Monsoons are seasonal changes in surface wind and precipitation patterns over the tropical and subtropical continents and surrounding oceans. These changes are due to the differences in thermal properties between land and ocean that give rise to different responses to seasonal changes in insolation.

The specific heat of land is typically less than that of ocean. That is, it requires less thermal energy to raise the temperature of a given amount of land by a given amount than it does to raise the temperature of the same amount of ocean by the same amount. As a result, the summer Sun warms the land more quickly and to a greater extent than it warms the oceans, which remain relatively cool.

Typically, low-pressure weather systems form over warm surfaces and high-pressure systems form over cool surfaces. Thus, in the summertime, landmasses often become centers of low pressure, whereas the adjacent oceans become centers of high pressure. Wind blows from high-pressure centers to low-pressure centers, so in summer winds typically blow from the ocean to the land. This phenomenon is called "wind convergence" over land (Duan, 2006).

When wind converges over land, convection causes clouds to form. Precipitation typically follows the development of these clouds. Therefore, summer monsoons are characterized by winds blowing from the ocean to the land, over which clouds then form and rain falls.

In the winter, the entire process reverses. The lower specific heat of land, which caused it to heat more quickly than the oceans in the summer, causes it to cool more quickly than the oceans in the winter. As insolation decreases, the land quickly loses heat, becoming cold relative to the oceans, which retain much of their heat. As a result, the oceans become centers of low pressure, landmasses become centers of high pressure, and winds begin to blow from the land to the oceans. Clouds and precipitation also move from land to the oceans.

The contrast between the thermal properties of land and seawater is the basic cause of monsoons, but certain topographic features can enhance this effect. Larger landmasses and higher-altitude land surfaces increase the thermal and pressure differentials between land and water. As a result, the world's strongest monsoons are all related to the world's largest mountain ranges: The East Asian and South Asian monsoons are related to the Tibetan Plateau, the North American monsoon is related to the Rocky Mountains, and the South American monsoon is related to the Andes Mountains.

While monsoons are thus caused by contrasts between land and ocean, the atmosphere plays a crucial role in the system as well (Ahrens, 2008). The atmosphere couples with both land and sea to mediate and react to the thermal differences between them. It forms different pressure systems over the two bodies based on their temperature differential, reversing the direction of the winds. It also transports large amounts of water from sea to land, first by absorbing evaporated water vapor and then by precipitating the water onto the land.

Monsoons form a central part of many regional climate systems. In many parts of the world, monsoonal precipitation constitutes a major rainfall system, providing water resources for regional ecosystems.

Many tropical and subtropical climates are dependent upon the monsoonal rains. Any significant climate change will inevitably affect global monsoon circulations, causing changes in the patterns and intensity of monsoonal rainfall. However, it is a matter of debate among climatologists as to whether global warming will intensify or weaken the monsoons. On one hand, generally warmer climates may reduce the temperature contrast between land and ocean, resulting in weaker monsoons. Evidence to support this hypothesis exists in studies of Himalayan ice cores. Data for the last three hundred years indicate that for every 0.1œ Celsius increase in temperature in the Himalayas, there was a decrease of about 100 millimeters in monsoonal rainfall.

On the other hand, global warming may enhance evaporation from the ocean's surface, because warmer air holds more water vapor than does cooler air. An increase in the amount of atmospheric water vapor would likely lead to greater monsoonal rainfall and stronger monsoons. Data from the Tibetan Plateau--where Earth's strongest monsoon is located-- indicate that over the last fifty years, total atmospheric water vapor content and total surface rainfall both increased (Duan, 2006).

Some scientists argue that, rather than strengthen or weaken monsoons, global warming might simply redistribute rainfall without significantly altering average amounts. Such redistribution would entail more severe events at both extremes, as heavy precipitation and flooding would occur in some areas and drought would occur in others. Thus, life in monsoonal areas is likely to change significantly if global warming continues, but the nature of those changes remains uncertain.

References

1. Ahrens, C. Donald. Essentials of Meteorology: An Invitation to the Atmosphere. 5th ed. Belmont, Calif.: Thomson Brooks/Cole, 2008.

2. Duan, Keqing, Tandong Yao, and Lonnie G. Thompson. "Response of Monsoon Precipitation in the Himalayas to Global Warming." Journal of Geophysical Research 111 (2006).

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