| Focus 1/1 | Monsoons

The discovery of the monsoons


The term “monsoon” appeared in French at the beginning of the 17th century, from the Dutch “monssoen ” or the Portuguese “monçao”, terms that these navigating nations had borrowed from the Arabic “mausim” designating the season of winds favourable to navigation towards India. But the history of the monsoon is much older. More than 4,000 years ago, Mesopotamian tablets already mentioned maritime exchanges with the Harappan civilization of the Indus Valley. Closer to our time, Aristotle evokes the alternation of monsoon winds in “The Meteorologicals” [1] (Book II, Chapter 5), written around 334 BC: “… in the southern sea beyond Libya (i.e. the mouth of the Red Sea, the Gulf of Aden and the western Arabian Sea), the winds from the east and west always blow unceasingly in alternation”. His contemporary Nearchus, who was commissioned by Alexander the Great in 325 BC to pilot a fleet of 120 ships from the mouth of the Indus to the Persian Gulf, had to wait several months for the headwinds of the summer monsoon to subside. Nearchus is often considered as the initiator of the Indian Ocean maritime route which was followed after him, in both directions, by Greek, Phoenician, Byzantine, Indian, Chinese and especially Arab navigators.

During the Abassid dynasty (8th-12th centuries), dhows of three to four hundred tons provided regular trade links between Arabia and India. They took advantage of the alternating monsoon winds on the Indian Ocean, going out in spring and summer with a south-westerly tailwind and returning in autumn and winter with a north-easterly wind. The knowledge of navigation in the Indian Ocean is described at the end of the 15th century in several works by the poet, navigator and cartographer Ahmad Ibn Mâdjid [2]. Legend has it that he was the Arab pilot entrusted by the Sultan of Malindi (on the coast of present-day Kenya) to the Portuguese navigator Vasco da Gama, who was inexperienced in this unfamiliar ocean, enabling him to arrive in Calicut, in southern India, on 20 May 1498, after a 27-day crossing. The intensification of maritime trade between Europe and Asia from the 16th century onwards led to a better knowledge of the meteorological and oceanic conditions of the Indian Ocean crossings. The information recorded in the logbooks then became jealously guarded trade secrets.

Dhow. [Source: Pearson Scott Foresman, Public domain, via Wikimedia Commons]
The first developments of modern science in Western Europe during the 17th and 18th centuries encouraged the objective description of natural phenomena in order to provide a theoretical interpretation based on established physical and mathematical laws. In 1666, the Royal Society in London made recommendations for the collection of data by merchant ships. Based on this information, Edmund Halley [3] (who identified the comet bearing his name) published in 1686 a map of the Indian Ocean showing the distribution of strong southwest winds during the summer monsoon, and weaker northeast winds during the winter monsoon. He notes that in summer the winds are from the south near the African coast, indicating a flow across the equator from the southern hemisphere. In 1735, George Hadley [4], a lawyer and member of the Royal Society, showed that the trade winds blowing from the east in the tropics were not, as was then thought, the result of the atmosphere chasing the local noon (the sub-solar point) as it moved westward during the day, but rather of the conservation of planetary angular momentum (due to the Earth’s rotation) as the air moved from the higher latitudes toward the equator. In contrast, the movement of air from lower latitudes to the northern Indian Ocean during the summer monsoon generates a westerly component that increases with latitude.

During the 19th century, the development of steam shipping made maritime trade less dependent on wind patterns. In 1817, the German geographer and climatologist Alexander von Hümboldt [5] identified the changing thermal contrast between India and the adjacent ocean (Arabian Sea and Bay of Bengal) as a determining factor in the alternation between southwestern summer monsoon and northeastern winter monsoon. In 1841, the American meteorologist James P. Epsy [6] completed the scheme by showing the importance of the latent heat released during the condensation of atmospheric moisture into liquid water and ice within the rainstorm systems associated with the monsoon. In 1875, the Indian Meteorological Department (IMD) was established by the British government to monitor and, if possible, forecast the arrival of monsoon rains in southern India and their progress northwards. Published in 1877 by Henry F. Blanford [7], the first director of the IMD, the “Indian Meteorologist’s Vade Mecum”, based on numerous observations and on the physical knowledge of the time, remained for decades the reference work on the Indian monsoon.

During the 20th century, experimental studies based on data from permanent observation networks or intensive measurement campaigns of limited duration, on theoretical analyses based on the physical laws governing the atmosphere, the oceans and the tropical continents, and on increasingly accurate numerical simulations thanks to the growing capacity of computing resources, have led to a better understanding of the monsoon. The information deduced from images taken by meteorological satellites from 1960 onwards and the observations collected during the International Indian Ocean Experiment [8] (1962-1965) gave rise to the reference book ” Monsoon Meteorology ” by C.S. Ramage [9], published in 1971. More recently, the international campaigns MONEX [10] (MONsoon Experiment) in 1978-1979 and JASMINE [11] (Joint Air-Sea Monsoon Interaction Experiment) in 1999 should be mentioned. Outside the Indian Ocean, cooperative experiments targeting other monsoon regimes – e.g. Australia: AMEX [12] (Australian Monsoon Experiment) in 1986-87, Asia: GAME [13] (GEWEX Asian Monsoon Experiment) in 1998, North America: NAME [14] (North American Monsoon Experiment) in 2004, West Africa: AMMA [15] (African Monsoon Multidisciplinary Approach) in 2006, … – have led to significant progress in our understanding and forecasting capabilities.


Cover image. [Source: Image from https://www.freepik.com/vectors/background (copyright free)]

[1] Aristotle, -334: “The Meteorologicals”, translated by J. Tricot, Librairie Philosophique Vrin, Paris, 1976, 299 pp.

[2] Ahmad Ibn Mâdjid, ca. 1490: “Kitab al-fawa’id fi ‘usul ‘ilm al-bahr wa-l-qawa’id” (“Book of useful information on the principles and rules of navigation“), copied by Nedjm eddin Bey in 1926 from an older copy in the Arab Academy of Damascus, Library of Congress, Washington, 174 p., available at https://www.loc.gov/item/2008401696/

[3] Halley, E., 1686: “An historical account of the trade winds, and monsoons, observable in the seas between and near the Tropicks, with an attempt to assign the physical cause of the said winds”, Phil. Trans. Roy. Soc. 16, no. 183, p. 153-168, doi : 10.1098/rstl.1686.0026

[4] Hadley, G., 1735: “Concerning the cause of the general trade-winds”, Phil. Trans. Roy. Soc. 39, no. 437, pp. 58-62, doi: 10.1098/rstl.1735.0014

[5] von Hümboldt, A., 1837 : “Examen critique de l’histoire de la géographie du nouveau continent: et des progrès de l’astronomie nautique aux 15 me et 16 me siècles, Volume 2”, Librairie de Gide, Paris, 373 p..

[6] Epsy, J.P., 1841: “Philosophy of storms”, Charles C. Little and James Brown, Boston, 552 pp.

[7] Blanford, H.F., 1877: “Indian Meteorologist’s Vade Mecum”, Thacker, Spink and co, Calcutta, 281 pp.

[8] Snider, R.G. , 1961: “The International Indian Ocean Expedition”, Discovery (March 1961), pp . 114-117, available at https://scor-int.org/Historical%20Documents/Snider-Discovery.pdf

[9] Ramage, C.S., 1971: “Monsoon meteorology “, Int. Geophys. Series, vol. 15, Academic Press, New York, 296 pp.

[10] Murakami, T., 1979: “Scientific objectives of the Monsoon Experiment (MONEX)”, GeoJournal, vol. 3, p. 117-139, doi 10.1007/BF00257701

[11] Webster, P.J., et al. 2002: “The JASMINE pilot study”, Bull. Amer. Meteor. Soc. 83, pp. 1603-1630, doi: 10.1175/BAMS-83-11-1603

[12] Holland, G.J., et al, 1986: “The BMRC Australian Monsoon Experiment: AMEX”, Bull. Amer. Meteor. Soc, vol. 67, pp. 1466-1472, doi : 10.1175/1520-477(1986)067%3C1466: TBAMEA%3E2.0.CO;2

[13] Yasunari, T., 1994: “Gewex-related asian monsoon experiment (GAME)”, Adv. Space Res., vol. 14, pp. 161-165, doi: 10.1016/0273-1177(94)90365-4

[14] Higgins, W., and D. Gochis, 2007: “Synthesis of Results from the North American Monsoon Experiment (NAME) Process Study”, J. Clim. J. Clim. 20, p. 1601-1607, doi: 10.1175/JCLI4081.1

[15] Redelsperger, J.-L., et al, 2006: “African Monsoon Multidisciplinary Analysis: An International Research Project and Field Campaign”, Bull. Amer. Meteor. Soc. 87, p. 1739-1746, doi : 10.1175/BAMS-87-12-1739