov (1957). He understands a monsoon as a wind phenomenon in which between January and July there is a change in the direction of the prevailing wind directions of at least 120 °. This angle is also known as the monsoon angle . The main wind directions must have certain averaged frequencies in January and July so that one can speak of a monsoon. At over 60% the term monsoon applies , at 40% to 60% one speaks of a monsoon of low persistence and at below 40% the main wind directions only show a monsoon tendency .
A further restriction of the monsoon climates took place in 1971 by Ramage, which is why the resulting criteria of the monsoon definition are also known as the Ramage-Chromov criteria, or Ramage criteria for short . In addition to a monsoon angle of at least 120 ° and an averaged frequency of the main wind direction of over 40% in January and July, a wind speed of at least 3 m / s in the resulting wind direction must occur in these months and only one cyclone - anticyclone - Changes occur every two years on an area five degrees of latitude times five degrees of longitude.
This comparatively strict definition ensures that many extra-tropical weather and climate phenomena are not counted among the monsoons, since their monsoon-like phenomena, such as a wind reversal or seasonal drought, usually have causes alien to the monsoon. The monsoon classification made in the article relates to this definition, whereby only the Indian or Southeast Asian, North Australian and African monsoon phenomena are also classified as monsoons (see section regional monsoon phenomena).
- G. Borchert: Climate geography in brief. (Hirts index books) Berlin 1993, ISBN 3-443-03105-6
- SP Chromov: The geographical distribution of the monsoons. Peterm. Geogr. Mitt. 1957, pp. 234-237.
- C. Ramage: Monsoon Meteorology. International Geophysic Series, vol. 15. Academic Press, San Diego, CA 1971
- A. Goudie: Physical Geography: An Introduction. (4th ed.). Heidelberg 2002, ISBN 3-86025-159-7
- E. Heyer: The monsoon term. Geographic reports. Vol. 13, pp. 218-227. 1958
- H. Malberg: Meteorology and Climatology. An introduction. (4th, updated and expanded edition). New York 2002, ISBN 3-540-62784-7
- W. Weischet: Introduction to General Climatology. (6th, revised edition) Borntraeger, Berlin 2002, ISBN 3-443-07123-6
- JR Holton et al .: Encyclopedia of Atmospheric Sciences. San Diego, London 2002, Academic Press, ISBN 0-12-227090-8
For individual aspects of the monsoon see the web links in their articles.
- More detailed descriptions of some regional monsoons (English)
- Monsoon in Myanmar
- Monsoon in Thailand
Cultural and historical significance
- Documentation "Every wind has its journey" on Arab dhow sailing, trailer can be downloaded ( Memento from September 4, 2011 in the Internet Archive )
Climatic change and its consequences
- IPCC 2001: Monsoons , Monsoons and Teleconnections
- Relationship between mountain formation and monsoons