RONALD SMITH: We are starting a brand
new subject today.
It's one though that is integral
with the course, and
that's the study
of the oceans.
As we'll see, not only the
atmosphere and the ocean
interact, each influences the
other which makes it necessary
to understand both.
But also, to some extent, they
obey similar laws of physics.
For example, questions of static
stability, when will a
column of air turn over, when
will a column of water turn
over, what makes the winds blow,
what makes the ocean
currents move, Coriolis
plays--
Coriolis force plays a
similar role in both.
So this would be a time not only
for you to learn a few
things about the ocean, but
to establish more linkages
between the things you've
learned before and the things
you're learning now.
Now that we're halfway, or
almost halfway through the
course, linkages are a big
part of the course.
Where you find connections
between things we've already
done, and things we're
learning now.
So try to flag those whenever
you come across them, I think
it'll help you learn the
material more and it'll
establish a sense, I hope,
of unity in the course.
Where things begin to gel, and
become easier, because a very
limited set of physical
principles can apply to a wide
range of geophysical
phenomena.
OK, so what we're going
to do today.
I'll probably just do the first
two, we'll talk about
the bathymetry of the oceans.
The word bathymetry here means
the study of the ocean depth.
So it's pretty straightforward,
just how deep
is the ocean.
That's the subject
of bathymetry.
We're going to see, however,
that ties very closely to
plate tectonics.
Now if you were taking a course
in geology, which my
department offers a number
of, you would study plate
tectonics in quite
some detail.
It's the modern paradigm to
understand how the continents
were formed, and the oceans are
formed, and a number of
the things you see in rocks.
We're just going to touch on it
briefly, because this isn't
a course in geology, but I
need to do it because you
can't understand the bathymetry
of the oceans
without understanding a bit
about plate tectonics.
Has anybody had a course in
geology where they talked
about plate tectonics?
Sarah has--So you're going to
see that stuff coming back,
but fairly quickly.
I'm not going to spend
too much time on it.
We'll spend most of today
talking about temperature and
salinity, and we might get into
ocean currents today, but
if not that'll be the subject
of next time.
And then biological productivity
will eventually,
maybe Friday or next week,
get into El Nino.
Sarah reminded me that there's
no chapter in your book on
oceanography, so we're kind
of on our own here.
These notes will be posted, but
there is a section on El
Nino, and you're going to want
to read that, and that'll give
a little background information
about the oceans.
And there may be a few little
other bits and pieces
scattered throughout the
textbook on oceanography.
So take advantage of what you
have there, but realize there
isn't a great deal, and so
you'll have to rely fairly
heavily on the notes here.
OK, so some of you have
seen this, probably
most of you seen this.
The basic idea behind plate
tectonics is that the
spherical skin of the earth,
this shallow layer called the
crust that's rather rigid
floating above a deeper
semi-liquid mantle, that crust
is broken into several
discrete plates.
The Indo-Australian plate, the
African plate, the South
American plate, and those
plates, as they move around
remain rigid.
And so the interactions, or the
interesting parts, occur
at the boundaries where one
rigid plate butts up against
another and some kind of
interaction occurs.
Maybe it's a subduction zone,
maybe it's a new--new plate
material being formed, but
that's the way we try to
understand the structure of
the earth these days is to
understand where are these
plates, and how do their edges
deform as the plates
move around.
So for example, that boundary
there is a mid-ocean ridge, or
a so called mid-ocean spreading
center, you can tell
that from the arrow.
So new ocean crust is being
formed at that point, and so
this plate is moving away from
that plate at a certain rate.
Up here in the North Atlantic as
well, you've got the plates
pulling away from each
other, and new ocean
crust is being formed.
In a line like that look, this
artist has used kind of a cold
front symbol, I don't know where
he got that idea from.
But the basic idea here is that
that symbol, for this
artist, is referring to a
subduction zone, where one
plate is being drawn
down under another.
And so crustal material is being
disappeared, it's being
returned down into the
mantle of the earth.
Notice that some plates are
consisting of ocean only, some
plates are consisting of a
combination of continental
crust and ocean crust. OK, so
that's the basic idea behind
plate tectonics.
The plates are shown also in
this cartoon, and once again
note that for example, this
large Pacific plate, fairly
rigid and giant in size,
is ocean crust only.
Whereas most of the others, not
the Nazca plate, but the
Australian plate, has ocean
crust plus continental crust.
The Eurasian plate has both,
North American plate has a big
chunk of ocean in it.
So when you think
about plates.
Remember the earlier, the thing
that preceded this, was
the theory of continental
drift.
Continental drift was the idea
that people noticed this nice
jigsaw fit, for example, between
South America and the
bite of Africa.
Or the way this coast kind of
could fit in against here.
Or the fact that the rocks
here, were similar to the
rocks there up in Scotland.
So the idea was that, early on,
that the continents may
have moved.
But the early idea was that they
moved through the ocean,
the continents plowed their
way through ocean crust to
move around on our planet.
However that was soon found to
be incorrect, because the
physics of trying to push a
continent through ocean crust
was shown to be impossible.
Instead this is the vision that
now seems to be the right
one, where you've got plates
sometimes consisting of
continents and oceans.
They move relative to another,
relative to one another, and
all the action is right at
their boundaries, where
there's creation of new crust
or the destruction of crust.
So this is the conceptual model
that seems to fit all
the data that we have.
And what would drive that kind
of motion, well it's basically
part of the mantle convection.
So there's heat being generated
in the interior of
the earth, by the decay of
radioactive elements, like
uranium and so on, decays
naturally, releases heat.
The interior of the earth
heats up, and that
destabilizes the lapse
rate if you like.
It basically, if you're heating
this fluid from below,
you're destabilizing it
and convection begins.
And as part of that convection
cell, then you get spreading
centers for the crust and then
subduction zones where some of
that crustal material is drawn
back into the mantle, and
melted, and returned.
So it's not a crust only
phenomenon,, it's driven by
mantle convection, but for our
purposes we're interested
primarily in what it does to
the crust of the earth.
So going back into
geologic time, we
can see this happening.
So here's the present day, and
then we go back in time to the
Cretaceous, the Jurassic, the
Triassic, and Permian.
Here's a geologic time scale,
the age of the earth, this is
in thousands--in millions of
years, the age of the earth is
back here around five, roughly
five billion years ago.
And the first one of these
diagrams that the artist is
showing is the Permian,
which is about 255
million years ago.
At that point all the continents
were together, in a
giant super continent
called Pangaea.
And then as time progressed it
split up, first with a seaway
that came through this way, and
then eventually you begin
to get the Atlantic ocean
opening up, and today you have
something like this.
So we're going from 255 million
years ago for the
Permian, and then stepping
forward to the Triassic, the
Jurassic, the Cretaceous.
So when the dinosaurs
roamed the earth,
it was in this stage.
And when humans evolved,
well it was already
looking like this.
So humans never saw this
configuration, humans evolved
just in the last couple
of million years.
So we've been looking at
that structure for our
evolutionary history.
Now, so what does this mean
for the structure of the
oceans and the continents?
So here's a section through
let's take it through here I
guess an east-west, section.
I think it's in the southern
hemisphere, let's check that.
Yeah, so there's South America,
so this is an
east-west, section through
the South Atlantic ocean.
It shows the Pacific Ocean
plate, which is continental
crust being subducted below the
mountains of South America.
As that material is drawn down
into the mantle, it melts.
As it melts, some lava, some
magma, has come off of that.
That then make their way upwards
to cause the volcanoes
along the west coast
of South America.
Otherwise the rest of that
material is just lost down
into the mantle.
When you get over into the
Atlantic Ocean, there's a
rigid boundary there that's
part of the same plate.
Continental crust, and ocean
crust, part of the same plate,
but there's a spreading
center.
That's where molten material
is coming up again and
solidifying as it cools to form
new ocean crust. So this
is moving away, while new ocean
crust is being created
right at that point.
And then over here, that's a
rigid-rigid connection, so
that once again is part of the
same plate with no crust being
lost or gained at that
particular boundary.
But the point I want to make
here is that there are two
types of crust. Continental
crust is generally of a
lighter material, and floats a
bit higher in the semi-molten
parts of the mantle, whereas
the ocean crust is a little
bit denser and floats
a bit lower.
So you've got basically a lower
floating ocean crust
here, and a less dense higher
floating continental crust.
And then when you fill that
with water the water has
nothing to do with this of
course, but the addition of
water makes it seem like
an ocean to us.
There happens to be enough
water in the ocean to
generally cover the ocean crust,
but not to cover the
continents.
Now if there were twice as much
liquid water available on
the planet, that distinction
would be less important
because the water level would
be here, and it would cover
both the ocean crust and the
continental crust. But with
the amount of water that we
have, it means that the
continental crust usually sticks
above sea level and the
ocean crust does not, it's
submerged below sea level.
Remember that amount
of water is quite
unconnected with any of this.
The amount of water we have on
the planet probably did come
out of the interior of the
planet over geologic time, but
it's just an accident that it
happens to be deep enough to
cover the ocean crust but not
the continental crust. And
we'll find a few exceptions
to that when we look
more closely as well.
Any questions on this?
Yes.
STUDENT: So what are the
differences between the ocean
crust and continental crust
besides just their density?
PROFESSOR: Well, they're made of slightly
different chemical
compositions.
Normally there's more, I think
quartz generally in these
rocks, a light mineral allows it
to be a little bit denser,
a little bit less dense.
And other denser minerals are
found more prolifically in the
ocean crust, which makes
it a little bit denser.
OK, we'll come back to that.
But I wanted to make this point,
so when you then take
a, what's called, make a
hypsometric curve, I'm going
to focus on the bar graph
over here on the left.
This is elevation in meters
above and below sea level, sea
level is marked at zero here.
Which I remind you is a somewhat
arbitrary choice, it
depends on how much water
we have in the oceans.
And over geologic time,
that has probably
changed a little bit.
For example, when you have an
ice age you store some of the
water up on the continents in
the form of glaciers, and sea
level drops a little bit.
So that is kind of an arbitrary
reference point, but
it's commonly used and
so we'll use it here.
Whats shown in the bar graph
then, is the percent of the
earth's surface that lies, for
example, between sea level and
one kilometer above.
And it's about 20% of
the earth's surface.
Between one kilometer and two
kilometers above sea level,
it's about 5% of the
earth's surface.
And you find some parts on the
continents that are even
higher, even up to four, but you
know in fact Mount Everest
is up here somewhere, there's
even a little bit of land that
lies 9000 meters above
sea level.
Going down below sea level, you
find there's not much land
at one kilometer, two
kilometers, and three
kilometers below sea level.
But a lot at four,
five, and six
kilometers below sea level.
Now this is a bit of a surprise
because if the earth
was just a rough surface, had
been roughened by some
process, it would have kind of a
normal distribution for this
hypsometric curve.
It would have some average
height, and then less above,
and less below.
But actually no, this has
a double peak, a very
interesting double peak.
And of course that has to do
with the point I already made.
There are two types of crust
here, this is continental
crust and this is ocean crust.
So this plate tectonics that
gives us the two types of
crust, ocean crust and
continental crust, is the cause
for this double peak in
the hypsometric curve
for land elevation.
And again it just so happens
that we have an amount of
water that puts most of this
down below sea level, and some
or most of the continents
just at sea
level or slightly above.
So now we can turn to the
particular features,
bathymetric features
in the ocean.
I'm going to talk about the
abyssal plane, which are these
flat lying parts of
the ocean bottom.
And then some of these other
things that have to do with
plate tectonics like the
mid-ocean ridges, the
trenches, and then some other
features as well.
The way we know all of this
by the way is from
acoustic depth profiling.
So you take a ship, and it sends
out an acoustic signal,
a sound wave, and you bounce it
off the bottom and you time
how long it takes that signal
to go down to the bottom of
the ocean and back up again.
And in the old days you just
had a single pinger going
right directly down, so you'd
have to take the ship back and
forth on a very complicated
long route
to map out the ocean.
But now, they can send it out
in a fan with different
acoustic beams going in
different directions, so you
can do a single swath as you
go along and get depth over
some range to your left
and to your right as
the ship sails along.
Also in some cases, but I won't
be emphasizing it here,
you can look for other
reflections
off subsurface layers.
For example, that sound wave
may go down into the ocean
crust or the sediments a little
bit, and bounce back
up, giving you ideas of what's
going on below the
bottom of the ocean.
And that'd be useful if you're
doing geological surveys of
the ocean crust. But for our
purposes we're just going to
be using that to map out the
actual ocean depth itself.
Stop me if you have
questions here.
Sound moves rapidly in seawater,
the speed of sound
in air is about 300 meters per
second, speed of sound in
water I think is three or four
times that, it's really--it
goes quite rapidly.
Nonetheless it's still a finite
speed, and you can
easily time how long it takes
for that acoustic signal to
get back to your receiver and
get the depth from that.
So this is a cartoon just
showing some of the features.
For example, there's a section
of abyssal plane, that kind of
flat lying section.
Flat in part, because it's
composed of sediments that
have fallen from above, and as
they fall they fill in the
cracks first, and then as you
get quite a pile of sediments
it tends to give you
a flat surface.
A mid-ocean ridge, one of these
spreading centers where
magmas are coming up and forming
new ocean crust, tends
to be elevated because those
rocks are still hot, and less
dense, and they float a bit
higher than cold ocean crust
until they cool down.
And then they sink a little
bit away from
the spreading center.
But that could come up a bit,
if this abyssal plane is at
five kilometers, then this
mid-ocean ridge might only be
two or three kilometers below
the ocean surface.
You can have undersea volcanoes,
that'll start
building from the ocean floor,
just like volcanoes on land
start to build from
the continental
surface and build upwards.
Undersea volcanoes build up from
the ocean floor, and in
some cases they will not reach
the ocean surface, in which
case they're called seamounts.
I don't know why they've drawn
this the way they have,
indicating that these have
penetrated the earth's
surface, the ocean surface.
Usually the word seamount is
confined to an undersea
volcano that has not reached
the earth's surface, so I
disagree with the artist
a little bit here.
On occasion you find them with
flat tops, which means that at
one point they reached the ocean
surface and were leveled
by wave action.
And now they've settled back
down a little bit, so you'll
find undersea volcanoes some of
them with a flat surface,
those are called guyots.
And then the ocean trenches,
where you get the subducting
plates, are the deepest parts
of the ocean generally.
Then I wanted to make
this point about
a continental shelf.
So there's a continent, there's
a continental shelf,
there's the drop down to
the abyssal plane.
You see then geologically, a
continental shelf is really
part of the continent.
It just so happens that the
water level is high enough so
it's covered up slightly some of
this, making it appear on a
map of the earth that its ocean,
but geologically it is
continental.
And there should be very little
confusion about the
two, because this is going to
be a very shallow ocean,
probably only 100 or
200 meters deep.
Whereas this is five kilometers
deep, so it's going
to be pretty clear to separate
geologic continental
structures from geologic ocean
crustal structures.
Because they really are at a
very different level, even
though there's enough water at
the present time, especially
with the glaciers mostly melted
at this time to make a
little thin layer of
water covering
those continental shelves.
So now here's the whole, the
whole world ocean, and the
color scheme is not quantified
on here, but I can tell you
basically what's going on.
All the deep blue is abyssal
plane, about
five kilometers deep.
And you see it a
lot of places.
These little lines that run up
through the middle of oceans,
for example in the South Pacific
and all the way up
through the Atlantic, the south
and the north Atlantic,
those are spreading centers,
mid-ocean ridges.
And then the trenches don't show
up well on this diagram,
but they are the darker
blue still.
And you see a little thin line
along there that's a trench,
you see one here, you see an
important one here, and up
along here, and even along the
tip of Aleutian Islands.
So those are the subduction
zones.
You also see a scattering of sea
mounts various places, and
other features.
But I'd say oh and you see the,
for example, right along
there, you see a nice example
of continental material with
just enough water over it so we
would call that ocean, but
geologically it is continent.
That's the continental shelves,
continental shelf area.
So this is the geometry in which
then we'll be studying
ocean currents and so on.
It's going to be constrained by
this pattern of depth, and
the thing that's going to be
very important to us is the
way that these continents
tend to break up
the oceans into segments.
For example, Asia with Australia
included, and North
and South America, break up the
Pacific ocean into kind of
a north, south, oriented
ocean.
North America, South America,
compared with Europe and
Africa, break up the Atlantic
again into a north, south,
oriented ocean.
The Indian Ocean is a little
bit different because Asia
fills the northern hemisphere,
most of it, down to about say
20 degrees north latitude.
So the Indian Ocean is primarily
an ocean just in the
southern hemisphere.
And then, the one gigantic
exception to this is the
so-called Southern Ocean.
The term for that strip of ocean
that goes all the way
around the globe, it's usually
called the Southern Ocean in
oceanography.
And if you go far enough south
in the Pacific you join onto
it, the Atlantic you join onto
it, the Indian you join on.
And as we'll see, you can have
ocean currents here that go
right around the globe.
Whereas at all these higher
latitudes, any ocean current
that moves east-west is going
to hit a continent and is
going to have to wrap
back around.
So you get what are called
gyres, in most of the oceans,
because they're confined
by these
north-south oriented barriers.
So gyres are the things we'll
be studying for most of the
oceans, but not in the
southern ocean.
There we'll be able to look at
a current that goes all the
way, all the way around
the globe.
Questions on this?
So we'll zoom in a little bit
to--so all the major oceans,
there's the Atlantic ocean.
A little bit of a trench here,
but generally it's abyssal
plane, and it is with
a mid-ocean
spreading center here.
By the way, so that's spreading
which now these
continents were originally
joined together, and you can
see how well they fit from a
jigsaw puzzle point of view.
That spreading is at about the
rate the way I remember this
it's at about the rate that your
fingernails grow, so it's
a couple of centimeters
per year basically.
So this is still widening today,
and at the end of the
year it's going to be
a few centimeters--
the Atlantic ocean is going to
be a few centimeters wider
than it is today.
And that's a slow process, but
when you take that speed and
multiply it over millions of
years, you can see how you can
get hundreds or even thousands
of kilometers of ocean width
generated by that
slow spreading.
And then once again, you see
some continental shelf area up
in here, covered by water, but
generally part of the continent.
And you see a big piece of that
down here as well, around
Africa a little bit too.
And all of the Mediterranean
sea, most of it is you'd
probably call it
ocean--continental crust
rather than ocean crust.
Pacific ocean, vast areas of
abyssal plane, but there are
trench systems hard to see,
but the black lines.
Mid-ocean ridge that comes down
here, was spreading, and
then lots of ocean crust here.
And then up along California,
you're having some motions.
It's more of a transform fault,
where things are moving
parallel to one another, not
exactly a spreading center,
not exactly subduction, but some
complicated combination
of the two including
lateral slip.
And so we'll see, once again,
we'll get gyres in the
Northern and Southern Pacific
constrained by the barrier
affect of the continents
to the east and west.
Indian Ocean bathymetry, big
continental a big abyssal
planes, and then some curious
old, these appear to be old
spreading centers that
are no longer active.
Maybe Erin can tell us
more about that.
But anyway there's some
structures there, but they
don't appear to be active
spreading centers.
And here's some of the
subduction zones over in the
Pacific there.
The Arctic Ocean has some
interesting structure, but
it's pretty passive too.
There's two deep basins--most of
this is continental shelf,
pretty shallow, depth the order
of a few hundred meters.
But there are a couple of deep
parts as well, that go down to
several kilometers.
And then a ridge, I think a
passive one, that's called the
Lomonosov Ridge, but I don't
think that's an active
spreading center.
So this has got some interesting
structure, but
it's more of a passive structure
at the moment, it's
not an active spreading
center.
OK, so that's an introduction
to the shape
of the ocean basins.
Are there any questions
on that?
I want to turn now to ocean
properties, ocean water
properties.
Sea surface temperature, that
we measure from ships, from
instruments below the ships,
and from satellites.
We can measure salinity from
ships, to get into the deep
ocean we need to put something
down into the ocean, I'll show
you how we do that.
For ocean topography, which I'll
define later, a little
irregularity in the ocean
surface, we use
satellites for that.
And then for ocean currents we
use all those things ships,
floats, and moorings.
So before we're done, I'm going
to talk about how we
measure all these things, and
understand a bit about the
ocean water and how
it's moving.
So here is a map taken in late
August, I think of the year
2000 from satellites, showing
the sea surface temperature,
SST. The color is in Celsius,
and you see that in the
tropical regions you're getting
up to temperatures 28
and higher.
Remember 27, 28, was the
threshold for hurricanes.
So you are getting a lot of warm
ocean temperature that
support hurricanes, some of that
however is right at the
equator and so you couldn't have
hurricanes forming there
because you don't have
the Coriolis force.
But in other parts you get the
warm water extending far
enough away from the equator, so
you could have hurricanes.
I pointed out when we were
talking about hurricanes, two
interesting places.
The Western tropical South
Pacific, where you don't have
hurricanes because it's too
cold, and you see it there.
It's a cold ocean current, the
Humboldt Current coming up
here, taking cold water from
the Southern Ocean, peeling
some of it off, bring it up
here, and keeping that part of
the world ocean cool.
And you see something very
similar here, where cold
waters being peeled off and
come up here to keep the
southern tropical Atlantic
a bit cool as well.
Otherwise it's pretty warm in
the tropics, except where
you're getting these
cool currents.
The California current does a
little bit of cooling in this
region, and the return from
the Gulfstream does some
cooling on the eastern side of
the north Atlantic there, so
that's that now.
But if you get a mental picture
of this, be very
careful what you do with
it, because you could
be very much misled.
This is sea surface temperature,
when I go down
even just one kilometer in the
ocean, or especially if I go
down to the ocean
bottom, it looks
nothing at all like this.
So temperature is not vertically
homogeneous.
As I'll show you later on, this
warm water that can form
near the tropics, forms a rather
thin layer floating on
the cold water that fills
most of the world ocean.
So this is not a picture that
can then be transferred down
into the ocean very
deep at all.
In fact in some cases, you may
only be able to go down a few
hundred meters before this
picture changes rather
dramatically.
Salinity is the other important
property we track in
ocean water, and here's a map
of the sea surface salinity.
So just the surface, just the
salinity you would measure
from a ship if you took a
surface sample of water and
analyzed it.
And well, the most remarkable
thing is the narrow range.
And you get some salinities as
fresh as maybe 31 or 32 parts
per thousand.
And some, perhaps in the
Mediterranean Sea, and the Red
Sea, Gulf of Aden perhaps,
getting up to 38, 39.
But generally that's the full
range of ocean salinity.
So you've heard me say,
occasionally, that sea surface
salinity is 35 parts per
thousand, of course that's not
a very precise statement.
But it only goes about plus or
minus four parts per thousand
around that mean value of
35 parts per thousand.
The reason for this must be that
the ocean mixes itself
occasionally, to maintain
this kind of
rather homogeneous salinity.
At least compared to the rate
at which you're adding
freshwater, or the rate at
which you're adding or
subtracting salt.
So the basic story behind this
narrow range of salinity, is
that the ocean is, at
least for salt,
relatively well mixed.
Now if it were completely well
mixed, it would have the same
salinity everywhere.
If it were vigorously being
stirred, like a vigorous spoon
stirring in a pot, any
differences would be
immediately removed.
So it's not perfectly well
stirred, but is reasonably
well stirred, giving you this
rather homogeneous salinity
over the world ocean.
So that's lesson number one, but
at the next level though
of detail, we can notice that
there are some variations.
And they probably make sense
to us because it's in
these--it's in the so called
Intertropical Convergence
Zone, or the belt of tropical
rainforests, that you get a
little bit lower salinity.
That's right through here, and
you see it here, and a little
bit down through here.
So there's a lot of rain falling
on the ocean there,
fresh water coming down and
diluting the salt a little
bit, giving you a smaller
salinity along the equator.
And then as you move north and
south from that into the belt
of deserts, the descending
branch of the Hadley cell,
with very little precipitation
and some evaporation.
Remember when you evaporate
seawater, you leave the salt
behind, and so the salinity
is going to be increased.
And you see the increased
salinity there in both the
Northern and Southern
hemisphere, connected with the
belt of deserts.
And then you get up in the
mid-latitudes and once again
you've got the frontal storms,
cold fronts, warm fronts,
bringing rain and
that once again
dilutes the surface salinity.
So a lot of things we've spoken
about before, in terms
of the general circulation of
the atmosphere and where it
rains and where it doesn't, are
reflected in this plot of
sea surface salinity.
Here again it'd be dangerous
though, to try to imagine that
pattern would extend down
to the oceans very far.
Other things will take place
that will prevent this from
being the pattern deeper
down in the ocean.
Questions on that?
OK, now I want to tell you
about how we get how we
measure ocean properties
down into the ocean.
The old way to do this, and I'll
talk about the new ways
too, the old way for about 50
years, the predominant way for
sounding the ocean, for getting
temperature and
salinity profiles with depth,
was the Nansen bottle.
And here's a diagram, here's a
picture of one, mounted on a
cable that's about to be put
down into the ocean.
And here is a cartoon of what
happens after it's put down
into the ocean and is triggered,
so that it tips
over and the valves
close on it.
Now I have one of those with
me, and I want to show how
this thing works because it's
kind of a clever gadget.
And although it's not being
used much anymore, much of
what we know about the
world ocean came
from this simple device.
Again it's called a Nansen
bottle, it was designed by the
famous Norwegian explorer
Fridtjof Nansen.
And it is composed of a hollow
metal tube, fairly thin and
its steel, with a valve at the
top and the bottom, that is
currently If I look through this
it's open, I've got both
valves open.
And there is a tie rod that
connects the two valves, and
so when this tie rod shifts
relative to the bottle itself,
it'll close both valves
simultaneously.
So there's also a couple of
housings for thermometers on
the outside, so you can get the
temperature of the water
at that depth as well.
So imagine that you've come
you've taken your ship to a
given location, and you've got
a long cable coming up to a
winch and a pulley, that then
takes that cable overboard and
down to whatever depths
you want to go to.
So how do we get started
with that?
Well the winch operator gets
some of the cable in the
water, and then the scientist
would lean over and attach the
first one of these Nansen
bottles to this cable.
So imagine you've got a cable
coming down like this, and
you're leaning over the side of
the ship, and you reach out
and you fasten this thing in
by putting the cable right
down in there and pushing
that little pin over.
And then down here the cable
comes through there, and you
lock it with a turn buckle.
So it's locked securely
at the bottom, not so
securely at the top.
Once you get it on there, you
wave to the winch operator,
and he puts that cable down.
And you'd repeat that about 20
times over the next hour or
two, and when you're all done
you've got a cable overboard,
perhaps going down to
five kilometers.
You can do this all the way down
to the abyssal plane, and
along this cable you have Nansen
bottles, perhaps as
many as 20 of them.
When that's all in place, then
you reach over one more time
and you fasten a little thing
called a messenger, a little
brass cylinder that slides
down the cable, hits this
little plate, knocking free this
pin, and now the weight
of the bottle begins to act.
Let's see if I can do this now,
OK I'm having trouble
doing it with my bum arm here.
But I'm pushing this thing down,
and as I do so it is has
fallen away to an orientation
like this, and now both valves
are closed, trapping water
from that level.
So when it's brought up, you've
got samples of the
water at each of these depths.
Also, when you flip over these
thermometers, they're so
called reversing thermometers,
and when they're flipped over
the mercury column breaks, and
so you lock in the temperature
at that depth.
And when you bring it up, you've
got a record of what
the temperature was
at that depth.
So by flipping it over, you lock
in the temperature, and
by flipping it over and closing
the valves, you lock
in the sample of water
from that depth.
Then over the next couple of
hours you bring that cable
back to the surface, and each
time when a Nansen bottle
emerges, you reach over,
disconnect it,
and put it in a rack.
And after several hours of work,
you've got data from the
whole sounding.
And over a period of 50 years
or so throughout the 40's,
50's, 60's, 70's, and 80's, this
was used to map out most
of the world ocean in terms
of its temperature
and salinity structure.
Questions on that?
You can pass that around if you
like, so people can get a
sense for it.
Now my first oceanographic
cruise I managed to embarrass
myself, by failing to get that
Nansen bottle properly secured
before I waved to the
winch operator to
tell him to go down.
So the very first one of these
I did, one of the Nansen
bottles came up totally
destroyed, crushed.
And can you figure out how
that would have happened?
So what happened is, it wasn't
secured properly at the top.
It had flipped, I didn't notice
I turned my back away,
but it flipped before it
entered the water.
So both valves had closed with
air inside, and then it went
down to depth.
And of course, the pressure is
enormous at the bottom of the
ocean, you can do a quick
hydrostatic calculation to
understand you can get hundreds
of atmospheres of
pressure down there.
And it just took that bottle
with air inside, and just
crushed it like a Coke can in
your fist. And it came up all
wrinkled, and my chief scientist
was not very happy
with me but I learned to do
better the next time.
Anyway it's important
historical, but also to
understand how that was done.
Today it's done a bit
differently, it's done with a
CTD and a rosette.
Now CTD is an abbreviation
meaning conductivity
temperature depth.
It's an electronic device, the
conductivity part is similar
to what we did on
the river lab.
As you lower this thing down
into the ocean, it measures
the electrical conductivity
and from that you can
determine the salinity.
It also has a thermistor on
there, so you can measure
temperature, and it's got a
pressure sensor on there so
you can measure depth.
So as you lower this down, in
real time because there's a
cable coming back to the
surface, you're getting a
detailed profile
of temperature,
salinity, and depth.
That's a great system.
However that of course would not
give you a water sample,
so if you want to do any kind of
water chemistry a CTD would
not be sufficient.
The beauty of the old Nansen
bottle was, you got a water
sample as well.
So if you later on decide you
want to do any kind of
chemistry, you had that
water sample.
The CTD is a great advance on
that, but it doesn't give you
the water sample.
So what's usually done today,
is that they have a set of
water collecting bottles
arranged around the perimeter,
and that's called the rosette.
And they've got an electronic
control, where the valves on
these can be closed
on command.
So in addition so you send this
down to the bottom the
ocean, profiling conductivity,
temperature, depth.
And then every, I don't know,
500 meters or so, you might
close one of the valves on these
bottles and you get a
water sample from that
depth as well.
So if you were to go out on an
oceanographic cruise today,
you'd be largely using the CTD
and the rosette to do vertical
profiling of temperature
and salinity.
Questions on that?
So there's one, they brought it
back on board and they're,
I guess, drawing samples off
of the, from the different
water bottles there.
Now technology is advancing
even beyond that, however.
Remember even with that one, you
have to get a ship there,
you've got to get a cable over
that can go all the way to the
bottom, and that can take a
couple hours to do, and ship
time is very expensive.
So now the field is moving
in the direction of these
autonomous explorers, where it's
an unmanned vehicle that
you launch from a ship but then
it pretty much goes its
own way, and you
can control it.
You talk to it with sonar,
sending acoustic signals to it
which it can receive, and it
will then cruise around in the
ocean up, or down, or laterally,
measuring.
Of course you don't get water
samples from this but you get
temperature, conductivity,
and depth, because this
has a CTD on board.
And you can control it and move
it wherever you want,
back and forth.
In one area, if you think
conditions might be changing,
or traversing larger parts of
the ocean, if you want to map
out a big piece of
the ocean volume.
So this is a new thing coming
on, it's quite exciting to
have this, and you can be back
in your office actually in a
way, and this data is coming
out on your screen.
It's pretty remarkable what
this will mean for
oceanography.
This is just getting started,
so this is really opening up
brand new doors for
understanding the oceans with
these autonomous explorers.
Any questions on that?
OK.
So what do you get
from this then?
We're almost out of
time, but here's a
typical ocean sounding.
Temperature, salinity, and
density, versus depth.
So on this plot, zero is taken
to be sea level, and this goes
down to about 4000 meters which
is almost the depth to
the abyssal plane.
Very often you find rapid
changes at first, and when the
temperature drops from warm
surface to colder values
beneath, that region of strong
temperature gradient is
referred to as the
thermocline.
And you'll hear that term over
and over again, it's very
important in the ocean,
this thermocline.
In this particular sounding,
it started about 200 meters
below the surface, and by the
time you got down to 500 or
600 meters, you're at a
temperature of about four
degrees Celsius, and then
eventually down to just one
degree Celsius.
Whereas at the surface you had
25, 26, 27 degrees Celsius.
The salinity in this case was
large at the surface, 34.9,
became a bit fresher as you
dropped down through what's
called the halocline, and then
became slowly a little bit
saltier beneath.
Now the density of seawater is
controlled primarily by the
temperature and the salinity.
The density is a very important
quantity, but if you
know temperature and you know
salinity, you can compute or
you can measure the density.
So what's plotted in this final
panel, is the density
derived from the measured
temperature and salinity.
And it shows a lower density
water near the surface by the
way, the units are in grams
per cubic centimeter here,
remember fresh water has
a density of about
one in those units.
So this is a little bit denser
than fresh water, and gets
even denser by the time you
get down to the bottom
primarily because of the
temperature in this case.
The colder temperature is giving
rise to the lower--to
the higher density in
the deep ocean.
I think I'm out of time, so
we're going to continue this
next time and talk about the
concept of static stability.
When will water remain
in layers?
When will it overturn
to form convection?
And this will be the starting
point for that discussion.
