- Welcome to science for Teachers.

I'm your host, Larry Crum.

Today we begin the first of four programs on oceanography with the topic Ocean Circulation and Waves.

Our guest is Dr.

Ron Johnson from Old Dominion University.

We're glad to have you with us today, Ron.

Let me tell the folks a little bit about your background.

Ron has his bachelor's and master's degree in civil engineering with his PhD in physical oceanography from Oregon State University.

He was on staff at Lockheed and at Oregon State University prior to coming to Old Dominion in 1968.

Ron is a scuba instructor and an oceanographer in the Navy, in addition to being graduate program director of the oceanography program at Old Dominion University.

Ron, we're looking forward to today's presentation.

The first question, does oceanic circulation in any way resemble atmospheric circulation?

- Well, thank you, Larry.

I'm happy to be here.

And yes, indeed, the atmospheric circulation equations of motion are essentially the same as we use in the ocean.

There are a couple of outstanding differences, one being we have continents, land masses that constrain the circulation, and that seawater density is a complicated function, not only of temperature and pressure, but we must include salinity effects.

The first force in the equation we want to look at is the choriolis effect.

The coriolis effect is the effect of the Earth's rotation and because the Earth is round, you must include the fact the Earth is round.

You'll notice that with the Earth spinning at the equator much faster than it spins at the pole in a tangential sense, that a force, that a, a particle of water that is sent to the north would be deflected to the right in the Northern Hemisphere and deflected to the left in the Southern Hemisphere.

And as we go along today, we'll have to keep that in mind that the currents want to be deflected to the right in the Northern Hemisphere and to the left in the Southern hemisphere.

The next force that we must consider is the pressure gradient force.

This is due to differences in density, creating a difference in horizontal pressure.

And you can think of it, if you will, as a mound of water.

The water tries to run down the hill, but Coriolis wants to deflect it to the right.

So when you look at ocean circulation, anytime you see a closed loop of water, the water in the center is going to be a little higher.

And it turns out in the northern hemisphere, this is slightly off center of the ocean, but it comes out to be a little bit less than one meter of elevation difference as you go from Florida out to the middle of the Atlantic and then go back over to Spain, let's say.

The third force is the one that actually starts everything in motion, and this is the wind stress.

Without the wind stress, we would have very little circulation.

The wind stress is shown here.

We have primarily the trade winds in the low latitudes blowing to the west.

We have the westerlies in our latitude and a little higher blowing to the east, and then there are also easterlies even higher.

So the energy then to drive this system is put in at these trade wind latitudes and at the westerly latitudes.

The Coriolis parameter wants to deflect the currents to the right.

The pressure gradient says, "Hey, you've got to have a mound of water in the middle."

So all three of these forces are continually acting, but without the wind stress, we wouldn't have anything.

Now, let's look at the equator for a minute and look at surface temperature and see the heating effect that we can have near the equator, because this is also extremely important.

The equatorial waters heat up because they are receiving almost direct sunlight every day.

And when you look at surface temperature maps, you'll see that they are slightly cooler on the east side.

And as these currents move across the ocean, they warm up.

And this, of course, is the source of energy for hurricanes, which is not the topic for today, but is, is where the energy comes from that drives hurricanes.

What we're going to look at now is where this heat goes via the ocean circulation.

And if we now look at the Gulf Stream in, in general, take a look at the schematic of the Gulf Stream.

The red line on here represents this tremendous amount of heat that is being transported northward via the Gulf Stream and then across the open Atlantic Ocean.

The slope water and the continental shelf water to the north are seasonally heated by local effects, but mostly they're cooler.

And so we get a very strong boundary between the gulf stream and the slope water.

And this is exactly analogous to an atmospheric front.

But in the ocean, these fronts don't move anywhere.

The gulf stream really doesn't go anywhere.

It stays in the same location, transferring heat northward via its current.

And the boundary between it is a very strong shift in current speed.

The slope water, the shelf water doesn't move very fast.

The gulf stream water can move up to four or five miles per hour.

The gulf stream, however, does have a lot of meanders in it.

It's not a nice straight current, which would be very simple if it was.

So this next film here is an infrared film loop showing the gulf stream as it actually is, starting from Florida down the lower left corner, meandering up off of Cape Hatteras where it actually leaves the shoreline and then going across the North Atlantic.

The Gulf Stream here is the very dark water.

On infrared film, warm is dark.

And north of the Gulf Stream, you will see some cold water that shows up as a light color.

This is water that intrudes from the, the shelf.

You'll also notice to the right of this picture a loop of very warm water up to the north.

So the gulf stream then can shed eddies to either side of the gulf stream.

You will get warm water eddies that are on the north side and very cold water eddies that are on the south side.

Let's go back and take a look then at the general circulation once more, and we can talk about some of the other features that we find.

Here's a schematic to show you the Atlantic Ocean.

The land on the left being North and South America and the land on the right being Africa.

And then of course the British coastline, Scandinavia way to the north.

And the red down at the very bottom would be Antarctica.

You'll notice what we call the subtropical jar is this circulation where we get the trade winds, the Gulf Stream, the North Atlantic current blowing by the westerlies, and then the water coming back around again.

That last shot also showed another circulation jar to the north of that.

And we'll come back and talk about that when we get to the deep water circulation.

You may have noticed on there a couple of other currents down low, the west wind drift, the Antarctic convergence zone.

These are very important, Larry.

- What is the significance of the west wind drift and the Antarctic convergence?

- Well, the west wind drift, for those of you that have a globe handy and take a look at it, is the only major current that actually circumnavigates the earth, the globe.

This current then is carrying continuously a tremendous amount of energy all the way around the entire world.

And it is this current that tries to make all three oceans uniform.

So the temperature and salinity characteristics that we find in the west wind drift, we find that in all southern oceans, makes it a little harder to identify the waters of these oceans.

The Antarctic convergence is also due to the westerly winds in this area, forcing together in a line currents that converge.

This gives you then this deep circulation effect.

The antarctic convergence is located on this chart immediately over the words that say Antarctic intermediate water.

We also have convergence zones in the far north around the southern part of Greenland where we get the North Atlantic deep water sinking down and flowing south.

And we also get a convergence area in the Waddell Sea area, which is really far south, which has super cold water that, that flows to the south.

Sorry, flows to the north.

The waters at depth are identified by a word we call thermohaline circulation.

Thermohaline circulation means not only temperature, but haline refers to the salt content.

The intermediate water that we showed is low salinity.

The North Atlantic deep water has very high salinity because of the gulf stream and the water that it carries, carries north.

- Now before the program began, you were talking about the effect of wind on creating some of the the waves on the ocean.

Could you explain more about this?

- Right.

The the wind waves are caused by exactly the same wind stress that causes the ocean circulation.

But in putting the energy into the water to drive the currents, they also put energy in to make waves.

And I think we need first look at a wave tank picture to show surface characteristics of waves.

See what these waves look like.

You'll notice the wavelength is the distance between crests.

The height, of course, is the measuring the wave height between the trough and the crest.

The depth of water is important as we'll see later.

The period of the wave is the time it takes excessive crests to pass a point.

And the speed of the waveform, if you watch one of these crests go by, that speed is called solarity.

And we do have to differentiate solarity from particle motion.

The particle motion behaves entirely differently than the wave crest.

We need to make sure that that is very clear that the wave speed moving along does not carry with it wave particles.

We can see here three little tufts that have been glued onto the wall.

And you'll notice the tuft closest to the surface is making a very large orbit as the wave goes by, forward with a crest and back in a trough.

And as you go deeper in the water, these orbits will get smaller and smaller until they would eventually die away if the water were deep enough.

We now see in this view that under the crest, these tufts are compressed.

And under the troughs, these tufts are spread apart, which is what you would imagine.

Okay.

The waves that we've been looking at can be classified as to whether they're in deep water or as to whether they're in shallow water or even intermediate water.

The deep water waves behave differently from shallow water waves.

The deep water waves do not feel the bottom.

This is what we mean by deep water.

The water is so deep that the effect of the wave is completely gone by the time we get down to approximately one half of the wavelength.

If we look at the surface tuff, we see that the orbit is essentially circular.

And by the time we get down to a depth equal or greater to one half of the wavelength, the motion is gone.

So as a diver I know that if I can get down to a depth at least half of the wavelength, then I don't have to worry about any wave motion.

I'm not going to get seasick when I'm out there being a diver.

If we now go to the other extreme and look at these long waves or these shallow water waves, we'll find that the motion continues all the way to the bottom.

The waves are so long that they feel the bottom all the way down.

The orbital motion is elliptical.

It's not circular anymore.

But as I think all of us recognize, the motion right on the bottom is only parallel to the bottom.

You can't have water motion on the bottom that's leaping up and down off the bottom.

That, that would violate all the rules that we've got.

So the water motion then in a shallow water wave is strictly parallel on the bottom and these orbits decay.

There would be another area of waves that we could talk about would be for another time if you'd have me back on your show.

And this would be a standing wave, which is a wave that reflects.

But I think that it's sufficient to talk now about these progressive waves and then move on.

- Ryan, would you elaborate more on the particle motion?

- Yes.

Laurie, we need to do that before we get into the tides and the tsunami part of this section.

Remember I mentioned that long waves feel the bottom everywhere.

And we need to notice that these waves at the crest compress, the tufts come together.

And in the trough, they expand out.

They, they actually lower the height of the wave.

They spread out.

The wave form is still progressing in a progressive wave.

Remember I mentioned a standing wave or the wave is reflected off of a wall?

Well, this view that we're looking at is of a standing wave.

So the orbits are not circular anymore.

They're more of a washing machine motion.

They're back and forth.

And this type of motion, you find at in the heads of all estuaries or at the ends of all tidal creeks, the wave must change from this progressive nature to a standing wave nature.

We can't have the water moving forward on a crest.

When we get to the end, it's got to change to an up and down motion.

You can see this very nicely in a small fish tank.

If you don't disturb the fish, you can slosh the water and you'll see it oscillating in an up and down motion at the ends and horizontally in the middle.

You can also do it in a bathtub.

So.

- Well, you mentioned wind-generated waves.

What about the tides themselves?

- Well, the tides are extremely long, progressive waves out in the open ocean.

And when we say long waves, we mean that they feel the bottom everywhere.

So they are affected by the depth of water.

Now to generate tides, we need to realize that the sun and the moon are exerting a gravitational pull on the earth.

Won't move the earth very much, but they will indeed move the water.

We also have the effect of the earth going around the sun and the moon and the earth revolving about a common point, which we call the centrifugal force effect.

Well, these two forces are balanced for the entire earth, but they are not balanced on the surface of the earth.

And this net pull is what gives us the, the tides.

We must couple that, of course, with the rotation of the earth to give us the two tides that we have per day.

Out in the open ocean, tidal theory allows us to predict that we will get a, a tide range of about two feet, a little bit something less than a meter.

But when we get near shore, this fact that the water is shoaling and when we force the tide up onto the continental shelf and into the bays, the tidal wave changes its characteristics.

Not the period.

The period stays the same, 12 hours, 24 hours, but the height will change.

And because we can't predict that, we need to have very accurate measurements of the tide.

And this is where the federal government has come in for a long period of time.

And we've invited Jim Dixon, who's chief of the water levels branch at the Atlantic Marine Center, which is part of the National Ocean Service, to explain and talk to us a little bit about this measurement of tides.

- Why don't we pause and take a look at that right now?

- This is a tide station.

We've come to find out how we collect tides and how we gather those tides and convert those bits of value and pieces of information and measuring tides and convert it and get it back to a product that you can use.

This particular station that we're standing in is part of the National Ocean Services National Tide Observation Network.

The National Tide Observation Network consists of some 165 tide stations along the East Coast of the United States and some 84 tide stations on the West Coast.

The responsibilities for measuring tides since 1853 has been part of NOAA's responsibility, which falls under the US Department of Commerce.

In particular, the National Ocean Service under NOAA is the agency that has been charged with this responsibility.

The collection of tides since 1853 has changed quite a bit over the years.

As time moves along, we've developed equipment that is telemetry equipment that goes over phone lines.

We have computerized systems, binary code decimal systems.

But the basic principles and the basic understanding of tides has not really changed that much since the 1800s.

The method for collecting tides is really rather simple.

Below us here, we have a large 12 inch stilling well, what we refer to as a float well, that the tide level is recreated inside of the pipe.

And a float is connected to this instrument, which is the, what we refer to as the analog to digital rear carder.

The float as it rises and falls inside of the tube generates the friction or the motion of the tide rising and falling on the instrument here.

And once every six minutes, the gauge punches a binary code decimal value on this paper tape.

At the end of the month, the paper tape is removed from the instrument.

The tape is then forwarded onto our headquarters office in Rockville, Maryland where it's processed.

And the end product, what we end up with is every six minutes, the height of the tide, reference to the electric tape gauge, which is on my right.

Now, given that the tide is so high so many times a day and so low so many times a day, we're able to come up with the mean highs and the mean lows.

Once we derive what those mean highs and mean lows are, particularly at this station, which has been here for 10 years now, we're able to transfer that value onto this electric tape gauge.

Then our team of surveyors use class one first order genetic levels and run precise levels from this reading mark here, which is a fine hash mark on this instrument, to a network of benchmarks which are located up and down the East Coast of the United States and go right across the country.

The datams, the title datams themselves are used for many different resources.

One of those resources is for the publication of this tide table.

This is a publication that's put out by the National Ocean Service and it's called East Coast of North and South America, 1985 for this year, 86 on and forth for every year.

High and low water predictions.

These values are used for the local mariner, for shipping, navigation, oceanographic support, engineering support, and so forth.

One other thing that we've found that has been a very strong indication of the use of tides in this area in particular is the system of telemetering the information real time to agencies such as the National Weather Service, the US Pilot Association and the Lower Chesapeake Bay here, the Defense Department.

All of these areas receive this information from this telemetry system, which is by real phone line.

And in some of our stations are in remote, extreme remote areas.

And in areas where we're very cautious about hurricane force winds, we have what we call a satellite transmitter, which goes to the Go satellite system.

This is valuable, extremely valuable information for an area such as in tide water where most of your elevations are very low.

And when we get storm warnings or storm waters, this information off of this gauge is extremely valuable.

Since we have a network of tide recording stations located on the East Coast and the West Coast of United States, it gives us the ability to also collect further types of information.

Simple as this may be, a seawater sampling bucket, we also collect temperature density at many of our stations, primarily the control tide stations.

This gives us a historical record on water surface temperatures and salinity particles in the water also.

This information is kept in a historical database at our headquarters office and is used greatly and largely by our academic communities and our National Fishery Services, Environmental Protection Agency, and many other federal and state local agencies.

The data has been collected at control tide stations for an established period, and we try to keep these stations maintained for at least a minimum of 19 years.

That is the one complete tidal cycle.

This particular station, as I said, had only been here 10 years, but many of our stations have been there for 50 and 75 years.

It is very important that we in the community, you as earth science instructors, ensure that data of this nature is collected on a continual basis and that we educate people in the future so that we don't lose what we've worked so hard to develop for some 200 years.

- Ron, that was a fascinating field trip.

What, what does the weather forecaster mean when he says that in reference to the tides that they're running three feet above normal?

- Well, Jim alluded to this in his talk, and this is one of the primary functions of having these tide gauges.

We need to establish mean sea level so that we know what the normal range of tides are, so that when we get a strong, intense storm, such as a northeaster that we have occasionally around here, that we then can predict what that storm is going to do on top of the tides.

So if you can think then of a storm surge as being a single wave where the water is physically pushed into the bay by the wind, this will raise the elevation of the water.

The tides are still operating on top of that.

But now perhaps the tide level is three or four feet above normal in a rather extreme case.

And I think the highest case we have on record is something like nine feet above normal.

And this of course floods everybody.

The tide then is still operating in a tide range of three feet in our area, plus or minus three feet.

I'm sorry, plus or minus a foot and a half for a total of a three foot tide range.

And then if you add on top of that this storm surge of four or five feet.

And then on top of that, you put these wind waves that we've already talked about, you can see then that you can get some pretty destructive situations.

Well, by having this study of the tides, we can then make better models of what this storm surge is going to do for us.

So that is just one aspect.

- You know, you mentioned tidal waves, and I'm sure there's some folks out there that are wondering, you can't be referring to these large ocean waves that we call sometimes you know, commonly called tidal waves.

Can you explain the difference?

- Yeah, that's an extremely important point and that one that I continually call tides tidal waves specifically because of that.

Tidal waves are caused by the effects of the moon in the sun.

Tsunamis are caused by earthquakes that actually force the water level to change suddenly.

And this effect is primarily found in the Pacific Ocean.

And because of the ring of fire that we have there, that we have these major earthquakes that will cause tremendous waves that have the same speed as the tide, but are much shorter in period, probably around 15 seconds, 15 minutes, sorry.

The wave acts like an ordinary wave that breaks on the beach, but because it's got a period of about 15 minutes, it will cause a tremendous run up.

And you can see here this house being inundated by a tsunami that a person was very fortunate in taking these pictures in Hawaii a few years ago during the 1964 earthquake that was in Alaska.

So a tsunami then is totally separate from the tidal wave.

It has a period of around 15 minutes, but because it is also a shallow water wave, it feels the bottom.

It travels at the same speed as the, as the tide or the tidal wave, but it's much more destructive.

The tide takes 12 hours, 24 hours, whatever its period is.

The water rises and falls very, very slowly.

But a tsunami arriving every 15 minutes, the rise of water is very fast.

You will get very little warning if it wasn't for a tsunami network that we have.

- And would you explain the network?

Is, is this a means that we can detect these things in time to perhaps avoid the damage or destruction?

- The the governments of the world that have countries that border on the Pacific Ocean have cooperated, believe it or not, to form a tsunami early warning system.

They have installed pressure gauges on the floor of the ocean, in the deep ocean.

And you remember I mentioned that tsunamis are shallow water waves, which means they feel the bottom everywhere.

So if an earthquake occurs anywhere, a tsunami warning is sent out saying there's a possibility that we could have a tsunami or seismic sea wave, then the people watch these gauges very carefully.

And if the wave passes over, it will cause a change in pressure on these gauges that are mounted on the deep ocean floor and then can send out a tsunami watch.

And the tsunami, the length of it can reach several hundred miles in the open ocean.

And the height in the open ocean is only a, a foot or two.

So you don't really notice it when you're out in the open ocean.

But as I mentioned, as the wave gets into shallow water, the height goes up.

Or I mentioned this for tides, but it also happens for tsunamis.

The height of the wave goes up as you get into shallower and shallower water.

There's one other effect called wave refraction, which is a focusing of wave energy.

You can either focus it or not focus it depending on the shape of the bay you're in or the shape of the estuary like Chesapeake Bay.

And if you get a focusing of this energy or concentration, then the height of the wave will go up even more.

And you simply then have a gigantic disaster on your hand.

- You sound like the kind of waves that you run away from the beach rather than running to the beach to see.

Un- - Unfortunately many people don't.

They go down to see what's happening and then they get caught and they're drowned.

- Ron, thanks for such an interesting review of ocean circulation and waves.

- Thank you, Larry.

I enjoyed being here.