On the previous page, y'all learned about the different layers of the bounding main: the surface ocean, the deep ocean, and seafloor sediments. Hither, we'll elaborate on these layers, specifically the major ocean currents and how they operate in the surface and deep sea.

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 power to affect weather and long-term climate patterns
  • Deep bounding main currents are driven by density differences amidst water masses
  • Deep body of water water is formed in the Atlantic Ocean in 3 different locations from cold, salty waters
  • These sinking h2o masses aid mix the entire body of water and take gases from the atmosphere to the deep ocean
  • Increased warming of World 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 tin can inhibit ocean mixing, leading to decreased oxygen levels in the deep ocean, warmer surface waters, and intensification of bounding main acidification

Surface Sea Circulation

Recall from the Modern Atmosphere folio that at that place are ii main factors that drive surface currents in our oceans: one) differential heating between the equator and the poles, which leads to the wind patterns, and ii) the Coriolis Effect (the invisible forcefulness that deflects objects as they motion over the surface of the World). In improver to these two forces, gravity also plays a part in the evolution of surface currents. More specifically, surface ocean currents are currents that move water in the upper layer (few hundred meters) of the h2o column, and can have localized affects on conditions and climate, which collectively tin can have a global impact on climate (the long-term weather patterns).

Major surface currents of the earth's oceans. Notice that whorl systems operate in every body of water 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 iv.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, every bit they carry warm water from the equator towards the poles into cooler areas.

Specifically, the western 'limb' of the scroll systems, chosen western purlieus currents, transfer warm h2o to higher latitudes, which vents oestrus and moisture to the lower atmosphere . The moisture brought further n and due south from the equator past western boundary currents is what provides rain for areas forth the coasts . Some examples of western boundary currents are the Gulf Stream in the Northern Hemisphere that flows along the eastward coast of North America, and the East Australian Electric current in the Southern Hemisphere that flows along the eastward 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 purlieus currents are colored red.

A lamb poses majestically against a heaven of depression-hanging clouds and fog on a rainy day in western Republic of ireland.

As an case of a western boundary current carrying moisture north, call back virtually Ireland's weather. The country is located at rather loftier latitudes (nigh 53° N), just does not receive much snowfall (for reference, New York is at 40° N), but information technology 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 wet that is rained out over Ireland.

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

Another of import current of our oceans is the Antarctic Circumpolar Current, or the ACC, which flows clockwise effectually Antarctica in the Southern ocean. The ACC flows almost totally uninhibited effectually Antarctica (in that location is no land mass blocking its path), and thus is a very strong electric current. In fact, it is the largest current in the earth with regards to the amount of water it transports (100-150 1000000 cubic meters of h2o per second!!). Therefore, the ACC blocks warm western boundary currents traveling south from the equator, which is i reasons why Antarctica has maintained its large ice sheets.

Deep Body of water Circulation

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

A cross section from the n Atlantic Ocean (on the right) to the due south Atlantic Ocean (on the left). Here, at that place are four water masses represented. The Gulf Stream electric current is a surface current, specifically a western boundary current. Image from 'Physical Geology' by Steven Earle used under a CC-Past four.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 once again. It is often idea of as a conveyor belt by geoscientists for this reason. In the cantankerous section at left, there are four principal water masses illustrated. Find that the 1 at the top of the h2o column, colored red and labeled 'Gulf Stream', represents the surface current (western purlieus current).  This diagram is looking at the Atlantic Ocean from the Chill (on the right) to Antarctica (on the left).

Deep water is firstly formed in the northern Atlantic Ocean (see figure at correct). The water hither becomes denser than surrounding h2o considering of alkali rejection . Brine rejection occurs when seawater freezes but leaves behind the salt. Thus, the water around the water ice becomes more dense due to increased salt content, so it sinks underneath the less dumbo water brought north past the Gulf Stream.

Global thermohaline circulation. Red paths bespeak warm surface currents, whereas blue paths indicate common cold, deep ocean currents. Discover there are two areas of deep h2o formation in the northern Atlantic Ocean, and ane in the southern Atlantic Bounding main near Antarctica. The scale at the lesser indicates salinity of the sea h2o, measured in units of the Practical Salinity Scale (PSS; a dimensionless number, but the higher the number, the saltier the seawater). Image from 'Concrete Geology' past Steven Earle used under a CC-BY iv.0 international license.

Deep water is also formed off the coast of Antarctica by this same procedure in southern Atlantic Bounding main. Notice in the cross section effigy above, Antarctic Bottom Water is much colder, and thus denser, than North Atlantic Deep Water, then it sinks and flows below it. In other words, the deep water that is formed in the N Atlantic flows above the deep h2o that forms in Antarctica . Some of the North Atlantic Deep Water mass eventually resurfaces nigh the coast of Antarctica . It is of import that deep water masses sink, because as previously mentioned, this leads to the eventual mixing of the oceans on thousand-yr timescales, and it also brings oxygen and other atmospheric gases (such as CO2 ) into the deep bounding main.

Simply earlier resurfacing, the deep water masses circulate effectually the entire body of water basin, from the northern Atlantic Ocean, into the Indian Ocean, then into the Pacific Ocean. Thus, it is in the Pacific Body of water where the bottom waters are oldest and resurface.

It is important to suspension here earlier reading farther 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 germination of water ice . The germination 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 organisation when climate begins to warm? Well, nosotros're glad you asked! Continue reading below to discover out.

Ocean Stratification

It'southward not a big stretch to realize that every bit our Earth warms, our oceans are warming likewise. A warming bounding main 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. Yet, this is OK, because there is sinking and thus mixing of deep, cold waters in the Atlantic.

It seems unreasonable to remember that the thermohaline circulation arrangement would terminate or dull down, but in that location is evidence that this has happened in the geologic past.

But how would something like this happen, and what would this mean for our Globe? Retrieve that in order to grade a denser water mass, ocean ice needs to form, which requires cool climates. If the climate becomes warm, body of water water ice formation will cease to form and will begin to cook. The water 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 dumbo because information technology does not contain table salt.

Considering the melt water is less dense, it floats in the surface sea, where it is apace warmed. This warm water also leads to the melting of water ice, which puts more freshwater into the sea that is warmed. Eventually, plenty ice is melted to create a 'freshwater lens' over the surface of the ocean in the areas effectually ice, where deep water is formed. Thus, considering the water is one) warm, and two) less dumbo, it inhibits the germination of deep water due to the increased stratification, or layering, of the oceans.

In one case 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 organization slows downwards. Increased melting of water ice will further prohibit deep water germination, which would lead to the abeyance of thermohaline apportionment.

In improver, deep h2o that is and has been sinking in the north Atlantic Ocean is warming due to a warming climate.

Antarctica

You may think 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 germination would continue in the southern Atlantic Ocean, correct? Wrong.

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

Recall in the above section 'Deep Ocean Apportionment'  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 n Atlantic deep water that is sinking today is warmer than it should be.

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

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

ii. The meltwater is causing a freshwater lens on the surface bounding main, which is causing stratification and as well inhibiting deep-h2o formation in the southern Atlantic Ocean

In the figure at correct, the concept of Antarctic ice shelf melting is illustrated.At Fourth dimension 1, the ice shelf is prominently projecting into the southern Atlantic Bounding main, just deep, warm waters accept begun to melt the ice from the bottom. In Fourth dimension ii, as global warming continues, the ice shelf is melted back further. In Fourth dimension 3, the water ice shelf is almost completely gone, which causes water ice sheet instability. This means that the entire ice sheet will begin to menses 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:

Commencement,sea level is first to rise due to the loss of ice sheets and ice shelves. This will affect coastal cities profoundly.

Second, the slowdown of thermohaline circulation will lead to an increase in the time it takes for the ocean to mix. This will accept huge implications for the rate at which the sea can absorb CO2 from the atmosphere (it will ho-hum down). Retrieve from the 'Body of water Layers & Mixing' page that  the surface ocean completely mixes with the deep body of water 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 go very acidic very quickly (remember from the 'Sea Chemical science & Acidification' folio 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 non make information technology to the deep body of water, which could lead to oceanic anoxic events, or OAEs. OAEs accept been identified in the geologic past, and occur at times when the bounding main 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 inquiry suggests that western purlieus currents are condign stronger, and they are beginning to motility towards higher latitudes. This would modify conditions patterns over the continents, such equally increased storms and increased rut.

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

  • Windows to the Universe: Surface Sea Currents
  • NOAA Surface Ocean Currents
  • University of Hawaii: Ocean Surface Currents
  • NASA Ocean Conveyor Belt
  • How Stuff Works: Deep Bounding main Currents
  • NOAA Thermohaline Apportionment
  • WHOI Interactive Deep Bounding main Circulation
  • PBS Short Video: Global Bounding main Apportionment
  • University of Southampton Ocean Anoxia: Can the oceans suffocate?
  • Climatic change and Ocean Stratification Blog

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