GEOL 2503 Introduction to Oceanography
Dr. David M. Bush
Department of Geosciences
University of West Georgia
Topic 14: Density Driven Currents
POWERPOINT SLIDE SHOW NOTES
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Topic 14: Density Driven Currents
Thermohaline circulation is the name given to vertical movement of water driven by
density. We’ve seen vertical motions in the air driven by density. In that case, density
differences were created by variations in solar heating. In other words, temperature was
the controlling factor. In the ocean, there is another factor that creates density
differences—salinity. Thus the term “thermohaline.” Thermo- for temperature and –
haline for salt or salinity. Of these, temperature has a greater effect. That being true,
and knowing that the sea surface is heated by solar energy, how does water sink? We
know water sinks. That’s how oxygen is carried to all ocean depths.
Recall downwelling and upwelling from the El Niño discussion.
Downwelling and upwelling are controlled by surface processes. That’s where the
salinity and temperature differences originate.
Recall the zones of rising and falling air in global air circulation patterns.
And recall precipitation and evaporation patterns driven by the rising and falling air.
Precipitation and Evaporation Patterns
Recall that ice is fresh water. So when sea ice forms, salt is left behind which increases
the surface water salinity and thus density.
Note the latitudinal zones of ocean surface salinity. Lower salinity in the tropics, higher
around 30° N and S, lower again around 60° N and S, controlled by the precipitation and
evaporation patters driven by global atmospheric circulation.
Ocean Surface Water Density Summary
Controls on salinity
This is called a temperature-salinity or T-S graph. The blue lines are lines of equal
density. Follow any of the lines across the graph and see that there many combinations
of salinity and temperature that can produce the same density. Recall that 1.028 g/cm3
is the average density of sea water.
How the ocean becomes layered.
Big picture of ocean water density layers.
Same layers for temperature and salinity (which control density). They are all the same
layer, just given different names depending on whether you are studying temperature,
salinity, or density.
The mid-latitude temperature vertical structure of the oceans. The depth of the
thermocline shown here is a little different than what is given on the previous two slides,
but we’ll go with the thermocline (and halocline and pycnocline) extending from 1001,000 meters.
Typical oceanic temperature profiles. Note the amount of change in temperature within
the thermocline depends on latitude, and that in polar regions (high latitudes) there
really is no thermocline. The ocean temperature is pretty much constant with depth.
Formation of the pycnocline.
Latitudinal variation of average depth of vertical density zones in the ocean. The surface
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layer of water accounts for only about 2% of ocean volume, the pycnocline about 18%.
and the vast majority of water in the ocean is the deep zone with pretty much constant
temperature, salinity, and density.
Again, thermohaline circulation is density driven, and two factors control density in the
ocean.
Upwelling and downwelling. Continuity is a term that means matter must be continuous.
That is, if overall volume does not change, then water sinking somewhere must be
balanced by water rising somewhere else. Continuity (or conservation of matter).
Continuity applies everywhere. It is the reason we can have convection cells in the
mantle, in the atmosphere, in the ocean, and in a pot of soup boiling in the stove.
Upwelling and downwelling can be driven by density as we have seen, but there is also
another way to drive upwelling and downwelling.
Surface divergences occur when water is pushed apart, and surface convergences occur
when water is pushed together. Surface divergence drives upwelling, and surface
convergence drives downwelling. Somewhat reminiscent of plate tectonics, no? Note
the change in surface elevation under the two conditions.
An example of surface divergence and convergence on a large scale. We’ll see this in
more detail in the next topic, but for now think back to global atmospheric circulation,
trade winds, and the Coriolis Effect. The trade winds (solid black arrows) push on the
water surface (dashed black arrows), driving ocean surface currents. Note that the
surface currents move to the right of the trade winds in the northern hemisphere and to
the left in the southern hemisphere because of the Coriolis Effect.
Ekman transport (the big blue arrows) is something we’ll discuss in detail in the next
topic, but for now just know that Ekman transport involves the entire thickness of
surface water pushed by the wind (approximately the upper 100-150 meters of the
ocean), and that the overall water movement pushed by the wind ends up being 90° to
the right in the northern hemisphere and to the left in the southern hemisphere (notice
the big blue arrows are at 90° angles to the solid black arrows). The result is surface
water being pushed apart (diverging) in the tropics (the tropical divergence) and pushed
together (converging) centered around 30° N and S (the elevated convergence).
Water can also be pushed away from a continent (driving divergence) or toward a
continent (driving convergence). In the El Niño topic we learned about trade winds
pushing water across the Pacific Ocean. This drives a surface divergence off the west
coast of South American and forces upwelling. In the west, water is pushed against
Australia driving a surface convergence and forcing downwelling. These processes are
often referred to as continental divergence and continental convergence.
The big circulation cell shown here above the thermocline is not a convection cell
because it is not driven by density. It’s driven by upwelling and downwelling that are
being driven by surface divergence and surface convergence.
Think back to the discussion of the Southern Ocean. We said that it extends from the
coast of Antarctica north to 60 degrees south latitude, but that the northern boundary
defined by an ocean current, not by land. We see now that there is a major surface
convergence, called the Antarctic Convergence, that is the northern boundary of the
Southern Ocean.
The Antarctic Convergence separates cold Antarctic water from warmer Subantarctic
water. Note the sinking of water at the Antarctic Convergence as the surface water of
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the Subantarctic zone collide with Antarctic zone surface water.
Antarctic Convergence
Coastal upwelling and downwelling is a smaller-scale version of upwelling and
downwelling caused by continental divergence and convergence. Earlier in this topic we
mentioned that wind pushes water to form ocean surface currents, and that something
called Ekman transport causes the surface water to move 90° to the direction of the wind
(to the right in the northern hemisphere and to the left in the southern hemisphere).
Thus, alongshore winds can push water toward the coast or away from the coast. In this
northern hemisphere example, a wind blowing from the north pushes surface water
away from the coast causing a surface divergence and coastal upwelling. Likewise, a
wind from the south pushes surface water toward from the coast causing a surface
convergence and coastal downwelling.
Now back to true thermohaline circulation. Sinking water and no thermocline in the
polar regions. Surface heating toward the equatorial regions means a thicker
thermocline. Convection circulation is driven by differences in density.
Water masses. There are even more layers to the ocean than the simple thermocline,
halocline, pycnocline model.
The water masses in the Atlantic Ocean are similar to those of other oceans. This is a
simplified view of Atlantic water masses. The previous slide mentions that water masses
are named for where they’re from and where they’re found. Notice Antarctic bottom
water. It’s from the Antarctic and found on the bottom of the ocean. North Atlantic
Deep Water is from the North Atlantic and found deep in the ocean, but mostly above
bottom water. Surface water is found at the surface, above the thermocline.
Intermediate water is above deep water, but mostly found below surface water. And the
most unusual water mass in the Atlantic is Mediterranean Intermediate water.
Temperature dominates over salinity in controlling density so even though it is high
salinity it is warm enough that it can only sink to about 1,000-1,500 meters.
Mediterranean outflow water is an interesting story. Remember the term “sea?” A semienclosed basin, part of a larger ocean. The Mediterranean Sea is part of the Atlantic
Ocean, connected to it by only one opening—the Strait of Gibraltar between Europe
(Spain) and Africa (Morocco).
The Mediterranean Sea is in a warm, dry climate. It is centered around the sinking
subtropical air mass discussed in global atmospheric circulation. Evaporation dominates,
so much so that there is more evaporation off its surface than is replaced by rainfall and
river flow. The salinity of the Mediterranean is higher than that of average sea water,
between 36-40 ‰. The salty water sinks to the bottom of the Mediterranean, builds up,
and eventually spills over the Gibraltar Sill out of the Mediterranean into the Atlantic
Ocean proper. Look back to the T-S diagram. Even though the Mediterranean water is
very salty, it is so warm that its density stays low and it can only sink to about 1,0001,500 meters.
Because water is spilling out of the Mediterranean, and because so much water is
evaporating from its surface, water has to flow in from the Atlantic to take its place. The
result is a set of currents through the Strait of Gibraltar. One flowing to the west just
over the sill, and another flowing east at the surface.
Both the Allied and Axis powers took advantage of this physical oceanographic setting
during World War II. The Strait of Gibraltar is very narrow (only about 8 miles) and
shallow (a few hundred meters). It was relatively easy for one side or the other to
monitor submarine traffic through the strait simply by using underwater microphones
called hydrophones and listening for the sub’s engines. In order to enter without being
detected, the submarine crew would turn off the engines and let the incoming current
carry them in. When safely inside, on go the engines and away they go. In order to exit
safely they would use the same tactic, only diving deeper to take advantage of the
outflowing current.
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