Video 3.2.1: How the oceans work - Dr. Gregory Lane-Serff

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Из курса от партнера University of Manchester

Our Earth: Its Climate, History, and Processes

85 оценки

University of Manchester

Our Earth: Its Climate, History, and Processes

85 оценки

Develop a greater appreciation for how the air, water, land, and life formed and have interacted over the last 4.5 billion years.

Из урока

Water in Earth’s Climate System: Oceans, Atmosphere, and Cryosphere

Video 3.1.1: Where Did the Oceans Come From?6:55

Video 3.1.2: Are the Oceans in Steady State?13:36

Video 3.2.1: How the oceans work - Dr. Gregory Lane-Serff9:36

Video 3.2.2: The oceanic conveyor belt - Dr. Gregory Lane-Serff12:57

Video 3.3.1: What is the Atmosphere Made Of?6:41

Video 3.3.2: What Controls the Temperature Profile of the Atmosphere?5:49

Video 3.3.3: Three Radiation Laws4:48

Video 3.3.4: What if the Earth had no Atmosphere?7:37

Video 3.4.1: How does the Atmosphere Work?9:04

Video 3.4.2: How do the Jet Streams Control the Weather?7:05

Video 3.5: Extratropical cyclones8:11

Video 3.6: The Rise and Fall of Ice on Earth12:44

Video 3.7: How do Glaciers Control the Height of Mountains? – Dr. Simon Brocklehurst10:46

Video 3.8: Why the Arctic is Crucial to Earth's Climate - Dr. Bart Van Dongen and Dr. Robert Sparks 7:48

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Prof. David M. Schultz
Professor of Synoptic Meteorology
School of Earth, Atmospheric and Environmental Sciences
Dr Rochelle Taylor
Postdoctoral Research Associate
School of Earth, Atmospheric & Environmental Sciences
Dr Jonathan Fairman
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences

Hello, my name is David Schultz.

Welcome to Our Earth, It's Climate, History, and Processes.

In this lecture, I want to talk about how the ocean works.

Now, most of the sun's energy that's retained by the earth

is stored in the oceans.

The reasons for this are really quite simple.

First, there is a lot more ocean surface than there is land surface on the Earth.

Second, the heat capacity of water is four times larger than that of air.

So, although air can move very quickly and move energy from the tropics to the poles

more quickly than water does, because of the faster wind speeds than the ocean

currents, the fact that the water is carrying more energy per unit mass

makes this an effective way to transport heat from the tropics to the poles.

Now, in fact, in the mid latitudes,

the atmosphere transports about twice as much energy as the oceans.

So, we got about four petawatts in the atmosphere and

compare that to two petawatts in the ocean.

As we'll see in the next lecture,

what controls the circulation is the density of seawater.

Two things control it's density, it's temperature and it's salinity.

The salinity is essentially the salt content in the water.

Now if we look at the map of January average sea surface temperatures,

sometime abbreviated SST, from Build Your Own Earth, the pattern is pretty clear.

Green and yellow represent relatively high sea surface temperatures,

whereas blue represents relatively low sea surface temperatures.

As we would expect, the tropical oceans receive more solar energy so

they tend to be warm.

The temperature can be as high as 30 degrees Celsius and

it drops as you move poleward in each hemisphere

to temperatures of 0 degrees Celsius, or freezing.

But even in the mid latitudes, you'll see temperatures 10, 15,

20 degrees across much of the oceans.

Now let's look at the corresponding map of average salinity in January

from Build Your Own Earth.

Yellow and orange represent relatively high salinity, or more salty water.

The green represents relatively low salinity or less salty water.

We have high salinity in tropical regions, particularly in the Atlantic Ocean,

where the values can be as high as 37 parts per thousand, or 3.7%.

The values are high here because the amount of evaporation

from the ocean to the atmosphere exceeds the amount of precipitation of

fresh water from the atmosphere to the ocean.

This makes the ocean water in this location particularly salty.

You can see very high salinity values in the Mediterranean Sea as well for

exactly the same reason.

Now as we move poleward in each hemisphere,

the salinity generally decreases due to fresh water input from polar ice sheets.

Also, as you move from the center of the oceans towards the coast line of

the continents, the salinity also tends to decrease

here due to the increase of the fresh water input from the rivers.

Then there are the surface currents, these are largely driven by the winds.

Here's a map of the generic ocean surface currents around the world.

You can see relatively warm currents in red,

moving from the equator to the polar regions,

particularly along two enhanced regions of flow in the Northern Hemisphere.

There's the Gulf Stream on the East Coast of North America and

the Kuroshio Current on the East Coast of Asia.

In particular, these two strong ocean currents

transport much of the energy from the tropics to the Northern Hemisphere pole.

In each ocean basin in the Northern Hemisphere, you see a clockwise gyres

circulation of which one component is either the Gulf Stream or the Kuroshio.

In the Southern Hemisphere,

we see the same thing except moving in the opposite direction.

In this case,

counter-clockwise gyres in the Southern Hemisphere in each ocean basin.

In the next lecture, we'll show how these surface currents are part of the global

network of flow in the ocean that we call the global oceanic conveyor belt.

Now one more aspect of the structure of the ocean that we need to talk about,

this is the vertical structure of the ocean temperature in salinity.

To help explain this, I'm joined by Dr.

Gregory Lane Surf, a senior lecturer here in the school of Mechanical, Aerospace,

and Civil Engineering at the University of Manchester.

So Gregory, can you explain to us about this vertical structure of the oceans?

>> Yeah, so the oceans are generally stably stratified,

that is they get denser as you go down in depth.

However, the surface layers tend to be mixed up by wind and wave action, and so

on, and also heated somewhat by the sun, giving lower sea surface temperatures and

lower densities there.

The depth of this upper mixed layer varies during the year.

So in the summer when there is strong solar radiation and

relatively little wind, you have a shallow relatively warm buoyant mixed layer.

And in the winter, as there's some cooling,

which results in lower surface temperatures, smaller density contrast,

but also much stronger mixing from the wind and the waves, resulting in a mixed

layer that can extend to 50 meters or sometimes even 100 meters depth or so.

So we can do a simple experiment with a straight mixed layer deepening.

We fill a pint glass two thirds full of water and

then mix in about 3 teaspoons of salt, and in a separate tumbler,

I've got some fresh water with about 1 cm cubed of food coloring mixed in.

Now what I'm going to do is carefully spoon or

pour some of this water onto the salty water in here.

And I'm just going to stick an ice cube there because it just makes it

a little bit easier to spoon this on without it going everywhere,

so let's just see how again.

So if you just start slowly.

And just by directing onto the ice cube, we can reduce the amount of mixing and

what you should be seeing is I'm gradually building up

a layer of fresh water which is colored on top of this.

If I get a bit braver I can start to cover it a bit more vigorously.

[SOUND] And it won't mix quite so much.

Let's see, here we go.

Okay, so let's see if I, oops, I'll pour it a bit more slowly to start with.

Okay, so I now have a layer of relatively fresh water on top colored green here,

and a layer of salty, denser water on the bottom and

actually quite a strong interface in between.

And if I just move it around a bit, you might be able to see some interesting

internal waves generated at the interface between these two.

The ice cube, I should say, was, as I say, just there to help me introduce fresh

water without mixing it up so much, but there are regions of the earth, for

example, in the polar regions, where there's some ice in the system, as well.

Now, we can pretend to be a winter storm.

So we're going to add some energy to the top by making up this surface layer.

So start slowly, just use the spoon and

you'll start to see some mixing extending

down into the saltier water below, and as I keep on going,

You'll see that I've increased the mix layer to a much more substantial depth.

And if I stop there you'll see, again,

these interesting internal waves going on in the bottom.

By mixing, we've actually increased the potential energy of the system.

Some of the salt that was originally in the lower two thirds

has been raised to occupy part of the upper part of the glass.

These mixing places are actually very inefficient,

typically only about 3% of the energy put in

by the storm actually increases the potential energy of the water column.

Most is dissipated to heat via turbulent processes or

carried away by those internal waves that I mentioned.

>> Well thanks, Gregory.

So, to summarize this lecture, we learned that the oceans are an important

contributor to moving the excess energy received from the sun poleward.

We've also seen the variability in temperature,

salinity in the ocean currents that exist around the world.

We also have seen a dramatic illustration of how the thermocline forms and

how it separates warm, less salty water from the surface, from deeper,

colder and more saline water below.

Finally, Gregory showed us a dramatic illustration of how storms

can lead to mixing in the surface layer, deepening this layer and

increasing the depth of the thermocline.

In the next lecture, we'll see more on how these characteristics

of the ocean change on longtime scales.

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