West African monsoon system

from Wikipedia, the free encyclopedia

The West African monsoon ( WAM ) is a coupled atmosphere-ocean-land system that controls summer precipitation and winter drought over the regions of West and Central Africa . The processes in this system are characterized by interacting space and time scales. The word monsoon is derived from the Arabic mausim ( موسم ) and originally means " season ". Arab merchants used it to designate the time of year when they sailed across the Arabian Sea to India on their dhows . The regions in the zone of influence of the monsoons have a pronounced dry and rainy season over the course of the year .

The West African monsoon is characterized by a sharp, spatio-temporal meridional gradient , i. H. its northern border falls between 50 and 150 mm of precipitation in July / August , while its southern border falls between 1200 and 1600 mm. The time span also varies: the four months from June to September are rainy in the south of the area of ​​influence , in the north there is usually only one.

At the time of its greatest activity, the rain belt of the West African monsoons extends from Cape Verde across the Chad Basin to the foot of the Abyssinian highlands , making it the second major monsoon system on earth after the Australian-Indian monsoon . It is the primary weather phenomenon that provides the African regions of the central Sahara , the Sahel , the Sudan and the windward side of the Upper Guinea threshold with precipitation in the summer months.

Precipitation in West Africa is influenced by the location of the Innertropical Convergence Zone (ITCZ). It shifts every six months and ensures higher levels of precipitation in West Africa, especially from May to July. In the winter months the ITCZ ​​is close to the equator over the Gulf of Guinea, so that the dry season prevails on the mainland.

In the north of the area influenced by the West African monsoons, mesoscale, highly organized gust line systems (storm fronts) are the most important source of the rain precipitation there. As mesoscale atmospheric phenomena over West Africa, weather phenomena such as storm fronts and thunderstorm cells are described, which can reach a horizontal extent between 2 and 2,000 kilometers. In the main activity zone of the monsoons, thunderstorm systems , so-called MSC ( Mesoscale Convective Systems “mesoscale convection systems”) develop . These represent a combination of individual thunderstorm cells. They can cover an area of ​​more than 100,000 km² and reach cloud heights of up to 18 km within the cloud cluster . In locally restricted active core areas, up to 50 liters of rain per square meter fall in half an hour. Towards the damp south coast of West Africa, other types of organized and unorganized wet convection also contribute to annual precipitation.

The investigations of the West African Monsoon Multidisciplinary Analysis Project (AMMA) have shown that the inflow of cold water into the Guinean Gulf plays a decisive role in this system. Similar to the meteorological conditions in the Mediterranean or the Indian Ocean , the water temperatures are considered to be the key factors in the variability, intensity and the retreat of monsoon precipitation in autumn. The annual cycle of the sea surface temperature in the Gulf of Guinea is asymmetrical, with a rapid cooling in April to the lowest water temperatures in August and a gradual increase in water temperature until next April . The resulting near-surface equatorial cold water tongue is created by the swelling of cold water from depths of about 100 meters and originates from an equatorial undercurrent that extends from Brazil to the eastern Atlantic. An average of 20 million cubic meters of water flow per second and cool the surface temperature of the Atlantic to 20 to 25 ° C.

Other decisive factors are the formation of stable high pressure systems over the sea and low pressure systems over the continent. The precipitation over West and Central Africa is essentially characterized by the approach of humid air masses , which leads to a horizontal movement of the air over the marine boundary layer to the various temperature anomalies over West and Central Africa. Due to the development of the high air pressure at sea level, which favors the development of the ground wind , to the low pressure anomalies in the secondary intra-tropical convergence . The research also showed that the seasonal changes in solar radiation result in a net increase in the amount of energy in the column of atmosphere that controls these seasonal changes. This excess energy leads to a horizontal energy export, through which the thermal circulation of the humid air masses leads to a collection and convection of the thermally charged air humidity in the area of ​​the intertropical convergence zone (ITCZ). The resulting difference in atmospheric pressure and temperature between the high and low pressure systems is a decisive factor for the intensity and extent of the monsoons. These processes ultimately modulate the productivity of the monsoon rainfall inland.

The West African monsoon is more variable than its Asian counterparts, both in its origin and in its intensity. In the Indian subcontinent , for example, no monsoons could be observed for several years , while the West African monsoons in the second half of the 20th century were extremely weak for several years and led to the great famine in the Sahel during the 1970s and 1980s. This development towards ever drier climatic conditions over West and Central Africa led to the establishment of the AMMA program ( African Monsoon Multidisciplinary Analysis "Multidisciplinary analysis of the West African monsoons") in order to better understand the changes and the theoretical basis for understanding and the possibilities for predicting the To improve the intensity and extent of the monsoon system. This work is of particular importance for the farmers of African agriculture who are largely dependent on rain-fed agriculture. The right time to sow is an important factor that determines a high-yield harvest. The temporal forecast of the monsoon rain is therefore of particular importance for the food security of the African population. In 2017, a study also made the (negative) influence of the Greenland ice melt on the West African monsoons and, depending on this, on the Sahel zone.

Web links

Individual evidence

  1. ↑ Serge Janicot, Jean Philippe Lafore, Chris Thorncroft: The West African Monsoon System ( Memento of the original from March 25, 2012 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. (English) PDF document 11.6 MB @1@ 2Template: Webachiv / IABot / www.amma-eu.org
  2. ↑ Linkage of the Boreal Spring Antarctic Oscillation to the West African Summer Monsoon ( Memento of the original from October 13, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.3 MB) Authors: Jianqi Sun and Huijun Wang Publication by the Nansen-Zhu International Research Center (NZC), Institute of Atmospheric Physics, Chinese Academy of Sciences, Beijing and China Meteorological Administration Training Center, Beijing, China (English ) PDF document 1.24 MB  @1@ 2Template: Webachiv / IABot / nzc.iap.ac.cn
  3. ↑ Scinexx researches heavy rain in Africa (German)
  4. ↑ AH Fink: The West African Monsoon System ( Memento of the original from September 23, 2015 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. from PROMET Volume 32, Issue 3/4, 2006 Topic: Climate and weather in the tropics, pp. 114–122, publication by the German Weather Service (PDF document) (German) @1@ 2Template: Webachiv / IABot / www.dwd.de
  5. ↑ Cold water at the equator influences monsoons over West Africa - International research project examines causes and mechanisms - , IMF-GEOMAR, November 26, 2007 (German)
  6. ↑ Overview of the West African monsoons on Amma-International.org (English)
  7. ^ Benjamin Sultan, Serge Janicot, Arona Diedhiou : The West African Monsoon Dynamics. Part I: Documentation of Intraseasonal Variability . In: Journal of Climate . tape 16 , no. 21 , January 1, 2003, p. 3389–3406 ( PDF file; 2.5 MB - English).
  8. ↑ History of AMMA (English)
  9. ↑ Michael Vellinga, Alberto Arribas, Richard Graham: Seasonal forecasts for regional onset of the West African monsoon . In: Climate Dynamics . tape 40 , no. 11/12 , June 2003, pp. 3047–3070 , doi : 10.1007 / s00382-012-1520-z ( PDF file; 4.6 MB; available at SpringerLink - English).
  10. ↑ Jean-Paul Vanderlinden, Jorge Alvarez-Solas, François Gemenne, Christophe Dumas, Didier Swingedouw: Consequences of rapid ice sheet melting on the Sahelian population vulnerability . In: Proceedings of the National Academy of Sciences . tape 114 , no. 25 , June 20, 2017, ISSN  1091-6490 , p. 6533-6538 , doi : 10.1073 / pnas.1619358114 , PMID 28584113 ( pnas.org [accessed December 16, 2018]).