A tropical monsoon climate is a warm tropical climate characterised by high temperatures throughout the year, a very wet season and a comparatively short dry season. In the Köppen climate classification it is designated Am. It occupies an intermediate position between the continuously wet tropical rainforest climate and the more strongly seasonal tropical savanna climate. The defining feature is a pronounced seasonal concentration of rainfall caused largely by the migration of tropical atmospheric circulation and monsoonal wind systems. Temperatures remain high because the climate is tropical, but rainfall changes considerably through the year.
The tropical monsoon climate develops where seasonal changes in atmospheric circulation bring moist maritime air over land during the wet season and relatively drier air during part of the dry season. Heating differences between continents and oceans contribute to monsoon circulation, while the seasonal migration of the Intertropical Convergence Zone is also important. During the wet season, moist air can produce frequent heavy rainfall and thunderstorms. In some regions, tropical cyclones contribute substantial rainfall. The resulting precipitation regime is strongly seasonal, but the dry season is usually shorter or less severe than in a tropical savanna climate.
The Köppen Am category has a specific precipitation relationship rather than simply meaning any tropical place affected by a monsoon. Under the standard classification, the driest month receives less than 60 millimetres of precipitation but also receives at least a threshold amount related to the total annual rainfall and the length of the wet season. This distinguishes Am from Af and Aw. Classification details can vary slightly between versions and interpretations of the Köppen system. The important geographical principle is that tropical monsoon climates have a substantial dry interval but retain enough annual moisture and wet season intensity to remain distinct from climates with a longer, more pronounced tropical dry season.
Tropical monsoon climates are common in South and Southeast Asia, especially parts of India, Bangladesh, Myanmar, Thailand, Vietnam and the Philippines, although similar climates occur in parts of Africa, northern Australia and tropical coastal regions elsewhere. Local geography strongly influences their distribution. Mountains can force moist monsoon air upward, producing exceptionally heavy rainfall on windward slopes, while rain shadow areas may be much drier. Coastal proximity also matters because oceanic moisture supplies the water vapour required for intense wet season precipitation.
Temperatures in tropical monsoon climates are generally high throughout the year, but the hottest period is not necessarily the same as the wettest period. In some areas, maximum temperatures occur immediately before the arrival of the main monsoon rains because cloud cover and evaporation later reduce daytime heating. The wet season can therefore be thermally different from the pre monsoon period even when annual temperature variation is relatively small. Humidity commonly rises substantially during the monsoon, while the dry season can be considerably less humid.
Tropical monsoon climates support vegetation adapted to a strong seasonal water supply. Natural vegetation can include tropical seasonal forests, moist deciduous forests, palms, grasslands and other communities depending on rainfall duration and intensity. Trees may shed leaves during the drier part of the year to reduce water loss, although vegetation can remain considerably denser than in many savanna environments. Agriculture often follows the seasonal rainfall cycle. Rice, for example, is extensively cultivated in suitable monsoon regions because abundant wet season water can support flooded or irrigated fields.
Monsoon climates have major economic and social importance because seasonal rainfall supplies water for agriculture, reservoirs, rivers and groundwater recharge. At the same time, excessive rainfall can produce river flooding, landslides, urban flooding and crop damage. A delayed or weak monsoon can create agricultural drought even when annual rainfall appears substantial. A very intense monsoon can cause destructive floods while still being beneficial to water supplies over the longer term. Effective management therefore depends on seasonal forecasting, reservoirs, irrigation, drainage, flood planning and agricultural systems adapted to rainfall variability.
Tropical monsoon climate is distinguished from tropical rainforest climate by its defined dry season and from tropical savanna climate by its generally shorter or less severe dry season and much stronger wet season rainfall. The word monsoon refers fundamentally to a seasonal reversal or major seasonal change in wind circulation, although popular usage often uses monsoon to mean the rainy season itself. Tropical monsoon climate should therefore be understood as a climatic regime shaped by seasonal atmospheric circulation, not merely as any location that receives heavy summer rainfall.
