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Instruction 5-2


Circulation Patterns in the Earth's Atmosphere and Oceans | The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers | The Origin and Effects of Temperature Inversion | Properties of Ocean Water | Location of Deserts and Rain Forests | Features of ENSO

The Relationship Between the Rotation of the Earth and the Circular Motion of Ocean Currents and Air in Pressure Centers
CCSTD Earth Science 5.b.

In our last Instruction, we told you about circulation patterns and pressure centers in both the Earth's oceans and its atmosphere. But we concentrated mostly on the atmosphere.

In this Instruction, we're going to concentrate mostly on the oceans.

Approximately 70 percent of the Earth's surface is ocean.

The waters of the ocean are in constant motion.

These moving waters are called currents.

These currents flow in complex patterns and are affected by many things, including wind, salinity (saltiness), the topography of the ocean floor, the rotation of the Earth and heat (temperature).

Let's start with temperature.

Scientists divide the ocean into three different temperature zones.

Temperature Zones

The temperature zones in the ocean are the Surface Zone, the Thermocline and the Deep Zone.

The Surface Zone

The Surface Zone begins at the ocean's surface and extends to a depth of about 400 meters. Under ideal conditions, sunlight can reach to the bottom of this zone -- but it usually only gets down to about 100 meters.

The water in this zone is well mixed by currents, winds and waves. This means that it is uniformly dense and salty. Wave energy is concentrated on headlands and dispersed in bays by wave refraction -- a process by which waves approaching the shore change direction because of a slowing of those parts of the wave that enter shallow water first. These waves often end up roughly
parallel to the coast. The Surface Zone makes up about 10% of the Earth's oceans.

Although temperature throughout the Surface Zone varies, the worldwide year-round average is a warm 22 degrees Celsius (71.6 degrees Fahrenheit).

The Thermocline

The Thermocline is the zone of separation between the Surface Zone and the Deep Zone. It begins at 400 meters and extends down to 800 meters. It is sometimes also referred to as a transmission zone.

Temperatures in the Thermocline drop rapidly from warm surface conditions to frigid deep-water conditions. This is not a mixed layer, but haloclines occur throughout.

A halocline is a vertical zone in which salinity (degree of saltiness) changes rapidly with depth. Especially well-developed haloclines occur in the Atlantic Ocean, in which saltiness decreases by several parts per thousand from the base of the Surface Zone to a depth of 1000 meters (well within the Deep Zone).

The Deep Zone

The Deep Zone makes up about 90% of the ocean. It starts at about 800 meters and extends all the way down to the ocean floor (which is several miles deep in many places).

Conditions in this zone are very harsh. There is no sunlight and temperatures hover just above the freezing point of water (0 to 4 degrees Celsius). Water pressure is enormous.

This means that the unique, highly adapted creatures who live here live by a different set of rules than creatures who live anywhere else.

Oceanic trenches where one plate moves beneath another comprise the deepest parts of the ocean, with water depths up to 8000 meters. The topography of the ocean floor is a constantly changing dynamic environment -- which has a profound effect on currents, climate and living things.

No-Zone Zones

The Arctic and Antarctic oceans have no temperatures zones. They are uniformly cold throughout.

So, since the colder the water the more gas it can hold, these waters are filled with gasses like oxygen and carbon dioxide.

That's why they contain such an astonishing abundance of marine life, which is attracted to oxygen.

Other Influences

How do temperature differences and other factors -- wind, salinity, the topography of the ocean floor and the rotation of the Earth -- affect the circulation of ocean waters (the currents) in these zones?

There are two kinds of ocean currents: Surface Currents and Deep Water Currents. Both are affected by primary forces like heat from the Sun, winds, gravity and the Coriolis Effect.

They are also affected by secondary forces that determine where the currents flow.

Surface Currents (Surface Circulation)

The Sun's heat causes water to expand. So, since the Sun's heat is hottest at the Equator, the water rises higher here than it does in other latitudes. This causes a slight slope -- and water flows down the slope.

Winds blowing on the surface of the ocean also push the water. A wind blowing for 10 hours across the ocean will cause surface waters to flow at 2% of the speed of the wind. Water will pile up in the direction the wind is flowing.

Gravity then pulls the water down the slope across the pressure gradient. (A pressure gradient is a change in pressure from one area to another.)

This results in a force that is directed from high to low pressure. This is called the pressure gradient force – and it is what triggers the initial movement of air or water.

In other words, winds want to blow from high pressure areas to low pressure areas. But the Coriolis Effect intervenes (we'll explain more about that in a minute). This causes the water to move in a circular motion around the mound of water.

These large mounds of water (and the circular currents around them) are called Gyres.

To see a helpful diagram of this phenomenon, click: http://earth.usc.edu/~stott/Catalina/Oceans.html

The North Atlantic Gyre is separated into four different currents: The North Equatorial Current, the Gulf Stream, the North Atlantic Current and the Canary Current.

Before we talk about these currents, we'd like to refresh your memory about the Coriolis Effect.

The Coriolis Effect

The effect of the rotation of the Earth is called the Coriolis Effect. It is named after French mathematician Gaspard de Coriolis (1792-1843), who discovered it.

As we told you in our last Instruction, the Coriolis Effect is the tendency for a moving body on or above the Earth's surface (like an ocean current) to drift sideways from its course because of the motion of the Earth beneath it.

This happens because the Earth is bigger at the Equator than it is at the poles, so its surface rotates faster at the Equator than it does further north or south.

The Earth rotates eastward. Therefore, an ocean current traveling toward the Equator with a slower high-latitude speed will drift to the west (in relation to the more-rapidly-rotating Earth underneath it).

And a body traveling toward either pole with a faster low-latitude speed will drift to the east (in relation to more-slowly-rotating Earth beneath it).

This may sound complicated – and it is.

But if you remember just one thing, remember that things drift to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

It is important to take the Coriolis Effect into consideration when dealing with wind systems, ocean currents or the trajectories of bullets (as they do on the TV show CSI).

To see an excellent diagram of the Coriolis Effect, click:

The Gulf Stream

As we said, there are two types of ocean currents:

Surface Currents (Surface Circulation), and
Deep Water Currents (Deep Water Circulation)

As we told you, the North Atlantic Gyre is separated into four different currents: the North Equatorial Current, the Gulf Stream, the North Atlantic Current and the Canary Current.

The Gulf Stream is mostly a surface current -- a Western Boundary Current. It is one of the most intensively studied currents in the world, and is the strongest, warmest, deepest and fastest of all the Western Boundary Currents.

It begins in the Caribbean and ends in the North Atlantic. It transports a significant amount of warm water and salt toward the North Pole and eventually warms the European subcontinent.

The core of the Gulf Stream is about 90 km wide and travels along a meandering route at speeds between 40 and 120 km a day. It separates open-ocean water from coastal water and fluctuates with the seasons, carrying more water in the fall than in the spring.

Once it reaches the Grand Banks, an international fishing ground southeast and south of Newfoundland (Canada), the Gulf Stream changes from a single, meandering front into multiple branching fronts, one of which is the North Atlantic Current.

To see an excellent illustration of the Gulf Stream, click:
http://oceancurrents.rsmas.miami.edu/atlantic/gulf-stream.html (The Gulf Stream)

Other Major Currents

The Earth's rotation and its seasonal winds push surface water away from some western coasts -- so water rises on the western edges of the continents to replace it. This is called an upwelling.

Colder and/or saltier water tends to sink. So when seawater enters the Polar Regions, it cools or freezes. This makes it more salty and dense. A global "conveyor belt" is formed and set in motion when this deep dense current is formed in the North Atlantic, sinks, moves in a southernly direction and circulates around Antarctica.

From Antarctica, this current moves northward to the Indian, Pacific and Atlantic
basins. It takes approximately 1,000 years for a drop of water in this conveyor belt to make the journey from Antarctica to the North Pacific.

On a global scale, large ocean currents are held in check by continental masses bordering the three ocean basins: the Atlantic, the Pacific, and the Indian Ocean.

Besides the Gulf Stream, other Western Boundary Currents include the Kuroshio, the Brazil, the East Australia and the Agulhas.

There are Eastern Boundary Currents, too. The California Current is one of them. Others include the Benguela, the Peru and the West Australia. All are broad and shallow and move at speeds between 3 and 7 kilometers per day.

To see an excellent diagram of the Earth's ocean currents, click:

Video Instruction
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for Students, Parents and Teachers

Now let's do Practice Exercise 5-2 (top).

Next Page: The Origin and Effects of Temperature Inversion (top)