Full disclosure, I was initially planning on writing about how coastal deserts are possible after last month's page on deserts got me interested in diving a bit deeper into that ecosystem.  Turns out that understanding how they exist is tied up in one of the Earth's largest environmental systems.  As can be the case with large systems, this one operates as a sort of harmonization of a number of smaller (but still quite big) pieces unique to the Earth's characteristics and a bit due to its current form.

That system, and the topic of this Eco Extra, is ocean currents.  I hope you're ready for a crash course in thermo-, hydro-, and aerodynamics because this one is a doozy.  Don't worry too much though, we'll be doing our best to keep the concept high-level while still building a solid understanding of how and why this works.

Let's dive in!

Wind, Water, Rotation

Before we jump into the systems supported by ocean current, we should take a big picture look at how the Earth's ocean currents are influenced.  To understand that, we have to understand how matter moves.  Over the course of this topic we'll return to a few of these, but matter (like air and water) likes to move from area to area based on certain criteria.  The biggest of these for ocean currents are heat, density, and salinity (the saltiness of a solution).
When it comes to heat, matter will move from areas of higher heat to areas of lower heat.  When you add ice cubes to a warm drink, it might be easy to think of the ice cubes making the drink cooler, but what is actually happening at an energy level is the heat moving to the ice cube, causing it to melt and spread cooler water throughout the drink.  This is why the drink also tastes more diluted as the the ice melts.  There's just more plain old water in the drink now.

This principle applies to all things, not just your unreasonably warm soda.  For ocean water, the area around the Earth's equator is the warmest as it is receives the sun's energy more directly than the rest of the planet's water.  This warmer water naturally moves toward cooler water, toward the poles of the Earth.  This water also wants to take the most direct path toward the poles.  After all, water isn't a thinking organism.
The Coriolis Effect caused by the Earth's rotation is how currents inherit their circular motion
But it doesn't travel in a straight line to the north and south poles.  Why is what?

The reason is the Earth's rotation on its axis.  As the Earth spins, this causes the path of wind and water to change its course to a more generally circular pattern moving away from the equator.  In the northern hemisphere currents travel clockwise, while the southern hemisphere inherits a counterclockwise movement.  This phenomenon is known as the Coriolis Effect and is a fundamental piece to understanding the Earth's wind and water systems.
Ocean current move clockwise in the northern hemisphere and counter-clockwise in the southern hemisphere

Gyres: The Ocean's Stream Systems

Okay, so the rules of heat energy transfer coupled with the Earth's rotation make our ocean currents move in circular patterns away from the equator.  At least, that'd be the end of the story if not for a third of the planet being a collection of giant landmasses.  Our planet's continents play a role in how the currents are directed by way of their coastlines.  Ocean currents can't just jump onto land, so they follow the curves of the land instead.

Based on the current formation of the Earth's land, there are 5 major currents across the oceans.  These can be found in the north Pacific ocean, south Pacific ocean, north Atlantic ocean, south Atlantic ocean, and the Indian ocean.  These currents form massive circular streams covering the entirety of the planet's ocean called gyres, moving warm water to the poles and cool water to the equator.
The gyre we hear a lot about around North America and Europe is the one in the north Atlantic, also known as the Gulf Stream System.  It's thanks to this circular current that warm surface water travel up to the western coast of Europe, warming the atmosphere creating a mild climate for a majority of the year.  Because of this, places like the United Kingdom, despite being at the same latitude range (distance from the equator) as Canada, don't have consistently harsh wintry conditions.

But wait, hold on a second.  We said earlier that heat dictates how energy and associated matter move.  We also said in these gyres, warm water moves toward the poles and cold water moves toward the equator.  How is that happening since the water is colder than where it is moving toward?

Glad you asked!

The Global Conveyor Belt

If you recall, we mentioned that heat is one of the factors that influence how the currents move, not the only one.  The density and salinity of water also play a major role in how water moves.  And while these pockets of circular motion do exist thanks to our current continental formation, this water is still doing its best to make it to the north and south poles of the Earth.  What happens when water reaches its destination?

When warm water finally reaches a pole, it has cooled a significant amount.  When the water approaches freezing, this is when sea ice begins to form.  What's interested about sea ice formation is that the salt in the water (remember this is salty ocean water) doesn't get encased in the ice.  It's left in the water that didn't freeze.  When this happens, the water becomes denser, sinking into the deep sea.
Western Europe is able to enjoy mild climates at high latitudes all thanks to the north Atlantic gyre, also known as the Gulf Stream System
This downward current also causes the warmer surface water to be pulled strongly toward the sea ice as the surface temperature decreases.  As the heat leaves the water, it is pulled under and undergoes the same transformation into the underwater sea ice.  This builds up massive amounts of sea ice over time, with underwater temperature plunging as this happens, causing more and more ice to form.  This is why icebergs have much more mass underwater than the warmer surface water.

Once the water reaches depth due to its density that it is warmer than the deep sea, it starts to move toward the equator.  This is what completes the cycle.  The cooler deep ocean current travels in the opposite direction as the warm surface water toward the equator.  Fun fact, if the water started from the north pole, it usually doesn't make it to the surface again when it passes the equator and continues toward the south pole.  Water on its way northward toward the equator warms up enough in the Indian ocean and north Pacific ocean gyres to rejoin the surface water circulation and move about the globe.
The Global Conveyor Belt moves water all around the world.

​It takes around 1,000 years for one cubic meter of water to finish one lap!
This is the global conveyor belt.  It's responsible for our functioning water and carbon cycles and influences the local climate of many a coastline around the world (including the coastal desert I initially thought I was going to write about!).  It's a massive, complex piece of environmental machinery and a ton of what this planet provides for us in its current form is thanks to it humming along.​

A Large But Fragile System

However, bigness doesn't necessarily translate to resilience.  As with a number of the systems the Earth provides us, it relies on a careful balance fine tuned over eons of transformation.  Going back to the Gulf Stream System for a bit, the reason it comes up so much is that it's noticeably slowing down.  Europe is experiencing cooler climates now than it ever has because the warm surfaces waters aren't as steadily making it to their shores.

That would seem to indicate there's something wrong with the north Atlantic gyre, but it's a bit more complicated than that.  Due to climbing global temperatures, ice melt has been at a record high near the arctic pole.  Without the sea ice pushing the warm surface water to a depth it can travel back toward the equator, the warm surface water has been clogging near the pole, slowing the entire system.
So what we're dealing with isn't just Europe having colder climates as we'd expect at their latitude.  We're seeing the beginning of what an Earth without the global conveyor belt would look like.  Given we're only seeing it from a local climate perspective.  There will be extensive resonating effects caused by a breakdown of how the Earth's water and carbon cycles operate on the surface and depths of its ocean.

That sounds rather alarmist, and to be quite frank, it most certainly is.  The climate crisis is one of immeasurable unknown that we're trying to piece together as we influence our planet.  We mentioned before that humanity doesn't control the Earth's systems.  Despite what we know, it's only a fraction of the big picture and all we really manage to do is influence it and manage the resulting rippling effect.

Understanding our planet's systems is a key part of realizing just where humanity is in its climate trials.  Knowing the benefits we receive from the gifts our home gives us and that they cannot be rebuilt by our own hands should encourage us to protect what's there and more intentionally do what we can to make our home continue to support us.  On top of being incredibly important, these things are just simply fascinating.  Who wouldn't want to keep living here to unlock even more of the wonder our planet holds?  So keep on learning, keep on caring, and advocate for a planet we can live and learn from for eons to come.
~ And, as always, don't forget to keep wondering ~
Sources
Misachi, J. (2021). How Do Ocean Currents Affect Climate?. Retrieved 14 March 2021, from https://www.worldatlas.com/articles/how-do-ocean-currents-affect-climate.html

​Ocean currents. (2021). Retrieved 14 March 2021, from https://www.noaa.gov/education/resource-collections/ocean-coasts/ocean-currents

Currents: NOAA's National Ocean Service Education. (2021). Retrieved 14 March 2021, from https://oceanservice.noaa.gov/education/tutorial_currents/
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