The United States, arriving in the North Atlantic

The movement of air and water across the planet is the driving force of climate and weather patterns.

There are many factors that affect ocean and atmospheric circulation, and here we will explore what they are and how they function. At the end, you can test your knowledge with a brief quiz.

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A Fluid Planet

The Earth’s surface is covered with fluids, namely air and water.

These fluids are separated by density, with the dense, cold waters making up the bottom, and the thinnest, lightest air masses making up the top of the atmosphere, where air meets space. The movement of air and water across the planet is known as circulation and has many different variables driving it.

The Coriolis Effect

Because Earth is a spinning ball, the gases that make up the atmosphere and the water that makes up the oceans are forced into motion. As Earth rotates to the east, the air sitting on top of it is pulled along, but more slowly than the planet itself.This means that air seems to be moving westward along the Equator and then changes direction in different latitudes to the north and the south. This deflection of air due to Earth’s rotation is called the Coriolis effect. The Coriolis effect also applies to ocean surface currents, which deflect in the same manner along the Equator, creating surface currents moving westward.

Rising and Sinking

Air and water do not just move laterally. Because of differences in temperature among the fluids, air and water masses also rise and sink vertically. If you have ever used a convection oven, you have witnessed the process of hot air rising and cool air sinking. This happens in Earth’s atmosphere and in the oceans: warm fluids rise; cool fluids sink. Vertical air movement combined with lateral movement makes up cells of circulating air, known for bringing consistent wind patterns in various parts of the world.

This rising and sinking is not just about the temperature – the composition of the fluids also affects their densities. Sea water salinity is a measure of the amount of dissolved particles, or the ‘saltiness’, of the water. The higher the salinity, the denser the water becomes. So, the densest water on Earth is very cold and very salty.

Water moves vertically in a process called thermohaline circulation, a system of currents driven by changes in temperature (‘thermo’) and salinity (‘haline’).When surface water is at the Equator, some of it evaporates, leaving saltier water behind, which changes the water’s density. When warm currents move towards the poles, they cool off, also changing the density…in this case, causing those currents to sink.

This also involves the release of latent heat, the heat given off as a water mass cools, which has a big impact on the climate of whatever land mass or region the body of water is near. For example, Great Britain has a relatively temperate climate given how far north it is. This is thanks to the warm Gulf Stream current, which runs along the East Coast of the United States, arriving in the North Atlantic where it cools and sinks, giving off the latent heat that keeps Northwestern Europe from being much, much colder. The changes in thermohaline circulation throughout Earth’s history have marked extreme climate changes, often happening very quickly.

Land

If Earth’s surface was covered exclusively with water, no land masses, the surface currents would flow in very consistent directions with nothing to get in their way. But Earth has many land masses: continents.

This means that surface currents are also deflected when they run into land. Atmospheric circulation is also influenced by land masses. As we’ve already seen, warm air rises and cool air sinks, just like that convection oven. Because they differ in conductivity, land tends to reflect heat, whereas water tends to absorb heat. Similar to how sea water rises and sinks with changes in temperature and salinity, air masses will rise and sink with changes in temperature caused by the presence of land or water. Glider pilots are experts at analyzing these changes, riding the updrafts over warm land and downdrafts over cool water.

Lesson Summary

Oceanic and atmospheric circulation drives climate and weather patterns across the globe. The lateral and vertical movement of air and water can partly be attributed to the Coriolis effect, creating air cells thanks to Earth’s constant spinning. It is also controlled by differences in temperature and composition, affecting the densities of these fluids. The vertical movement of ocean currents, known as thermohaline circulation, transports latent heat around the surface and largely determines regional climates.

The presence of land influences how air rises and sinks in the atmosphere, as well as how ocean currents deflect and change direction. Circulation patterns are definitely complex and a critical part of weather and climate patterns, as well as sea water movement!

Learning Outcomes

After you are done, you should be able to:

  • Explain what causes oceanic and atmospheric circulation
  • State the Coriolis effect
  • Explain the importance of thermohaline circulation and what causes it
  • Discuss how global heat transfer is affected by land masses
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