Global Patterns of Atmospheric Heating and Circulation

The primary cause of general circulation in the atmosphere is caused by differential heating
by the sun. The pattern of atmospheric heating is, therefore, determined by the relative geometry
of the earth and the sun as well as regional differences in the terrestrial controls over solar
energy. The circulation of wind in the atmosphere is driven by the rotation of the earth and the
incoming energy from the sun. Global circulation facilitates the transport of heat from the
equator to the poles. This circulation and heating is controlled by three cells: Ferrel, the Hadley,
and Polar Cell. The Polar Cell starts off from lower altitudes to higher altitudes, where the high
pressure causes the cooled air to descend. The Ferrel Cell facilitates the flow of air to the tropics
near the Hadley Cell, which is closest to the equator. The movement of these winds is influenced
by solar energy, with warm air rising as much cooler air sinks.
In the Hadley Cell at the equator, winds are like as a result of weak pressure gradients.
Eastward angular momentum is transported to the middle altitudes from the equatorial latitudes.
This kind of circulation results in two Hadley Cells in each hemisphere. The rising air is
facilitated by low level convergence that is caused by ageostrophic motions for relatively low
surface pressure. This circulation is not only caused by heating but also eddies, which are
fluctuations in the flow that shape the structure and intensity of the Hadley cell. The combined
effect amplifies the subtropical part of the Hadley Cell. Some studies have revealed that
conservation of angular momentum of the upper tropospheric zonal wind makes the vertical
shears unstable (Chaplain III, Matson & Vitousek, 2011). Although both theories have been
widely applied in determination of the location of Hadley cell in global circulation, the
instability-based theory is more applicable to the terminus of the cell. The explanation captures
the sensitivity of the Hadley cell width base on changes in the climate. Furthermore, the

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instability causes thunderstorms that release copious amounts of latent heat. As some of the air
sinks to flow back to the equator, distinct winds- the southeast trade winds in the southern
Hemisphere and the northeast trade winds in the northern hemisphere are produced.
The Ferrell Cell takes over from 30 0 latitude to 60 0 latitude and is considered an intermediate
between the Hadley and Polar cells. According to Chaplain III, Matson and Vitousek (2011),
“the Ferrell cell is actually the long-term average air movement of mid-latitude weather systems
rather than a stable permanent atmospheric feature.” Some of the air sinking at the Hadley cell
continues to travel northward, and it is bent by the Coriolis force to the right. At around 60 0
latitude, this air converges with cold air from the poles, resulting in low pressure at the surface
and subsequently rising to form clouds. High precipitation occurs in latitudinal zones with rapid
and continual ascension of air mases. In the tropics, low pressure at the ITCZ causes
convergence of trade winds. The moist air rises as it cools, condenses and, therefore, holds more
water that results in high precipitation. At the temperate latitudes, the convergence of cold polar
air and moist subtropical air leads to condensation. Large lakes and oceans have a high heat
capacity, which makes them cool or heat slower than land. The contrasting heating processes
results in land and sea breezes and monsoons that influence climate on adjacent land.
Although some regions of the tropics receive high precipitation, other mechanisms can cause low
precipitation on other areas, especially those located on leeward side of mountains. Precipitation
is low where air descends. The low precipitation in the tropics is along the high-pressure belts
that are dominated by three major low-pressure zones: the equator, 60 0 north and 60 0 south.
Warm prevailing winds encounter mountains and reduce the moisture in those winds. As the
winds descend the other side of the mountains, they are dry, hence, low precipitation. Vegetation
is also important in determining surface albedo, which in turn affects the amount of heat

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absorbed by the earth surface, as well as the turbulent fluxes of sensible and latent heat.
Longwave radiation and sensible heat fluxes heat the atmosphere while latent heat transfers
water vapor in ascension, therefore, affecting the moisture sources for precipitation and the local
temperature.

Question 2: Tropical Dry Forests and Savanna Biomes

Multiple biomes occur in the dry tropics with highly varied physiognomy and ecology.
Savanna biomes and tropical dry forests are found in the Inter-Tropical Convergence Zone.
Savanna biomes are also known as grasslands with few scattered vegetation covers while
Tropical dry forest have more vegetation that dries up during the dry season. Savannas occur
between deserts and the tropical dry forests. Typically, wind movements cause the ITCZ to shift,
and as the warm air rises, pressure decreases (Allen, 2009). This mechanism leads to high
precipitation by the seasonal variations between the Southern and Northern hemispheres cause
changes in precipitation and pressure. In the tropical grasslands, distance from the Equator is one
of the main significant factors affecting the amount of precipitation. Furthermore, savannas are
war and have highly seasonal low precipitation.
The climate of the tropical dry forests resembles that of the savanna. However, they are
deciduous in the dry season and have closed canopies. The lack of strong lows, highs, and fonts
pushing the air around and influencing its ascend, rain mainly results from spontaneous
convective thunderstorms. Tropical dry forests occur south and north of tropical wet forests
(Ågren & Andersson, 2009). The seasonal movement of ITCZ over the forests and away from
the forests. Vegetation structure changes with climate within and among the biomes. While
tropical wet forests are characterized by evergreen trees, tropical dry forests are characterized by
deciduous trees that thrive both during the rainy and dry season.

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Since biomes are not discrete units, conditions vary along different climatic gradients. For
instance, vegetation changes from tall evergreen trees in the wettest regions to deciduous trees in
areas that experience seasonal drought. The climate becomes dryer towards the savannah and
eventually the dessert gradient. There is large-scale atmospheric circulation in the tropics since
the region receives more energy from the sun than they emit to space. Around the poles, the earth
loses more energy to space than it receives from the sun. As a result, the tropical air mass,
temperate, and polar air mass affect precipitation. Notably, long-term changes in climate are
greatly influences by variations in atmospheric composition and solar input. As such, the
predictable season and daily patters in the tropics as well as patterns associated with Southern
Oscillation and El Nino. The oscillations lead to widespread alterations in global geographic
pattern of climate over a long period. Without doubt, climate change can be witnessed in the
changing large-scale climate gradients.

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References

Ågren, G. I., & Andersson, F. O. (2011). Terrestrial ecosystem ecology: principles and
applications. Cambridge University Press.
Allen, P. A. (2009). Earth surface processes. John Wiley & Sons.
Chapin III, F. S., Matson, P. A., & Vitousek, P. (2011). Principles of terrestrial ecosystem
ecology. Springer Science & Business Media.