(upbeat music)
>>Hello, I am Dr. Deanna Hence,
and I'm Assistant Professor
at the University of Illinois,
Department of Atmospheric Sciences.
In this video, I'll be talking about atmospheric
and oceanic circulation and the role they play in climate.
When we stand outside, we will often feel the wind
blowing against us.
When we swim in the ocean, we may feel currents
trying to push us away from our spot on the beach.
We're going to talk about how these two fluids,
the air and the ocean, move on a global scale
and how this leads to different climate regimes
around the world.
So why do the atmosphere and ocean move the way they do?
The sun's radiation doesn't hit the Earth evenly.
The tropics receive direct energy from the sun year round,
while the poles not only get less direct solar radiation,
but also spend part of the year not getting any direct
solar radiation at all.
This lead to a big imbalance of energy
with the tropical regions of the ocean and atmosphere
receiving more energy and thus being warmer and less dense
than the poles.
The atmosphere and ocean circulations exist to remove
this imbalance by moving the heat towards the poles.
The air in the tropics is warmer and less dense
than the poles, so the air tends to rise in the tropics,
known as conduction.
Air at the poles is colder and denser, so it tends to sink.
This drives the circulation between the tropics
and the poles in each hemisphere.
This would be how our atmosphere works,
if our Earth didn't rotate.
When we add rotation, the large overturning circulation
breaks up into cells or three smaller circulations
in each hemisphere.
The tropical circulation is called the Hadley Cell.
Air rises in the Intertropical Convergence Zone
or ITCZ, near the equator.
The ITCZ is very rainy and thus many of the world's
luscious rainforests tend to be located in this region.
The descending branch of the Hadley Cell is located
over the subtropics.
This subsiding air tends to be very warm and dry
and is at the latitude where many
of the world's deserts are.
A band of easterly winds known as the Trade Winds,
form between the rising and the descending branches
due to the rotation of the Earth.
The second cell, in the mid latitudes,
is called the Ferrel Cell.
The Ferrel Cell causes westerly winds in mid latitudes,
and there's a reason why we look to the west
when we make weather forecasts.
The third cell at the poles is called the Polar Cell.
This is the smallest and the weakest of the three cells
in the atmosphere.
Air in these cells sink over highest latitudes
and flows towards the lowest latitudes at the surface.
Now the ocean is also working to move the excess energy
it receives in the tropics towards the poles
to remove the energy and balance we've discussed.
The atmosphere and ocean are obviously not the same,
and these differences have a lot to do why the atmosphere
and the ocean move differently.
The ocean is much more dense than the atmosphere.
It also has a higher heat capacity than the atmosphere,
so it takes a lot more energy to raise its temperature,
but also loses that heat much more slowly.
Because the ocean has a high heat capacity,
it can also serve as a sink and reservoir
for heat that the atmosphere and the land cannot.
Also, let us not forget that the ocean isn't just water.
All of that salt means that the density of the ocean
relies on both the temperature and the salinity.
Plus, on top if it all, the oceans have continents
to deal with.
The surface currents in the ocean move mostly
in the same direction as the surface winds
and are similarly deflected by the Earth's rotation.
As water moves towards the poles, it cools,
and becomes more saline as the water freezes.
This cold, salty water is very dense and so it sinks
near the poles.
This deep water moves along the bottom of the ocean
towards the equator.
This ocean circulation is known
as the Thermohaline Circulation.
Upwelling is a process of deep water from the bottom
of the ocean being brought to the surface.
In regions of upwelling, like in the Pacific near Peru,
the oceans seems to be colder and rich with sea life
as nutrients are brought up from the ocean floor.
The ocean circulation happens on a much slower timescale
than what happens in the atmosphere.
Although we see the atmosphere respond quickly
to big changes such as increased greenhouse gas
concentrations, it can take years or decades
for those changes to be obvious in the oceans.
So as you can see, these large circulations
in the atmosphere and the ocean play a key role
in maintaining our climate system.
As the energy balance of our Earth's system changes,
it is essential for us to understand how these circulations
will behave.