Understanding Monsoon Winds

Nothing grows without water and for the people of India, agriculture yields 25% of the country’s income. Their crops of cotton, rice, oilseeds, and coarse grains receive 80% of their moisture from the annual summer monsoon. If the rains arrive even a few days late, their crops are damaged. Thus, the beginning of the monsoon is anticipated by those in the countryside and also in the city as it provides life-giving water and ends the heat climax of June. But in years when the rains are especially heavy, the poorly constructed cities with shoddy roads are flooded. Sewers and drains back up and toxic sewage seeps into the city, claiming additional lives by disease.

The word “monsoon,” derived from the Arabic word “mausim” means season or wind shift. Although we associate the word “monsoon” with rain, a winter monsoon in India brings dry winds from the North and East that cause drought. We don’t hear much about that; instead it is the summer monsoon that makes the news. In the case of India, the large Thar Desert heats to extreme temperatures in the summer. The hot air rises, causing a low pressure cell which changes the normal wind pattern from the north. Instead, the wind swirls out over the oceans to the west, south, and east, and comes back in over land, heavy with rain.

A monsoon is not a brief event like a hurricane but rather a seasonal prevailing wind that lasts for months. Since, the winds bring rain with them, the rains also last for months. Any area of the world which receives the majority of its rainfall during a certain season is considered to have monsoons. Thus, even in Arizona and Mexico which get their rain in intense summer storms, the possibility for monsoon-like events such as flash floods exists. Because of the high density of population in South Asia, it is this area that incurs the most damage and loss of life from the heavy rains.

It is believed that the potential for the strong Asian monsoon happened 50 million years ago when the Tibetan Plateau was uplifted after the collision of India and Asia. Through geological testing of fossils and long duration sediment records from the South China Sea, more accurate timing of the monsoon in history is possible. Over time, the strength of the monsoon has varied greatly due to climate change especially during the ice ages of the Pleistocene Epoch. Of particular interest in modern times is the weakening of the Leeuwin Current near Western Australia. Changes in the Leeuwin Current in any given year can lead to decreased or increased ocean temperatures which have a great influence on how much moisture is drawn up into the atmosphere by the monsoon winds. The effect of El Nino and other current patterns which affect ocean temperature is also being studied.

In North American, the monsoon is also a summer event from late June to September. As well as Arizona, it can affect New Mexico, Nevada, Utah, Colorado, West Texas, and California. Sometimes called the Desert Monsoon, the rain generated over arid land ill-equipped to absorb it can cause sudden (flash) floods which sweep through dry riverbeds or roads overcoming any cars or people in the way. Often monsoon storms in the Southwest U.S. start with dust storms or violent thunderstorms. The dust particles in the air help to attract more moisture. Downpours are heavy, often over 2 inches in a few hours, and account for 1/3 of the area’s annual rainfall.

The monsoon of western sub-Saharan Africa is caused by the huge seasonal temperature differences between the Sahara Desert and the equatorial Atlantic Ocean. In South America, summer monsoon rains often bring flooding especially to Brazil.

Climate change, whether it is man-caused or cyclical, has taught us one thing for sure: the weather of today may not be the weather of tomorrow. In areas like India where the balance between nourishing rainfall and devastating destruction is delicate, it is increasingly important to understand all we can about the monsoon.