Indian monsoon

from Wikipedia, the free encyclopedia

The Indian monsoon is the most important regional monsoon and is therefore often referred to simply as the monsoon for short , which is not clear due to the variety of different monsoons. It essentially extends over the Indian subcontinent , but also belongs to a larger network of monsoon phenomena in the area of ​​the Indian Ocean . Its foothills extend into the South , Southeast Asian , North Australian , but also East African area.

Indian subcontinent

Special features of the Indian monsoon

Summer monsoon: A heat dip beyond the tropic is effective. The intra -tropical convergence has shifted so far to the north that “extra-tropical convergence” would be more appropriate.
Winter monsoon: Dry winds coming from the mainland cause periods of drought.

Because of the large continental surface, especially the Tibetan plateau , the monsoon phenomenon in India appears very clearly with an ITC shift up to 30 ° north latitude . The Indian monsoon is also the only monsoon that has an effect up to the upper layers of the troposphere . Here there is a deep reversal of the meridional temperature gradient over the Tibetan plateau and thus a tendency towards pronounced advection phenomena. In addition, a seasonal reversal of the wind directions beyond the 700-hPa isobars becomes clear on altitude weather maps . Neither the North American nor the West African monsoons show such a height effect, although the layers of air near the ground warm up very strongly. The development of a comparatively deep, moist, deep air layer in connection with the advective ascent and the adiabatic cooling of the air, which lead to the monsoon rain , are therefore typical phenomena of the Indian monsoon, while in the other monsoon regions there are only much weaker monsoon rains.

The strong contrast between the seasons, which are characterized by the dry winter monsoon and the rain-bringing summer monsoon, has far-reaching effects on people's lives. Since the majority of India is still an agricultural country , this rainfall is of vital importance. This applies primarily to the rural population, but the Indian cities are also dependent on the monsoon rains for their water and food supplies on the one hand and are threatened by its extreme precipitation on the other.

This enormous dependency in connection with the rapid growth of the Indian population and an increasingly unstable climate in the course of global warming results in a dramatic potential for crisis. This is particularly evident in the extensive deforestation and the resulting susceptibility to soil erosion and flooding . From this dualism between the dependence on and the dangers of the monsoon, the characteristic attitude of the Indian population to it developed, which is closely interwoven with many aspects of Indian culture and attitude to life. The role of the monsoons in connection with Indian culture or cultural history, especially in relation to the Indus culture , is explained in more detail in the section on the meaning of the monsoons .

Origin and course of the year

Basic article: Formation of a monsoon

For the regional weather pattern , the large-scale thermal control must be combined with cellular structures such as the monsoon depression . Terrestrial conditions, such as the accumulation of winds on the lee of mountains, also play a major role. During the winter monsoon season at the Pamir junction , the jet stream branches into a north and a southeast stream , which is established over the Himalayan drop and plays an essential role in the formation of a stable high pressure area over north-central India. The leeward convergence of the two jet streams shows a cyclogenetic tendency, which climatically affects southern Japan and China .

The Himalayas and the Tibetan Plateau

In summer, the southern branch of the jet stream is initially blocked by a high altitude in the air over Tibet , whereupon it jumps to the northern jet stream at Kunlun Shan . The Tibetan plateau, responsible for the high altitude above Tibet, is ideally suited to heating up the layers of air above it due to its size, altitude, isolation and dryness. As explained in the section on surface heating, a thermal depth of the soil is formed. This stable bottom now has a very strong impact on the Indian subcontinent and intensifies the phenomenon of convergence towards the continent. As a result, the air at the foot of the Himalayas is less stable and the wintry subtropical high above India is disappearing. After the winter monsoon has subsided, the summer monsoon can now penetrate unhindered ( burst of monsoon ). The Tibet high and the resulting temperature discrepancies create a strong pressure gradient, which forms an eastern jet stream ( Tropical Easterly Jet ), which is weather-effective as far as the Sahara . So the Indian monsoon has global climatic effects.


Climate diagram of Kanpur
Extremely high rainfall is measured in the Cherrapunji region .

The Indian summer monsoon is a southwest wind, because when the ITC is in the northern hemisphere, the southeast trade wind crosses the geographical equator and is deflected into a southwest wind by the changing Coriolis force . The summer monsoon starts in June / July due to the ITC shift and trade diversion (SE trade winds to SW monsoons) explained above from the southwest and lasts until September / October. Moist oceanic air masses first reach the Western Ghats and cause an unstable atmosphere stratification (for an explanation see article Föhn ), which results in monsoon rains. This high rainfall lasts for several months, which is why one speaks of a rainy season . Especially on morphological obstacles ( mountains ) of monsoon rains can be used as orographic rainfall reach very high rainfall which even places the limit of 10,000 millimeters annual rainfall exceeding. Extremely high precipitation due to monsoon rains is measured in the region around Cherrapunji with an average annual total precipitation of approx. 9000  mm , where the global single annual record of 26461 mm was also registered. In Mawsynram in the Indian state of Meghalaya , the monsoon rain brings the world's highest mean annual precipitation of 11,872 mm. Large parts of this precipitation run off as surface runoff and regularly lead to flood disasters in Bangladesh .

The winter monsoon is the Northeast - Passat identical, delivering cold, dry air masses from the cold high over Siberia ( Ref : Goudie 2002). It starts in September / October and lasts until June / July, this arid period being called the dry season . In years with a weak summer monsoon, this can escalate into a drought and in the past often led to great famine .

Expansion and retreat of the Indian summer monsoon

In the picture on the right, the temporal spread of the Indian monsoon is illustrated by some markings. The dark blue lines stand for the expansion of the summer monsoon (monsoon front) or the retreat of the winter monsoon in June / July and the light blue lines for the expansion of the winter monsoon or the retreat of the summer monsoon in September / October, whereby each one is sufficiently accurate but by no means reliable Line can assign a specific point in time in the course of the year. It should be noted here that the spread of the monsoon influences is not reliable and the daily information is only a rough guide, as the advance of the monsoon is a dynamic process that is characterized by oscillations on the monsoon front. The occurrence or disappearance of the monsoon rain was always used as a feature of the times.

Variability and change

The Indian monsoon as a whole is a reliable climate phenomenon with only relatively minor differences over the course of several years. The mean annual rainfall of 852 mm with a standard deviation of 84 mm reflects a humid, but also low-fluctuation climate. Weather observations , but also climatically evaluated trend analyzes of the same, indicate an increase in monsoon precipitation in the Indian region. These are associated with an increasing risk of flooding, which is already demonstrably increasing in frequency and intensity. The causes of this development are attributed to a link with the global average temperature, i.e. also with global warming .

Interactions with ENSO

Anomalous sea surface temperature (° C), observed in December 1997 during the last strong El Niño (Source: NCEP , NOAA )

The occurrence of an El Niño phenomenon in the East Pacific has an effect even as far as the Indic and thus also on the Indian monsoons. Very early on, the Southern Oscillation Index , a parameter for the probability of the occurrence of an El Niño phenomenon, based on the measurements made by the observatory of the British colonial administration in India by its director, Gilbert Walker , was able to deal with the absence or weakening of the Indian summer monsoon and the resulting precipitation. It was shown here that both phenomena are closely interwoven and influence each other, which says nothing else than that the global atmospheric circulation does not contain any individual elements and therefore their global character is in the foreground. So it is not enough to look at the monsoons in isolation to understand its dynamics and how it occurs. But even a direct connection between the two phenomena is not sufficient, as there were normal monsoon rains during a strong El Niño in 1997, for example. For example, the variability of the warming of the air over the Tibetan plateau and thus above all the local albedo , which is decisively influenced by the presence of snow-covered areas and their snow depth , is very important . In general, however, a decreasing influence of the El Niño on the Indian monsoon can be ascertained in the last few decades, although it is still largely unclear why this occurs and, above all, how sustainable this change is.

Significance for cultural history

The cultural and historical significance of the monsoons is particularly pronounced in the case of the Indian monsoons. In addition to the role of the monsoon winds as a mediator of cultural exchange in the area of ​​the Indic (see also monsoon ), this is particularly evident in the example of the Indus culture.

Indus culture

Sites of the Indus culture

The Indus culture is characterized by the fact that it developed advanced hydraulic engineering very early on and was dependent on it due to the fluctuating monsoon rains. The infrastructural developments in relation to water storage, transport and distribution reached in the period from 3500 to 1500 BC. A level similar to that of the millennia later in Roman high culture . In some areas it even exceeded this and has not yet been achieved in many regions of India. The issue of water and water supply had a high priority here and the many innovations already included things such as bathrooms, flush toilets, a well-supported sewer and pipe system up to the residential buildings and a sophisticated sewer system . Although the archaeological finds so far are of limited informative value, there are also many indications that there is a pronounced reservoir economy, which could have contributed to securing the water supply even in the dry winter months. This in turn represents a basic requirement for the establishment of a stable high culture and at the same time requires a large number of hydraulic engineering knowledge, as well as a "water elite" entrusted with its application and preservation. The need to implement a very productive agriculture with the very different rainfall in the course of the year could therefore have given the decisive impetus for the development of such a hydraulic engineering and the associated “water culture”.
The same applies to other regions and not only to India, although the fundamental questions of the interrelationship between monsoons, agriculture and humans have changed little to this day.


The Western Ghats in the dry season
The Western Ghats in the rainy season

The monsoon in India claims large numbers of victims every year. In the 2005 season, around 1,300 people died from floods and storms. In 2006, as of July 31, there were already 480 victims. However, if the monsoon did not occur, it would have enormous consequences for agriculture. Since the fields would have to be irrigated, there would be droughts and up to 95 percent crop failure.

The 2007 monsoons had exceptionally severe effects. Around 21 million people lost their homes in India, Nepal and Bangladesh. In the Indian states of Bihar , Uttar Pradesh and Assam alone , more than 120 people had died by early August.

After weeks of monsoon rains, over 1.2 million homeless people and several hundred deaths occurred in the Indian state of Bihar at the end of August 2008 .

Additional information

For general literature sources and web links see the article Monsun .

Individual evidence

  1. ( Memento from March 24, 2009 in the Internet Archive )


  • Vidya Sagar Katiyar: The Indian Monsoon and Its Frontiers. Inter-India Publications, New Delhi 1990, ISBN 81-2100245-1 .
  • Klaus Leßmann: The Asian summer monsoon circulation. Sensitivity studies with a zonally symmetrical base model. Kovač, Hamburg 1997, ISBN 3-86064-569-2 .

Web links