Hi marine biology students.
In this video we're going to talk about ocean
circulation and how water moves from one ocean
basin to another.
[Intro Music]
So let's talk about ocean circulation.
Some of the differences seen in different
parts of the ocean are due to circulation
patterns.
Circulation can occur in the form of waves,
tides, currents, and gyres, which are large
circular currents that are found in ocean
basins.
Oceanic circulation is significantly driven
by wind patterns and wind is ultimately the
result of being driven by solar energy.
Because the Earth is rotating, both winds
and currents are affected by the Coriolis
effect.
Because the Earth spins continuously, anything
that passes over the Earth is deflected.
In the Northern Hemisphere, winds and currents
are deflected to the right whereas in the
Southern Hemisphere winds and currents are
deflected to the left.
Since the earth is spinning, it causes this
deflection of wind and currents.
This deflection often causes currents to travel
in circular patterns called gyres.
Here we can see some of the major oceanic
gyres, including those in the north and south
Pacific, north and south Atlantic, and also
within the Indian ocean.
Now, because of the hemisphere that these
gyres are found on, the gyres on the Northern
Hemisphere circulate in a clockwise direction
whereas the gyres in the Southern Hemisphere
circulate in a counterclockwise direction.
Because of this current, the west coast of
the United States will have primarily cold
water from the Arctic coming down along that
coast, whereas the east coast of the United
States will have warm equatorial and gulf
water going up its coast.
So why do these gyres happen?
Well, sunlight heats the air above the equator
and that air rises.
As that air rises, it becomes an area of low
pressure and other cooler air rushes in to
fill that open spot.
This movement is the source of winds.
Near shore winds are also generated as there
are differences
in air temperature over land versus water.
The air over the land heats up quicker and
so will rise and draw ocean breezes onto the
land.
Likewise, if the air on land cools down but
the ocean is still warm you'll have breezes
from shore blowing out to sea, often during
the evenings.
Winds that are continuously created in the
matter described by the equator are known
as the trade winds.
When we look at different latitudes, the westerlies
are formed in the mid-latitudes while the
easterlies 
are formed closer to the poles, yet both the
westerlies and the easterlies are less consistent
than the trade winds.
As solar energy heats up the air at the equator,
that air rises and that area of low pressure
is filled in by cooler air coming from other
places.
This forms the basis of the trade winds.
Now we can see multiple areas of pretty consistent
wind formation along with areas with very
slight wind formation neighboring them.
It's these wind patterns that promote the
formation of these oceanic gyres.
Despite being repeatedly mixed by winds and
surface currents; the ocean is stratified
into three layers.
The surface layers are from the surface to
about 200 meters.
This layer stays well mixed most of the year
and these are also the surface currents which
are moved by wind.
Deeper than 200 meters, in fact from 200 to
1500 meters, we have our intermediate layer.
A noticeable feature of the intermediate layer
is a sharp temperature change or thermocline.
The water is more uniformly dense here and
so there's very little mixing.
After the intermediate layer we have the bottom
layer which is below 1500 meters.
There's very low mixing and it's normally
uniformly cold down there.
Sometimes this cold deeper water will up well
in certain areas, but for the most part it
is consistently denser than the surface and
intermediate waters.
Here in this diagram we can see how salinity
varies in the surface waters, but less so
in the intermediate and deeper layers.
We can also see the temperature change known
as the thermocline that happens throughout
the depth of the intermediate waters and we
can also see how density can vary between
those layers.
In some locations, large volumes of water
may sink or rise.
Water sinks due to changes in temperature
and salinity and this is known as downwelling.
Downwelling brings gases from the surface
to deeper layers of the ocean and this becomes
very important for these marine organisms
living at these deeper layers.
Here we see a seasonal downwelling event,
where during the summer and even the autumn
the surface temperatures are too warm and
therefore less dense than the deeper waters,
but in the winter as the surface layers are
cooled to temperatures even lower than the
deep water, those surface waters will go down,
taking with them dissolved gases like oxygen.
Areas of upwelling come from currents that
push deeper waters towards the surface.
Nutrients are much more plentiful in these
deeper waters they have been fortified with
nutrients over time while they've been down
deep in these lower layers, and so, often
areas of upwelling are beneficial for surface
organisms.
It's an influx of nutrients and usually there'll
be a lot of algae growth and that can drive
whole food webs.
Now, when we look at the global patterns of
upwelling and downwelling within the ocean
basins, it actually turns out that all of
our ocean basins are connected to each other
in what's known as the Great Ocean Conveyor,
and this Great Ocean Conveyor is driven by
thermohaline circulation.
The areas of deep water formation include
the Arctic ocean and north Atlantic and also
around the Southern ocean, where areas of
upwelling are going to be in the northern
Pacific and also in the Indian oceans.
Water can pass all the way through this global
conveyor system, but it takes over a thousand
years to do so.
So ocean water cycles through this system
anywhere from one thousand years to eighteen
hundred years.
Water that is being upwelled right now in
the north Pacific had last been surface water
more than one thousand years ago.
Likewise, the deep water being formed right
now in the Arctic and north Atlantic and its
associated dissolved gases will not have contact
with the ocean surface for some time.
There are concerns that human-mediated climate
change may have an impact on global ocean
circulation with unknown and potentially significant
consequences, so understanding the system
of global ocean circulation is important as
we try to provide insight to the rest of the
world.
So the world oceans really are connected.
Well that's the end of this video.
Something I want you to think about before
our next video is “Have you ever noticed
how waves seem to get larger as they come
into shore?”
We're going to talk about that.
All right, I'll see you in the next video.
