Atmospheric Circulation & Ocean Currents (DP IB Environmental Systems & Societies (ESS))

Tricellular Model of Atmospheric Circulation

Global atmospheric circulation

• Global atmospheric circulation can be described as the worldwide system of winds that move solar heat energy from the equator to the poles to reach a balance in temperature

Wind formation

• Air always moves from areas of higher pressure to lower pressure and this movement of air generates wind

  • Winds are large scale movements of air due to differences in air pressure

  • This pressure difference is because the Sun heats the Earth's surface unevenly

  • Insolation that reaches the Earth's surface is greater at the equator than at the poles

    • This is due to the Earth's curvature and the angle of the Earth's tilt

• This irregular heating of the Earth’s surface creates pressure cells

  • In these pressure cells, hot air rises and cooler air sinks through the process of convection

• Air movement within the cell is roughly circular and moves surplus heat from equatorial regions to other parts of the Earth

• In both hemispheres (the Northern hemisphere and the Southern hemisphere), heat energy transfer occurs where different atmospheric circulation cells meet

  • There are three types of cell

  • Each cell generates different weather patterns

• These are the Hadley, Ferrel and Polar cells

  • Together, these three cells make up the tricellular model of atmospheric circulation:

The tricellular atmospheric wind model

• Each hemisphere has three cells (the Hadley cell, Ferrel cell and Polar cell) that circulate air from the surface, through the atmosphere, and back to the Earth's surface again

• The Hadley cell is the largest cell and extends from the equator to between 30° and 40° north and south

  • Trade winds blow from the tropical regions to the equator and travel in an easterly direction

  • Near the equator, the trade winds meet, and the hot air rises and forms thunderstorms (tropical rainstorms)

  • From the top of these storms, air flows towards higher latitudes, where it becomes cooler and sinks over subtropical regions

  • This brings dry, cloudless air, which is warmed by the Sun as it descends: the climate is warm and dry (hot deserts are usually found here)

• The Ferrel cell is the middle cell, and generally occurs from the edge of the Hadley cell to between 60° and 70° north and south of the equator

  • This is the most complicated cell as it moves in the opposite direction from the Hadley and Polar cells; similar to a cog in a machine

  • Air in this cell joins the sinking air of the Hadley cell and travels at low heights to mid-latitudes where it rises along the border with the cold air of the Polar cell

  • This occurs around the mid-latitudes and accounts for frequent unsettled weather

• The Polar cell is the smallest and weakest of the atmospheric cells. It extends from the edge of the Ferrel cell to the poles at 90° north and south

  • Air in these cells is cold and sinks creating high pressure over the highest latitudes

  • The cold air flows out towards the lower latitudes at the surface, where it is slightly warmed and rises to return at altitude to the poles

Influence on terrestrial biomes

• The tricellular model influences the distribution of precipitation and temperature across latitudes

• Near the equator, rising warm air leads to high rainfall and high temperatures

  • This creates tropical rainforests and savannas

  • Tropical rainforests thrive in regions of high precipitation and warmth within the Hadley cell

• Mid-latitudes experience variable weather due to interactions between warm and cold air masses, resulting in temperate climates with moderate precipitation

  • This creates temperate forests and grasslands

  • These biomes occur in areas within the Ferrel cell, with moderate precipitation and temperatures

• High latitudes, influenced by descending cold air, have low temperatures and limited precipitation

  • This creates polar deserts and tundra

  • These biomes occur due to the cold, dry conditions within the Polar cell

• These climatic factors, in turn, influence the structure and productivity of terrestrial biomes by affecting plant growth, water availability and average temperatures

• The tricellular model therefore helps us to:

  • Understand the global distribution of biomes

  • Understand the ecological characteristics of biomes

  • Predict biome shifts due to climate change and global warming

Ocean Currents

Solar radiation absorption

• Oceans act as vast heat reservoirs

  • This is because they absorb the solar radiation that penetrates their surface layers

  • Solar energy is absorbed primarily in the top layer of the ocean

    • Here, it warms the water and results in thermal energy being stored

Ocean currents and heat distribution

• Ocean currents play an important role in distributing the heat absorbed by the oceans around the world

  • Surface ocean currents, driven by winds and Earth's rotation, transport warm water from the equator towards the poles and cold water from the poles towards the equator

  • These currents redistribute heat horizontally across the ocean surface

    • This movement of heat affects regional climates and weather patterns

Impact on climate and ecosystems

• The redistribution of heat by ocean currents helps regulate global climate

  • This is because it helps to moderate temperature extremes

• Warm ocean currents can bring milder, warmer weather conditions to coastal regions, while cold currents cool down coastal regions

• Oceanic heat transport also affects marine ecosystems

  • They affect patterns of ocean productivity, distributions of marine species and levels of marine biodiversity