Atmospheric Heating and Circulation
Strictly speaking, the Earth is considerably inhomogeneous; that is, there are large mountain ranges that interfere with the flow between the land and sea. For this paper, these variations in surface conditions are neglected, and the Three-Cell Theory is used to explain atmospheric heating and circulation. According to the Three-Cell Theory, the Earth is divided into three circulation cells including the tropical cell, mid-latitude cell, and polar cell ( Jones et al., 2007 ).
Tropical Cell
The tropical cell lies between the equator and a latitude of 30 0 . Due to convection, the air at the equator heats and rises until it reaches the upper layers of the troposphere. At this point, the air tends to flow toward the poles. In the northern hemisphere, for example, the air flows towards the North Pole. At 30 0 N, the Coriolis Effect deflects the air so that it is flowing eastward ( Jones et al., 2007 ). Consequently, there is a convergence and subsidence of air streams near latitude 30 0 N, creating high pressure. On reaching the surface, the air diverges outward, part of it toward the North Pole to form the mid-latitude cell, and the other portion toward the equator to form the northeast trades.
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Mid-latitude cell
The mid-latitude cell is found between latitudes 30 0 and 60 0 . In the northern hemisphere, for example, the air currents in this region, both near the surface and high above blow from the west. This phenomenon may also be attributed to the Coriolis Effect ( Jones et al., 2007 ). The air current far above the surface is driven by the west winds in the two neighboring cells.
Polar cell
The polar cell is located between latitude 60 0 and the poles. In this region the air currents at high altitudes flow towards the poles and descend at the pole due to convection, thereby creating high pressure in the Polar Regions. Considering the northern hemisphere, the air reaches the ground and flows towards the equator. The Coriolis Effect then deflects this air so that it flows from the northeast ( Jones et al., 2007 ). On reaching the margin of the mid-latitude cell, the air is deflected upward with part of it flowing toward the region and the rest of it toward the equator.
Mechanisms of Precipitation in the Tropics and Temperate Latitudes
There are two primary mechanisms associated with the development of a tropical climate regime, which is characterized by low precipitation in the low sun season and high precipitation in the high sun season. On the western parts of a continent, the subtropical high-pressure system dominates during the low sun season causing clear skies, subsiding air, and stable conditions ( Hastenrath, 2012 ). On the other hand, during the high sun season, the high pressure tends to shift toward the pole and is often replaced by the Inter-Tropical Convergence Zone, which is linked with the occurrence of convective storms. The high precipitation in the tropics is brought about by warm air from the northern and southern hemispheres which converge at the ITCZ and the ascension of this air as a result of low pressure. The high precipitation in the temperate latitude may be associated with several factors including cold fronts, low-pressure systems, stationary fronts, and warm fronts.
The other mechanism, more dominant in the eastern parts of continents, is the monsoon regime where winter conditions are subject to cool, dry air often coming from the deep interiors of these continents ( Hastenrath, 2012 ). However, this flow is reversed during the summer, and the air becomes moist and warm, originating from the tropical oceanic regions.
The presence of mountain ranges in the tropic areas, however, tends to complicate these mechanisms and may bring about rain shadows ( Hastenrath, 2012 ). The mountains impede the warm, prevailing winds thereby forcing the moisture to come down as precipitation on the side blocking the wind. The other side, identified as the leeward side, therefore, receives low precipitation.
Seasonal Rainfall in the Tropical Dry Forest and Tropical Savanna Biomes
The Inter-Tropical Convergence Zone represents the geographical region that experiences the most of the direct sunlight at a given time of the year ( Murphy and Lugo, 2006 ). The direct sunlight in the ITCZ leads to heating of air immediately above the ground. This air rises and creates low pressure in this area. Once the air is at a much high altitude, it condenses and forms clouds, which bring about precipitation often in the form of rain.
The shift of ITCZ throughout the year also influences areas that do not experience overhead sunlight. These are high-pressure areas and do not produce rain-bearing clouds; hence, low precipitation. The areas north and south of the ITCZ, commonly referred to as doldrums, are responsible for dry seasons in the tropics. Thus, there is high precipitation in the tropical rainforest and tropical monsoon since they are close to the equator ( Murphy and Lugo, 2006 ). Meaning, they are close to or at the ITCZ during most of the year which favors the production of clouds, and consequently, precipitation in these areas.
The climatic zones where tropical dry forests and tropical savannas are located dictate their biomes. Given that the tropical dry forests are significantly north and south of the equator, they experience more climatic shifts as compared to the tropical rain forests ( Murphy and Lugo, 2006 ). The tropical dry forests have both dry and wet seasons, but the precipitation is lower than that of the rainforests.
The tropical savannas have similar seasons but, unlike the tropical dry forests, they are dominated by grasslands ( Bond, 2008 ). It is also important to note that the dry forests are under the ITCZ for longer periods of the year than the savannas. This is because the savannas are found north of the dry forests in the northern hemisphere and south of the dry forests in the southern hemisphere. As a result, the savannas experience the tropical wet and dry climate which is the opposite of the normal monsoon and rainforest climate ( Beerling and Osborne, 2006 ). While the precipitation in the tropical savanna is also season, it is considerably lower. Additionally, it experiences dry conditions during the low-sun season.
References
Beerling, D. J., & Osborne, C. P. (2006). The origin of the savanna biome. Global change biology , 12 (11), 2023-2031.
Bond, W. J. (2008). What Limits Trees in C ₄ Grasslands and Savannas?. Annual Review of Ecology, Evolution, and Systematics , 641-659.
Hastenrath, S. (2012). Climate and Circulation of the Tropics (Vol. 8). Springer Science & Business Media.
Jones, P. D., Trenberth, K. E., Ambenje, P., Bojariu, R., Easterling, D., Klein, T., ... & Zhai, P. (2007). Observations: surface and atmospheric climate change. IPCC, Climate change , 235-336.
Murphy, P. G., & Lugo, A. E. (2006). Ecology of tropical dry forest. Annual review of ecology and systematics , 67-88.
