- Education and Science
About the oceans, deep ocean circulation
This article contains information of the chain of events that must take place in order for the deep ocean circulation, as we know it today, to occur and the driving forces behind it. You will get a glimpse of what is happening deep down in the oceans over and over again without we even notice it. And this constant move of water masses is of major importance for all living on Earth!
My soul is full of longing
for the secrets of the sea,
And the heart of the great ocean
Sends a thrilling pulse through me.
Henry Wadsworth Longfellow
One way to look at our planet: The Earth!
When I think about the planet Earth I think about a whole. A total and fantastic whole where everything is in order and everything is present because of a complex chain of events and development. Nothing on Earth can exist without the other and we are all dependent on the fact that all these events actually take place. If one part in this fragile chain is broken it will affect other parts with severe force. Maybe not visible at first, since nature on our planet has a fantastic ability to adapt to new circumstances, but eventually it will effect the other parts in the chain. The effect on other part can go slowly, in a human perspective, but yet firmly and often be irreparable.
Besides thinking about our planet as a whole I think about it as a living creature. This may sound very strange but let me explain!
I have worked in health care and gain knowledge about the fantastic human body where everything is dependent on the other body functions in order to have a healthy life. You need most of your organs to stay and feel healthy and most of all; you need your vital functions such as the blood circulations, the lungs that gives you oxygen and you need your heart to pump blood to all parts of your body. If something in these vital functions is damaged or disturbed the body will not function.
During my studies in Earth sciences and marine geology I found that much of what I learned about the Earth and the contexts have similarities with the human body but at a much bigger scale. This is what I mean by saying I think about planet Earth as a living creature. We should all think about Earth in this way and not as something static that will always stay the same no matter how we damage the planet or use the resources.
Now, after this rather philosophical introduction I think it is time to move on to the deep ocean circulation. And if you don’t look upon Earth the way I do by now, as something living and fragile, I can help you by telling you that, to me, the deep ocean circulation is as the human circulatory system! And to understand the system is a beautiful journey!
The deep ocean circulation affects, as you might have guess, the whole planet. It is a vital part in the water circulation between the atmosphere and the oceans, the climate on Earth as well as transporting water and heat. The deep ocean circulation transport heat from the equatorial regions to the poles and is also very important when it comes to even out the temperature differences over the globe. Without the deep ocean circulation many places at Earth would be impossible for man to live in. The equatorial regions would be so hot that nothing could exist there and the Polar Regions would be even colder than today. How it would affect the climate in other parts of the Earth is mostly theories because of the complexity of the system but it would certainly be a world that we wouldn’t recognise at all.
The deep ocean circulation is also vital in order to understand what goes on in the oceans and why the oceans are the way they are today. By knowing the driving force behind the circulation we also gain an understanding of how important this present circulation is to our present climate and we also get a glimpse of how fragile the whole system is. Deep ocean circulation is the study field for an oceanographer as well as a marine geologist since the study field for a marine geologist is much influenced by seawater and currents that have a major effect on the ocean floor.
Lets start with the driving force behind the deep ocean circulation!
The Earth doesn’t have a heart pumping like the human body but still the deep ocean circulation is steadily going on over and over again, transporting water in a pretty steady pattern around the Earth in our present time.
The force behind this deep ocean circulation is to simplify it: density, which depends on salinity and temperature. The wind is important when it comes to water movement in the oceans but the wind mostly concerns the surface water. At the bottom of the oceans the currents flows for other reasons. We will look into this in a moment.
Even though the water in the oceans looks to be uniform, in fact they are not! In the oceans one can distinguish different water masses due to their density and the water depth of their path. These small changes make the water masses to “behave” differently and are important in the deep ocean circulation. The biggest water masses are:
Deep bottom water masses, surface water and the water masses in between that are called the intermediate water masses.
Dens and cold surface water sinks, fills the ocean basins, and is replaced by water moving towards the poles, which rises from intermediate depths. These water masses are different from surface water because once they are formed they spread widely and displace the surrounding water.
First, a little about the wind!
I can´t skip the wind totally in this hub even though the wind doesn’t affect the deep ocean waters. But even so, the wind is of importance as a participant in the Global movement of the water in the oceans and can´t be neglected. The wind is the force behind the surface currents and the wind creates the surface currents in the oceans by frictional drag on the water surface. But the wind only affects the surface and the water layers just underneath the surface. For the wind to affect deeper in the oceans it takes a very strong and persistent wind. How deep the wind can influence the ocean water is much dependent on the water column itself. If the water column consists of water masses with highly different density the influence is weaker and the opposite can occur in water masses with more uniform water columns. (I will soon explain about the density of water masses so just keep this in mind for now!) You can also read more about the surface waves created by wind here!
It may look as if the density of water in the oceans is the same but it isn’t! The density of the ocean water is nothing but uniform and varies between 1.02 to 1.07 g/cm3.
This might sound like small variations but the difference have a major importance on the enormous water masses in the oceans.
The density of seawater depends on several things like temperature, salinity and pressure and in general one can say that density increases as salinity and pressure increase and temperature decreases.
Remember the parable with the human circulation system, human blood looks like it contains of one single object but it doesn’t! And the same goes for common seawater; it can be very different although it looks the same!
Surface temperatures range from freezing point in the Polar Regions at high latitudes in the winter, to more than 28° C in low latitudes. Due to the location of continents on Earth the mean annual temperature in the oceans in the Northern hemisphere is 16°C and 19°C in the Southern hemisphere. Water has the highest density at about 3,98 ° C.
The surface water salinity differs between 33 ‰ to 37‰ with average salinity being about 35 ‰. The ocean with the highest salinity is the Atlantic Ocean with average salinity at about 35,37 ‰ due to inflow of high salinity water from the Mediterranean Sea. Evaporation, rain- and snowfall and ice melting affect salinity in surface water. In regions with warmer climate the surface seawater has a higher salinity than in the Polar Regions due to increased evaporation and decreases rainfall.
The highest salinity of the surface water in the oceans is located to gyres in the middle latitudes where evaporation exceeds rainfall. The lowest salinity in the surface waters of the oceans can be found along the coasts particularly near the mouths of large rivers that transport large amounts of freshwater into the sea.
So now we know:
The most dense water masses can be found as deep bottom currents in the ocean with cold water with high salinity and high pressure.
The lightest water masses can be found as warm surface current near continents with big contribution of freshwater.
This is a somewhat simplified explanation since there are always exceptions due to other parameters but let us keep it at this level for now.
Why is this interesting?
Because the differences of salinity and temperatures in water masses and currents can define the distinct water masses and ocean currents! By measuring these parameters the scientist could describe and also name the different deep water currents and trace their origin and their movement across the oceans, even if we can´t see them in reality!
Here is a video that shows an overview of the deep ocean circulation so you get a picture over the scale of this circulation. It is amazing and also beautiful!
The different water masses in the oceans!
AABW Antarctic Bottom Waters
AADW - Antarctic Deep Water
AAIW - Antarctic Intermediate Water
CoW - Common Water
ECW - Equatorial Central Water
MIW - Mediterranean Intermediate Water
NABW - North Atlantic Bottom Water
NACW - North Atlantic Central Water
NADW - North Atlantic Deep Water
NPCW - North Pacific Central Water
NPIW - North Pacific Intermediate Water
PSW - Pacific Subarctic Water
RSIW - Red Sea Intermediate Water
SACW - South Atlantic Central Water
SICW - South Indian Central Water
SPCW - South Pacific Central Water
Now lets move on to:The deep circulation!
This is where we where heading initially! The deep ocean circulation is also called the Thermohaline circulation where “Thermal” stands for temperature and “haline” stand for salinity. The deep bottom currents in the oceans have, as we said before, high density and low temperature. But they need to be at the surface somewhere on the planet, otherwise it wouldn’t be any circulation! And there are two places on Earth where the major part of the dense and cold water masses can occur at the water surface: at the Polar regions!
Other places on Earth where minor deep bottom currents are created are in the enclosed basins like the Mediterranean Sea and in the Red Sea.
But the greatest bottom currents are the AABW and the NABW!
In the Arctic Ocean, in the Norwegian and the Greenland seas, the NABW (North Atlantic Bottom Water) is formed and at the South Pole is the AABW (Antarctic Bottom Water) formed. The reason these water masses is so dense is of course due to the cold climate that creates the low water temperatures. The high salinity of the water masses in this regions is due to the fact that icebergs is formed by freshwater but only with a smaller amount of sea salt which leaves the salt in the water.
The AABW influences vast areas of the world oceans
All water in the Indian and Pacific Oceans with temperatures less than 3° C is AABW!
All water in the Atlantic Ocean with temperature less than 2° C is AABW. (except in the Arctic and Greenland and Norwegian seas.
The AABW and the AADW
The AABW is formed by the formation of sea ice around Antarctica that leaves a cold and dens water that sinks to the bottom of the ocean as a bottom current.
The formation of AADW is as follows:
It begins with dense and cold surface water that because of its density starts to sink downwards in the water column. There the sinking cold water mixes with the “warmer” water with high salinity in underlying water (Intermediate water) which result in an even denser water that then sinks to the ocean floor. The bottom water from The Wedell Sea and the Ross Sea flows north, transporting oxygenated and cold water to a big part of the Pacific, the Atlantic and the Indian oceans.
NADW is generated in such a high rate over such a large area of the ocean that it is the most common water in the North Atlantic.
The NADW has a temperature of 2 °C and a salinity of about 34.9 ‰
The NABW and NADW
The formation of bottom waters in the Northern Hemisphere is formed in the Arctic Ocean and the Norwegian and the Greenland seas. The NABW is the water mass with highest density and sinks to the bottom. The formation of NADW is made from North Atlantic central waters that flow north into the Norwegian and Greenland seas. The North Atlantic central water enters the Polar Region as a relatively warm and salty water but cools off. Sea ice is formed and the now cold and salt water mixes with cold Arctic water and sinks. The NADW then begins its journey and flows south between island and Great Britain into the North Atlantic.
In the Atlantic Ocean there are three main subdivisions of NADW:
The upper part, derived from the Mediterranean Sea, the middle part from the areas near Greenland and the lower part from the Arctic basin.
This is in a way the “pump” that sets the circulation in motion because without the formation of dense water in the Polar Regions there wouldn’t be any deep ocean circulation in the way we know it.
The Global Conveyor Belt
Because of its density it sinks to the bottom and because of the new formation of dense water the NABW and the AABW are strong currents at the bottom of the oceans. Salinity and temperature exhibit little change once the water has sunk to the ocean floor. Deep-water masses don’t behave like surface water. Once they are formed, they sort of seek their respective layer in the ocean where they fit according to density of the other different water layers and then they spread widely and displace the surrounding water. By time, they mixes with other water masses and get some new properties as they go along. When density decreases due to the mix with less dens water the former bottom water will change depth and find a new level according to the surrounding water.
The deep ocean circulation is due to many small parameters that all have to be just right for the ocean circulation, as we know it, to occur. Small changes in temperature and salinity of the vast water masses in the ocean can cause very severe climate change if the fragile equilibrium is disturbed. Even though we know a great deal about the deep ocean circulation there is so much more to know and learn about these systems and about the oceans. One can’t help being humble and amazed over the beauty and the magnificent system in the oceans of the World!
Look at the video below and see for your self!