47: Ocean Stratification

The ocean isn’t just one giant uniform puddle of water on the earth’s surface. Within it’s waters there are lots of different of layers, which have, and have the potential to have big impacts on us and our planet. Also they can contain briny pools of toxic death in the middle of the ocean floor, with life surrounding their ‘shore’ like on land. This week we will be looking at ocean currents and the layers within the ocean, what causes them, what they do, and what these odd little lakes of death are. Enjoy!

The separation of salt and freshwater from ice formation. (Image: Oceanus Magazine)

Ocean currents

The oceans have two different kinds of currents. The first is the surface currents, the kinds that carry boats along with them. These currents are created by a combination of prevalent wind patterns and the Coriolis Effect, the movement of water in relation to the rotation of the earth. These surface currents are really important, such as the well-known Gulf Stream current bringing warm water to northern Europe meaning that places like the very north of the UK have a completely different warmer climate to southern Alaska despite being at the same latitude. These currents also have historically allowed sailing boats to move along with the moving water in the absence of wind, allowing trade routes such as the Atlantic Trade Triangle which brought good from the UK to West Africa, then slaves from West Africa to the US and finally tobacco and cotton from slave plantations in the US back to the UK for sale. Historically significant though of course horrific in impact…

There is however another type of ocean current occurring deeper down in the water known as Thermohaline circulation. This current is driven by varying water density because of differences in temperature (thermo) and saltiness (haline). This is a much slower current than those on the surface. To illustrate how the layers form, let’s first focus on the Atlantic Ocean.

In the north of the Atlantic Ocean, the surface of the water freezes into ice each year. This means the freshwater is extracted and becomes solid, but the salt in it is left behind, making the remaining water more salty and so denser. This causes this water to sink. It is also cold water, and as with air heat rises so cool water sinks to the bottom. The same thing happens in the Antarctic but here the water is even cooler and saltier due to the extent of the ice, so this water sinks even further below. So far then we have cold water at the Antarctic flowing down from the surface and along the bottom of the ocean floor quite a long way across the planet’s, beyond the equator north. The same is happening with the north Atlantic waters flowing south though this is on top of the Antarctic waters, making a new layer on top of it. Above this is a section known as intermediate water. It’s not too hot or too cold, and is a middling amount of salty. Finally on top of that towards the north are currents like the Gulf Stream or the Mediterranean outflow, these being warm currents of water which have been heated up by the sun. Due to their heat these are at the top of the water column. I’ve said all this in many words, but this diagram explains it quite well.

Layers of water in the Atlantic Ocean from South to North, with cold waters from the Antarctic and North Atlantic on the bottom, and warmer waters such as the Mediterranean outflow and Gulf Stream on top.

Mixing and brine pools

These massive slow currents of stratified, separated water actually play an important role in mixing, bringing oxygen and atmospheric gases into the deepest parts of the ocean, and creating mixing of entire oceans on a more north-south plane over a period of thousands of years.

As an aside, it’s not all smoothly mixed. You can get what are called brine pools in the ocean, where water sometimes 5 to 8 times saltier than the rest of the ocean congregates and sink to the ocean floor, forming almost like a really salty lake underwater. These pools can be so dense that fish can float on their surface and submarines have been able to ‘land’ on them, having to increase their thrust to break through the salty surface. As they are so salty there is very little oxygen contained in these pools, and they can also contain toxic chemicals like hydrogen sulphide meaning they can kill any sea creatures trying to enter them. Creatures that do fall into them are preserved by the salty water- a bit like pickling something.

An image from Planet Earth 2 showing an eel diving into a brine pool deep underwater, with molluscs living along the shore of this toxic lake. (Picture: BBC)

Increased meltwater

The whole of the thermohaline circulation depends on one crucial thing, ice. It is both the cold temperature of ice in melting water as well as the separation of freshwater from salty brine left behind, that creates the difference in water density. As the planet continues to warm, more ice will melt diluting the waters nearby. This is expected to create more layers within the water column, increasing ocean stratification as well as slowing down the thermohaline circulation. The ocean itself is expected to be warmer in general too, meaning less will be sinking to the deepest depths and so there will be less mixing of water as the already cold and salty stuff will occupy the ocean floor like a milder brine pool. This pushes the now warmer water to flow along the surface all the way to the Antarctic heating up the ice there and increasing melting, amplifying the whole process.

What does all this mean? Less ice and more water leading to sea level rise is one. Reduced mixing of gases in the water will mean that the surface will have more CO2 making the ocean more acidic whilst deep oceans will have less oxygen meaning life like whales and many fish may not be able to survive. Finally the changes in these deep currents might start to affect surface water currents such as the Gulf Stream. If, for example it stops the Gulf Stream this would have major implications on the climate of Europe which could be plunged into a cold period.

Map showing the thermohaline circulation, with blue lines being cold water and red lines warm water. The red current flowing from the Caribbean to northern Europe is the Gulf Stream. (Image: NASA/Public Domain)

All these potential implications aren’t firmly understood yet as changes like these that have happened in the Earth’s past have happened deep in its history. However as we learn more about the way the ocean works, it is becoming clear that ocean circulation matters hugely, and the importance of preserving it shouldn’t be taken lightly. On a lighter note, it’s also pretty interesting to know you can get lakes within ocean, even if they are brine pools of toxic doom…

For more info:

Scientific paper on the consequences of increased meltwater on ocean circulation: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2015JC011156

Clip from Blue Planet 2 about the brine pools:

Music: kongano.com

Cover photo: a stream of freshwater that has frozen within the surrounding saltwater, known as a brinicle.

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