The motion of the ocean: The concentration gradient - Sasha Wright

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The constant motion of our oceans represents a vast and complicated system involving many different drivers. Sasha Wright explains the physics behind one of those drivers -- the concentration gradient -- and illustrates how our oceans are continually engaging in a universal struggle for space.

Lesson by Sasha Wright, animation by Andrew Foerster.

If you've ever floated on an ocean swell,

you'll know that the sea moves constantly.

Zoom out, and you'll see the larger picture:

our Earth, covered by 71% water,

moving in one enormous current around the planet.

This intimidating global conveyor belt

has many complicated drivers,

but behind it all is a simple pump

that moves water all over the earth.

The process is called thermohaline circulation.

And it's driven by a basic concept:

the concentration gradient.

Let's leave the ocean for one moment

and imagine we're in an empty room

with lots of roombas sardined together

in one corner.

Turn them all on at once

and the machines glide outwards

bumping into and away from each other

until the room is filled with an evenly spaced distribution.

The machines have moved randomly

towards equilibrium.

A place where the concentration of a substance

is equally spread out.

That's what happens along a concentration gradient.

As substances shift passively from a high,

or squashed, concentration,

to a lower, more comfortable one.

How does this relate to ocean currents and thermohaline circulation?

Thermo means temperature and

haline means salt

because in the real-world scenario of the sea,

temperature and salinity drive the shift

from high to low concentrations.

Let's put you back in the ocean

to see how this works.

Snap!

You're transformed into a molecule of surface water,

off the temperate coast of New York

surrounded by a zillion rowdy others.

Here, the sun's rays act as an energizer

that set you and the other water molecules

jostling about, bouncing off each other

like the roombas did.

The more you spread out,

the less concentrated the water molecules

at the surface become.

Through this passive motion,

you move from a high to a lower concentration.

Let's suspend the laws of physics for a moment,

and pretend that your molecular self

can plunge deep down into the water column.

In these colder depths,

the comparative lack of solar warmth

makes water molecules sluggish,

meaning they can sit quite still at high concentrations.

No jostling here.

But seeking relief

from the cramped conditions they're in,

they soon start moving upwards

towards the roomier situation at the surface.

This is how temperature

drives a shift of water molecules

from high to low concentrations,

towards equilibrium.

But sea water is made up of more than just H2O.

There are a great deal of salt ions in it as well.

And like you, these guys have a similar desire

for spacious real estate.

As the sun warms the sea,

some of your fellow water molecules

evaporate from the surface,

increasing the ration of salt to H2O.

The crowded salt ions left behind

notice that lower down,

salt molecules seem to be enjoying more space.

And so an invasion begins,

as they too move downwards in the water column.

In the polar regions,

we see how this small local process

effects global movement.

In the arctic and antarctic,

where ice slabs decorate the water's surface,

there's little temperature difference

between surface and deeper waters.

It's all pretty cold.

But salinity differs,

and in this scenario,

that's what triggers the action.

Here, the sun's rays melt surface ice,

depositing a new load of water molecules

into the sea.

That not only increases the proxmity

between you and other water molecules,

leaving you vying for space again,

but it also conversely dilutes

the concentration of salt ions.

So, down you go,

riding along the concentration gradient

towards more comfortable conditions.

For salt ions, however,

their lower concentration at the surface,

acts like an advertisement

to the clamoring masses of salt molecules below

who begin their assent.

In both temperate and polar regions,

this passive motion, along a concentration gradient,

can get a current going.

And that is the starting point

of the global conveyor

called thermohaline circulation.

This is how a simple concept

becomes the mechanism underlying

one of the largest

and most important systems on our planet.

And if you look around,

you'll see it happening everywhere.

Turn on a light, and it's there.

Concentration gradients govern

the flow of electricity,

allowing electrons squashed together in one space

to travel to an area of lower concentration

when a channel is opened.

Which you do, by flipping a switch.

Right now, in fact, there's some gradient action going on

inside you as you breath air into your lungs

letting the concentrated oxygen in that air

move passively out of your lungs

and into your blood stream.

We know that the world is filled

with complex physical problems,

but sometimes the first step

towards understanding them can be simple.

So when you confront the magnitude

of the ocean's currents,

or have to figure out how electricity works,

remember not to panic.

Understanding can be as simple as flipping a switch.