2.4C The tricellular mode of atmospheric circulation Notes

The Tricellular Model of Atmospheric Circulation

1. This model divides the Earth's atmosphere into three distinct cells at different latitudes: the Hadley cell, the Ferrel cell, and the Polar cell.
2. These cells help explain how energy (heat) from the Sun is distributed and how it drives the Earth's weather patterns and biomes.

Latitude and Insolation

1. Latitude is the angular distance north or south of the equator, measured in degrees from the Earth’s center.
2. Solar energy is distributed unevenly across latitudes:
   1. Near the equator (0°), the Sun’s rays strike almost vertically, so energy is concentrated on a smaller area.
   2. Near the poles (>60°), solar rays spread over a larger area and must pass through more atmosphere, causing cooler conditions.
3. These differences in heating create zones of rising and sinking air, which is the foundation of the tricellular model.

The Tricellular Model: Three Distinct Atmospheric Cells

1. The Hadley Cell

1. Strongest and largest circulation cell.
2. Intense solar heating at the equator causes warm, moist air to rise (Intertropical Convergence Zone - ITCZ).
3. As it rises, the air cools and condenses, forming cumulonimbus clouds and heavy rainfall, which is a characteristic of tropical rainforests.
4. The cooled, dry air moves poleward at high altitude and descends around 30° N/S, creating high-pressure belts (subtropical highs).
5. Descending air is dry and stable, producing arid climates and desert biomes.

2. The Ferrel Cell

1. Lies between the Hadley and Polar cells (30°–60°).
2. Operates in the opposite direction to the other two cells, like a cogwheel interlocking with them.
3. Warm air from the subtropics meets cold polar air at about 60° N/S, forming a low-pressure zone (the polar front).
4. Rising warm air creates frequent storms and variable weather.
5. Associated biomes include temperate forests and grasslands with moderate precipitation.

3. The Polar Cell

1. Extends from 60° to 90° latitude.
2. Cold, dense air sinks at the poles, creating high pressure.
3. Surface winds (polar easterlies) move toward 60°, where they meet warmer air and rise again.
4. Produces low precipitation and cold temperatures, typical of tundra and polar desert biomes.

Linking the Tricellular Model to the Distribution of Biomes

Ocean Currents and Heat Distribution

1. The oceans cover ~70 % of Earth’s surface and act as a thermal regulator for the planet.
2. They absorb, store, and redistribute solar energy through currents, moderating climate and influencing the distribution of marine and coastal ecosystems.

Solar Energy Absorption

1. Sunlight penetrates the upper 100 m of the ocean.
2. Water absorbs heat efficiently and releases it slowly, giving oceans enormous thermal inertia.
3. This moderates climate by preventing rapid changes in air temperature.

Surface Ocean Currents

1. Warm currents (e.g., Gulf Stream, Kuroshio) move water away from the equator toward the poles.
2. Cold currents (e.g., Peru Current, Canary Current) move water from polar regions toward the equator.