6.1B Tricellular model of atmospheric circulation Notes

1. The Earth receives uneven heating from the Sun because sunlight strikes different latitudes at different angles.
2. The equator receives direct, concentrated solar radiation, which warms the surface and the air above it more intensely.
3. The poles receive sunlight at a much lower angle, spreading the same energy over a larger area, which results in much cooler temperatures.
4. This unequal heating creates pressure differences and sets the atmosphere in motion, forming a global circulation system known as the tricellular model.
5. The tricellular model redistributes heat from the equator toward the poles, helping to balance global temperatures and maintain stable climate patterns.

Why Differential Heating Occurs

1. The Earth is spherical, so sunlight strikes the equator more directly than the poles.
2. Tropical regions absorb significantly more solar radiation than polar regions.
3. Snow and ice in polar regions have higher albedo, meaning they reflect much of the incoming radiation.
4. Heat input exceeds heat loss between about 38°N and 38°S, creating a surplus of energy at low latitudes.
5. At higher latitudes, heat loss exceeds heat input, producing an energy deficit.
6. Atmospheric circulation develops to redistribute this energy and reduce extreme temperature contrasts.

The Tricellular Model

1. The tricellular model divides each hemisphere into three large atmospheric circulation cells:
   1. The Hadley Cell
   2. The Ferrel Cell
   3. The Polar Cell
2. Together, these cells create a global pattern of rising and sinking air, wind belts, and pressure zones.

1. The Hadley Cell (0° to 30° latitude)

1. Strong heating at the equator causes warm, moist air to rise, creating a zone of low pressure known as the Intertropical Convergence Zone (ITCZ).
2. As this air rises, it cools and loses moisture, producing heavy equatorial rainfall and tropical rainforests.
3. The rising air spreads toward the subtropics and sinks at around 20°–30° latitude, creating regions of high pressure, dry conditions, and major deserts.
4. The sinking air then flows back toward the equator at the surface, forming the trade winds.

2. The Ferrel Cell (30° to 60° latitude)

1. The Ferrel cell lies between the Hadley and Polar cells.
2. It is indirectly driven by the movements of the other two cells rather than by temperature differences.
3. Air at the surface flows from subtropical high pressure toward subpolar low pressure, forming the westerly winds.
4. Rising air at around 60° latitude leads to cloud formation and moderate precipitation.
5. This zone supports temperate forests and grasslands.

3. The Polar Cell (60° to 90° latitude)

1. Very cold, dense air sinks at the poles, creating high-pressure systems.
2. Surface air flows outward from the poles toward lower latitudes.
3. At around 60° latitude, it meets warmer air from the Ferrel cell and rises, creating low pressure and frontal weather systems.
4. The rising air cools and sinks again toward the poles, completing the cell.

How the Tricellular Model Redistributes Heat

1. Warm air rises at the equator and carries heat upward and poleward.
2. As air travels toward higher latitudes, it cools and eventually sinks, bringing cool air into lower latitudes.
3. This continuous cycle:
   1. Reduces the extreme heat of equatorial regions, and
   2. Warms higher latitudes, preventing them from becoming even colder.
4. Without this circulation, equatorial regions would continue to heat indefinitely, and polar regions would become increasingly cold.

Atmospheric Systems

1. Storages

1. The atmosphere stores gases such as nitrogen, oxygen, carbon dioxide, methane, and water vapour.
2. Concentrations of these gases change over time due to natural processes and human activities.
3. Greenhouse gas concentrations influence Earth's energy balance and climate.

Flows

1. Winds, convection currents, jet streams, and turbulence all represent flows of energy and gases.
2. These flows connect atmospheric layers and transport gases, heat, and particles around the planet.

Inputs

1. Volcanic eruptions release gases and ash into the atmosphere.
2. Wildfires and vegetation release gases such as carbon dioxide.
3. Human activities such as combustion, industrial emissions, and agriculture add pollutants and greenhouse gases.

Outputs

1. Removal of gases through processes such as photosynthesis, respiration, chemical reactions, and deposition (wet and dry).
2. Pollutants can leave the atmosphere through precipitation or settling

Interactions with Other Earth Systems

1. The atmosphere exchanges gases with the biosphere through photosynthesis and respiration.
2. It interacts with the hydrosphere through evaporation, condensation, and precipitation.
3. It influences the lithosphere through weathering and is affected by volcanic emissions.

Relationship Between Atmospheric Circulation and Climate

1. The tricellular model helps explain the distribution of:
   1. Tropical rainforests
   2. Deserts
   3. Temperate forests
   4. Polar regions
2. High rainfall at the equator and at 60° arises from rising air and condensation.
3. Dry climates at 30° and at the poles result from sinking air and high pressure.