9.4.2: Tropical Monsoon Climate

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Page ID: 16092

Michael E. Ritter
University of Wisconsin-Stevens Point via The Physical Environment

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The tropical monsoon climate experiences abundant rainfall like that of the tropical rain forest climate, but it is concentrated in the high-sun season. Being located near the equator, the tropical monsoon climate experiences warm temperatures throughout the year.

Geographical Distribution

The monsoon climate is found along the coastal regions of southwest India, Sri Lanka, Bangladesh, Myanmar, Southwestern Africa, French Guiana, and northeast and southeastern Brazil.

Controlling Factors

The major controlling factor over the monsoon climate is its relationship to the monsoon circulation. Recall that the monsoon is a seasonal change in wind direction. The "classic" monsoon circulation of Asia exhibits an onshore flow of air (air moving from ocean towards land) during the summer or high-sun season, and offshore air flow (air moving from land toward water) during the winter or low-sun season. The change in direction is due to the difference in the way water and land heat.

Changing pressure patterns that affect the seasonality of precipitation also occur in Africa. During the high-sun season, the ITCZ induces rain while the subtropical high creates dry conditions. The monsoon climate of Africa, and South America for that matter, are typically located along tradewind coasts.

Figure \(\PageIndex{2}\): Climograph for Mangalore, India
Lat/Long = 12.53^o N, 74.52^o E
Average Annual Temperature (^oC) = 27.05
Annual Temperature Range (^oC) = 3.6
Total Annual Precipitation (mm) = 3409.2
Summer Precipitation (mm) = 3115.9
Winter Precipitation (mm) = 293.3

Distinguishing Characteristics

Temperature

Like in the tropical rain forest climate, temperatures remain high all year in the monsoon climate. As shown in the climograph for Mangalore, India (Figure \(\PageIndex{1}\)), the average annual temperature is 27.05^oC (80.7^oF) but only has an annual temperature range of 3.6^oC (2^oF). The monsoon climate's temperature range is somewhat similar to that of the rain forest, but it exhibits a slightly different temporal pattern. In the rain forest we noted two periods of maximum temperature in association with the migration of the Sun's vertical rays. The monsoon climate tends to have its highest temperature just before rainy period. Once the rainy period starts, clouds block incoming solar radiation to reduce monthly temperatures.

Precipitation

Seasonality of its precipitation is the hallmark and most well-known characteristic of the monsoon climate. Many think that the term "monsoon" means wet weather, when in fact it describes an atmospheric circulation pattern. Though the annual amount of precipitation is quite similar to that of the rain forest, monsoon precipitation is concentrated into the high-sun season. Maritime equatorial and maritime tropical air masses travel from the ocean on to land during the summer, where they are uplifted by either convection or convergence of air to induce condensation. Locally, orographic uplift is an important mechanism for promoting precipitation. As air travels into the Indian subcontinent, it is uplifted by the Himalayas, causing cloud development and precipitation.

The low-sun season is characterized by a short drought season when high pressure inhibits precipitation formation. In the case of the Asian monsoon, the replacement of the thermal low with the subsidence of the Siberian High suppresses uplift. Air masses that dominate this period are dry given their continental origin (cT, cP) or stability (mTs).