Ocean Circulation & Stratification

On the previous page, you learned about the different layers of the ocean: the surface ocean, the deep ocean, and seafloor sediments. Here, we’ll elaborate on these layers, specifically the major ocean currents and how they operate in the surface and deep ocean.

Main Points

  • Surface currents, namely western boundary currents, are important currents that bring heat and moisture from the equator to higher latitudes. They have the ability to affect weather and long-term climate patterns
  • Deep ocean currents are driven by density differences among water masses
  • Deep ocean water is formed in the Atlantic Ocean in three different locations from cold, salty waters
  • These sinking water masses help mix the entire ocean and take gases from the atmosphere to the deep ocean
  • Increased warming of Earth is warming the ocean, which leading to a freshwater lens in the northern and southern Atlantic Ocean
  • This is inhibiting the formation of deep waters, and leading to increased stratification of our oceans
  • Stratification slows and can inhibit ocean mixing, leading to decreased oxygen levels in the deep ocean, warmer surface waters, and intensification of ocean acidification

Surface Ocean Circulation

Recall from the Modern Atmosphere page that there are two main factors that drive surface currents in our oceans: 1) differential heating between the equator and the poles, which leads to the wind patterns, and 2) the Coriolis Effect (the invisible force that deflects objects as they move over the surface of the Earth). In addition to these two forces, gravity also plays a role in the development of surface currents. More specifically, surface ocean currents are currents that move water in the upper layer (few hundred meters) of the water column, and can have localized affects on weather and climate, which collectively can have a global impact on climate (the long-term weather patterns).

Major surface currents of the world’s oceans. Notice that gyre systems operate in every ocean basin. Red lines indicate warm-water currents, and blue lines indicate cool-water currents. Image from ‘Physical Geology‘ by Steven Earle used under a CC-BY 4.0 international license.

Some of the major features of the surface ocean currents are the gyres systems. Gyres, such as the North Atlantic and North Pacific, are very important ocean features, as they carry warm water from the equator towards the poles into cooler areas.

Specifically, the western ‘limb’ of the gyre systems, called western boundary currents, transfer warm water to higher latitudes, which vents heat and moisture to the lower atmosphere. The moisture brought further north and south from the equator by western boundary currents is what provides rain for areas along the coasts. Some examples of western boundary currents are the Gulf Stream in the Northern Hemisphere that flows along the east coast of North America, and the East Australian Current in the Southern Hemisphere that flows along the east coast of Australia. These currents are fast-flowing, occur to greater depths in the water column, and tend to be very narrow. In the figure at left, western boundary currents are colored red.

A lamb poses majestically against a sky of low-hanging clouds and fog on a rainy day in western Ireland.

As an example of a western boundary current carrying moisture north, think about Ireland’s weather. The country is located at rather high latitudes (about 53° N), but does not receive much snow (for reference, New York is at 40° N), but it does rain a lot. This is due to the Gulf Stream and Norwegian currents that bring warm water north from the equator, thus providing the moisture that is rained out over Ireland.

The opposite limb of the gyres are eastern boundary currents, which are slower, broader, and wider than western boundary currents. Eastern boundary currents generally carry colder water from higher latitudes back towards the equator.

Another important current of our oceans is the Antarctic Circumpolar Current, or the ACC, which flows clockwise around Antarctica in the Southern Ocean. The ACC flows almost totally uninhibited around Antarctica (there is no land mass blocking its path), and thus is a very strong current. In fact, it is the largest current in the world with regards to the amount of water it transports (100-150 million cubic meters of water per second!!). Therefore, the ACC blocks warm western boundary currents traveling south from the equator, which is one reasons why Antarctica has maintained its large ice sheets.

Deep Ocean Circulation

Deep ocean currents, which collectively are referred to as thermohaline circulation, are much different than surface ocean currents. The term ‘thermohaline’ refers to density differences in temperature (thermo) and salinity (haline) in different bodies of water (often called water masses). Unlike surface currents, which are driven by gravity, winds, and the Coriolis Effect, thermohaline circulation is driven by differences in density. These currents are slow and occur deep within the water column.

A cross section from the north Atlantic Ocean (on the right) to the south Atlantic Ocean (on the left). Here, there are four water masses represented. The Gulf Stream current is a surface current, specifically a western boundary current. Image from ‘Physical Geology‘ by Steven Earle used under a CC-BY 4.0 international license.

The Earth’s thermohaline circulation system generally affects the entire ocean, and is important in transporting water and heat from the surface to the deep ocean and back again. It is often thought of as a conveyor belt by geoscientists for this reason. In the cross section at left, there are four main water masses illustrated. Notice that the one at the top of the water column, colored red and labeled ‘Gulf Stream’, represents the surface current (western boundary current).  This diagram is looking at the Atlantic Ocean from the Arctic (on the right) to Antarctica (on the left).

Deep water is firstly formed in the northern Atlantic Ocean (see figure at right). The water here becomes denser than surrounding water because of brine rejection. Brine rejection occurs when seawater freezes but leaves behind the salt. Thus, the water around the ice becomes more dense due to increased salt content, so it sinks underneath the less dense water brought north by the Gulf Stream.

Global thermohaline circulation. Red paths indicate warm surface currents, whereas blue paths indicate cold, deep ocean currents. Notice there are two areas of deep water formation in the northern Atlantic Ocean, and one in the southern Atlantic Ocean near Antarctica. The scale at the bottom indicates salinity of the ocean water, measured in units of the Practical Salinity Scale (PSS; a dimensionless number, but the higher the number, the saltier the seawater). Image from ‘Physical Geology‘ by Steven Earle used under a CC-BY 4.0 international license.

Deep water is also formed off the coast of Antarctica by this same process in southern Atlantic Ocean. Notice in the cross section figure above, Antarctic Bottom Water is much colder, and thus denser, than North Atlantic Deep Water, so it sinks and flows below it. In other words, the deep water that is formed in the North Atlantic flows above the deep water that forms in Antarctica. Some of the North Atlantic Deep Water mass eventually resurfaces near the coast of Antarctica. It is important that deep water masses sink, because as previously mentioned, this leads to the eventual mixing of the oceans on thousand-year timescales, and it also brings oxygen and other atmospheric gases (such as CO2 ) into the deep ocean. 

But before resurfacing, the deep water masses circulate around the entire ocean basin, from the northern Atlantic Ocean, into the Indian Ocean, then into the Pacific Ocean. Thus, it is in the Pacific Ocean where the bottom waters are oldest and resurface.

It is important to pause here before reading further and reflect on the information you have read thus far. The entire thermohaline circulation system depends on the sinking of dense, saline waters in the Atlantic Ocean. The formation of dense water depends on the formation of ice. The formation of ice depends on cool climates.

It is because of the thermohaline circulation system, along with surface currents, that our oceans are able to mix on longer timescales, and thus absorb more CO2 from the atmosphere.

So, what happens to the entire circulation system when climate begins to warm? Well, we’re glad you asked! Continue reading below to find out.

Ocean Stratification

It’s not a big stretch to realize that as our Earth warms, our oceans are warming as well. A warming ocean has huge implications for climate change, some of which we’ve already discussed.Another of these implications is ocean stratification, or the increased layering of our oceans due to differences in temperature. Recall from the ‘Ocean Layers & Mixing’ page that our oceans are already a bit stratified due to differences in temperature and salinity. However, this is OK, because there is sinking and thus mixing of deep, cold waters in the Atlantic.

It seems unreasonable to think that the thermohaline circulation system would stop or slow down, but there is evidence that this has happened in the geologic past.

But how would something like this happen, and what would this mean for our Earth? Recall that in order to form a denser water mass, sea ice needs to form, which requires cool climates. If the climate becomes warm, sea ice formation will cease to form and will begin to melt. The ice that formed in the first place does not incorporate the salt from seawater, so this meltwater is essentially freshwater. Thus, the melted water from the ice is less dense because it does not contain salt.

Because the melt water is less dense, it floats in the surface ocean, where it is rapidly warmed. This warm water also leads to the melting of ice, which puts more freshwater into the ocean that is warmed. Eventually, enough ice is melted to create a ‘freshwater lens’ over the surface of the ocean in the areas around ice, where deep water is formed. Thus, because the water is 1) warm, and 2) less dense, it inhibits the formation of deep water due to the increased stratification, or layering, of the oceans.

Once deep water formation in the Atlantic Ocean is inhibited by a layer of warm and fresh water over the northern Atlantic, the entire global thermohaline circulation system slows down. Increased melting of ice will further prohibit deep water formation, which would lead to the cessation of thermohaline circulation.

In addition, deep water that is and has been sinking in the north Atlantic Ocean is warming due to a warming climate.

Antarctica

You may recall that we mentioned in the ‘Surface Currents’ section of this page that Antarctic ice is somewhat protected from warm surface currents by the Antarctic Circumpolar Current. So surely, deep water formation would continue in the southern Atlantic Ocean, right? Wrong.

Three times illustrating how warming deep waters, that are brought to the surface by upwelling, causes melting of ice shelves on Antarctica. Figure modified after Griggs et al. (2017).

Recall in the above section ‘Deep Ocean Circulation’  that some of the deep water that sinks in the north Atlantic resurfaces in the south Atlantic. This is due to a major upwelling zone off the coast of Antarctica that pulls the deep water to the surface. Also recall from the above information that the north Atlantic deep water that is sinking today is warmer than it should be.

So, warmer waters (warmer by 1-2 degrees C) are being brought to the surface ocean under the ice shelves on Antarctica, shelves that project into the ocean. This is major, for two reasons:

1.The ice shelves on Antarctica that project into the ocean are melting from the warm, deep water that is being upwelled

2. The meltwater is causing a freshwater lens on the surface ocean, which is causing stratification and also inhibiting deep-water formation in the southern Atlantic Ocean

In the figure at right, the concept of Antarctic ice shelf melting is illustrated. At Time 1, the ice shelf is prominently projecting into the southern Atlantic Ocean, but deep, warm waters have begun to melt the ice from the bottom. In Time 2, as global warming continues, the ice shelf is melted back further. In Time 3, the ice shelf is almost completely gone, which causes ice sheet instability. This means that the entire ice sheet will begin to flow into the ocean (due to an imbalance between the mass of the ice sheet and mass of the ice shelf), and the melting will continue.

Future Implications

Melting of the ice shelves and thus increased stratification of our oceans have some huge implications:

First, sea level is beginning to rise due to the loss of ice sheets and ice shelves. This will affect coastal cities greatly.

Second, the slowdown of thermohaline circulation will lead to an increase in the time it takes for the ocean to mix. This will have huge implications for the rate at which the ocean can absorb CO2 from the atmosphere (it will slow down). Recall from the ‘Ocean Layers & Mixing’ page that  the surface ocean completely mixes with the deep ocean on a scale of about 1,000 years. Thus, the amount of CO2 in the surface oceans will greatly increase.

Third, with decreased mixing time of the ocean layers and increased CO2 amounts in the surface ocean, this layer will become very acidic very quickly (recall from the ‘Ocean Chemistry & Acidification’ page that the surface ocean has a limited buffering capacity by itself).

Fourth, with decreased mixing due to cessation of the thermhaline circulation, oxygen and other dissolved gases will not make it to the deep ocean, which could lead to oceanic anoxic events, or OAEs. OAEs have been identified in the geologic past, and occur at times when the ocean became nearly stagnant from lack of oxygen reaching the deep ocean. No oxygen in the ocean? No problem; but that means we won’t have a fishing industry, or whales, dolphins, seals, penguins, etc.

Fifth, with increased warming, surface currents are becoming affected. Recent research suggests that western boundary currents are becoming stronger, and they are beginning to move towards higher latitudes. This would modify weather patterns over the continents, such as increased storms and increased heat.

To learn more about surface currents, the thermohaline circulation system, and ocean stratification, visit these sites:

Proceed to ‘CO2: Past, Present, & Future’

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