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Ocean currents are moving rivers of water. In this highly generalized bathtub cross-section of the ocean,
the equator is in the center, the south pole to the right, and the north pole to the left; we also
see three basic ocean layers: the surface layer, the deep layer, and the boundary between the two
called the pycnoline. As discussed in an earlier video tutorial, the pycnocline is the boundary
between two water layers of different density. Surface currents are primarily caused by winds
blowing across the surface of the oceans. They, therefore, happen in the surface layer.
Thermohaline currents are caused by density differences, which are themselves caused by
differences in the salinity or temperature of the water, hence the name thermohaline. Thermohaline
currents are found in the deep layer. They move vertically due to density and then spread out
horizontally to minimize pressure differences. For example, cold water at the polar ocean surface is
denser than the water around it. When that water sinks, it spreads out laterally across the bottoms
of all the world’s oceans and becomes the deep water layer. At the poles therefore, the surface
water and deep water are basically the same – same density; same temperature. They are not
separated by a pycnocline. The depth profiles of these waters are therefore called isothermal and
isopycnal. If they also have the same salinity, we’d call them isohaline. Do they? Depends on
the seasons. In the polar winters, ice formation increases, leaving the surface water more saline.
In the polar summers, ice melting increases, leaving the surface waters fresher. So salinity
might change slightly with depth at the poles. What about at the equator and tropics? You can
see from this image that in these zones, the pycnocline is well developed. Surface waters
are heated up by the strong direct sunlight in these and become quite warm. Even though the
surface waters at subtropical highs have higher-than-average salinity due to high
evaporation rates (as discussed in previous video tutorials), these waters are still so warm that
they end up being less dense than the deep polar water that sits under them. So in the subtropics
and equator, the depth profiles show that with depth, density increases, temperature decreases,
and salinity decreases either a little or a lot, depending whether you’re descending at the equator
(only a little saltier than polar water) or the subtropics (a lot saltier than polar water).
What about at the midlatitudes? In these zones, the pyncocline comes and goes with the seasons.
During the summers, the surface waters are heated up enough to separate from the deep
water and stop mixing. In the winter, however, the pycnoclines disappear as the surface waters
cool. The surface and deep waters mix, and the depth profiles are more isopycnal.
Why does the separation of surface and deep waters matter so much? It’s a major barrier
to mixing in the world’s oceans, including mixing and resupply of nutrients necessary for biological
productivity. When photosynthesizing autotrophs living at the ocean’s surface (otherwise known
as phytoplankton) die, their remains rain down to the bottom of the seafloor, where
they decompose. The nutrients in their bodies are released into the water. Where pycnoclines exist,
these nutrients are trapped at depth. Phytoplankton will eventually use up
all available nutrients at the surface, and their populations diminish until nutrients can be added
back to the area through increased river runoff or through the disappearance of the pycnocline.
In this graph, water depth is increasing as we move down the Y axis. The dashed lines represent
the boundaries between water masses. Within each layer of water characteristics like temperature,
salinity, and density are pretty uniform, but as each layer may have different characteristics than
the one above or below it, travel across the pycnocline not only means changes in density,
but it also means changes in temperature and/or salinity. In this example there are
three water masses sitting atop each other, the one on top the least dense; the one at
the bottom the most dense. Three layers means two boundaries or two pycnoclines. What causes
the density differences? Between the top layer and the middle layer, salinity increases. The
same is true between the middle and the lower water layer. Because salinity changes at each
of these boundaries, these pycnoclines are also called haloclines. When we look at temperature,
we see it’s decreasing between the top and the middle layer and there’s no change between the
middle and bottom layer. Because temperature changes at the first boundary, that pycnocline
and halocline is also a thermocline. However, the middle and bottom layers are isothermal,
so there’s no thermocline between them.
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This picture shows cross-sections through the three major oceans and their major thermohaline
currents. Each of these cross sections shows a surface layer atop a number of different deep
water currents. AAIW means Antarctic Intermediate Water. It originates in the oceans north of
Antarctica at about 55°S latitude and descends down below the surface waters of the Atlantic,
Pacific, and Indian Oceans. It sits atop all the other thermohaline currents,
which means it must be the least dense. This birds-eye view of the surface waters
around Antarctica show the origination points of the Antarctic Intermediate Water. In these
zones two surface currents are converging and pushing water toward each other, making surface
water pile up. Some of that water pile pushes downward (otherwise known as downwelling),
where it can descend below the pycnocline and slide equatorward at depth. Although the Antarctic
Intermediate Water is cold, it is not as cold as the water underneath it. It is also a bit fresher
due to the high precipitation rates at this latitude where low pressure systems dominate.
The deepest densest water in all the world’s oceans is the Antarctic Bottom Water. It is
densest because it is the coldest. Its salinity and proximity to freezing temperatures allow it
to cool below the freezing point of freshwater and average -1 to 1°C. The Antarctic Circumpolar
Current is a surface current that isolates Antarctica from surrounding weather and
current systems and allows it to get colder than almost anywhere else on the
planet. That cold water then feeds the deep and intermediate waters of all the world’s oceans.
Arctic Ocean waters are also isolated, but because they are trapped under sea ice most
of the year and surrounded by shallow continental shelves on all sides, they are not able to enter
all the world’s oceans. Their primary exit is through a narrow shallow passageway between
Greenland and Svalbard in the North Atlantic. After exiting, these waters sink to the bottom
of the North Atlantic and spread southward until they meet the Antarctic Bottom Water,
which they then ride over. Eventually these waters travel atop the Antarctic Bottom Water
all the way down toward Antarctica at which point they rise up to the surface. Why? Surface currents
in that region are being pulled away creating an area of surface water depletion that allows deep
water to well up (otherwise known as upwelling). The North Atlantic Deep Water then mixes at the
surface and can become part of both the Antarctic Bottom Water and the Antarctic Intermediate
Water. Watch the Ocean Mixing & Pollution video tutorial to learn more about how these currents
help spread and mix pollutants worldwide. The Mediterranean Sea, south of Europe and
north of Africa, contributes its own intermediate water or thermohaline current to the Atlantic
Ocean. High evaporation rates in the area make that water super warm, but also super salty.
That means increased density which makes it sink. A shallow sill at the opening of the sea isolates
the sunken dense water and keeps the majority of it from mixing with the nearby Atlantic Ocean.
When enough of it piles up to reach above the sill, it exits its basin and due to its
density sinks down to about 1,000 meters depth and spreads out across the Atlantic Ocean just under
the pycnocline. It sits in the Atlantic Ocean bathtub at about 40°N as a small lens of water.
Imagine being a whale or submarine traveling through this area – you’d go from cold North
Atlantic Deep Water, derived from the Arctic, and about 3°C, to 10 to 12°C very salty water.
The same thing happens in the Red Sea, where evaporation rates are even higher. This sea
feeds an intermediate current found in the Indian Ocean, called the Red Sea Intermediate Water,
which can be as warm as 23°C and much more salty. It sits as a lens of super warm super salty water
within the coldest water of the world’s oceans. Let’s compare the temperatures, salinities,
and densities of all these thermohaline currents. In this chart, the y-axis shows temperature in °C
increasing as we move upwards. The x-axis shows salinity in parts per thousand getting higher as
we go to the right. Each curved line represents a specific density; the lines in the lower right
are the densest; upper left, the least dense. The least dense water in this plot is the North
Atlantic Central Surface Water, with salinities greater than the 35 ppt average for seawater,
but temperatures warmer than most of those shown on this plot. The warm temperature is what makes
it least dense, and hence surface water. Antarctic Intermediate Water is the next lowest in density.
It is much colder than the surface water, but also fresher. The densest water in this picture
is the Antarctic Bottom Water, followed closely by North Atlantic Deep Water. The Antarctic Bottom
Water is slightly fresher than the North Atlantic Deep Water, but also much colder. Mediterranean
Intermediate Water sits in the middle density position – both warmer and saltier than any of
the other thermohaline currents shown in this image. But where is the Red Sea Intermediate
Water? It is SO warm and SO salty that it doesn’t even show up on this graph. In fact,
we have to shrink the graph to see it.
Pause now. [music]
For more information and more detail, continue on to the next video in the series.
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