Ocean Currents and Gyres

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Podcast Transcript

The surface of the Earth is 70 percent water. 

If you just looked at a map and saw a sea of blue, you might think that the water is just sitting there, but it’s not. 

The oceans are constantly moving, and it isn’t just waves and tides that move. There are enormous rivers of water flowing through the oceans, near the surface, and near the seafloor, which influence the Earth’s climate and its weather patterns. 

Learn more about ocean currents and how they affect the planet on this episode of Everything Everywhere Daily.

All over the oceans of the world, you will find permanent currents of water. These currents move incredible amounts of water, as well as nutrients and heat.

These currents are responsible for weather patterns, dictate what and where ocean life can be found, and how fast ships can travel between points. 

Before I get into the details of how ocean currents affect the world, I need to explain how they work and why they exist. 

Rivers flow due to gravity. Water simply flows from a higher elevation to a lower one. However, all the oceans in the world are at sea level. Ocean currents have to be driven by a different process. 

The process that drives ocean currents is known as thermohaline circulation. 

I readily admit that thermohaline is probably not a word you encounter every day, but it is just a combination of thermo-, meaning heat, and -haline, meaning salt. 

The relevant thing you need to know about the heat aspect of this phenomenon is that cold water is more dense than warm water and, as such, will sink. 

Likewise, the saltier the water is, or the higher the salinity it has, the denser it is, and the more it will sink. In particular, salt water is more dense than fresh water. This is why it is so easy to float in the Dead Sea.

The reason why salt water is denser is simply because salt plus water is more heavy than just water. 

So, what do these facts about heat and salinity have to do with ocean currents?

Thermohaline circulation gets its start in the polar regions, usually the North Atlantic or the Southern Ocean near Antarctica. 

The first thing is that water that flows to the polar regions gets cold, and when it becomes cold, it will sink.

However, there is more to it. When sea ice forms, it expells most of the salt in the water as the ice becomes a solid. The salt that is expelled is pushed into the surrounding water, which increases its salinity.

The very cold, salty water will sink, causing other water to flow in behind it. This is how the process gets started. Cold salty water sinks and begins to flow deep near the bottom of the ocean. 

The flow goes away from the source of the downwell, generally traveling towards the equator. Warm water flows on the surface to the polar region to replace the water sinking down. 

The cold, salty, sinking water is only half of the story. If there is to be a complete cycle, it eventually has to come to the surface. How does that happen if the water is already cold and more saline near the seafloor?

This is known as upwelling. It occurs at the edge of landmasses. Upwelling, like downwelling, has two primary causes. 

The first is simply physical. On a continental shelf, water depths decrease the close you get to shore. As the water becomes shallower, the cold water is literally being pushed up a ramp. 

The second cause is wind. When the wind blows from the land to the sea, or parallel to the shore, it pushes surface water away. Water from below is literally pulled up to replace the water that was pushed away, sort of acting as a type of siphon. 

This cold water, now at the surface, will warm up and start the process anew. These ocean currents serve as a giant conveyor belt. 

I’m really simplifying the upwelling process. The Coriolis effect can be involved, and there are other methods of upwelling available, but the end result is that cold water from the sea floor comes up to the surface. 

I should note that this process of wind on the coast can act in reverse as well. When wind blows from the sea to land, it can result in surface water being pushed down. 

This entire process of water sinking in polar regions, traveling across the bottom of the sea, and then being brought up to repeat the cycle can take centuries. 

So why are these ocean currents important? 

Well, for starters, it is responsible for an enormous amount of life in the sea, and on land for that matter. 

If you remember back to my episode on the element iron, iron is one of the key nutrients in the ocean for phytoplankton. 

Most of the deep ocean is a desert of marine life because there is no iron. Without iron, phytoplankton can’t grow, and without phytoplankton, you don’t have the basis of the food web for the ocean. 

Iron does enter the ocean from rivers and other land runoff, but by far, the largest source is from upwelling. When water is brought up from the bottom of the ocean, it brings with it a host of nutrients, including iron. 

Where you find cold water upwelling to the surface, you will almost always find abundant sea life and productive fisheries. 

The phytoplankton in the ocean is responsible for the production of about half of the oxygen in our atmosphere. 

No ocean currents mean not enough nutrients, which means not enough phytoplankton, which means not enough oxygen. 

Ocean currents play another vital role. It helps distribute heat throughout the planet. Warm water currents can warm up places that would otherwise be colder and cool down places that would otherwise be much hotter. 

Weather systems and the overall climate of the Earth are dependent on these ocean currents. 

So, what are some specific examples of these currents? There are dozens of identified permanent ocean currents that exist around the world. Some are more important than others. 

The two most important downwells are the North Atlantic Deep Water and the Antarctic bottom water. 

The North Atlantic Deep Water forms in the area off the coasts of Northern Canada and Greenland.  

Antarctic bottom water forms all around the continent of Antarctica. This is by far the largest source of high-salinity water in the oceans due to the sheer amount of sea ice that is produced. 

Much of the cold salty water that is created here winds up in some of the lowest points in the ocean. 

You might be asking, what about the Arctic Ocean? There are currents in the Arctic Ocean, but it isn’t as connected to the rest of the world’s oceans as the Southern Ocean is.  There is a narrow opening between Alaska and Russia, a couple of straits through the Canadian archipelago, and one major opening between Norway and Iceland. 

So, it plays a part, but it isn’t as big of an effect as the downwelling found near Antarctica and the North Atlantic.

There are several places on Earth where you can very clearly see the effects that ocean currents have. 

Perhaps the most obvious that I have ever observed has been in South Africa. The city of Durban on the east coast of South Africa has a very warm climate. This is due to the warm Agulhas current, which travels south down the coast of East Africa. 

However, if you go to Cape Town, you will find conditions are much cooler. That is because it has colder waters flowing past it, which came up from Antarctica. This is the Benguela Current, which flows up the coast of West Africa. 

The Agulhas current doesn’t make it to Cape Town as it goes south to Antarctica before it gets there. 

I experienced this effect firsthand when I was camping in the sand dunes of the Namib Desert in Namibia. Despite the fact that we were in the middle of a desert, in topics, temperatures at night and in the morning would often be much cooler than you would expect. The cool waters and the hot air would often result in heavy fog, which we experience almost every morning. 

South America has analogous currents on both of its coasts. On the east coast, there is a warm current that flows south known as the Brazil. On the west coast, there is a cold current known as the Humbolt Current that flows north. 

In the Northern Hemisphere, these are reversed. The west coast of North America has a south-flowing cold current known as the California Current.

On the East coast of North America is the very important Gulf Stream. The Gulf Stream has uniquely warm water due to the fact that water tends not to circulate as much in the Gulf of Mexico and the Caribbean Sea. 

The Gulf Stream goes up the east coast of North America and reaches Western Europe. 

The Gulf Stream is largely responsible for the temperate climate of Europe, which is far warmer than it should be given its latitude. For example, London is at a similar latitude to Calgary, Alberta, even though it has much warmer temperatures. 

The westernmost point of England is the Isles of Scilly, a collection of islands off the coast of Cornwall. Because they are out in the middle of the Gulf Stream, the Scilly Islands actually have palm trees, even though they are in England. 

The Gulf Stream is also why it takes less time to sail from North America to Europe than vice versa. This discrepancy in sailing times was one of the things that tipped off scientists to the existence of ocean currents.

I should note that these major currents which follow the coasts of continents usually do not cross the equator due to the Coriolis effect.

For example, the cold Benguela Current goes up the west coast of Africa and flows west in the Gulf of Guinea, where it warms up considerably and becomes an equatorial current, and then goes back down the coast of Brazil. 

Because they tend to loop around at the equator, there exist ocean gyres. The Earth has five great ocean gyres. 

When the warm Gulf Stream goes up, a colder Canary current goes down the northwest coast of Africa before heading west to the Caribbean north of the Equator. 

The North Atlantic Gyre has at its heart a region known as the Sargasso Sea. The Sargasso Sea gets its name from the large amount of seaweed that can be found in it. 

The other major gyres are the Indian Ocean Gyre, the North Pacific Gyre, the South Atlantic Gyre, and the South Pacific Gyre.

Water in the middle of a gyre tends to be more stagnant, and floating objects, including sailing ships, can become stuck there. 

In the North Pacific Gyre, this has become a major problem. Millions of tons of floating plastic have caused this region to be dubbed the Great Pacific garbage patch. 

Most of the plastic in the garbage patch is quite small, and if you sailed through it you might not even notice more plastic. While the density of plastic is much greater than almost anywhere else, it isn’t as if plastic is so dense you could walk across it.

There is a similar garbage patch in the North Atlantic as well. 

Ocean currents are fundamental to the study of oceanography. The natural forces behind thermohaline circulation (changes in the density of water due to temperate and salinity) drive the currents that shape our world. 

It is a good thing, too, because without ocean currents, the Earth would be a very different place.