Climate as a Global System (Edexcel GCSE Geography B)

Global Atmospheric Circulation

• The global atmospheric circulation can be described as a worldwide system of winds moving solar heat energy FROM the equator TO the poles to reach a balance in temperature

Wind formation

• Air always moves from high 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 due to the Earth's curvature and angle of the Earth's tilt

• Hot air rises and cooler air sinks through the process of convection

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

• Each cell generates different weather patterns

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

• In both hemispheres, heat energy transfer occurs where 3 atmospheric circulation cells meet

• These are the Hadley, Ferrel and Polar cells and are shown via the tri-cellular model:

The tri-cellular atmospheric wind model

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

• 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)

• 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 (particularly in the UK)

• 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

Coriolis effect

• Each cell has prevailing winds associated with it 

• These winds are influenced by the Coriolis effect

• The Coriolis effect is the appearance that global winds, and ocean currents curve as they move

• The curve is due to the Earth's rotation on its axis, and this forces the winds to actually blow diagonally

• The Coriolis effect influences wind direction around the world in this way:

  • In the northern hemisphere it curves winds to the right

  • In the southern hemisphere it curves them left

• The exception is when there is a low-pressure system:

  • In these systems, the winds flow in reverse (anti clockwise in the northern hemisphere and clockwise in the southern hemisphere)

Global wind belts - surface winds

• The combination of pressure cells, the Coriolis effect and the 3-cells produce wind belts in each hemisphere:

  • The trade winds: Blow from the subtropical high-pressure belts (30 degrees N and S) towards the Equator's low-pressure zones and are deflected by the Coriolis force

  • The westerlies: Blow from the sub-tropical high-pressure belts to the mid-latitude low areas, but again, are deflected by the Coriolis force

  • The easterlies: Polar easterlies meet the westerlies at 60 degrees S

• Global atmospheric circulation affects the Earth's climate

• It causes some areas to have certain types of weather more frequently than other areas:

  • The UK has a lot of low-pressure weather systems that are blown in from the Atlantic Ocean on south-westerly winds, bringing wet and windy weather

Ocean conveyor belt

• Ocean currents redistribute heat energy around the globe

• Currents (warm or cold) act like 'rivers' of water in the sea 

• Cold currents move towards the equator and warm currents towards the poles

• Each ocean has its own pattern of current:

  • E.g. the warm Atlantic Ocean waters of the low latitudes are moved to high latitudes via the North Atlantic Drift

• All ocean currents are triggered by the prevailing surface winds of the global atmospheric circulation, with the Coriolis effect pushing the water direction

• Circulation is through convection currents driven by cold water freezing into ice at the poles

• The polar cold waters are denser, saltier sea water which sinks to the ocean floor

• Water then flows in behind it at the surface, which forms a current

• The deep ocean currents then flow towards Antarctica along the western Atlantic basin, then splits off into the Indian and Pacific Oceans where the water begins to warm up

• The warming makes the water less dense so it loops back up to the surface in the South and North Atlantic Ocean

• The warmed surface waters continue to flow around the globe and eventually return to the North Atlantic and the cycle begins again

• This movement of water is known as the thermohaline circulation and drives the ocean conveyor belt

High & Low Pressure Areas

Pressure differences

• Air moves in the atmosphere either towards the ground (subsidence) or up into the atmosphere (convection)

• These movements influence air pressure and rainfall

• The sea and land heat up differently

  • Sea:

    • Forms high pressure in summer and low pressure in winter

    • Takes longer to heat and cool

    • Air is denser and cooler in summer but warmer in winter

  • Land:

    • Generally forms areas of lower pressure in summer and higher in winter

    • Heats quickly in summer and the air is lighter and rises

    • Cools quickly in winter

Table Showing Influence of Air Movement on Weather Conditions

• A broad pattern of latitudinal high and low pressure belts are created via the horizontal bands of the Hadley, Ferrel and Polar cells

• However, the distribution of land and sea, affects the location of these pressure zones and so the pattern is not symmetrical in each hemisphere, despite the mirroring of the cells