Monsoons

Monsoons are seasonal wind systems that reverse directions biannually and are crucial for the economic stability and agricultural productivity of affected geographic areas.

Origins of Monsoons

The term “monsoon” originated from the Arabic word mausim, meaning season, used by sailors to comment about changing winds above the Arabian Sea. Solar heat produces winds that shift north and south according to the Sun’s position each season. Scientists cite geological evidence that suggests conditions favorable for monsoons began millions of years ago when the Indian subcontinent collided with the Asian plate, eventually creating the Himalayan Mountain range and the Tibetan Plateau. Warm and cool ocean water and landmasses affect atmospheric circulation, creating recurring wind systems that sweep over large regions. Most monsoons occur in tropical areas adjacent to the Indian Ocean, although seasonal winds also affect Africa, northeast Asia, Australia, and North and South America. Monsoons are described by geographical terminology; the major monsoon systems are the South Asian or Indian monsoon, the East Asian monsoon, the Malaysian-Australian monsoon, the North American monsoon, and the West African monsoon. Monsoons are also characterized as either boreal or austral, according to their location in the Northern or Southern Hemisphere, respectively.

For thousands of years, monsoons have been incorporated into numerous cultures' literature, folklore, and religious rituals. The monsoon motif is a universal symbol of rebirth, fertility, devastation, and death. Monsoons are predictable to the extent that it is known that they probably will occur during specific seasons. However, the winds are often erratic, being delayed or appearing prematurely, precipitating extreme or minute amounts of rainfall, and sometimes bypassing regions entirely. Usually, between April and October, monsoon winds develop in the southwest, shifting direction to originate from the northeast between October and April. Monsoons are a prolonged series of winds and are not restricted to a single storm.

Monsoons manipulate the planet’s climate, often proving beneficial and occasionally detrimental. The winter monsoon blowing from land to sea is usually associated with dryness, while the summer monsoons storming from the sea onto land produce torrential, sustained rainfall vital for agricultural activity. Sufficient precipitation assures the growth of ample crops to nourish domestic populations, export for economic profits, and provide employment for farm laborers. The absence of monsoon rains can result in famines and impoverishment.

Monsoon Dynamics

Scientists have analyzed monsoons for several centuries to determine the physical forces that generate the winds and regulate their behavior. Researchers have agreed on basic explanations regarding fundamental aspects of monsoons, such as their relationship to atmospheric and oceanic conditions and how they tend to vary instead of conforming to exacting standards. Scholars continue to seek answers to complex questions about monsoons, especially concerning fluctuations displayed in periods ranging from seasons to decades. Such information might enable meteorologists to predict occurrences and outcomes that could impact local and global populations and economies. Researchers want to understand more about the onset of monsoons and their active and break periods, which are thought to be caused by shifting troughs. The actual beginning of a monsoon is often disputed, with some scientists saying that increased humidity indicates the monsoon’s start, while others say precipitation or the formation of a vortex signals the beginning.

Monsoons are caused by sea and land breezes that create temperature and air-pressure differences between landmasses and bodies of water. Land absorbs heat to a different extent than water does, and the difference between land and water temperature is one factor in the instigation of monsoons. Areas in low latitudes near the equator undergo circulatory and precipitation changes because of temperature deviations on adjacent continents and seas. The amount of solar radiation emitted each season affects the temperature and air pressure over continents. During the Northern Hemisphere’s summer, when the planet's northern half tilts toward the Sun, high-pressure systems move from the cooler ocean into the land’s low-pressure area. As the landmass cools during winter, its high-pressure air mass moves from the land to the low-pressure air mass over the warmer ocean.

The jet stream moves south during winter and north in the summer, transporting air masses to and from monsoon regions. In winter, the Siberian high-pressure system causes air to circulate clockwise. Winds move from the northeast, down the Himalayas, and across land cooled during shortened days toward the sea, creating dry conditions and causing monsoon rains in Indonesia and Australia. In summer, the winds reverse direction because the land is heated by increased solar energy. Moving counterclockwise, the winds carry moisture from the sea in the southwest toward land. For example, the Somali jet stream near Africa moves across the equator to the Arabian Sea. It alters the direction of ocean currents, which causes cold water to rise from the depths and surface temperatures to drop. Humid winds shift into India, rising when they reach the Himalayas. As the winds are lifted over the Tibetan Plateau, the air cools sufficiently to become saturated, triggering thunderstorms and convectional rainfall. The variation and strength of the Indian monsoon also seems to be related to the Southern Oscillation, which is characterized by a reversal of air pressure at opposite ends of the South Pacific Ocean at irregular intervals of three to seven years. The mechanics of this relationship are not yet well understood.

Scientists have designated three monsoon circulation patterns. The lateral component indicates a monsoon that circulates across the equator. Transverse circulation moves from North Africa and the Middle East into southern Asia. The Walker circulation, also a transverse pattern, moves across the Pacific Ocean. The size, shape, coastal positions, and elevation of landmasses influence these patterns, and the shifting of the low-pressure Intertropical Convergence Zone (ITCZ) between north and south can determine how monsoon systems move. These factors contribute to variations in monsoon intensity and duration. In areas such as Australia, monsoon winds do not rise over or descend from mountains, and high-pressure masses undergo geostrophic adjustment as they ascend over monsoon troughs. Orography influences the nature of monsoons globally. African and Australian monsoons tend to be weaker because they are not elevated as high as those lifted by Asian winds. Because the Rocky Mountains and the Sierra Madre lift air masses somewhat like the Himalayas, though not as high, some researchers claim that a monsoon circulation pattern occurs in North America, causing increased precipitation in Mexico and the southwestern United States every summer.

Impact on Civilization

The effects of monsoons permeate the civilizations of regions where they occur. Approximately 60 percent of people worldwide are economically dependent on the climate affected by the Malaysian-Australian monsoon system. Humans, animals, and plants rely on monsoons to provide essential moisture. The absence of monsoon rains can cause droughts and famine, killing millions of people by starvation. Any fluctuation in the monsoon cycle, whether within one season, year, or decade, can be detrimental. Billions of people rely on monsoons to irrigate rice and wheat crops. In India, grain production is essential to feed the growing population that expands annually at a rate greater than agricultural yields increase. Between 15 and 18 percent of the Indian economy is based on agriculture, and 43 percent of laborers are engaged in agricultural employment. Monsoon rain is critical to maintaining this balance.

Ironically, monsoons flood areas, drown people, cause landslides that destroy communities, and inundate crops. Millions of acres often remain underwater for months. Several thousand people die annually during monsoons, and thousands more become homeless. Many people are reported missing after a monsoon deluge. Floodwaters wash away unstable dams, buildings, and graves in cemeteries. Extreme humidity is stifling for most people. Humanitarian and relief agencies, such as the International Red Cross and Red Crescent Society, provide emergency food rations and shelter. Shipping along trade routes in monsoon regions is often both helped and hindered by winds.

Folklore, proverbs, and prayers provide insights into personal experiences with monsoons. Indigenous peoples and visitors to monsoon regions have documented their encounters with monsoons. Some tourists purposefully travel to Asia during monsoons because hotels and businesses are not crowded, prices are lower, and reservations are easier to secure. However, some visitors note the inconvenience of always carrying an umbrella, traveling on monsoon-damaged roads, and encountering storm-related delays. Astronomers can observe celestial phenomena because of clear skies during the winter monsoons. Oceanographers also consider the monsoons useful because nutrient-rich waters rise to the surface, allowing scientists to study how plants, animals, the sea, and the atmosphere exchange carbon dioxide.

Modeling Monsoons

Because monsoons are essential for affected populations to thrive despite the wind’s uncertain behavior, scientists have initiated cooperative research programs regarding monsoons. They hope to collect sufficient data to develop computer models to predict when monsoons will occur and how they will impact landmasses. Such forecasting efforts have often frustrated researchers because of the monsoons’ capricious nature. Intraseasonal oscillations between monsoons' active and passive precipitation phases primarily hinder speculation. The Center for Ocean-Land-Atmosphere Studies (COLA) examines the relationship between the ocean, the atmosphere, and heat sources. While empirically based forecasts often proved more reliable than modeled simulations during the twentieth century, scientists seek to perfect their experimental methods in the twenty-first century.

Researchers recognize that regional topographic differences and varying hydrodynamic situations impede monsoon modeling attempts. Incorporating information about variables, such as the temperature of the sea surface and snow cover, scientists try to understand how these factors alter monsoon behavior. They also want to comprehend how the monsoons affect ocean and atmosphere interactions and how the water-air pressure system relationship influences monsoons. Studies have been designed to expand knowledge about the role of convection in monsoons. Researchers are also interested in studying the impact of monsoons on climates inside and outside the monsoons’ immediate zone. Scientists disagree on whether monsoons affect or are affected by El Niño; studies have been done to evaluate global precipitation data and drought conditions to determine any correlations.

Monsoon variability hinders forecasting and potential benefits to agriculture based on information concerning the onset of rainfall. For example, with advanced knowledge, farmers could plant crops that require less water in case a weaker monsoon is forecast. Monsoons, however, do not always begin when expected. Variations can happen within the yearly cycle or fluctuate over several years. Using different global circulation models, researchers seek to comprehend the fundamental physical processes of monsoons and then apply this knowledge to create specific models representing a seasonal cycle based on their observations, statistics, and hypotheses. Interpreting results produced by these models compared with satellite data, researchers become aware of how monsoons vary according to the landmasses they traverse and unique oceanic and atmospheric conditions.

Realizing these factors are linked, researchers create models to consider numerous variables concurrently. Yet, they realize that more sophisticated modeling of factors, such as sea-surface temperature, solar radiation, water vapor, cloud cover, soil moisture, and terrain, are necessary to improve prediction methods. Such models must accurately simulate a monsoon’s average seasonal rainfall and behavior, considering how it varies within one season and one year. The models must also consider anomalies with other documented monsoons in that area to isolate how external conditions, such as altered topography, may affect the monsoon’s internal dynamics.

Future Research

Monsoons fascinate the researchers who strive to acquire a more complex understanding of the seasonal wind systems. Scientists hope to predict monsoons more precisely because the winds impact global economies and populations. Meetings of international monsoon experts occur to share knowledge of monsoon cycles, coordinate research methods, and set future goals. Primarily, scientists want to explain why monsoons vary in behavior and how such variables as sea-surface temperature and the location of land-based heat sources interact with and influence the monsoons’ fluctuations. Researchers also want to explore the relationship between monsoons and the El Niño/Southern Oscillation (ENSO) phenomenon.

Scientists realize that to achieve accurate predictive techniques, a complete understanding of sea-surface temperature anomalies that affect monsoon circulation must be obtained through an analysis of oceanic processes that currently remain vague. Future models will assess these temperature patterns in different geographic regions. Models will also further evaluate water surface fluxes, land surface coverings of snow and vegetation, and global climate change, all affecting temperatures and altering monsoon cycles. Until such comprehension is attained through enhanced model simulations designed to analyze numerous dynamic variables simultaneously, predictions will be minimized, slowing endeavors to manage the winds, and monsoons will continue to affect life both advantageously and harmfully in tropical zones.

In the twenty-first century, scientists have made great strides in modeling and predicting monsoons, especially through a comprehensive approach considering several factors. Integrated Earth system models can evaluate the interactions between the atmosphere, oceans, land, and cryosphere, allowing for a more complete understanding of monsoon formation. Technology has played a significant role in scientists' ability to more accurately model monsoons. These innovations include satellite data, radar observations, other atmospheric measurements, and the ability to produce higher-resolution models. Artificial intelligence is also being integrated into monsoon modeling. Finally, these and other innovations in monsoon modeling have allowed scientists to pinpoint the arrival of monsoons far more accurately, allowing them to narrow their arrival window to weeks rather than months. 

Principal Terms

austral: referring to an object or occurrence that is of the Southern Hemisphere

boreal: referring to an object or occurrence that is of the Northern Hemisphere

convection: heat transfer by the circulating movement that occurs in fluid materials as warmer, less dense material rises above cooler, denser material

geostrophic: descriptive of wind that occurs when the Coriolis force is in exact balance with the force of a horizontal pressure gradient and, therefore, blows in a straight line

Intertropical Convergence Zones (ITCZ): low-pressure areas where southern and northern trade winds meet

orography: the study of mountains that incorporates assessment of how they influence and are affected by weather and other variables

oscillation: variation of some physical property or condition between two opposing states, much like the rising and falling of a wave between its maximum and minimum heights

troposphere: the level of the atmosphere closest to the ground, extending from the surface to an altitude of eleven kilometers

trough: a long and relatively narrow area of low barometric pressure

vortex: the central locus of a whirling liquid or gas, about which the fluid mass circulates

Bibliography

Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. Belmont: Brooks, 2012.

Bamzai, A. S., and J. Shukla. “Relation between Eurasian Snow Cover, Snow Depth, and the Indian Summer Monsoon: An Observational Study.” Journal of Climate, vol. 12.10, 1999, pp. 3117–32.

Bernardi, Dan. “Using Monsoons of the Past to Predict Climate Conditions of the Future.” Syracuse University News, 14 Nov. 2022, news.syr.edu/blog/2022/11/14/using-monsoons-of-the-past-to-predict-climate-conditions-of-the-future. Accessed 15 Apr. 2023.

Chang, Chih-Pei, editor. East Asian Monsoon. Hackensack: World Scientific, 2004.

Chang, Chih-Pei, et al., editors. The Global Monsoon System: Research and Forecast. 2nd ed., Hackensack: World Scientific, 2011.

Cheung, Chan Chik. Synoptic Patterns Associated with Wet and Dry Northerly Cold Surges of the Northeast Monsoon. Hong Kong: Royal Observatory, 1997.

Clift, P. D., R. Tada, and H. Zheng, editors. Monsoon Evolution and Tectonic-Climate Linkage in Asia. Bath: Geological Soc., 2010.

Douglas, Michael W., et al. “The Mexican Monsoon.” Journal of Climate, vol. 6.8, 1993, pp. 1665–77.

Fein, Jay S., and Pamela L. Stephens, editors. Monsoons. New York: Wiley, 1987.

Gettelman, Andrew, et al. "The Future of Earth System Prediction: Advances in Model-data Fusion." Science Advances, 2022, doi.org/abn3488. Accessed 20 July 2024.

Hodges, Kip. “Climate and the Evolution of Mountains.” Scientific American, Aug. 2006, pp. 72–79.

Lighthill, James, and Robert Pearce, editors. Monsoon Dynamics. New York: Cambridge UP, 2009.

McCurry, Steve. Monsoon. New York: Thames, 1988.

Oshima, Harry T. Strategic Processes in Monsoon Asia’s Economic Development. Baltimore: Johns Hopkins UP, 1993.

Shu, Hailong, et al. "Forecasting the Future with Future Technologies: Advancements in Large Meteorological Models." ArXiv, 2024, arxiv.org/abs/2404.06668. Accessed 20 July 2024.

Vesilind, Priit J. “Monsoons.” National Geographic, Dec. 1984, pp. 712–47.

Wang, Bin. The Asian Monsoon. Chichester: Praxis, 2006.

Wang, Bin, Chunhan Jin, and Jian Liu. "Understanding Future Change of Global Monsoons Projected by CMIP6 Models". Journal of Climate, vol. 33.15, 2020, pp. 6471-6489, doi.org/10.1175/JCLI-D-19-0993.1. Accessed 20 July 2024.

Webster, P. J., et al. “Monsoons: Processes, Predictability, and the Prospects for Prediction.” Journal of Geophysical Research: Oceans, vol. 103.C7, 1998, pp. 14451–510.