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Properties of Seawater & Currents
1. Water & Climate:
The ocean extends over 71% of the earth's surface. The ocean holds 98% of the 1.4
billion cubic kilometers of water on the planet, divided within three major basins (The
Atlantic, Pacific and Indian Oceans).
Seawater fills the basins separating the continents (Fig. 1) with an average depth of
3795 meters.
The continental margins, extending from the seashore to around 2500 meters depth,
cover 40.7% of the ocean (29% of Earth surface).
The deep ocean covers about 59.3% of the ocean's surface (42% of Earth's surface).
The extensive flat plains of the deep basins range 4000 to 5000 meters in depth.
The mid-ocean ridge, marking the spreading of tectonic plates, is marked by
mountains; the ridge crest is about 2500 m from the ocean surface.
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The deepest ocean is found in the trenches where the plates are subducted, the
Mariana Trench is 11,035 meters deep (compared to the 8848 meter height of Mount
Everest).
If the solid earth were made into a flat plain, the seawater would cover the entire
earth to a depth of 2440 meters. If all of the water vapor in the atmosphere were
converted to liquid it would cover the smoothed earth surface by about an 1-inch.
Water has a very high heat capacity.
This means that it holds a lot of energy.
It takes a lot of energy to warm it up,
but it also gives off that energy slowly.
The upper 700 m hold the most heat.
The lower levels of water also hold a lot
of heat; much more than the land. The
high capacity of water makes the ocean
a powerful and stabilizing force of the
Earth's climate system. The stored
energy in the ocean helps to keep the
land temperature moderate.
One obvious consequence of the ocean's influence is the "marine effect", which acts to
reduce the extreme differences between winter & summer and day & night air
temperatures.
Another is that the ocean circulation transfers a significant amount of heat from the
equator to the Northern and Southern latitudes, helping the climate system to attain an
approximately steady state condition.
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2. Sea Surface Temperature:
The temperature of the sea surface is high (27-30°C) near the equator, often the
maximum value occurs a few degrees of latitude north of the equator. The temperature
is low (at the sea water freezing point of -1.9°C due to the amount of salt in it) within
the polar oceans.
However, there are also changes of sea surface temperature with longitude
(Northerly/Southerly directions). Warmer water moves poleward along the western
sides of the ocean. The eastern tropical regions of each ocean are cooler than the
western tropical margin. These are due to the movement of seawater in the horizontal
(ocean currents) and vertical (upwelling/sinking) directions.
Temperature and density of ocean water are inversely related: warm water means low
density, cold water means higher density seawater. The salt content of the water also
affects Ocean density.
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3. Sea Surface Salinity:
Sea water is about a 3.49% (or 35 ppt) salt solution, the rest is freshwater. The more
salt, the denser the seawater. As the range of salt concentration in the ocean varies
from about 3.2 to 3.8% (32 ppt - 38 ppt). Oceanographers refer to salt content as
'salinity', express salt concentration as parts per thousand.
A region of excess evaporation, such as the subtropics tends to become salty, while the
areas of excess rainfall become fresher.
When seawater evaporates the salt remains behind, only the freshwater is transferred
from the ocean to the atmosphere. That means that salinity is higher near the poles
where water freezes. Sea ice formation also removes freshwater from the ocean,
leaving behind a more saline solution.
Along the shores of Antarctica this
process produces dense water. This
causes the water to sink and push the
lower water out of the way starting a
major deep water current that will travel
around the globe.
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Below the Sea Surface:
Waters warmer than 10°C dominate the sea surface but do not extend much below 500
m within the ocean.
Tropical and subtropical surface water is just a veneer of warmth over a cold ocean;
typical deep ocean temperature vary from -1° to 3°C. The sharp drop off in
temperature with depth is called the thermocline.
The warm surface water is generally saltier than the cooler deep or polar waters. The
halocline marks the drop of salinity with depth that accompanies the thermocline. The
surface water warmth overrides the saltiness in governing density, so that the warm
surface water regions coincide with buoyant (less dense) water.
In polar regions buoyancy of the surface layer is mainly a consequence of the freshness
of the surface water. Deep cold waters derive their properties at the sea surface during
winter at high latitude.
4. Deep Water Masses:
The deep Atlantic is relatively salty (34.9). This is due to the sinking of chilled saline
surface water in the northern North Atlantic. The cooling makes the surface water
dense, forcing it to sink, or convect into the deep ocean, and spread southward along
the bottom. It is called North Atlantic Deep Water (NADW).
In contrast the deep Pacific is lower in salinity (34.7), as it experiences no deep
convection of cooled salty surface water, its surface layer is too fresh and buoyant to
sink. Pacific deep water is due to the lower salinity water column of the southern ocean.
Towards the sea floor, temperatures are near 0°C marking the presence of Antarctic
Bottom Water (AABW) due to the very cold (-1.9°C; 34.65), dense water along the
shores of Antarctica.
At the base of the thermocline is the low salinity Antarctic Intermediate Water
(AAIW) derived from sinking of cool (3° to 4°C), low salinity waters (34.4) from 50° 60°S marking the Antarctic Circumpolar Current and ocean polar front zone.
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Ocean Currents and Climate
Name: ___________________________________________
Date: _____________________________
There are two type of Ocean Currents:
1. Surface Currents -- Surface Circulation
These waters make up about 10% of all the water in the ocean.
These waters are the upper 400 meters of the ocean.
2. Deep Water Currents -- Thermohaline Circulation
These waters make up the other 90% of the ocean.
These waters move around the ocean basins by density driven forces and gravity.
The density difference is caused by differences in temperatures and salinity.
These deep waters sink into the deep ocean basins at high latitudes (top and bottom of the globe)
where the temperatures are cold enough to cause the density to increase.
Ocean Currents are influenced by two types of forces
1. Primary Forces--start the water moving
The primary forces are:
1. Solar Heating
2. Winds
3. Gravity
4. Coriolis
2. Secondary Forces--influence where the currents flow
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1. Surface Circulation
Solar heating causes water to expand. Near the equator the
water is about 8 centimeters higher than in middle latitudes.
This causes a very slight slope and water wants to flow down
the slope.
Winds blowing on the surface of the ocean push the water.
Friction creates a connection between the wind and the
water's surface.
A wind blowing for 10 hours across the ocean will cause the
surface waters to flow at about 2% of the wind speed.
Water will pile up in the direction the wind is
blowing.
Gravity will tend to pull the water down the "hill" or
pile of water against the pressure gradient.
The force of the Coriolis effect intervenes and causes
the water to move to the right (in the northern
hemisphere) around the mound of water.
These large mounds of water and the flow around them are called
Gyres. Gyres are large circular currents in each of the 5 main
ocean basins.
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Gyres
The North Atlantic Gyre
Note how the North Atlantic Gyre is separated into
four distinct Currents:




The North Equatorial Current,
The Gulf Stream,
The North Atlantic Current, and
The Canary Current.
But why doesn't the water spin towards the center of
the ocean? Why does it flow around the hill in this
circular motion.
Remember the hill of water-- This hill is formed by the
inward push of water through a process call Ekman
Transport
Remember the Coriolis Force moves objects to the right
in the northern hemisphere (and the left in the Southern
Hemisphere)
Ekman Transport
Wind blowing on the surface of the
ocean has the greatest effect on the top
layers of the water. However, for the
lower layers of the ocean to move they
must be pushed by the friction between
the layers of water above. Consequently,
the lower layer moves slower than the
layer above. With each successive layer
down in the water column the speed is
reduce. This leads to the spiral affect
seen in the above diagram.
The net movement of water (averaged
over the entire upper 330 meters of the
ocean) is 90o to the right of the wind
direction (in the northern hemisphere).
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The effect of winds on the vertical movement of water
Upwelling along the coast caused by Ekman transport of
waters (waters move to the right of the wind).
The waters moved offshore are replaced by waters from
below. This brings cold, nutrient rich waters to the surface.
The nutrient-rich upwelled water stimulates the growth and
reproduction of primary producers such as phytoplankton. Due to the abundance of
phytoplankton and presence of cool water in these regions, upwelling zones can be identified by
cool sea surface temperatures (SST) .
The increased plant life in upwelling regions results in high levels of animals production.
Approximately 25% of the total global marine fish catches come from five upwellings that
occupy only 5% of the total ocean area. These areas if the ocean have the greatest impact on
nutrient-enriched waters and global fishery yields.
This occurs along the coast of Eastern USA and Canada.
Downwelling caused by Ekman
transport onshore (movement of
water to the right of the wind
direction).
Downwelling also occurs where
the water density increases,
causing the denser water to drop
down to the ocean bottom
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Investigative Activity #1:
Currents and Marine Life
Currents are powerful physical forces in the seas. They move water and heat
around the globe, and help determine the chemical make-up of the water column.
Currents also are a major factor in ocean ecosystems. Two types of current motion,
upwelling and downwelling, strongly influence the distribution and abundance of
marine life.
Upwelling
Currents play a huge role in marine productivity, through a process called
upwelling. Sea life is concentrated in the sunlit waters near the surface, but most
organic matter is far below, in deep waters and on the sea floor. When currents
upwell, or flow up to the surface from beneath, they sweep vital nutrients back to
where they're needed most.
Nowhere is the link between ocean circulation and productivity more evident than
around Antarctica. There, strong currents pump nitrogen and phosphate up from
the deep sea to fuel vast blooms of algae and other plants. These plankton are
eaten by swarms of shrimp-like crustaceans called krill. Because of upwelling
nutrients, krill are abundant enough to feed the largest animals on earth, baleen
whales, as well as myriad penguins, seals, and seabirds. In fact, despite the harsh
conditions, the biomass of Antarctic krill is thought to be greater than that of any
other animal on Earth.
Downwelling
The importance of upwelling to surface organisms is matched by the need of sea
bottom life for downwelling, or the sinking of surface water. Surface water can be
forced downward by the pressure of the “pile” of water that forms where currents
converge or wind drives the sea against a coastline. But for bottom dwellers, the
sinking of water caused by density changes is especially noteworthy. The global
conveyer belt takes oxygen-rich surface water and flushes it through the deep sea.
Without this renewal, the dissolved oxygen in bottom sediments and waters would
quickly be used up by the decay of organic matter. Anaerobic bacteria would take
over decomposition, leading to a build up of hydrogen sulfide. Few benthic animals
would survive such toxic conditions.
In the most extreme cases, a lack of downwelling may lead to mass extinctions.
Paleontologists have suggested that 250 million years ago, deep circulation slowed
nearly to a stop, and the ocean began to stagnate. Low oxygen, sulfide and
methane-rich waters filled the ocean deeps and then spread onto the continental
shelves, wiping out 95% of all marine species in the greatest extinction event in
Earth history.
Instructions: In this activity, you will explore the differences between upwelling and
downwelling. Study the graphics and photographs illustrating upwelling and downwelling,
then answer the questions about each process. Maps of the world’s major surface and
deep currents are included as resources to help you understand where and how upwelling
and downwelling occur.
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Examine the following images related to Upwelling
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Examine the following images related to Downwelling
Resources
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Questions
Upwelling
1. Most primary productivity in the oceans occurs in surface waters, but most of the organic matter is at
the bottom of the sea. Explain.
Almost all food chains begin with photosynthesis, and photosynthesis occurs in sunlight. So marine
plants will only live near the surface where light can penetrate, and of course that will keep
herbivores, and their predators, up at the surface too. But once these organisms die, or shed scales,
eggs, leaves, shells, or feces, they and their by-products will sink down toward the sea floor.
2. What causes upwelling?
Upwelling occurs when surface currents move away from one another, or when winds push surface
water away from the shore. This draws deeper water upward to replace the surface water.
3. Why is nutrient upwelling so powerful around Antarctica?
The deep water in the global conveyer belt flows across the seafloor all the way from the North
Atlantic before it reaches Antarctica, so it has a lot of time to pick up a lot of nutrients. There are a lot
of surface currents flowing around the continent - the southern limbs of the Pacific, Atlantic, and
Indian Ocean gyres and the Circumpolar and Sub polar Currents - to stir up the water. And there are
likely to be strong polar winds blowing offshore, which would also drive upwelling.
4. What other conditions near the South Pole help stimulate lush plant growth?
In summer, the sun shines non-stop, which would allow photosynthesis 24 hours a day.
Downwelling
1. What causes downwelling?
Down welling occurs when the water on the surface of the sea becomes denser than the water
beneath it and so it sinks. Seawater gets denser when it gets colder or saltier.
2. Where does most downwelling occur?
Most down welling happens at the poles. There, cold air chills the water. The water brought in by the
surface gyres is pretty salty already, because it comes from the tropics, where evaporation increased
salinity. And once it gets to high latitudes, the water becomes even more saline as ice forms and
further concentrates sea salts.
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Life on Earth nearly died out at the end of the Paleozoic Era 250 million years ago. At that time, all
the continents had just come together to form a supercontinent called Pangaea, and a single
superocean called Panthalassa. The global climate had warmed by several degrees, especially at the
poles.
3. How do you think these conditions might have affected ocean circulation, and led to low oxygen levels
in the sea?
The arrangement of the continents would certainly make circulation patterns different than they are
today. For example, it seems like on the surface, there wouldn't be so many gyres, since there was
only one giant ocean basin. There might have been more east-west movement and perhaps fewer
north-south currents, since there were fewer continental barriers to flows driven by the prevailing
winds.
If the air was warmer at the poles, then the water would be too. And without cold water and ice
formation at high latitudes, there would be little down-welling and not much of a global conveyer belt
in the deep sea. So all in all ocean circulation would probably have been much weaker than it is
today. Oxygen in the deep sea would have been used up, and without strong down-welling, there was
no way to replenish it.
Critical Thinking
Currents are important in marine ecosystems because they redistribute water, heat, nutrients, and oxygen
about the ocean. At the same time, currents inevitably sweep over and carry off living organisms. Discuss
how current flows might affect ocean organisms and species.
Scientists seek to understand and explain how the natural world works. Many of the questions raised in this
endeavor have no absolute answers.
Although many sea creatures are powerful, efficient swimmers, many others are ungainly or even immobile.
For these animals, currents could offer a free ride. Corals and sponges, for instance, are attached to the
bottom as adults, but when they reproduce, they release volumes of planktonic larvae into the water
column. These tiny creatures are free to "go with the flow" and could be carried long distances. This could
allow individuals to escape overpopulated areas with too much competition for resources, and allow the
species to spread into and colonize new habitat.
Once the juveniles settle down, currents will continue to bring other plankton and organic debris their way,
providing a steady supply of food.
Even the free-swimming animals can benefit from riding currents. Many marine vertebrates such as herring,
eels, and turtles, hatch in rivers or close to the coast. Small and weak, the juveniles need currents to carry
them to the open ocean feeding grounds where they grow and mature.
Because currents tend to concentrate food resources in limited areas, such as by upwelling, they make it
easier for predators to find prey.
But a reliance on currents - for transportation and for maintaining water conditions - could be the downfall of
individuals and populations. When currents shift, because of plate tectonics or climate change, ecosystems
would be thrown into turmoil. The young could be carried into areas where they could not survive, and adults
could be bathed in water whose temperature, salinity, or chemistry they cannot tolerate.
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The Global Conveyor Belt
Winds drive ocean currents in the upper 100 meters of the ocean’s
surface. However, ocean currents also flow thousands of meters
below the surface. These deep-ocean currents are driven by
differences in the water’s density, which is controlled by
temperature (thermo) and salinity (haline). This process is known as
thermohaline circulation.
In the Earth's polar regions ocean water gets very cold, forming sea
ice. As a consequence the surrounding seawater gets saltier,
because when sea ice forms, the salt is left behind. As the seawater
gets saltier, its density increases, and it starts to sink. Surface water
is pulled in to replace the sinking water, which in turn eventually
becomes cold and salty enough to sink. This initiates the deepocean currents driving the global conveyer belt.
Thermohaline circulation drives a global-scale system of currents
called the “global conveyor belt.” The conveyor belt begins on the
surface of the ocean near the pole in the North Atlantic. Here, the
water is chilled by arctic temperatures. It also gets saltier because
when sea ice forms, the salt does not freeze and is left behind in the
surrounding water. The cold water is now more dense, due to the
Thermohaline circulation begins in the
added salts, and sinks toward the ocean bottom. Surface water
Earth's polar regions. When ocean water in
moves in to replace the sinking water, thus creating a current.
these areas gets very cold, sea ice forms.
The surrounding seawater gets saltier,
This deep water moves south, between the continents, past the
increases in density and sinks.
equator, and down to the ends of Africa and South America. The
current travels around the edge of Antarctica, where the water cools
and sinks again, as it does in the North Atlantic. Thus, the conveyor
belt gets "recharged." As it moves around Antarctica, two sections split off the conveyor and turn
northward. One section moves into the Indian Ocean, the other into the Pacific Ocean.
These two sections that split off warm up and become less dense as they travel northward toward
the equator, so that they rise to the surface (upwelling). They then loop back southward and
westward to the South Atlantic, eventually returning to the North Atlantic, where the cycle begins
again.
The conveyor belt moves at much slower speeds (a few centimeters per second) than wind-driven
or tidal currents (tens to hundreds of centimeters per second). It is estimated that any given cubic
meter of water takes about 1,000 years to complete the journey along the global conveyor belt. In
addition, the conveyor moves an immense volume of water—more than 100 times the flow of the
Amazon River (Ross, 1995).
The conveyor belt is also a vital component of the global ocean nutrient and carbon dioxide cycles.
Warm surface waters are depleted of nutrients and carbon dioxide, but they are enriched again as
they travel through the conveyor belt as deep or bottom layers. The base of the world’s food chain
depends on the cool, nutrient-rich waters that support the growth of algae and seaweed.
The global conveyor belt is a strong, but easily disrupted process. Research suggests that the
conveyor belt may be affected by climate change. If global warming results in increased rainfall in
the North Atlantic, and the melting of glaciers and sea ice, the influx of warm freshwater onto the
sea surface could block the formation of sea ice, disrupting the sinking of cold, salty water. This
sequence of events could slow or even stop the conveyor belt, which could result in potentially
drastic temperature changes in Europe.
Currents: NOAA's National Ocean Service Education
http://oceanservice.noaa.gov/education/tutorial_currents/welcome.html