Global Patterns of Atmospheric Heating and Circulation
At any given point in time, there are various weather systems across the globe exhibiting variable heating and winds. Averaging the winds over several years has shown well-defined global circulation and heating patterns (Hastenrath, 2012). The global atmospheric heating and circulation refer to the large-scale air movement including ocean circulation, which also helps in the distribution of thermal energy (heat) on the earths surface. Atmospheric circulation occurs due to the mass movement of the atmosphere and can be either vertical or horizontal. The vertical movement entails warm air rising and becoming buoyant whereas horizontal movement entails the creation of wind by the movement of air from high-pressure areas to low pressure areas (Oliver, 2008). The horizontal movement of winds adheres to curved trajectories because of the earths rotation. Although the atmospheric circulation structure differs years, the underlying climatological structure is moderately constant. The global patterns of atmospheric heating and circulation are determined by the earths rotation and the inbound thermal energy from the sun (Hastenrath, 2012).
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The thermal energy from the sun heats the earths atmosphere at different intensities depending on the latitude, with the equator receiving the highest intensities, which then reduces in the middle latitudes, and the lowest in the poles (Oliver, 2008). In tropical regions near the equator, the high heat intensity results in the rising of warm air. When this air rises vertically to approximately 10-15 kilometers, it begins to move northwards from the equator to the poles, which results in a low-pressure system in the tropics (Robinson & Henderson-Sellers, 2014). The air in the north part of the equator moves northwards and that in the south of the equator moves southwards. As the air moves to higher latitudes having reduced intensities, it cools and sinks back to the ground creating a high-pressure system. The air then flows back to the equator and subsequently warms again (Hastenrath, 2012). The warmed air then rises again, resulting in a repeated pattern referred to as convention that occurs on a global scale.
Due to the rotation of the Earth, the air flowing northwards and southwards from the air is affected by the Earths spin. As a result, the air moving north changes direction to northwest whereas the south flowing air changes direction to southwest (Robinson & Henderson-Sellers, 2014). The effect of the Earths rotation on the movement of air is referred to as the Coriolis Effect. In a scenario whereby the Earth did not rotate, there would be a single convention cell stretching from the equator to the North Pole and another to the South Pole (Robinson & Henderson-Sellers, 2014). However, due to the spinning of the Earth, there are three convections north and south of the equator.
Mechanisms for High Precipitation in the Tropics
Precipitation occurs in areas having low pressure coupled with rising air which is the case in tropical regions adjacent to the equator. The areas close to the equator experience the highest precipitation thatcan be attributed to the rising warm air, which carries with significant amounts of water vapor (Robinson & Henderson-Sellers, 2014). It is essential to note that warm air from both hemispheres, carried by the trade winds and the ensuing air convention, meet at the tropical convergence zone (ITCZ), resulting in high precipitation in tropical areas (Oliver, 2008). This cycling is referred to as the Hadley Cell. Generally, it can be concluded that high precipitation in tropical areas is attributed to convective activity.
Mechanisms for High Precipitation at Temperate Latitudes
At the equator, the Suns rays hit the surface at a direct angle that ranges between 23.43719 S at the Tropic of Capricorn and 23.43719 N at the Tropic of Cancer, which depends on the period of the year (Oliver, 2008). Between these latitudes, the intensity of solar radiation is the highest. In other parts of the Earth, the sun is not directly overhead (the Suns rays hit the surface at an angle); as a result, the solar intensity is less. As one moves closer to the poles, the angle reduces, and so is the radiation intensity. The climate system of the Earth draws upon the location of cold and hot air-mass regions as well as the atmospheric circulation produced by the Westerlies and trade winds (Oliver, 2008). Trade winds in the north of the equator originate from the northeast whereas those from the south of the equator come from the south east (Robinson & Henderson-Sellers, 2014). Trade winds from the northern and southern hemisphere converge close to the equator, which results in the rising of air. With the cooling of the rising air, there is a formation of rain and clouds. The subsequent rainy and cloudy weather near the equator produces tropical conditions in the temperate zones. It is also imperative to note that the amount of precipitation in temperate zones is influenced by the amount of available water vapor, which depends on the source of air being uplifted (Robinson & Henderson-Sellers, 2014). Air that comes directly from subtropical oceans having high evaporation rates results in higher rates of precipitation. However, if the air comes from tropical deserts, the air is likely drier, resulting in lower rates of precipitation.
Mechanisms for Low Precipitation in the Tropics
Precipitation reduces as one moves from the equator towards subtropical regions to the poles. This decline in precipitation can be attributed to the changing locations of the global pressure system and wind (Hastenrath, 2012). According to Robinson and Henderson-Sellers (2014), the seasonal movement of the ITCZ contributes to low tropical precipitation, especially in the Tropic of Cancer. The ITCZ, found between the southern and northern Hadley cells, is created when the South East Trade winds and the North East Trade winds converge. During this convergence, a subtropical high-pressure belt is created in both southern and northern hemispheres (Hastenrath, 2012). In the southern hemisphere, the belt is moist and warm due to the Atlantic whereas in the northern hemisphere, the belt is hot and dry. The Earths spinning also results in the changing positions of the ITCZ. Low precipitation in the tropics has also been linked to the lack of mechanisms for creating air uplift and ensuring air saturation.
Highly Seasonal Rainfall in the Tropical Dry Forest and Tropical Savanna Biomes
The tropical savanna and the tropical dry forest biomes are found within the subtropical and tropical latitudes. According to Hastenrath (2012), these areas get several hundred centimeters of rain annually. Savannas are akin to the tropical version of the grasslands in the temperate zones and are found between the Tropics of Cancer and Capricorn. The bulk of the savannas have a strong dry season for the majority of the year. Large savanna tracks also exist in Australia, India, and South America (Oliver, 2008). Savannas receive an annual rainfall that ranges 50-.8-127 centimeters annually, although it is seasonal.
The ITCZ has the most intense rainfall. The ITCZ belt is often found about 6o N. The position of the ITCZ in the north of the equator can be attributed to the energy transported northward by the Atlantic Ocean northward through the equator, which makes the northern hemisphere warmer when compared to the southern hemisphere. Seasonally, the ITCZ usually moves following a warming hemisphere (Robinson & Henderson-Sellers, 2014). The rainy season in the tropical savanna and tropical dry forest biomes is experienced during the warmer months because of the Suns position and hot humid air. Oliver (2008) points out that, in the course of warmer months, the rising humid air collides with cooler air above which forms rain. The warm air experienced during the warmer months is required for this process to take place and form rainfall.
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