A tropical savanna climate is a tropical climate characterised by high temperatures throughout the year and a pronounced seasonal alternation between a wet season and a dry season. In the Köppen climate classification it is generally represented by Aw, although As is used in some locations where the dry season occurs during the warmer part of the year. Tropical savanna climates differ from tropical rainforest climates because they have a distinct dry season. They differ from tropical monsoon climates because the dry season is generally more substantial and the rainfall regime is more strongly seasonal.
Tropical savanna climates result primarily from the seasonal movement of atmospheric circulation belts. During the wet season, the Intertropical Convergence Zone or related tropical convergence systems move over the region, promoting rising moist air, clouds and thunderstorms. During the dry season, subtropical high pressure and descending air can suppress convection and reduce rainfall. The exact timing and strength of the wet and dry seasons depend on latitude, continental position, ocean temperatures, prevailing winds and topography. This seasonal atmospheric mechanism is responsible for much of the characteristic vegetation and water availability.
Rainfall in a tropical savanna climate is strongly seasonal and can vary substantially from year to year. The wet season may involve frequent thunderstorms, heavy downpours and rapid river responses, while the dry season can persist for several months. Annual rainfall can range from relatively modest tropical totals to amounts sufficient to support woodland and tall grass. The length of the dry season is particularly important because it affects soil moisture, plant growth, fire frequency and river discharge. Two places with similar annual rainfall can therefore have very different ecosystems if their rainfall is distributed differently through the year.
Natural savanna vegetation generally consists of grasses with scattered trees and shrubs, although the precise structure varies along rainfall and soil gradients. Frequent seasonal dryness limits the continuous development of closed humid forest, while grasses can recover rapidly when rains return. Fire is an important ecological process in many savannas, particularly where seasonal dryness creates abundant dry vegetation that can burn. Some trees have thick bark, deep roots or other adaptations that increase their tolerance of fire and drought. Grazing animals can also influence vegetation structure by consuming grasses and browsing shrubs.
Tropical savannas occur across large areas of Africa, South America, northern Australia and parts of Asia. African examples include extensive regions south and north of the equatorial rainforest belt. In South America, the cerrado and other seasonal tropical landscapes include areas with savanna characteristics, although not all are classified identically under every vegetation or climate system. Northern Australia contains extensive tropical savanna landscapes influenced by a strong wet and dry seasonal cycle. Local topography and soils create substantial variation within these broad regions.
Tropical savanna climates support important agricultural and pastoral systems. Seasonal rainfall can support crops such as maize, millet, sorghum, groundnuts and other drought tolerant or seasonally adapted crops, depending on soils and local conditions. Livestock grazing is widespread in many savanna regions. Water availability during the dry season is often a major limiting factor, and irrigation can substantially alter land use. Agricultural expansion can fragment habitats and modify fire regimes, while overgrazing can contribute to vegetation degradation and soil erosion when vegetation cover is insufficient to protect the soil.
Savanna climates are highly sensitive to changes in rainfall timing, intensity and duration. A small change in the length of the dry season can influence whether a landscape supports open grassland, wooded savanna or more continuous forest. Fire, herbivory, soil moisture and atmospheric carbon dioxide interact with rainfall to determine vegetation structure. Human burning can maintain open landscapes in some areas, while fire suppression can allow woody vegetation to increase. Climate change may alter drought frequency, extreme rainfall, fire conditions and growing seasons, although the direction and magnitude of change vary between regions.
Tropical savanna climate should not be equated simply with grassland. Savanna is a vegetation and ecological concept as well as a climatic association, and savanna vegetation can occur under different climatic conditions. Likewise, not every tropical dry forest is a savanna. The critical climatic distinction is the pronounced seasonal distribution of rainfall combined with year round tropical temperatures. The wet season supports rapid biological production, while the dry season imposes water stress and strongly influences vegetation, fire, agriculture, wildlife and river behaviour.
