Monsoon

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Monsoon

A monsoon is a seasonal change in the direction of the prevailing wind . This wind shift typically brings about a marked change in local weather . Monsoons are often associated with rainy seasons in the tropics (the areas of Earth within 23.5° latitude of the equator) and the subtropics (areas between 23.5° and about 35° latitude, both north and south). In these areas, life is critically dependent on the monsoon rains. A weak monsoon rainy season may cause drought , crop failures, and hardship for people and wildlife . The central role that monsoons play in determining climates around the world has made their study a high priority for meteorologists.

Many parts of the world experience monsoons to some extent. Probably the most famous are the Asian monsoons, which include the distinctly different monsoons that affect India, north China, and Japan, and south China and southeast Asia . Monsoons also affect portions of central Africa , where their rain is critical to supporting life in the area south of the Sahara Desert. Lesser monsoon circulations affect parts of the southwestern United States. These summer rainy periods bring much needed rain to the dry plateaus of Arizona and New Mexico.

General monsoon circulation

Monsoons, like most other winds, occur in response to the sun heating the atmosphere. In their simplest form, monsoons are caused by differences in solar heating between the oceans and continents, and they are most likely to form where a large continental land mass meets a major ocean basin . During the early summer, the increasing solar energy heats up the land surfaces fairly quickly. Water , on the other hand, heats much more slowly in response to the sun. This is one reason why we cool off by swimming in lakes during the summer—the water is still chilled from the recent winter and takes much of the summer to warm up. The enormous quantity of water in the oceans guarantees they will remain cooler than the nearby continents during the early summer. The relatively warm land surface will heat the air over it, causing it to rise, or convect. The convection of warm air produces an area of low pressure near the land surface. Meanwhile, the air over the cooler ocean will be more dense and tend to stay at the surface or sink downwards from aloft. Thus during the summer, oceanic air flows onshore toward the low pressure over land. This onshore flow is continually supplied by cooler oceanic air sinking from higher levels in the atmosphere. In the upper atmosphere, the rising continental air is drawn outward over the oceans to replace the sinking oceanic air, thus completing the cycle. In this way a large vertical circulation cell is set up, driven by solar heating. At the surface, the result is a constant wind flowing from sea to land.

The oceanic air moving onto the land is usually quite humid, due to its prolonged contact with the sea surface. As it flows on shore the moist marine air is pulled upward as part of the convecting half of the circulation cell. The rising air cools and eventually forms rain clouds . Rain clouds are especially likely when the continental areas have higher elevations (mountains , plateaus, etc.), because the humid sea air is forced upward over these barriers, causing widespread cloud formation and heavy rains. This is the reason why the summer monsoon forms the rainy season in many tropical areas.

In the late fall and early winter, the situation is reversed. Land surfaces cool off quickly in response to cooler weather, but the same property of water that makes it slow to absorb heat, called heat capacity , also causes it to cool slowly. As a result, continents are usually cooler than the oceans surrounding them during the winter. This sets up a new circulation in the reverse direction: air over the sea, now warmer than that over the land, rises and is replaced by winds flowing off the continent . The continental winds are supplied by cooler air sinking from aloft. At upper atmospheric levels the rising oceanic air moves over the land to replace the sinking continental air. Sinking air prevents the development of clouds and rain, so during the winter monsoon continental areas are typically very dry. This winter circulation causes a prevailing land to sea wind until it collapses with the coming of spring.

The Asian monsoons

While the thermal circulation described previously is a central part of monsoons, it is not sufficient to explain the world's most pronounced monsoons, those of Asia. Since they span such a huge area and affect over a billion people, the Asian monsoons have been studied for over a century to determine their causes and reasons for their variation. Although our understanding is not complete, it is clear that the monsoons of Asia are a complicated set of circulations, which combine the sun-driven winds with large scale circulations that span the entire planet . The extremely high mountains of the Himalayas also play a role in determining monsoon behavior.

An important factor in the development of the Asian monsoons is the existence of jet streams. These are great rivers of air, that ring the Earth at levels in the atmosphere ranging from 7-12 mi (12–20 km) above the surface. Jet streams are part of the global wind circulation, brought about by the large differences in temperature between the equator and the poles. Jet stream winds blow at several different latitudes, and play a major role in determining the weather beneath them. The monsoons of southern Asia are affected by two jet streams called the subtropical and the tropical jets. The subtropical jet is a permanent feature, flowing westerly (from west to east) at an altitude of about 7 mi (12 km). It migrates over the year in response to the seasons, moving northward to higher latitudes in the summer and southward in the winter. It occasionally splits in two. The tropical jet is a weaker easterly (east to west) flow that forms near the equator at a height of nearly 12 mi (20 km). It is found only in the summer months.

The monsoon of India

The interplay of jet streams and monsoon winds is well illustrated in the Indian monsoon. During the early summer months, increased solar heating begins to heat the Indian subcontinent, which would tend to set up a monsoon circulation cell between southern Asia and the Indian Ocean. However, the subtropical jet stream occupies its winter position at about 30° north latitude, south of the Himalayan Mountains. As long as the subtropical jet blows over India, it inhibits the development of summer monsoon. As summer progresses, the subtropical jet slides northward. The extremely high Himalayas present an obstacle for the jet; it must "jump over" the mountains and reform over central Asia. When it finally does so, a summer monsoon cell develops, supported by the tropical jet stream overhead. The transition can be very fast—the Indian monsoon has a reputation for appearing suddenly as soon as the subtropical jet is out of the way. The retarding effect of the subtropical jet delays the Indian monsoon by up to one month compared with the rest of Asia.

During the Indian summer monsoon, winds blow from the southwest, bringing moist air on shore. As the air is forced to rise over the foothills of the Himalayas, it causes constant and frequently heavy rains. The town of Cherrapunji, India, located on the Himalayan slopes, receives an annual rainfall of over 36 ft (11 m), making it one of the wettest places on Earth.

The summer monsoon ends as the highlands of Tibet begin to cool in the fall. The cooling atmosphere over Tibet allows the subtropical jet to reappear south of the Himalayas, bringing the rainy summer monsoon to an end. The circulation shifts to a winter monsoon cell, with sinking air over India and surface winds that blow out to sea. The resulting winter weather is dry.

The monsoons of South China and Japan

The monsoons of China and Japan are strongly affected by the huge land mass of Siberia. During the winter, the interior of Siberia becomes extremely cold. Cold air is dense, so a cold area of high pressure forms, where the air sinks from aloft. When it reaches the surface, the air spreads outward in all directions. The result is a dry winter monsoon that blows from the north through south China and southeast Asia. The same circulation affects northern China, Japan and Taiwan, where the prevailing wind is from the northwest. Although originally dry, the northwest winds pass over the sea and pick up moisture. When the winds arrive at the islands of Japan and Taiwan, they are forced up over the land. As a result, the western slopes of these islands experience a rainy winter monsoon.

During the summer, strong heating in the interior of Siberia sets up the summer circulation, giving south China, Indonesia, and southeast Asia humid southerly winds from the equator. Japan experiences winds from the south or southeast, which bring in moisture from the north Pacific Ocean.

When the monsoon fails

The importance of monsoons is demonstrated by the experience of the Sahel, a band of land on the southern fringe of the Sahara Desert. This area would also be arid if it were not for the seasonal monsoon, whose rains normally transform it to a grassland suitable for grazing livestock . The wetter southern Sahel can support agriculture, and many residents migrated to the area during the years of strong monsoons. Beginning in the late sixties, however, the annual monsoons began to fail. The pasture areas in the northern Sahel dried up, forcing nomadic herders and their livestock southward in search of pasture and water. The monsoon rains did not return until 1974. In the intervening six years, the area suffered devastating famines and loss of life, both human and animal , and placed extreme stress on the countries of central Africa.

Because of their tremendous effect on many tropical areas, atmospheric scientists continue to study the formation and variability of monsoons. Monsoon variations are still not entirely understood, making the prediction of the monsoon a distant goal. However, researchers have shown that monsoons are affected by El Niño , the changes in winds and sea water that occur in the tropical Pacific Ocean. Research continues into modeling monsoon circulations with complicated computer simulations.

Resources

books

Frater, Alexander. Chasing the Monsoon. New York: Alfred A. Knopf, 1991.

McCurry, Steve. Monsoon. New York: Thames and Hudson, 1984.

Navarra, John G. Atmosphere, Weather and Climate. Philadelphia: W.B. Saunders Co., 1979.

periodicals

Meehi, Gerald A. "Coupled Land-ocean-atmosphere Processes and South Asian Monsoon Variability." Science (October 14, 1994): 263.

James Marti

KEY TERMS

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Circulation cell: —A circular path of air, in which warm air rises from the surface, moves to cooler areas, sinks back down to the surface, then moves back to near where it began. The air circulation sets up prevailing (constant) winds at the surface and aloft.
Convection: —The rising of warm air from the surface of the Earth.
Heat capacity: —The amount of heat required to raise the temperature of a substance; water has a high heat capacity, while land surfaces (soil, rock, etc.) and air have much lower heat capacities.
Jet stream: —High speed winds that circulate around the Earth at altitudes of 7–12 mi (12–20 km) and affect weather patterns at the surface. Two important jet streams are the subtropical jet, flowing from west to east, and the weaker tropical jet, which flows from east to west.
Monsoon: —A seasonal change in the direction of the prevailing wind, often associated with a rainy or a dry season.
Subtropics: —Regions between 23.5 and about 35 degrees latitude, in both the northern and southern hemispheres, which surround the tropics.
Tropics: —The region around Earth's equator spanning 23.5° north latitude to 23.5° south latitude.