Monsoon Systems

As monsoons have become better understood, the definition has been broadened to include almost all of the phenomena associated with the annual weather cycle within the tropical and subtropical continents of Asia, Australia, and Africa, and the adjacent seas and oceans. It is within these regions that the most vigorous and dramatic cycles of weather events on Earth take place.

Even more broadly, it is now understood that in the geological past, monsoon systems must have always accompanied the formation of supercontinents such as Pangea, with their extreme continental climates.

Northeast Winter Monsoon

In Asia, the northeastern winter monsoons take place from December to early March. The temperature over central Asia is lower, creating a zone of high pressure there. The jet stream in this region splits into the southern subtropical jet and the polar jet. The subtropical flow directs northeasterly winds to blow across south Asia, creating dry air streams which produce clear skies over India from the months of November to May.

Meanwhile, a low pressure system develops over northern Australia and winds are directed toward Australia.

During the Northeast Winter Monsoon, Australia and southeast Asia receive large amounts of rainfall.

Southwest Summer Monsoon

The Southwestern Summer Monsoons occur from June to September, and are drawn towards the Himalayas, creating winds blowing rain clouds towards India, some areas of which receive up to 10,000 mm of rain.

Indian Ocean Monsoon

The southwest monsoon is generally expected to begin around the middle of June and dies down by September. It begins first in the coastal state of Kerala and moves upwards at a rate of roughly 1-2 weeks per state. The monsoon accounts for 80 percent of the rainfall in the country. Indian agriculture (which accounts for 25 percent of the GDP and employs 70 percent of the population) is heavily dependent on the rains, especially crops like cotton, rice, oilseeds and coarse grains. A delay of a few days in the arrival of the monsoon can, and does, badly affect the economy, as evidenced in the numerous droughts in India in the 90s.

The monsoon is widely welcomed and appreciated by city dwellers as well, for it provides relief from the climax of summer in June. However, because of the lack of adequate infrastructure in place, most major cities are often adversely affected as well. The roads, already shoddy, take a battering each year; houses and streets at the bottom of slopes and beside rivers are waterlogged, slums are flooded, and the sewers and the rare hurricane drain start to back up. This translates into various minor casualties most of the time (although a large number of people in rural areas are struck dead by lightning while working in their fields); however, this lack of city infrastructure coupled with changing climate patterns also causes severe damage to and loss of property and life, as evidenced in the Mumbai floods of 2005.

North American Monsoon

The North American Monsoon (NAM) occurs from mid July into September, originating over Mexico and frequently spreading over and affecting Arizona, New Mexico, Nevada, Utah, Colorado, West Texas, and California. It pushes as far west as the Peninsular Ranges and Transverse Ranges of southern California but rarely reaches the coastal strip (a wall of desert thunderstorms only a half-hour's drive away is a common summer sight from the sunny skies along the coast).

The North American Monsoon is associated with an area of high pressure called the subtropical ridge that moves northward during the summer months and a Thermal low (a trough of low pressure which develops from intense surface heating) over the Mexican Plateau and the desert southwest of the United States. The monsoon usually extends into the southwest United States when an area of high pressure develops in the upper atmosphere over the four corners region, creating an easterly to southeasterly wind flow aloft. This wind flow pattern directs moisture originating in the Gulf of Mexico, the Gulf of California, and the tropical Pacific by way of northern Mexico into the region, setting off brief, but often torrential thunderstorms, especially over mountainous terrain.

Many desert plants are adapted to take advantage of this wet season. Because it is dangerous to be caught in the open when these storms suddenly appear, many golf courses in Arizona have thunderstorm warning systems.