Currents and Ocean Circulation

7/31/2019 Currents and Ocean Circulation

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Currents and Ocean Circulation

Surface Circulation

The surface circulation of the oceans is intimately tied to the prevailing wind circulation of the

atmosphere (see wind). As the planetary winds flow across the water, frictional stresses are set up

which push huge rivers of water in their path. The general pattern of these surface currents is a

nearly closed system of currents, called gyres, which are approximately centered on the horse

latitudes (about 30 latitude in both hemispheres). Major circulation of water in these gyres isclockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. In the

North Pacific and North Atlantic oceans, smaller counterclockwise gyres are developed partly due

to the presence of the continents. These are centered on about 50N lat. The most dominant

current in the Southern Ocean is the West Wind Drift, which circles Antarctica in an easterly

direction. The northern and southern hemispheric gyres are divided by an eastward flowing

equatorial countercurrent, which essentially follows the belt of the doldrums. This countercurrent is

caused by the return flow of water piled up along the eastward portion of the equatorial seas, and

its return flow is uninhibited by the weak and erratic winds of the doldrums. Analysis of current

records shows that a number of major currents, such as the Gulf Stream, have strong fast-moving

currents beneath them trending in the opposite direction to the surface current. Such

undercurrents, or countercurrents, appear to be as important and pervasive as the surface

currents. In 1952 the Cromwell current was found flowing eastward beneath the south equatorial

current of the Pacific. In 1961 a similar current was discovered in the Atlantic. See also tide.

Thermohaline Circulation

Thermohaline circulation refers to the deepwater circulation of the oceans and is primarily caused

by differences in density between the waters of different regions. It is mainly a convection process

where cold, dense water formed in the polar regions sinks and flows slowly toward the equator.

Most of the deep water acquires its characteristics in the Antarctic region and in the Norwegian

Sea. Antarctic bottom water is the densest and coldest water in the ocean depths. It forms and

sinks just off the continental slope of Antarctica and drifts slowly along the bottom as far as the

middle North Atlantic Ocean, where it merges with other water. The circulation of ocean waters is

vitally important in dispersing heat energy around the globe. In general, heat flows toward the

poles in the surface currents, while the displaced cold water flows toward the equator in deeper

ocean layers.

Read more: ocean: Currents and Ocean Circulation

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Surface Ocean Currents

The water at the ocean surface is moved primarily by winds that blow in certain patterns because

of the Earths spin and the Coriolis Effect. Winds are able to move the top 400 meters of

the ocean creating surface ocean currents.

Surface ocean currents form large circular patterns called gyres. Gyres flow clockwise in Northern

Hemisphere oceans and counterclockwise in Southern Hemisphere oceans because of theCoriolis

Effect. creating surface ocean currents. Near the Earths poles, gyres tend to flow in the opposite

direction.
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Surface ocean currents flow in a regular pattern, but they are not all the same. Some currents are

deep and narrow. Other currents are shallow and wide. Currents are often affected by the shape of

the ocean floor. Some move quickly while others move more slowly. A current can also change

somewhat in depth and speed over time.

Surface ocean currents can be very large. TheGulf Stream, a surface current in the North Atlantic,

carries 4500 times more water than the Mississippi River. Each second, ninety million cubic meters

of water is carried past Chesapeake Bay (US) in the Gulf Stream.

Surface ocean currents carry heat from place to place in the Earth system. This affects regional

climates. TheSun warms water at the equator more than it does at the high latitude polar

regions. The heat travels in surface currents to higher latitudes. A current that brings warmth into

a high latitude region will make that regions climate less chilly.

Surface ocean currents can create eddies, swirling loops of water, as they flow. Surface ocean

currents can also affect upwelling in many places. They are important for sailors planning routes

through the ocean. Currents are also important for marine life because they transport creatures

around the world and affect the water temperature in ecosystems.

http://www.windows2universe.org/earth/Water/ocean_currents.html

Deep Ocean Currents (Global Conveyor Belt)

Invisible to us terrestrial creatures, an underwater current circles the globe with a force 16 times

as strong as all the world's rivers combined [source: NOAA: "Ocean"]. This deep-water current is

known as the global conveyor belt and is driven by density differences in the water. Water

movements driven by differences in density are also known asthermohaline circulationbecause

water density depends on its temperature (thermo) and salinity (haline).

Density refers to an object's mass per unit volume, or how compact it is. A heavy, compact bowling

ball is obviously going to be denser than an air-filled beach ball. With water, colder and saltier

equals denser.

At the earth's poles, when water freezes, the salt doesn't necessarily freeze with it, so a large

volume of dense cold, salt water is left behind. When this dense water sinks to the ocean floor,

more water moves in to replace it, creating a current. The new water also gets cold and sinks,

continuing the cycle. Incredibly, this process drives a current of water around the globe.

The global conveyor belt begins with the cold water near the North Pole and heads south

between South America and Africa toward Antarctica, partly directed by the landmasses it

encounters. In Antarctica, it gets recharged with more cold water and then splits in two directions

-- one section heads to the Indian Ocean and the other to the Pacific Ocean. As the two sections

near the equator, they warm up and rise to the surface in what you may remember as upwelling.

When they can't go any farther, the two sections loop back to theSouth Atlantic Ocean and finally

back to the North Atlantic Ocean, where the cycle starts again.

The global conveyor belt moves much more slowly than surface currents -- a few centimeters per

second, compared to tens or hundreds of centimeters per second. Scientists estimate that it takes

one section of the belt 1,000 years to complete one full circuit of the globe. However slow it is,

though, it moves a vast amount of water -- more than 100 times the flow of the Amazon River.

[source: NOAA: "Currents"].
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The global conveyor belt is crucial to the base of the world's food chain. As it transports water

around the globe, it enriches carbon dioxide-poor, nutrient-depleted surface waters by carrying

them through the ocean's deeper layers where those elements are abundant. The nutrients and

carbon dioxide from the bottom layers that are distributed through the upper layers enable the

growth of algae and seaweed that ultimately support all forms of life. The belt also helps to

regulate temperatures.

Read on to learn about a current that isn't caused by winds or density differences but by forces

that are out of this world.

DEEP-OCEAN CIRCULATION

Methods devised to determine deep-ocean circulation have met with varying success, but all point

to a quite complex pattern of subsurface currents.

The deep-ocean currents differ from surface currents in that they (1) are density driven, (2)

are much slower, (3) move in a predominantly north-south direction, and (4) they cross the

equator. The deep-ocean circulation is often referred to as a thermohaline circulation, because

the circulation is controlled by differences in temperature and salinity. Varying combinations of

temperature and salinity produce water of varying densities, and it is these density differences thatproduce the deep-ocean circulation. Since the majority of the worlds water masses are formed at

the surface, our discussion of the deep-ocean circulation must start here. We will move through the

circulatory pattern, beginning and ending with the surface waters around Antarctica.

As the high density surface water around Antarctica sinks, it mixes with the warmer, more saline

circumpolar water to form Antarctic bottom water. See figure 1-2-5. Because Antarctic bottom

water is the most dense water found in the ocean, it sinks to the ocean floor and spreads, or flows,

northward into the deep-ocean basins of the Atlantic, Pacific, and Indian Oceans. This water mass

has been

tracked as far

north asthe 35th parallel

of the Northern

Hemisphere.

Figure 1-2-5.-

Typical flow

pattern of circulation within the ocean.

Figure 1-2-6.-Simplified general circulation pattern of

the Atlantic Ocean.

In the sub-Arctic regions of the Northern Hemisphere,

the same type of process occurs. The cold, dense

surface water sinks and forms North Atlantic deep and

bottom water. This water mass spreads southward and is in contact with the bottom, except where

it encounters Antarctic bottom water. (See figure 1-2-6.) Being less dense than Antarctic bottom

water, it is found above Antarctic bottom water wherever the two existtogether.


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The North Atlantic deep and bottom water eventually makes its way back to the Antarctic Ocean,

where it mixes with intermediate water masses and Antarctic bottom water to form Antarctic

circumpolar water. Here, the cycle begins again as the cold, dense surface water of Antarctica

sinks and mixes with the circumpolar water.

Above the deep and bottom waters, the intermediate water masses also show a basic equatorward

movement. Antarctic intermediate water actually crosses the equator and moves as far north as

20 to 35N. Its Northern Hemisphere counterpart, Arctic intermediate water, moves south but

does not cross the equator. Mediterranean and Red Sea water both cross the equator, and havebeen identified far into the Southern Hemisphere.

The Central and Equatorial water of low and middIe latitudes move poleward in their respective

hemispheres, while in high latitudes the near-surface waters move toward the equator. The

Atlantic circulation is considered much more vigorous than that of the Pacific, because surface-

density contrasts are much greater. However, even with the greater surface-density contrasts, the

circulation is SLOWVERY SLOW.

The deep-sea currents associated with the deep-ocean circulation flow at a rate of a

few centimeters per second or less. If we were able to free float a bottle at a designated depth, this

rate of speed would equate to the bottle moving less than 2 degrees of latitude (120 nmi) in ayear, or 0.06 nmi/hr.

In summary, and in its simplest form, we can say that the deep-ocean circulation consists primarily

of (1) equatorward-flowing subsurface water, which moves at an extremely slow rate of speed and

(2) the much faster poleward-flowing surface water.

This article contains information of the chain of events that must take place in order for the deep

ocean circulation, as we know it today, to occur and the driving forces behind it. You will get a

glimpse of what is happening deep down in the oceans over and over again without we even notice

it. And this constant move of water masses is of major importance for all living on Earth!

One way to look at our planet: The Earth!

When I think about the planet Earth I think about a whole. A total and fantastic whole where

everything is in order and everything is present because of a complex chain of events and

development. Nothing on Earth can exist without the other and we are all dependent on the fact

that all these events actually take place. If one part in this fragile chain is broken it will affect other

parts with severe force. Maybe not visible at first, since nature on our planet has a fantastic ability

to adapt to new circumstances, but eventually it will affect the other parts in the chain. The affect

on other part can go slowly, in a human perspective, but yet firmly and often be irreparable.

Besides thinking about our planet as a whole I think about it as a living creature. This may soundvery strange but let me explain!

I have worked in health care and gain knowledge about the fantastic human body where

everything is dependent on the other body functions in order to have a healthy life. You need most

of your organs to stay and feel healthy and most of all; you need your vital functions such as the

blood circulations, the lungs that gives you oxygen and you need your heart to pump blood to all

parts of your body. If something in these vital functions is damaged or disturbed the body will not

function.


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During my studies in Earth sciences and marine geology I found that much of what I learned about

the Earth and the contexts have similarities with the human body but at a much bigger scale. This

is what I mean by saying I think about planet Earth as a living creature. We should all think about

Earth in this way and not as something static that will always stay the same no matter how we

damage the planet or use the resources.

Now, after this rather philosophical introduction I think it is time to move on to the deep ocean

circulation. And if you dont look upon Earth the way I do by now, as something living and fragile, I

can help you by telling you that, to me, the deep ocean circulation is as the human circulatorysystem! And to understand the system is a beautiful journey!

Ocean circulation

The deep ocean circulation affects, as you might have guess, the whole planet. It is a vital part in

the water circulation between the atmosphere and the oceans, the climate on Earth as well as

transporting water and heat. The deep ocean circulation transport heat from the equatorial regions

to the poles and is also very important when it comes to even out the temperature differences over

the globe. Without the deep ocean circulation many places at Earth would be impossible for man tolive in. The equatorial regions would be so hot that nothing could exist there and the Polar Regions

would be even colder than today. How it would affect the climate in other parts of the Earth is

mostly theories because of the complexity of the system but it would certainly be a world that we

wouldnt recognise at all.

The deep ocean circulation is also vital in order to understand what goes on in the oceans and why

the oceans are the way they are today. By knowing the driving force behind the circulation we also

gain an understanding of how important this present circulation is to our present climate and we

also get a glimpse of how fragile the whole system is. Deep ocean circulation is the study field for

an oceanographer as well as a marine geologist since the study field for a marine geologist is much

influenced by seawater and currents that have a major effect on the ocean floor.

Lets start with the driving force behind the deep ocean circulation!

The Earth doesnt have a heart pumping like the human body but still the deep ocean circulation is

steadily going on over and over again, transporting water in a pretty steady pattern around the

Earth in our present time.

The force behind this deep ocean circulation is to simplify it: density, which depends on salinity and

temperature. The wind is important when it comes to water movement in the oceans but the wind

mostly concerns the surface water. At the bottom of the oceans the currents flows for other

reasons. We will look into this in a moment.

Even though the water in the oceans looks to be uniform, in fact they are not! In the oceans one

can distinguish different water masses due to their density and the water depth of their path.

These small changes make the water masses to behave differently and are important in the deep

ocean circulation. The biggest water masses are:

Deep bottom water masses, surface water and the water masses in between that are called the

intermediate water masses.


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Dens and cold surface water sinks, fills the ocean basins, and is replaced by water moving towards

the poles, which rises from intermediate depths. These water masses are different from surface

water because once they are formed they spread widely and displace the surrounding water.

First, a little about the wind!

I cant skip the wind totally in this hub even though the wind doesnt affect the deep ocean waters.

But even so, the wind is of importance as a participant in the Global movement of the water in the

oceans and cant be neglected. The wind is the force behind the surface currents and the windcreates the surface currents in the oceans by frictional drag on the water surface. But the wind

only affects the surface and the water layers just underneath the surface. For the wind to affect

deeper in the oceans it takes a very strong and persistent wind. How deep the wind can influence

the ocean water is much dependent on the water column itself. If the water column consists of

water masses with highly different density the influence is weaker and the opposite can occur in

water masses with more uniform water columns. (I will soon explain about the density of water

masses so just keep this in mind for now!) You can also read more about the surface waves

created by wind here!

Density!

It may look as if the density of water in the oceans is the same but it isnt! The density of the

ocean water is nothing but uniform and varies between 1.02 to 1.07 g/cm3.

This might sound like small variations but the difference have a major importance on the enormous

water masses in the oceans.

The density of seawater depends on several things like temperature, salinity and pressure and in

general one can say that density increases as salinity and pressure increase and temperature

decreases.

Remember the parable with the human circulation system, human blood looks like it contains of

one single object but it doesnt! And the same goes for common seawater; it can be very different

although it looks the same!

Temperature!

Surface temperatures range from freezing point in the Polar Regions at high latitudes in the winter,

to more than 28 C in low latitudes. Due to the location of continents on Earth the mean annual

temperature in the oceans in the Northern hemisphere is 16C and 19C in the Southern

hemisphere. Water has the highest density at about 3,98 C.

Salinity!

The surface water salinity differs between 33 to 37 with average salinity being about 35 .

The ocean with the highest salinity is the Atlantic Ocean with average salinity at about 35,37

due to inflow of high salinity water from the Mediterranean Sea. Evaporation, rain- and snowfall

and ice melting affect salinity in surface water. In regions with warmer climate the surface

seawater has a higher salinity than in the Polar Regions due to increased evaporation and

decreases rainfall.

The highest salinity of the surface water in the oceans is located to gyres in the middle latitudes

where evaporation exceeds rainfall. The lowest salinity in the surface waters of the oceans can be
http://thougtforce.hubpages.com/hub/About-the-oceans-and-the-cause-of-surface-waves-Coriolis-deflectionhttp://thougtforce.hubpages.com/hub/About-the-oceans-and-the-cause-of-surface-waves-Coriolis-deflection


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found along the coasts particularly near the mouths of large rivers that transport large amounts of

freshwater into the sea.

So now we know:

The most dense water masses can be found as deep bottom currents in the ocean with cold water

with high salinity and high pressure.

The lightest water masses can be found as warm surface current near continents with big

contribution of freshwater.

This is a somewhat simplified explanation since there are always exceptions due to other

parameters but let us keep it at this level for now.

Why is this interesting?

Because the differences of salinity and temperatures in water masses and currents can define the

distinct water masses and ocean currents! By measuring these parameters the scientist could

describe and also name the different deep water currents and trace their origin and their

movement across the oceans, even if we cant see them in reality!

Here is a video that shows an overview of the deep ocean circulation so you get a picture over the

scale of this circulation. It is amazing and also beautiful!

This is where we where heading initially! The deep ocean circulation is also called the

Thermohaline circulation where Thermal stands for temperature and haline stand for salinity.

The deep bottom currents in the oceans have, as we said before, high density and low

temperature. But they need to be at the surface somewhere on the planet, otherwise it wouldnt be

any circulation! And there are two places on Earth where the major part of the dense and cold

water masses can occur at the water surface: at the Polar regions!

Other places on Earth where minor deep bottom currents are created are in the enclosed basinslike the Mediterranean Sea and in the Red Sea.

But the greatest bottom currents are the AABW and the NABW!

In the Arctic Ocean, in the Norwegian and the Greenland seas, the NABW (North Atlantic Bottom

Water) is formed and at the South Pole is the AABW (Antarctic Bottom Water) formed. The reason

these water masses is so dense is of course due to the cold climate that creates the low water

temperatures. The high salinity of the water masses in this regions is due to the fact that icebergs

is formed by freshwater but only with a smaller amount of sea salt which leaves the salt in the

water

The AABW influences vast areas of the world oceans

All water in the Indian and Pacific Oceans with temperatures less than 3 C is AABW!

All water in the Atlantic Ocean with temperature less than 2 C is AABW. (except in the Arctic and

Greenland and Norwegian seas.

The AABW and the AADW

The AABW is formed by the formation of sea ice around Antarctica that leaves a cold and dens

water that sinks to the bottom of the ocean as a bottom current.


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The formation of AADW is as follows:

It begins with dense and cold surface water that because of its density starts to sink downwards in

the water column. There the sinking cold water mixes with the warmer water with high salinity in

underlying water (Intermediate water) which result in an even denser water that then sinks to the

ocean floor. The bottom water from The Wedell Sea and the Ross Sea flows north, transporting

oxygenated and cold water to a big part of the Pacific, the Atlantic and the Indian oceans.

NADW

NADW is generated in such a high rate over such a large area of the ocean that it is the most

common water in the North Atlantic.

The NADW has a temperature of 2 C and a salinity of about 34.9

The NABW and NADW

The formation of bottom waters in the Northern Hemisphere is formed in the Arctic Ocean and the

Norwegian and the Greenland seas. The NABW is the water mass with highest density and sinks to

the bottom. The formation of NADW is made from North Atlantic central waters that flow north into

the Norwegian and Greenland seas. The North Atlantic central water enters the Polar Region as arelatively warm and salty water but cools off. Sea ice is formed and the now cold and salt water

mixes with cold Arctic water and sinks. The NADW then begins its journey and flows south between

island and Great Britain into the North Atlantic.

In the Atlantic Ocean there are three main subdivisions of NADW:

The upper part, derived from the Mediterranean Sea, the middle part from the areas near

Greenland and the lower part from the Arctic basin.

This is in a way the pump that sets the circulation in motion because without the formation of

dense water in the Polar Regions there wouldnt be any deep ocean circulation in the way we knowit.

The Global Conveyor Belt

Because of its density it sinks to the bottom and because of the new formation of dense water the

NABW and the AABW are strong currents at the bottom of the oceans. Salinity and temperature

exhibit little change once the water has sunk to the ocean floor. Deep-water masses dont behave

like surface water. Once they are formed, they sort of seek their respective layer in the ocean

where they fit according to density of the other different water layers and then they spread widely

and displace the surrounding water. By time, they mixes with other water masses and get some

new properties as they go along. When density decreases due to the mix with less dens water theformer bottom water will change depth and find a new level according to the surrounding water.

Summary!

The deep ocean circulation is due to many small parameters that all have to be just right for the

ocean circulation, as we know it, to occur. Small changes in temperature and salinity of the vast

water masses in the ocean can cause very severe climate change if the fragile equilibrium is

disturbed. Even though we know a great deal about the deep ocean circulation there is so much

more to know and learn about these systems and about the oceans. One cant help being humble

and amazed over the beauty and the magnificent system in the oceans of the World!


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Look at the video below and see for your self!

Take away ideas and understandings

What drives ocean circulation? Seawater flows along the horizontal plane and in the vertical.

Typical speeds of the horizontal flow or currents are 0.01-1.0 m/s; vertical speeds within the

stratified ocean are much smaller, closer to 0.001 m/s. Two forces produce the non-tidal ocean

currents: the wind exerting a stress on the sea surface and by buoyancy (heat and freshwater)

fluxes between the ocean and atmosphere that alter the density of the surface water. The formerinduces what we call the wind driven ocean circulation, the latter the thermohaline circulation. The

wind driven circulation is by far the more energetic but for the most part resides in the upper

kilometer. The sluggish thermohaline circulation reaches in some regions to the sea floor, and is

associated with ocean overturning linked the formation and spreading of the major water masses

of the global ocean, such as North Atlantic Deep Water and Antarctic Bottom Water.

Wind induced upwelling: The wind stress acting on the surface layer of the ocean induces

movement of that water. This is called Ekman Layer transport, which extends to the surface 50 to

200 meters. The Ekman transport is directed at 90 to the direction of the wind, to the right of the

wind in the northern hemisphere, left of the wind in the southern hemisphere. As the wind varies

from place to place, Ekman transport can produce divergence (upwelling) or convergence (sinking)of surface water.

Geostrophic Currents: The surface layer is less dense (more buoyant) than the deeper layers,

therefore a spatially variable Ekman transport field acts to redistribute the buoyant surface water:

thinning the buoyancy surface layer in divergence regions, thickening the buoyant surface layer in

convergence regions. As the ocean is in hydrostatic equilibrium, the redistribution of the buoyant

surface layer induces sea level "valleys" in divergent regions and "hills" in convergence regions.

While these hills and valleys amount to only a 1.5 meter in amplitude, they are sufficient to induce

horizontal pressure gradients which initiate the wind driven circulation following the geostrophic

balance concept. The ocean currents are for the most part geostrophic, meaning that the Coriolis

Force balances the horizontal pressure gradients. The wind driven circulation is characterized by

large clock-wise and counter clock-wise flowing gyres, such as the subtropical and sub polar gyres.

The Antarctic Circumpolar Current is also a wind driven current; in contrast to the subtropical gyres

it reaches the sea floor.

Thermohaline Circulation: As surface water is made denser through the removal of heat or

freshwater, the surface layer descends to deeper depths. If the stratification is weak and the

buoyancy removal sufficient, the descent would reach the deep sea floor. Such deep reaching

convention occurs in the northern North Atlantic (North Atlantic Deep Water) and around Antarctica

(Antarctic Bottom Water). The thermohaline circulation engages the full volume of the ocean into

the climate system, by allowing all of the ocean water to 'meet' and interact directly theatmosphere (on a time scale of 100-1000 years).

The ocean would have a significant role in governing climate, even if it did not circulate. The

oceans surface layer ability to store heat in summer and the release of that heat to the

atmosphere in winter, would mitigate the seasonal extremes of the atmosphere temperature even

without an ocean circulation. The circulation shapes the sea surface temperature (SST) pattern

moving heat from the tropics to the polar regions (Fig. 1). Currents along the western boundary of

the ocean, such as the Gulf Stream are swift and form filaments of warm water projecting towards

higher latitude. Ocean currents also affect the salinity patterns, as the ocean circulation moves


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saline water formed in the excess evaporative regions to the excess precipitation regions,

subtropical to tropical respectively, as part of the global hydrological cycle.

Ocean circulation includes currents, which follow along an approximate horizontal plane, such as

flow along the sea surface, and flow in the vertical. Ocean currents may be slow, 0.01 m/s in the

ocean interior or swift >1.0 m/s as in the Gulf Stream. Movement of water in the vertical is much

smaller, ranging from a maximum of 0.001 m/s to more characteristic speeds of 0.0001 m/s. The

ocean circulation, both horizontal and vertical, is induced by two means (Fig. 2): (1) by the wind

exerting a stress upon the sea surface, and (2) by buoyancy fluxes between the ocean andatmosphere. The former is called the wind driven circulation, the latter is the thermohaline

circulation. There are also tidal currents, products of the Earth/Moon/Sun gravitational interaction.

While we do not include discussion of the ocean tidal currents in this course, some oceanographers

consider the dissipation of tidal energy within the ocean as an important factor in mixing the ocean

and in shaping the SST patterns.

Wind Driven Circulation

The wind driven circulation is by far is the more energetic, though confined mostly to the upper

kilometer or two of the ocean. The mean surface winds patterns (Fig. 3) are composed of specific

elements. The trade winds blow westward with a component of flow towards the equator. Thetrade winds of the two hemispheres meet at the intertropical convergence zone (ITCZ), where

updrafts of air induce high precipitation. The ITCZ falls over the warmest band of SST, which on

annual average occurs in the northern hemisphere, near 5N. Poleward of the trades are the

westerlies (air flow towards the east) with a slight poleward component, and still further poleward

are the polar easterlies. The westerlies are associated with the strongest wind, but often very

variable, as shaped by storms and atmospheric fronts.

The wind exerts a force or stress on the ocean surface, proportional to the square of the wind

speed. This produces not just ocean waves but also injects momentum into the surface layer of the

ocean. The wind makes the surface layer of the ocean move, though not in the way that intuition

might dictate - its not in the direction of the wind stress, but rather at an angle to it, in a spiral

across the surface layer (Fig. 4). This is because of the Coriolis Force. A balance is achieved

between the wind stress and the Coriolis Force. The surface Ekman layer (named after the person

who developed the theory in 1908) extends to about 50 to 200 meters depth. The mean transport

within the Ekman layer is 90 towards the right of the wind in the northern hemisphere, 90 to the

left in the southern hemisphere. Typically the wind induced surface current is around 2 or 3% of

the wind speed. One clear effect of the Ekman transport can be seen in along the coast of

California (Fig. 5), where cold subsurface water is brought up to the sea surface from a depth of

perhaps 100 meters to replace surface water forced offshore by the Ekman transport. These

regions are rich in nutrients and support important fisheries. Ekman divergence occurs along the

Equator (Fig. 6) and Ekman convergence occurs in the subtropics (Fig. 7).

As we know the wind changes in strength and direction from place to place. This causes a spatially

variable Ekman transport. In some regions the Ekman transport forces accumulation (convergence)

of surface water, in other regions it results in the remove (divergence) of surface water. As surface

water is less dense than deeper water (the ocean is stable, less dense more buoyant water at top,

denser water at the bottom) this has the effect of heaping buoyant surface water in the

convergence regions and removing it from the divergence regions. The buoyant surface layer

thickens in convergence, thins in divergences regions. Through the Ekman transport mechanism

the wind redistributed the buoyant surface layer of the ocean.


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The thickening or thinning of the buoyant surface layer by the Ekman transport produces hill and

valleys of the sea surface (Fig. 8). The total relief of the ocean surface (not counting waves or

tides) is only about 1.5 meters, but this is sufficient to drive the ocean circulation. To understand

how the wind stress distorts sea level, with hills at the convergences, valleys at the divergences,

one needs to invoke the hydrostatic relationship. Hills of buoyant water are balanced by a 'root' of

buoyant water (Fig. 9), much as with a ship or iceberg floats, with the part above the sea level is

balanced by the submerged part. Removal of buoyant water in the Ekman divergences induces a

depression in sea level and upward bowing of the deeper dense water. The vertical movement of

water at the base of the Ekman Layer act to distort the density fields within the deeper water.

How do hills and valley drive a circulation? The pattern of the induced currents is governed not just

by the wind stress and its Ekman transport but also by the earth's rotation, the Coriolis Force. The

Coriolis Force acts at right angles (90) to the ocean current (or wind) direction, to the right in the

north hemisphere to the left in the southern hemisphere. The Coriolis Force strength is proportional

to the strength of the ocean current. In short: the wind produces convergences and divergences of

surface water, which causes hills and valleys of sea level, which then produce a horizontal gradient

of pressure. Hills have an outward directed pressure gradient, depression an inward directed

pressure gradient (Fig. 10). As the pressure gradients make the water move from high pressure to

low pressure, the Coriolis Force starts its action, and eventually a balance is achieved in these twoforces, the horizontal pressure gradient equals the magnitude of the Coriolis Force, but is directed

in the opposite direction. This balance is called the geostrophic balance, and a current in such a

balance is called a geostrophic current. Ocean currents are very close to being in geostrophic

balance (Figs. 11-13).

Thermohaline circulation [also see Ocean Stratification & Sea-Air Coupling Lectures]:

The exchange of heat and freshwater between the ocean and atmosphere alters the density of the

surface water. Cooling of the ocean makes the ocean denser, removing buoyancy; evaporation

makes the ocean saltier and hence denser, again removing buoyancy. Heating and excess

precipitation has the opposite effect, they add buoyancy to the ocean. Surface water that is madedenser, through cooling or increase of salinity, sinks to an equilibrium depth (a depth where its

density matches that of the ambient ocean). It then spreads into the world ocean, displacing the

older resident water made less dense by ocean mixing. To reach a steady state condition the loss

of water from the surface layer must be replaced, the loop must be closed: the displaced water

must find its way back to the sinking region. This sets up the thermohaline circulation. The

thermohaline circulation is more sluggish than the wind driven circulation, generally less than 0.1

m/s. But it involved overturning of the full depth of the ocean, hence bringing deep water removed

from direct contact with the atmosphere for 100 to 1000 years back to sea surface.

A key part of the thermohaline circulation is the production of North Atlantic Deep Water.

Warm/salty surface water spread into the northern north Atlantic (Figs. 14, 15), where coolingoccurs. The dense water sinks, overflows the ridges spanning the distance from Greenland to

Iceland (Fig. 16) to spread as a salty water mass into the world ocean (Figs. 17, 18).

Along the margins of Antarctica very dense water is formed (Figs 19, 20). This water descends as

thin sheets or plumes down the continental slope into the deep ocean forming Antarctic Bottom

water that spreads along the sea floor over much of the world ocean.

ow will man-made climate change affect the ocean circulation? Is the present system of ocean

currents stable, and could it be disrupted if we continue to fill the atmosphere with greenhouse

gases? These are questions of great importance not only to the coastal nations of the world. While


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the ultimate cause of anthropogenic climate change is in the atmosphere, the oceans are

nonetheless a vital factor. They do not respond passively to atmospheric changes but are a very

active component of the climate system. There is an intense interaction between oceans,

atmosphere and ice. Changes in ocean circulation appear to have strongly amplified past climatic

swings during the ice ages, and internal oscillations of the ocean circulation may be the ultimate

cause of some climate variations.

Our understanding of the stability and variability of the ocean circulation has greatly advanced

during the past decade through progress in modelling and new data on past climatic changes. I willnot attempt to give a comprehensive review of all the new findings here, but rather I will

emphasise four key points.

Ocean currents have a profound influence on climate

Covering some 71 per cent of the Earth and absorbing about twice as much of the sun's radiation

as the atmosphere or the land surface, the oceans are a major component of the climate system.

With their huge heat capacity, the oceans damp temperature fluctuations, but they play a more

active and dynamic role as well. Ocean currents move vast amounts of heat across the planet -

roughly the same amount as the atmosphere does. But in contrast to the atmosphere, the oceans

are confined by land masses, so that their heat transport is more localised and channelled intospecific regions.

The present El Nio event in the Pacific Ocean is an impressive demonstration of how a change in

regional ocean currents - in this case, the Humboldt current - can affect climatic conditions around

the world. As I write, severe drought conditions are occurring in a number of Western Pacific

countries. Catastrophic forest and bush fires have plagued several countries of South-East Asia for

months, causing dangerous air pollution levels. Major floods have devastated parts of East Africa. A

similar El Nio event in 1982/83 claimed nearly 2,000 lives and global losses of an estimated US$

13 billion.

Another region that feels the influence of ocean c?????eurrents particularly strongly is the NorthAtlantic. It is at the receiving end of a circulation system linking the Antarctic with the Arctic,

known as 'thermohaline circulation' or more picturesquely as 'Great Ocean Conveyor Belt' (Fig. 1).

The Gulf Stream and its extension towards Scotland play an important part in this system. The

term thermohaline circulation describes the driving forces: the temperature (thermo) and salinity

(haline) of sea water, which determine the water density differences which ultimately drive the

flow. The term 'conveyor belt' describes its function quite well: an upper branch loaded with heat

moves north, delivers the heat to the atmosphere, and then returns south at about 2-3 km below

the sea surface as North Atlantic Deep Water (NADW). The heat transported to the northern North

Atlantic in this way is enormous: it measures around 1 PW, equivalent to the output of a million

power stations. If we compare places in Europe with locations at similar latitudes on the NorthAmerican continent, the effect becomes obvious. Bod in Norway has average temperatures of

-2C in January and 14C in July; Nome, on the Pacific Coast of Alaska at the same latitude, has a

much colder -15C in January and only 10C in July. And satellite images show how the warm

current keeps much of the Greenland-Norwegian Sea free of ice even in winter, despite the rest of

the Arctic Ocean, even much further south, being frozen.

Figure 1. Europe's heating system. This highly simplified cartoon of

Atlantic currents shows warmer surface currents (red) and cold north


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Atlantic Deep Water (NADW, blue). The thermohaline circulation heats

the North Atlantic and Northern Europe. It extends right up to the

Greenland and Norwegian Seas, pushing ba?????eck the winter sea ice

margin. Reproduced from Rahmstorf 1997.

We now have computer models that give fairly realistic simulations of the ocean circulation, and

these models can be used to examine the effects of the currents on climate. For the Atlantic

'conveyor belt' this task is made particularly straightforward by a peculiarity of the climate system:

there are two stable climate states, one with the Atlantic conveyor, one without it. Just by usingdifferent initial conditions, all else remaining the same, the models can come up with either of

these two different climates. This makes it easy to compare what the world would look like without

the ocean circulation that warms Europe. Manabe and Stouffer 1988 were the first to analyse what

happens when the familiar conveyor circulation is absent in an ocean-atmosphere circulation

model. They found that the sea surface temperatures in the northern North Atlantic dropped up to

7C in this case. Air temperatures dropped even more, up to 10C over the Arctic seas near

Scandinavia, even though the root cause for the atmospheric cooling was the lower sea surface

temperatures. The reason for this amplification of the cooling was the advance of sea ice, which

reflects sunlight back into space and thus led to further cooling. The air temperature changes in

the model are roughly consistent with the observed difference between Bod and Nome,confirming that this difference is indeed mainly caused by the warmth brought north by the

Atlantic ocean currents in the present climate.

Ocean currents were different in the past

Painstaking detective work involving sediments of the deep sea has enabled scientists to derive a

wealth of information on ocean currents of the distant ?????epast. What is just mud to a lay-

person, provides a valuable archive of past climate data to the expert. Like tree rings or the annual

layers of snow accumulated on glaciers, the sediments at the sea bottom preserve information on

the environmental conditions from the time they were formed. It is even possible to distinguish

between conditions at the sea surface and at the bottom, as the prime source of data are theshells of tiny organisms. Under the microscope, the abundance of different species can be counted

and identified as surface or bottom dwellers. The chemical composition of their shells has been

determined by the temperature, salinity and nutrient content of the waters the organisms lived in,

which in turn reveals information on the ocean currents of the time.

Looking back over the oceanic records of the past 100,000 years or so, it is striking how variable

the currents must have been. Only the last 8,000 years, i.e. most of the Holocene, were a relatively

stable period. Before then, throughout the last ice age, sudden jumps and jolts occur in the record

roughly every 1,000 years. These are consistent between different sediment cores, and what is

more, most of the spikes in the oceanic conditions correspond to synchronous climate shifts on

land as recorded in the Greenland ice cap (Bond et al. 1993). Some cold climate episodes startedwith a temperature drop over Greenland of 5C happening over a few decades or even less. The

most plausible explanation for these sudden climatic changes are rapid shifts or breakdowns in the

ocean currents of the North Atlantic. The exact timing and sequence of events and the ultimate

causes are still under investigation, but there is widespread agreement that the 'conveyor belt'

circulation of the Atlantic played an active and dynamic role in the climatic roller coaster of the

past.

Fairly detailed reconstructions of the Atlantic ocean circulation (Labeyrie et al. 1992; Sarnthein et

al. 1994) at the height of last Ice Age, the Last Glacial Maximum (LGM) around 21,000 years before


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present, show that North Atlantic Deep Water (NADW) then formed south of Iceland (today much of

it forms by convection to the north of Iceland). It sank to intermediate depths only, and Antarctic

Bottom Water (AABW), which comes in from the south below the NADW, pushed further north than

it does today, filling most of the abyssal North Atlantic. Recently the first coupled ocean-

atmosphere simulation of glacial climate was performed at the Potsdam Institute (Ganopolski et al.

1998), accurately reproducing these features of the glacial ocean circulation (Fig. 2). Through a

sensitivity experiment using the present-day ocean heat transports (instead of taking the glacial

circulation changes into account), the authors were able to demonstrate that the change in Atlantic

ocean currents played a major role in surface climate, amplifying the glacial cooling of the

Northern Hemisphere by 50%. In the North Atlantic the southward shift of deep water formation

sites and the corresponding advance of sea-ice led to an air temperature drop of 20C.

Figure 2. Stream

functions of

meridional

ocean transport

in the Atlantic,

for the present

climate (left)

and for the last

glacial

maximum

(right), from the

coupled ocean-atmosphere model simulation of Ganopolski et al. 1998.

Both model experiments and paleo-data thus demonstrate that the ocean circulation has

undergone important change?????es in the past, and that these have led to major perturbations ofthe climate, at least in the North Atlantic region. It is possible that there are other regions of the

globe, such as the Southern Ocean, in which the dynamic ocean caused major climate variations,

but until now they have not been studied nearly as well as the North Atlantic.

The thermohaline circulation is a strongly non-linear system

Understanding the role of the ocean in climate change requires an understanding of the dynamics

of ocean circulation changes. Systematic computer simulations have led to important advances in

our knowledge of circulation dynamics in recent years. We have found that there are two distinct

mechanisms that can cause non-linear transitions in the state of the Atlantic ocean circulation, a

'fast' and a 'slow' mechanism. The slow mechanism is quite well understood, and can be describedby a simple stability diagram (discussed below).

The modelling studies have confirmed Stommel's 1961 idea that there are two states which are

stable under present climatic conditions, namely with and without deep water formation in the

North Atlantic (see section 1). Stommel described the positive salt advection feedback responsible

for this strange behaviour: salinity in the high latitudes needs to be high enough for deep water to

form, but it is only high enough because the thermohaline circulation continually brings in salty

water from the south. The system is therefore self-maintaining.The flow depends on precariously

balanced forces: cooling pulls in one direction, while the input of freshwater from rain, snow,

melting ice and rivers pulls in the other. This freshwater threatens to reduce the salinity, and


15/26

therefore the density, of the surface waters; only by a conti?????enuous flushing away of the

freshwater and replenishing with salty water from the south does the conveyor survive. If the flow

slows down too much, there comes a point where it can no longer keep up at all and the conveyor

breaks down. This 'spin-down' takes many decades or even centuries: this is the 'slow' transition

mechanism.

Figure 3. Stability diagram showing how the meridional

transport in the Atlantic ('strength of the conveyor')depends on the amount of freshwater (precipitation and

river run-off minus evaporation) entering the Atlantic.

Note the bifurcation point S, beyond which no North

Atlantic Deep Water formation can be maintained. For a

detailed discussion, see Rahmstorf 1996.

A look at a simple stability diagram shows how it works (Fig. 3). The key feature is that there is a

definite threshold for how much freshwater input the conveyor can cope with. Such thresholds are

typical for complex, non-linear systems. The diagram is based on Stommel's theory, adapted for

the Atlantic conveyor, but experiments with global circulation models also show the same

behaviour (Rahmstorf 1996). Different models locate the present climate at different positions on

the stability curve - for example, models with a rather strong conveyor are located further left in

the graph, and require a larger increase in precipitation to push the conveyor 'over the edge'. The

stability diagram is a unifying framework that allows us to understand and compare differentcomputer models and experiments.

The model st?????eudies also revealed another kind of threshold where the conveyor flow can

change or break down (Rahmstorf 1995). While the vulnerability in Stommel's theory arises from

the large-scale transport of salt by the conveyor, this second type of threshold depends on the

vertical mixing in the convection areas (e.g. the Greenland Sea and the Labrador Sea). If the

mixing is interrupted, then the conveyor may break down completely in a matter of years, or the

locations of the convection sites may shift. This process is known as 'convective instability', and is

the 'fast' transition mechanism. We do not yet know where the critical limits of convection are, nor

what it would take to set off such an event. Current climate models are not powerful enough to

resolve such regional processes accurately. Convective instability could be the mechanism

responsible for some of the very fast climatic changes seen in the paleo-climate records. Both

mechanisms are summarised in table 1.

Advective Spindown Convective Instability

Time Scale gradual (~100 y) rapid (~10 y)

Mechanism large-scale salt advection local convection physics


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Cause

(forcing)

basin-scale heat and freshwater

budget/td>

local forcing in convection region

Effects conveyor winds down shift of convection locations or complete

breakdown of conveyor

Equilibria conveyor 'on' or 'off' several equilibria with different convection patterns

Modelling modelled quite well by climate

models

large uncertainty in forcing and response

Table 1: Overview over properties of the two instability mechanisms relevant to the Atlantic ocean

circulation.

The ocean circulation may change in the future

Given the past instability of ocean currents and our understanding of their non-linear behaviour,

the future of the Atlantic circulation in the changing climate of the next century is a natural

concern. Manabe and Stouffer 1993 published scenario simulations with a coupled ocean-

atmosphere model in which the carbon dioxide content of the atmosphere was gradually increased

to both twice and four times the pre-industrial value and then kept constant (Fig.4). With adoubling of CO2, the Atlantic conveyor circulation declined strongly but subsequently recovered. If

the CO2 content was increased fourfold, h?????eowever, the thermohaline circulation in the model

was interrupted completely. Other model scenarios for a greenhouse world generally show a

reduction in thermohaline circulation between 20% and 50% for a carbon dioxide doubling in the

atmosphere (Rahmstorf 1997). A systematic sensitivity study with a simpler model revealed that

not only the total amount of carbon dioxide, but also its rate of increase determines the effects on

the ocean (Stocker and Schmittner 1997).

Figure 4. Time series of meridional transport in the Atlantic

for two greenhouse scenarios of Manabe and Stouffer 1993.

Top panel: carbon dioxide forcing of the runs. For the

scenario leading to a quadrupling of carbon dioxide in the

atmosphere, the thermohaline ocean circulation winds down

almost completely.

The circulation changes in all these experiments happen on the slow, advective time scale over

one or two centuries; rapid changes as seen during the last glacial were not triggered in these

scenarios. This is the main reason why the effects on regional temperatures are only moderate in

these models; the reduced ocean heat transport then falls in a time of strong greenhouse warming

and is partly cancelled by this. The effects of such circulation changes on marine ecosystems are

largely unexplored and will probably be serious. Furthermore, a weakened circulation reduces the

ability of the ocean to absorb carbon dioxide, making the climate system even less forgiving of

human emissions (Sarmiento and Le Qur 1996).


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The lack of rapid circulation changes ?????e in the model scenarios does not rule out that they

could happen. Due to poor resolution, present climate models cannot capture the fast convective

instability very well; this process depends on regional details. The latest report of the

Intergovernmental Panel on Climate Change (Houghton et al. 1995) concluded: "Future climate

changes may also involve 'surprises'. (...) Examples of such non- linear behaviour include rapid

circulation changes in the North Atlantic." This is still a valid conclusion today.

Waves are the forward movement of the ocean's water due to the oscillation of water particles by

the frictional drag ofwind over the water's surface.

Waves have crests (the peak of the wave) and troughs (the lowest point on the wave). The

wavelength, or horizontal size of the wave, is determined by the horizontal distance between two

crests or two troughs. The vertical size of the wave is determined by the vertical distance between

the two. Waves travel in groups called wave trains.

Waves can vary in size and strength based on wind speed and friction on the water's surface or

outside factors such as boats. The small wave trains created by a boats movement on the water

are called wake. By contrast, high winds and storms can generate large groups of wave trains with

enormous energy.

In addition, undersea earthquakes or other sharp motions in the seafloor can sometimes generate

enormous waves, called tsunamis (inappropriately known as tidal waves) that can devastate entire

coastlines.

Finally, regular patterns of smooth, rounded waves in the open ocean are called swells. Swells are

defined as mature undulations of water in the open ocean after wave energy has left the wave

generating region. Like other waves, swells can range in size from small ripples to large, flat-

crested waves.

Wave Energy and Movement

When studying waves, it is important to note that while it appears the water is moving forward,

only a small amount of water is actually moving. Instead, it is the waves energy that is moving

and since water is a flexible medium for energy transfer, it looks like the water itself is moving. In

the open ocean, the friction moving the waves generates energy within the water. This energy is

then passed between water molecules in ripples called waves of transition. When the water

molecules receive the energy, they move forward slightly and form a circular pattern.

As the waters energy moves forward toward the shore and the depth decreases, the diameter of

these circular patterns also decreases. When the diameter decreases, the patterns become

elliptical and the entire waves speed slows. Because waves move in groups, they continue arriving

behind the first and all of the waves are forced closer together since they are now moving slower.They then grow in height and steepness. When the waves become too high relative to the waters

depth, the waves stability is undermined and the entire wave topples onto the beach forming a

breaker.

Breakers come in different types - all of which are determined by the slope of the shoreline.

Plunging breakers are caused by a steep bottom; and spilling breakers signify that the shoreline

has a gentle, gradual slope.

The exchange of energy between water molecules also makes the ocean crisscrossed with waves

traveling in all directions. At times, these waves meet and their interaction is called interference, of
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which there are two types. The first occurs when the crests and troughs between two waves align

and they combine. This causes a dramatic increase in wave height. Waves can also cancel each

other out though when a crest meets a trough or vice-versa. Eventually, these waves do reach the

beach and the differing size of breakers hitting the beach is caused by interference farther out in

the ocean.

Ocean Waves and the Coast

Since ocean waves are one of the most powerful natural phenomena on Earth, they have asignificant impact on the shape of the Earths coastlines. Generally, they straighten coastlines.

Sometimes though, headlands composed of rocks resistant to erosion jut into the ocean and force

waves to bend around them. When this happens, the waves energy is spread out over multiple

areas and different sections of the coastline receive different amounts of energy and are thus

shaped differently by waves.

One of the most famous examples of ocean waves impacting the coastline is that of the longshore

or littoral current. These are ocean currents created by waves that are refracted as they reach the

shoreline. They are generated in the surf zone when the front end of the wave is pushed onshore

and slows. The back of the wave, which is still in deeper water moves faster and flows parallel to

the coast. As more water arrives, a new portion of the current is pushed onshore, creating a zigzagpattern in the direction of the waves coming in.

Longshore currents are important to the shape of the coastline because they exist in the surf zone

and work with waves hitting the shore. As such, they receive large amounts of sand and other

sediment and transport it down shore as they flow. This material is called longshore drift and is

essential to the building up of many of the worlds beaches.

The movement of sand, gravel and sediment with longshore drift is known as deposition. This is

just one type of deposition affecting the worlds coasts though, and have features formed entirely

through this process. Depositional coastlines are found along areas with gentle relief and a lot of

available sediment.

Coastal landforms caused by deposition include barrier spits, bay barriers, lagoons, tombolos and

even beaches themselves. A barrier spit is a landform made up of material deposited in a long

ridge extending away from the coast. These partially block the mouth of a bay, but if they continue

to grow and cut off the bay from the ocean, it becomes a bay barrier. A lagoon is the water body

that is cut off from the ocean by the barrier. A tombolo is the landform created when deposition

connects the shoreline with islands or other features.

In addition to deposition, erosion also creates many of the coastal features found today. Some of

these include cliffs, wave-cut platforms, sea caves and arches. Erosion can also act in removing

sand and sediment from beaches, especially on those that have heavy wave action.

These features make it clear that ocean waves have a tremendous impact on the shape of the

Earths coastlines. Their ability to erode rock and carry material away also exhibits their power and

begins to explain why they are an important component to the study ofphysical geography.

How Waves are Generated

Apart from special waves such as tsunamis, the only thing that produces the waves we see on our

coasts is the action of the wind blowing over the sea surface. Waves arriving on a coast can be

generated by local wind, in real time, in which case the waves are called windsea, or they can be
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the result of a wind that blew over the surface of the ocean thousands of kilometres away, up to

several days before, in which case they are termed swell or groundswell.

To produce waves, the air moving over the surface of the water has to somehow transmit its

energy to the water. Just how this happens is a very complicated process, still not well

understood. The most accepted theory is the one proposed in the 1950s by J.W. Miles and O.M.

Phillips" the Miles-Phillips theory. The theory describes how waves are generated from a flat sea

using two mechanisms; the first of which produces tiny ripples called capillary waves, and the

second of which produces bigger waves called gravity waves (those we ride).

According to the Miles-Phillips theory, capillary waves first begin to grow from an entirely flat sea,

and then gravity waves are subsequently formed from a sea already containing capillary waves.

Gravity waves and capillary waves are named as such because the restoring force (the force that

returns the sea to an equilibrium position after the wind has lifted it up) is gravity in the case of a

gravity wave and capillary action, or surface tension, in the case of a capillary wave.

The initial generation of capillary waves is due to perturbations in the surface wind, causing

irregularities in the water surface. The wind does not blow completely horizontally all the time; it

will naturally contain random disturbances that give it small vertical motions as well. Sometimes,these vertical motions are enough to create tiny up and down motions on the surface of the water

itself. This is the vital beginning which triggers off further reactions and facilitates the flow of

energy between wind and water.

Once the sea contains capillary waves, there is an increase in surface roughness, which allows the

moving air to grip the surface of the water. There is no longer any need for small vertical

perturbations in the air flow; the horizontally-moving air will now push up the existing bumps in the

water surface. This second mechanism is self-perpetuating; the rougher the surface the more

grip, the more grip the bigger the waves, the bigger the waves the rougher the surface, and so

on. While the first mechanism causes the waves to grow at a rate which is linear with time, the

second mechanism is exponential with time; the bigger they are the quicker they grow.

The restoring force of these bigger waves is now gravity, not surface tension. Eventually a point

will be reached where the wind cant lift up the surface of the sea any more" the force of gravity

pulls the water back down again at the same rate as the wind lifts it up. This natural limit is

reached for a given windspeed, so, if the wind gets stronger, the waves will get higher.

Wave height as a function of time: The first part of

the curve shows the waves growing at a linear ratewith time, then the second part shows when gravity

waves take over from capillary waves and they start

to grow exponentially. Lastly, the waves reach

saturation point for a particular windspeed and the

growth curve tails off.

Where does the wind come from to generate the

waves?

To produce waves big enough to surf, all you need is a reasonable strength wind blowing over a


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fairly decent stretch of ocean for a good few hours. It doesnt really matter

where that wind comes from. In fact, it can come from a number of

different phenomena; for example, a tropical cyclone where surface winds

of immense strength blow around a tight centre; a long, straight zone

called the trade-wind belt, just to the equator-side of a large semi-

permanent high pressure; a local wind on the coast called the sea breeze

which blows in the afternoon to equalize a surface pressure difference

caused by hot air rising off the land, or a monsoon which is a kind of giant

sea breeze. But probably the most important phenomenon for producing

the waves we ride is the low pressure, sometimes called the mid-latitude

depression.

A low pressure is just a cell of air on the surface whose pressure happens to

be lower than its immediate surroundings. To try to equalize this pressure

difference, air will try to flow from outside the low in towards the centre, in

other words from high to low pressure. However, because of the rotation of

the Earth, this air will tend to get steered to the right in the Northern

Hemisphere and to the left in the Southern Hemisphere. As a result, the air around a centre of low

pressure will end up circulating in an anticlockwise direction in the Northern Hemisphere and in aclockwise direction in the Southern Hemisphere. Note that the direction of rotation of air around a

low pressure is always called cyclonic, regardless of hemisphere.

The exact way in which a low-pressure system is formed is also something that scientists are still

struggling to understand. A simple explanation is as follows: They are born initially from small

disturbances in the atmosphere. Given the right conditions, these small disturbances can grow

into large ones. The initial perturbation must be something that causes a local drop in pressure,

and one thing that can do this is the meeting of two air masses of different temperatures and

(hence) pressures. The warmer (lower-pressure) air will tend to slide over the top of the colder

(higher-pressure) air, producing a forced local drop in pressure at the surface. If all the conditions

are right, the rising air will start to spin, sucking in more air from the outside, lowering the surface

pressure further, and so on until the system grows into a full-blown mid-latitude depression.

In an effort to equalize the pressure, air tries to flow directly from a high pressure to a low

pressure, but the Coriolis force causes it to turn to the right. As a result, the air circulates

clockwise (anticyclonic) around a high pressure and anticlockwise (cyclonic) around a low

pressure. Note: Northern-Hemisphere systems shown; in the Southern Hemisphere, everything is

reversed.

Ocean currents are some of the mightiest and most important cycles on the planet. They carry vast

amounts of heat and water to various locations across the Earth, providing the continual changes

that allow most animals and plant life to survive. While certain qualities of the ocean itself, such asits saline content and the topography of the ocean floor, affect currents, there are also a number of

key outside forces responsible for move the water of our oceans.

Read more: How Are Ocean Currents Created? |

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Definition

Ocean currents are the movement of the water in oceans across the world, creating cycles of flow

and common ocean features such as waves and primary streams that move in set patterns across
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the Earth. There are two different kinds of ocean currents: surface and deep water. Surface

currents affect only the top layer of water, the part of the ocean most easily moved by outside

forces. Deep water currents affect the rest of the ocean, going down deep to the ocean basin itself,

and are much more slow-moving, propelled only by the most powerful forces.

Wind

One of the most common factors affecting surface currents is the wind. It may seem at first that

the wind does not have much power to move the waves or establish currents, but a wind streampushing at the water over many miles of ocean surface can have a powerful build-up affect. Not

only can the wind create powerful waves in the direction it is traveling, but it can also fight against

other forces and push water in the opposite direction it is trying to travel, creating complicated

current interactions.

Heating

The warming and cooling of the atmosphere and ocean water is one of the most powerful forces

creating ocean currents. Most of these changes are related to sunlight, which warms vast tracts of

the ocean and atmosphere alike, creating pockets of warm water amid cooler water. Shifts in

energy occur as the warmth tries to pass from warm water to cold water. In the atmosphere, thesechanges create wind, and in a similar manner ocean currents are created from the slower

interactions of water, creating seasonal cycles as the ocean shifts around itself.

Earth Rotation

The combined forces of the wind and heat transfer are always affected by the turn of the Earth,

which rotates so fast that ocean currents are almost always altered by its force. The result is

known as the Coriolis effect, the same effect that causes natural circling patterns to rotate in

opposite directions in the north and south hemispheres. Ocean currents are also forced into circle-

cycles by this effect, vast circles of water known as gyres. Which way the gyre turns also depends

on which hemisphere it is formed in.

Gravity

Lastly, gravity adds its own touch on all ocean currents. The results of the other forces can often

"pile" water up into gradual swells across the ocean. The force of gravity causes these swells to

"fall" outward, adding impetus to their movement and making them much more powerful.

Read more: How Are Ocean Currents Created? |

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A gyre in oceanography is any large system of rotating ocean currents, particularly those involvedwith large wind movements. Gyres are caused by the Coriolis Effect; planetary vorticity along with

horizontal and vertical friction, which determine the circulation patterns from the wind curl

(torque).[1] The term gyre can be used to refer to any type ofvortex in the air or the sea, even one

that is man-made, but it is most commonly used in oceanography to refer to the

major ocean systems.

Coriolis Effect:
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Coriolis effect is an inertial force described by the 19th-century French engineer-mathematician

Gustave-Gaspard Coriolis in 1835. Coriolis showed that, if the ordinary Newtonian laws of motion of

bodies are to be used in a rotating frame of reference, an inertial force--acting to the right of the

direction of body motion for counterclockwise rotation of the reference frame or to the left for

clockwise rotation--must be included in the equations of motion.

The effect of the Coriolis force is an apparent deflection of the path of an object that moves within

a rotating coordinate system. The object does not actually deviate from its path, but it appears to

do so because of the motion of the coordinate system.

The Coriolis effect is most apparent in the path of an object

moving longitudinally. On the Earth an object that moves

along a north-south path, or longitudinal line, will undergo

apparent deflection to the right in the Northern Hemisphere

and to the left in the Southern Hemisphere. There are two

reasons for this phenomenon: first, the Earth rotates

eastward; and second, the tangential velocity of a point on

the Earth is a function of latitude (the velocity is essentiallyzero at the poles and it attains a maximum value at the

Equator). Thus, if a cannon were fired northward from a point

on the Equator, the projectile would land to the east of its due

north path. This variation would occur because the projectile

was moving eastward faster at the Equator than was its target

farther north. Similarly, if the weapon were fired toward the

Equator from the North Pole, the projectile would again land to the right of its true path. In this

case, the target area would have moved eastward before the shell reached it because of its

greater eastward velocity. An exactly similar displacement occurs if the projectile is fired in any

direction.

The Coriolis deflection is therefore related to the motion of the object, the motion of the Earth, and

the latitude. For this reason, the magnitude of the effect is given by 2 sin , in which is the velocity

of the object, is the angular velocity of the Earth, and is the latitude.

The Coriolis effect has great significance in astrophysics and stellar dynamics, in which it is a

controlling factor in the directions of rotation of sunspots. It is also significant in the earth sciences,

especially meteorology, physical geology, and oceanography, in that the Earth is a rotating frame

of reference, and motions over the surface of the Earth are subject to acceleration from the force

indicated. Thus, the Coriolis force figures prominently in studies of the dynamics of the

atmosphere, in which it affects prevailing winds and the rotation of storms, and in the hydrosphere,in which it affects the rotation of the oceanic currents.

Excerpt from the Encyclopedia Britannica without permission.

The Gulf Stream, together with its northern extension towards Europe, the North Atlantic Drift, is a

powerful, warm, and swift Atlanticocean current that originates at the tip ofFlorida, and follows the

eastern coastlines of the United States and Newfoundland before crossing the Atlantic Ocean. The

process ofwestern intensification causes the Gulf Stream to be a northward accelerating current

off the east coast ofNorth America. At about 400N 300W, it splits in two, with the northern

stream crossing to northern Europe and the southern stream recirculating offWest Africa. The Gulf
http://en.wikipedia.org/wiki/North_Atlantic_Currenthttp://en.wikipedia.org/wiki/Atlantic_Oceanhttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Floridahttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/Newfoundland_and_Labradorhttp://en.wikipedia.org/wiki/Boundary_currenthttp://en.wikipedia.org/wiki/North_Americahttp://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://en.wikipedia.org/wiki/Northern_Europehttp://en.wikipedia.org/wiki/West_Africahttp://en.wikipedia.org/wiki/North_Atlantic_Currenthttp://en.wikipedia.org/wiki/Atlantic_Oceanhttp://en.wikipedia.org/wiki/Ocean_currenthttp://en.wikipedia.org/wiki/Floridahttp://en.wikipedia.org/wiki/United_Stateshttp://en.wikipedia.org/wiki/Newfoundland_and_Labradorhttp://en.wikipedia.org/wiki/Boundary_currenthttp://en.wikipedia.org/wiki/North_Americahttp://toolserver.org/~geohack/geohack.php?pagename=Gulf_Stream&params=40_0_N_30_0_W_http://en.wikipedia.org/wiki/Northern_Europehttp://en.wikipedia.org/wiki/West_Africa


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Stream influences the climate of the east coast of North America from Florida to Newfoundland,

and the west coast ofEurope. Although there has been recent debate, there is consensus that the

climate ofWestern Europe and Northern Europe is warmer than it would otherwise be due to

the North Atlantic drift, one of the branches from the tail of the Gulf Stream. It is part of the North

Atlantic Gyre. Its presence has led to the development of strong cyclones of all types, both within

theatmosphere and within the ocean. The Gulf Stream is also a significant potential source

ofrenewable power generation.

The Gulf Stream is a strong, fast moving, warm ocean current that originates in the Gulf of Mexicoand flows into the Atlantic Ocean. It makes up a portion of the North Atlantic Subtropical Gyre.

The majority of the Gulf Stream is classified as a western boundary current. This means that it is a

current with behavior determined by the presence of a coastline - in this case the eastern United

States and Canada - and is found on the western edge of an oceanic basin. Western boundary

currents are normally very warm, deep, and narrow currents that carry water from the tropics to

the poles.

The Gulf Stream was first discovered in 1513 by the Spanish explorer Juan Ponce de Leon and was

then used extensively by Spanish ships as they travelled from the Caribbean to Spain. In 1786,

Benjamin Franklin mapped the current, further increasing its usage.

Path of the Gulf Stream

Today, it is understood that the waters feeding into the Gulf Stream begin flowing off the west

coast of North

Documents

Currents and Ocean Circulation