The Vital Importance of Monsoon Rains

Monsoon Circulation
The word monsoon comes from an Arabic word meaning season. Monsoon winds are giant sea and land breezes produced by seasonal changes in circulation. One of the most interesting questions one can ask, with regard to climate change, is whether global warming will favor the zonal circulation (i.e. Trades and Westerlies) or the monsoon circulation.

To understand how the monsoon circulation works, let us first have a quick look at the phenomenon called the "sea breeze," which is familiar to those dwelling on the coast. It is a gentle wind that brings relief during hot summer days, by blowing marine air inland. In the later afternoon, the sea breeze dies, and sometime at night the wind reverses, producing a land breeze. Sea and land breezes are caused by the difference in heat capacity of sea and land surface (i.e. their temperatures go up different amounts for a given heat input). During the day, when the sun shines both on the ocean and on the land surfaces, the land heats up much faster than the sea. The air above the land is heated from below and expands. As it expands, it becomes lighter than the surrounding air masses, and it rises. Air moves in from the periphery to replace it; along the coast the source for this incoming air would be the sea.

One reason for the unequal heating of land and sea is obvious: the great heat capacity of water compared with that of rock or soil a factor of five. But another reason for the unequal heating is both more subtle and more important than specific heat. The surface of the ocean, as far as diurnal (i.e. daily) heating is concerned, is much thicker than that of the land. The sunlight readily penetrates the ocean surface, and light is absorbed within a layer more than 10 meters thick. The thickness of the soil layer that is warmed by the sun (by downward conduction of surface heat) is more like 10 centimeters. The land surface, then, is effectively over a 100 times thinner than the sea surface as far as heat penetration is concerned.


The impact of monsoon circulation on rainfall amounts in the winter (A), when it results in relatively dry weather, and in summer (B), when it results in some of the highest rainfall in the world. The rainfall scale on the right ranges from 50 mm/year (blue) 450 mm/year (white). (From: UC Santa Barbara)
In essence, the "summer monsoon" is a giant sea breeze where the summer season is analogous to daytime. Summer monsoons bring moisture from the sea as the land heats and draws in air from the ocean. Thus, with the monsoon comes rainfall. Winter monsoons, in contrast, tend to bring drought. (In Southern California, the "Santa Anna" winds are related to winter monsoon action.) The strongest monsoonal patterns are in the tropics, with the Indian monsoon being the outstanding example. These seasonal winds within the monsoon system extend over East Africa, Arabia, India, and the Arabian Sea. Lesser monsoon activity occurs in North America, with the Southwest drawing air from the California Current and the Gulf of California, and the Midwest drawing from the Gulf of Mexico.

The summer monsoon of India and the foothills of the Himalayas is a creature of the low pressure zone which begins to form early in summer, over the Asian highlands (China and Tibet). As the noon sun moves north, in spring and summer, the resulting low pressure region also moves north. Thus, the southwesterly monsoonal winds first appear in Sri Lanka (Ceylon), at the end of May, and arrive at the foot of the Himalayas by July. Here the monsoonal winds drop an enormous amount of rain, with the highest values of rainfall anywhere in the world. The winter monsoon in the region is essentially indistinguishable from the easterly trade wind pattern which is normal for this region. These winds produce strong upwelling and mixing in the coastal ocean of the Arabian Sea and off Pakistan and northwestern India.


Monsoons and Climate Change
Now we return to our question regarding the competition between zonal circulation and monsoon winds. Which will be favored by global warming? Overall, global warming will change conditions in the sea more slowly than conditions on land, because of the difference in the rate of response to heating, a concept familiar from the sea breeze. Thus, we would expect the summer monsoons to be amplified by stronger warming in summer. In addition, with more moisture in the air, extracted from a warming ocean, rainfall (and flooding) might be expected to increase. Conversely, in winter, we should expect less of a difference, as the land refuses to get quite as cold as previously. Hence, the winter monsoons should weaken, as should any of the nutrient-supplying coastal upwelling associated with winter monsoons.