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What Is a Monsoon?

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Women planting rice shoots under Monsoon Rain
Eitan Simanor/Photographer's Choice/Getty Images
Question: What Is a Monsoon?
Monsoon season is a welcome relief to drought conditions in many areas of the world. Monsoons can also bring about widespread famine and enough rain to kill hundreds of people in floods. While the Asia and India monsoons are famous, there are even monsoon season in the United States. So, what is a monsoon?
Answer: Monsoons, or rainy seasons, are a shift in wind direction which causes excessive rainfall in many parts of the world including Asia, North America, South America, and Africa. The primary mechanism behind a monsoon is a shift in global wind patterns.

During most of the year, winds blow from land to ocean making the air dry. Winds originating from land are called continental. During certain months of the year, the winds begin to blow from the ocean to the land making the air moist. Winds originating over a body of water are called maritime. This moist ocean air is what causes monsoonal rains over many countries.

Why Do Wind Patterns Shift in a Monsoon?

Differential heating occurs when the sun heats the land and oceans. Incoming solar radiation heats landmasses faster than large bodies of water. In tropical and sub-tropical climates, solar heating is most intense in the summer months. As the land heats throughout the summer, a large low pressure system builds over the land. The heat from the sun also warms the surrounding ocean waters, but the effect happens much more slowly due to the high heat capacity of water. Therefore, the ocean temperatures as well as the layer of air above the oceans stays cooler longer. The cooler air above the oceans is moist and more dense creating a high pressure zone relative to the pressure above the landmass.

Winds flow from high pressure areas to low pressure areas due to the pressure gradient. Once the temperature conditions on the land and oceans change, the resultant pressure changes cause the winds to change from a land-to-ocean direction to an ocean-to-land direction. Monsoon season does not end as abruptly as it begins. While it takes time for the land to heat up, it also takes time for that land to cool in the fall. This makes monsoon season a time of rainfall that diminishes rather than ends.

History of Monsoon Studies

The word monsoon is derived from the Arabic word mausim meaning season. The most famous monsoon is the Indian monsoon. The intense rainfall in these regions can cause massive flooding and destruction of crops. In dry climates, monsoons are an important replenishment for life as water is brought back into drought-stricken zones of the world. Part of the reason India gets such an intense monsoon season is due to its elevation. The higher the land mass, the higher the likelihood of the development of a low pressure zone. The Tibetan Plateau to the north of India is one of the largest and highest plateaus on Earth.

The earliest explanation for monsoon development came in 1686 from the English astronomer and mathematician Edmond Halley. Halley is the man who first conceived the idea that differential heating of land and ocean caused these giant sea-breeze circulations. As with all scientific theories, these ideas have been expanded upon.

Monsoon seasons can actually fail bringing intense drought and famines to many parts of the world. From 1876-1879, India experienced such a monsoon failure. To study these droughts, the Indian Meteorological Service (IMS) was created. Later, Gilbert Walker, a British mathematician, began to study the effects of monsoons in India looking for patterns in climate data. He became convinced that there was a seasonal and directional reason for monsoon changes.

It is a natural supposition that there should be in weather free oscillations with fixed natural periods, and that these oscillations should persist except when some external disturbance produces discontinuous changes in phase or amplitude.—Sir Gilbert T. Walker (Walker, 1925, pages 340–341)
According to the Climate Prediction Center, Sir Walker used the term ‘Southern Oscillation’ to describe the east-west seesaw effect of pressure changes in climate data. In the review of the climate records, Walker noticed that when pressure rises in the east, it usually falls in the west, and vice versa. Walker also found that Asian monsoon seasons were often linked to drought in Australia, Indonesia, India, and parts of Africa.

Jacob Bjerknes, a Norwegian meteorologist, would later recognize that the circulation of winds, rain, and weather were part of a Pacific-wide air circulation pattern he called Walker circulation.

New Theories on the Causes of Monsoons

Theories of the development of monsoons have stood firm for over 300 years. Classical thinking on monsoons is that their development is sparked by the differential heating of land and ocean as described above. But in a recent NASA Earth Observatory release, those ideas may be changing. Geoscientists at the California Institute of Technology have been working on new ideas as to exactly why monsoons develop.

Two researchers, Schneider and Simona Bordoni of the National Center for Atmospheric Research in Colorado, used computer models to re-create an Earth with no landmasses. Surprisingly, they found that differential heating was not a necessary component to creating monsoons. Instead, they concluded monsoons arise because of an interaction between tropical air circulation and large-scale turbulence in the middle latitudes. The large middle latitude disturbances modify circulation in tropical regions causing rapid circulation changes which can bring on the characteristic high surface winds and heavy rainfall of the monsoon.

Other Seasonal Weather Patterns

Hurricane season is another example of a seasonal weather pattern caused by the differential heating of land and ocean. During hurricane season, the ocean waters must reach a minimum average temperature of about 80 degrees Fahrenheit (~27 degrees Celsius) for hurricanes to occur. While you may think the highest temperatures occur in summer, ocean waters again take longer to heat. Therefore, hurricane intensity and frequency is typically greatest in the late summer and fall. Therefore, ocean temperatures must be ready for hurricanes to develop.

Monsoon Season Data

Global Monsoons The Climate Prediction Center offers real-time data and maps of all the global monsoons.

Sources:

NWS – What Is a Monsoon?
North American Monsoons: A Report to the Nation
Sir Gilbert Walker and a Connection between El Niño and Statistics
NASA Earth Observatory - Scientists Offer New Explanation For Monsoon Development
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