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.
3. The Coriolis effect and continental boundaries deflect currents, creating large circular gyres in each ocean basin.

Salinity and Density

1. Average salinity: 35 parts per thousand (ppt).
2. High evaporation in tropical seas increases salinity; melting ice decreases it.
3. Water density increases with higher salinity, lower temperature, and greater depth.

The Great Ocean Conveyor Belt (HL Only)

1. The thermohaline circulation is a deep-water global current driven by differences in temperature (thermo-) and salinity (-haline).
2. It connects major ocean currents in the Atlantic, Pacific, and Indian Oceans.
3. It acts as a planetary conveyor belt transferring heat, nutrients, and gases around the oceans.
   1. Cold, salty water sinks in the North Atlantic.
   2. Deep currents flow southward toward the Antarctic and into other basins.
   3. Warm surface currents replace this water from the tropics.
   4. The cycle mixes ocean layers and regulates climate globally.

Disruptions and Climate Change

1. Melting Ice Caps: The melting of polar ice (especially in the Arctic) is adding fresh water to the ocean, reducing salinity and potentially interfering with the sinking process.
2. If the thermohaline circulation weakens or slows, it could have serious consequences for regional and global climates, such as:
   1. Cooling in Europe (a potential drop in temperatures due to the disruption of the Gulf Stream).
   2. Changes in monsoon patterns affecting food production in tropical regions.
   3. Impacts on Marine Ecosystems: Changes in circulation could also impact marine ecosystems by altering nutrient upwelling and distribution.

Global Warming and Shifts in Biomes

1. As global temperatures rise due to global warming, climates are changing, leading to shifts in biomes.
2. One of the most significant patterns observed is that biomes are generally moving poleward (toward the poles) and upward (to higher altitudes) as temperatures increase.
3. This phenomenon is affecting ecosystems worldwide, with major impacts on biodiversity, ecosystem structure, and human activities.

1. Poleward Shifts

1. As average temperatures rise, climatic zones migrate toward the poles.
2. Tropical biomes expand into subtropical areas; temperate forests replace boreal zones.
3. Tundra and polar biomes shrink, losing habitat for cold-adapted species.

2. Altitudinal Shifts

1. On mountains, climate zones rise ~150 m for every 1 °C increase in temperature.
2. Alpine species are forced upward until they run out of habitat.

3. Ecosystem Disruption and Biodiversity Loss

1. Rapid climate change outpaces many species’ ability to adapt or migrate.
2. Results in habitat fragmentation, altered food webs, and increased competition.
3. Spread of tropical diseases (e.g., malaria, dengue) into higher latitudes.

Implications for Humans

1. Agriculture: Drought and shifting rainfall zones threaten traditional crop belts (e.g., U.S. wheat regions becoming unsuitable by 2050).
2. Water resources: Snowpack loss and river flow declines reduce freshwater availability.
3. Economies: Migration of fisheries, loss of arable land, and infrastructure damage from extreme events.