- [Instructor] One of my favorite things to do
is go camping.
For me, there's nothing better than getting outside,
breathing in some fresh air
and taking a swim in my favorite river.
Have you ever jumped into a river
and felt that the deeper, cooler water
closer to your feet was moving faster
than the shallow, warmer water at your knees?
That's a current,
which is the a word we use to describe
how water or even air
flows within a larger body of water or air,
but what causes a current?
Well, let's start with the Sun.
The Sun actually heats Earth unevenly.
We know that it's hotter near the equator
and it gets colder as you go towards the poles.
Near the equator,
the Sun's rays hit Earth surface more directly,
while near the poles,
the Sun's rays hit Earth surface less directly.
In both regions,
the same amount of solar energy is heading Earth,
but near the equator, this energy is concentrated
into a smaller area
and near the poles it's spread out over a larger area.
So the regions near the equator get more solar energy,
which makes them warmer
and the regions near the poles get less solar energy,
which makes them cooler.
This uneven heating at Earth also affects air pressure,
where it's cooler near the poles,
cool air will sink making the air pressure high,
but where it's warmer near the equator,
warm air will rise resulting in low pressure.
This is where the terms low-pressure cells
and high-pressure cells come from.
The low-pressure warmer air at the equator
rises into the upper atmosphere where it cools
and flows away towards higher latitudes
away from the equator.
Because the air is now cooler,
it starts to sink again
and creates a high-pressure band near these latitudes.
This process repeats and creates a pattern
of high and low-pressure bands
from the equator to the poles.
We know that air flows from areas of high pressure
to areas of low pressure.
This creates air occurrence or winds.
Now you might think that these winds
would blow in straight lines
from high to low-pressure areas.
But the global wind patterns,
which we call prevailing winds,
look like they curved the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
This curving has to do with the rotation of Earth
and is called the Coriolis effect.
As these prevailing winds
blow across the surface of the land and water,
they also pushed against the surface of the ocean
and produce wind-driven surface currents,
which helped to move ocean water.
Here's what the global pattern
of ocean surface currents looks like.
Like wind currents,
ocean surface currents are also curved
due to the Coriolis effect.
We can see that in these currents
that are traveling north and south,
which curved to the right in the Northern Hemisphere
and to the left in the Southern Hemisphere.
Together as these surface currents of the ocean connect,
they formed giant rotating systems
of ocean currents called gyres.
The currents that drive these gyres
extend from the surface to about one kilometer
down into the ocean
and helped move water all around the globe.
But these gyres aren't just moving water,
they're moving heat energy as well.
Water is pretty good at holding onto heat
it absorbs from the Sun.
So as the water in our oceans moves around the world
through this gyre circulation,
the water also carries heat.
Here, warm water generally moves
from the equator to the poles
and cold water moves from the poles to the equator.
But the ocean has other deeper currents
that are affected by differences in temperature and density.
Remember how we talked about areas heated directly
and less directly by the Sun?
And how that results in low and high-pressure areas?
Same thing with water, except that water density
is affected by both temperature and salinity,
which is a measure of how salty the ocean is.
Cooler and saltier water is more dense,
so it tends to sink, just like cool air.
Whereas warmer and less salty water is less dense
and tends to rise, just like warm air.
So with these deeper ocean currents,
water actually moves vertically or up and down.
For example, water near the poles gets very cold.
It also gets very salty, because when sea ice is formed,
the salt can't go into the ice.
Instead, the salt stays behind in the water
and so the water gets saltier or more saline.
Together, the coldness and salinity
makes the water very dense
causing it to sink deep into the ocean.
And other parts of the ocean,
wind drags deep water up to the surface
and a process called upwelling.
These vertical currents are connected
by horizontal currents at the surface and in the deep ocean.
Collectively,
this system of currents is known
as the overturning circulation.
You might also hear it called
the global ocean conveyor belts.
So here's a map showing the overturning circulation.
This map might look a little bit strange,
but here we're looking at Earth from the South Pole.
So Australia and the southern tips of Africa
and South America are closes to the center of the map,
while Europe, most of Asia and North America
are at the edges.
Now, if you follow the currents
in the overturning circulation,
you can see that they flow all over the world's oceans.
From the southern ocean around the South Pole,
to the Pacific, the Indian
and all the way into the North Atlantic.
Like the currents in the gyre circulation,
the currents in the overturning circulation
also carry and disperse heat energy all around the world.
Here I am, back in my favorite river, waiting around
and enjoying currents of cool water flowing around my feet.
And even though this river is small,
the currents that flow through it
are similar to the global wind and ocean currents
that flow all around the world.
So these currents connect in our atmosphere and oceans,
which means that we are all connected.
So currents connect us all.