# Presentation: 004094

## OCEANIC CURRENTS

**UPDATED: 24 Feb 2000**

## TOPICS

- Atmospheric Circulation
- Role of Wind in Ocean Circulation
- Ocean Heat Transfer
- Surface Currents
- Ocean Currents
- Specific Currents
- Ekman Spiral
- Eddies
- Drifting Bouy

**Notes:**
We will be discussing the following topics. 

## Atmospheric Circulation

- We can gain an understanding of how global circulation works by developing a graphical model of processes that produce this system.
- The global reception of _[solar](file:///..%5Cphysgeoglos/i.html)__[ insolation](file:///..%5Cphysgeoglos/i.html)_ and loss of _[longwave](file:///..%5Cphysgeoglos/l.html)__[ radiation](file:///..%5Cphysgeoglos/l.html)_ cause a temperature gradient of hotter air at the equator and colder air at the poles.
- Surface air flow is from the poles to the equator. The air reaches the equator is then lifted vertically by the processes of _[convection](file:///..%5Cphysgeoglos/c.html)_ and _[convergence](file:///..%5Cphysgeoglos/c.html)_. When it reaches the top of the troposphere it begins to once again flow horizontally. However, the direction of flow is now from the equator to the poles. At the poles the air in the upper atmosphere then descends to the earth's surface to complete the cycle of flow.
- Planetary rotation causes the development of three circulation cells in each hemisphere rather than one. These three circulation cells are known as the: _[Hadley](file:///..%5Cphysgeoglos/h.html)__[ cell](file:///..%5Cphysgeoglos/h.html)_; _[Ferrel](file:///..%5Cphysgeoglos/f.html)__[ cell](file:///..%5Cphysgeoglos/f.html)_; and _[Polar cell](file:///..%5Cphysgeoglos/p.html)_.

**Notes:**
The primary cause of ocean currents is the wind.  In order to understand how the currents form, you must understand how the wind is generated.  
Since the equatorial sections of the earth receive the most incoming solar radiation, they are normally the warmest parts of the planet.  Consequently, the air in the equatorial region is quite warm, and rises as a result of heating.  Due to this rising air, low pressure belts form near the equator.  
Where does all this air go? In order to remain in equilibrium, the air that has risen over the equator has to descend as it moves toward the poles. Once  it reaches the poles, it is now cool and dense and moves back towards the equator where it’s journey began. 
However, we must add another force to the equation, that force being Coriolis. Coriolis force is the apparent force imparted to a moving object by the rotation of the earth.  This force tends to deflect moving objects to the right in the northern hemisphere and to the left in the southern.  This results in three cells of air circulation, the Hadley, Ferrel and Polar cells. 

## Slide 4

**Notes:**
This slide graphically depicts a simplified diagram of the earth’s atmospheric circulation. 

## GENERALIZED SURFACE WIND PATTERN

**Notes:**
Which results in the surface winds we see on a daily basis.  

## CURRENTS

- AS THE WIND BLOWS OVER THE OCEANS, IT CAUSES  THE SURFACE WATER TO DRIFT ALONG WITH IT.  THE MOVING WATER GRADUALLY PILES UP, CREATING PRESSURE DIFFERENCES WITHIN THE WATER ITSELF.  THIS LEADS TO FURTHER MOTION SEVERAL HUNDRED
- METERS DOWN INTO THE WATER.  IN THIS MANNER, THE GENERAL WIND FLOW AROUND THE GLOBE STARTS THE MAJOR SURFACE OCEAN CURRENTS MOVING.

## Slide 7

**Notes:**
Besides wind, friction and Coriolis, there is another factor that must be taken into account when discussing ocean currents. That is the Sea Surface Topography (And what does SS Topo mean??) 
In simplified terms, it is hills and valleys in the ocean, caused by density differences between water masses.  The sea surface contains many hills and valleys, especially near the central portions of the ocean basins. A primary example is the Sargasso Sea (anyone know where that is?), where a hill of warm, light water is surrounded by colder, heavier water.  Keep in mind that these hills and valleys are only several centimeters in height and depth, and are measured via satellite altimeters. 
In the end assesment, currents will tend to flow around hills of lighter water or around valleys of denser water.

## ABOVE AVERAGE 

**ABOVE AVERAGE **
**OCEAN HEIGHT**

**BELOW AVERAGE**
**OCEAN HEIGHT**

## Slide 9

**Notes:**
A new concept in oceanography has been discovered recently, that being the global  deepwater current heat conveyor.  This conveyor belt of water, is the earth’s primary means of transporting heat around the planet.  Warm surface waters primarily in the northern hem sink and then travel to the southern hem and then back, taking up a thousand years to complete the circuit. .  

## 3-D VIEW OF THE GLOBAL CONVEYOR BELT

**3-D VIEW OF THE GLOBAL CONVEYOR BELT**

## IN THE NORTH ATLANTIC, THE PREVAILING WINDS 

**IN THE NORTH ATLANTIC, THE PREVAILING WINDS **
**BLOW CLOCKWISE AND OUTWARD FROM THE **
**SUBTROPICAL HIGH, WHILE THE OCEAN CURRENTS **
**MOVE IN A MORE OR LESS CIRCULAR , BUT STILL **
**CLOCKWISE PATTERN. **

## GENERALIZED SURFACE WIND PATTERN FROM SEMIPERMINENT HIGHS

**GENERALIZED SURFACE WIND PATTERN FROM SEMIPERMINENT HIGHS**

**H**

**H**

**H**

**H**

**H**

**H**

**Notes:**
Which results in the surface winds we see on a daily basis.  

## H

**ALASKAN**
**STREAM**

**NORTH EQUATORIAL CURRENT**

**CALIFORNIA C.**

**NORTH PACIFIC C.**.

**EQUATORIAL COUNTER CURRENT**

**SOUTH EQUATORIAL C.**

**EAST AUSTRALIAN C.**

**PERU **
**CHILE C.**

**SOUTH PACIFIC C.**

**KUROSHIO C.**

**OYASHIO C.**

**ALASKAN**

**CURRENT**

**ANTARTIC CIRCUMPOLAR CURRENT**
**(WEST WIND CURRENT)**

**SW MONSOON C.**

**SOMALI C.**

**SOUTH **
**EQUATORIAL**
** ****CURRENT**

**SOUTH INDIAN C.**

**AGULHAS C.**

**GULF STREAM**

**CANARY CURRENT**

**NORTH**

**EQUATORIAL C.**

**EQUATORIAL COUNTER C.**

**EAST GREENLAND C.**

**NORTH **
**ATLANTIC C.**

**          **

**LABRADOR C.**

**SOUTH **
**EQUATORIAL**
**CURRENT**

**        **

**BRAZIL C.**

**BENGUELA**
**CURRENT**

**SOUTH ATLANTIC**
**CURRENT**

**H**

**H**

**H**

**H**

**H**

**H**

**SURFACE OCEAN CURRENTS AROUND SEMIPERMANENT HIGH**

## Slide 14

## MAJOR OCEAN CURRENTS DO NOT FOLLOW

**MAJOR OCEAN CURRENTS DO NOT FOLLOW**
**THE WIND PATTERN EXACTLY; RATHER, THEY **
**SPIRAL IN SEMICLOSED CIRCULAR WHIRLS **
**CALLED *****GYRES.***

## SURFACE CURRENTS

**SHIPS’ COURSES ARE DEFLECTED BY SURFACE**
**CURRENTS;  THIS DEFLECTION CAUSES A **
**DISCREPANCY BETWEEN A SHIP’S INTENDED **
**POSITION AND ITS ACTUAL POSITION AFTER IT **
**HAS TRAVELED FOR A PERIOD.  THUS THE DIRECTION **
**AND SPEED OF THE LOCAL CURRENT CAN BE DEDUCED**
**FROM A SHIP’S ACTUAL POSITION AFTER IT HAS BEEN**
**STEERED ON A GIVEN COURSE.**

## BECAUSE OF THE LARGER FRICTIONAL DRAG IN WATER,

**OCEAN CURRENTS MOVE MORE SLOWLY THAN THE **
**	****	****	****	****	****PREVAILING WIND.  **
**	****	****	****	****	****TYPICALLY, THESE **
**	****	****	****	****	****CURRENTS RANGE **
**	****	****	****	****	****IN SPEED FROM SEVERAL **
**	****	****	****	****	****KILOMETERS PER DAY TO **
**	****	****	****	****	****SEVERAL KILOMETERS ****	****	****	****	****	****	****PER HOUR.**

## Slide 18

## In 1990, a container vessel lost five containers overboard in a storm,releasing 61,000 Nike running shoes into the Pacific Ocean. These shoes float, and several months later, hundreds of them began to wash ashore along the coast of North America from Alaska to Oregon. 

**In 1990, a container vessel lost five containers overboard in a storm,releasing 61,000 Nike running shoes into the Pacific Ocean. These shoes float, and several months later, hundreds of them began to wash ashore along the coast of North America from Alaska to Oregon. **
**Scientists tapped into the information network of beachcombers, and the points of landfall of at least1600 Nike shoes were recorded exactly. By the time the data were plotted, the drift of the North Pacific was recorded in finer detail than ever. Years later, far-floating Nike shoes from the spill are still being recovered as far away as Hawaii, because the surviving floaters were turned southward into the main Pacific clockwise circulation. **
**It was with even greater delight that oceanographers learned of the fate of 29,000 Chinese plastic bath toys during their voyage from Hong Kong to Tacoma, Washington. In a severe storm on January 10, 1992, thousands of blue plastic turtles, yellow plastic duckies, red plastic beavers, and green plastic frogs began their great adventure. **
**Ten months later, the first waves of plastic toys hit the beaches of Sitka, Alaska. Advertisements in newspapers and letters to lighthouse keepers brought in reports of landings, all of them on the Alaskan coast. This time, the higher-floating plastic toys were more affected by the wind than the low-floating Nikes, and their main trajectory took them north, out of the gyre circulation. Those that did not reach shore quickly were swept northeast, and by the winter of 1993-94, the survivors would have been frozen into sea ice in the Bering Sea. Some may now be heading back into the North Pacific, others may be on their way round the Arctic Ocean, neastward along the north coast of Siberia, heading for the North Atlantic. **

## Ocean Circulation

- Vertical- Primarily driven by regional differences in density, salinity, temperature, sediment content
- Horizontal- “Wind-driven”
- mean flow is perpendicular to wind direction
- Ekman Spiral- Surface current(Northern Hemisphere)

**Notes:**
Now that we have a basic understanding of atmospheric circulation, lets move on to Ocean circulations.  There are two different types of ocean circulations, vertical and horizontal. For the purposes of today’s lecture, we will focus on the horizontal circulation. 
Vertical circulations are driven by differences in density brought on by differences in temperature, salinity, or sediment content. Vertical circulation is important in coastal areas or near closed seas such as the Med.

Horizontal currents however are primarily wind-driven.  But contrary to what you might expect, the currents do not directly follow the wind direction. This is due to the Ekman transport effect, which we will discuss on the next slide.  

## OCEAN CURRENTS

**OCEAN CURRENTS**

## Slide 22

## H

**ALASKAN**
**STREAM**

**NORTH EQUATORIAL CURRENT**

**CALIFORNIA C.**

**NORTH PACIFIC C.**.

**EQUATORIAL COUNTER CURRENT**

**SOUTH EQUATORIAL C.**

**EAST AUSTRALIAN C.**

**PERU **
**CHILE C.**

**SOUTH PACIFIC C.**

**KUROSHIO C.**

**OYASHIO C.**

**ALASKAN**

**CURRENT**

**ANTARTIC CIRCUMPOLAR CURRENT**
**(WEST WIND CURRENT)**

**SW MONSOON C.**

**SOMALI C.**

**SOUTH **
**EQUATORIAL**
** ****CURRENT**

**SOUTH INDIAN C.**

**AGULHAS C.**

**GULF STREAM**

**CANARY CURRENT**

**NORTH**

**EQUATORIAL C.**

**EQUATORIAL COUNTER C.**

**EAST GREENLAND C.**

**NORTH **
**ATLANTIC C.**

**          **

**LABRADOR C.**

**SOUTH **
**EQUATORIAL**
**CURRENT**

**        **

**BRAZIL C.**

**BENGUELA**
**CURRENT**

**SOUTH ATLANTIC**
**CURRENT**

**H**

**H**

**H**

**H**

**H**

**H**

**SURFACE OCEAN CURRENTS AROUND SEMIPERMANENT HIGH**

## Slide 24

## NORTH PACIFIC CURRENTS

**THE CURRENTS OF THE NORTH PACIFIC OCEAN**
**ARE VERY SIMILAR TO THE CURRENTS OF THE**
**NORTH ATLANTIC OCEAN.  EVEN SO, THERE ARE **
**SOME DISTINCT DIFFERENCES.  THESE DIFFERENCES**
**ARE DUE MAINLY TO LARGE AMOUNTS OF**
**SUBARTIC WATER IN THE NORTH PACIFIC, **
**COMPARED WITH THE SMALL AMOUNT IN THE **
**NORTH ATLANTIC.**

## The Kuroshio Current

- The next slide depicts an image of the Northwest Pacific near Japan. Blue and purple represent cold water while red and yellow represent warmer waters.  This north-south gradient is seen clearly in the image. This image also shows a current that brings warm water from the south up to the north.  It is one of the strongest currents in the ocean with an average velocity of 1 m/s (3 ft/s).

## KUROSHIO CURRENT

## NORTH PACIFIC CURRENTS

**NORTH EQUATORIAL CURRENT**** - STARTS NEAR **
**THE WESTERN COAST OF CENTRAL AMERICA. **
**CROMWELL CURRENT**** - A NARROW, SWIFT **
**SUBSURFACE CURRENT CENTERED ON THE EQUATOR **
**BETWEEN 2 deg N and 2 deg S.  **
**KUROSHIO SYSTEM**** - BEGINS  WHERE THE NORTH **
**EQUATORIAL CURRENT LEAVES OFF.  IT FLOWS PAST **
**FORMOSA AND PROCEEDS NORTHEASTWARD IN THE**
**DEEP OCEAN AREA BETWEEN THE CHINA SEA AND **
**THE RYUKYU ISLANDS.**
***COMPOSED OF KUROSHIO CURRENT, KUROSHIO **
**EXTENSION AND NORTH PACIFIC CURRENT.**

## Slide 29

## Slide 30

## EKMAN SPIRAL

**The Ekman spiral is one of the oldest results in dynamical oceanography. It was first proposed (conceptually) by the great Norwegian explorer Fridtjof Nansen. As part of a polar expedition in the late 1890s, Nansen froze his ship Fram into the ice north of Spitzbergen Island and allowed it to drift for more than two years.**
**During the expedition he noticed that the drift of the boat was generally to the right of the wind. Nansen proposed that this motion was the result of the Coriolis force, which causes objects to veer to the right in the northern hemisphere and to the left in the southern hemisphere. He supposed further that as the ice pushed on the water immediately below it, that water would move still further to the right of the wind, though a little more slowly. Extended down through the water column, the result would be a spiral structure**.

**Notes:**
When a wind blows over the ocean surface, the water particles near the surface tend to move in the same direction as the wind.  Once this motion is set initiated, Coriolis force begins to act, deflecting the particles to the right in the northern hem and to the left in the southern hem. The motion of a thin layer of surface water will be about 45deg to the right of the wind.  The wind energy, however is transmitted even below the surface layer. Because of friction, the water at the surface tends to pull the underlying water with it but at a slower speed.  Coriolis force further deflects this deeper water. This reaction continues to some depth at which the frictional forces and the water movement due to this process become negligible.  The vertical pattern of moving water forms a sort of spiral called the Ekman Spiral.  The water at some depth may even be moving at opposite to the wind at the surface.  The overall effect is that a surface layer of water, up to several tens of meters, will have a net motion of 90 deg to the wind.  

## Areas of Upwelling

**Areas of Upwelling**

**Ekman Transport, ****The California Current and Coastal Upwelling**

**Notes:**
A prime example of Ekman Transport and it’s effect is the California current,which flows southward along the California coast.  Assume that a wind is blowing parralel to the coast, (which is normally the case along the cali coast). With the coast to the left of the wind, the net movement of water, 90deg to the right of the wind, is away from the coast. Deeper, colder water must move up to replace the surface water that has moved offshore. This is known as upwelling, and may transport water from as deep as 400 meters to the surface. The cold deep water is generally rich in nutrients and will increase surface productivity. 
What are some of the other effectst that upwelling has along the coast?
How does it affect the weather? 
What other areas of the world experience significant upwelling? 
What happens if upwelling stops?
What is the connection between upwelling and El Nino?

## NORTH ATLANTIC CURRENTS

**NORTH EQUATORIAL CURRENT**** - LOCATED IN THE **
**TRADE WIND BELT OF THE NORTH ATLANTIC OCEAN.**
**  ****THE CHIEF SOURCES OF FLOW ARE THE **
**NORTHEASTERLY CURRENTS OFF THE WEST COAST **
**OF NORTHWESTERN AFRICA.  **
**GULF STREAM SYSTEM**** - BEGINS IN THE FLORIDA **
**STRAITS AND FLOWS NORTHWARD AND EASTWARD **
**ALONG THE EAST COAST OF THE UNITED STATES.**
***COMPOSED OF THE FLORIDA CURRENT, **
**GULF STREAM & NORTH ATLANTIC CURRENT.**

## The Gulf Stream

- Here is an image of the North Atlantic from June of 1984. Blue and purple represent cold water while green represents warmer water.  This north-south gradient is seen clearly in the image. This image also shows a current that brings warm water from the south up to the north.  It is one of the strongest currents in the ocean with an average velocity of 1 m/s (3 ft/s).   This image also shows that the Gulf Stream does not follow a straight path. It has many twists and turns called meanders. If a meander becomes really sharp, it may pinch off and form what is called a ring. This is much like the formation of an oxbow lake by a river. Rings can be formed either to the north or to the south of the stream.

## Slide 35

## EDDIES

**Notes:**
Eddies are pockets of either colder or warmer water cutoff from there parent current. Eddies are common along both sides of the gulf stream and the kuroshio currents. 

## SATELLITE PICTURES REVEAL 

**SATELLITE PICTURES REVEAL **
**THAT DISTINCT TEMPERATURE **
**GRADIENTS EXIST ALONG THE **
**BOUNDARIES OF SURFACE **
**OCEAN CURRENTS.  **
**THE BOUNDARY SEPERATING **
**THE TWO MASSES OF WATER **
**WITH CONTRASTING **
**TEMPERATURES AND DENSITIES  **
**IS CALLED AN *****OCEANIC FRONT.***
**ALONG THIS FRONTAL **
**BOUNDARY, A PORTION OF THE **
**MEANDERING GULF STREAM **
**OCASSIONALLY BREAKS AWAY **
**AND DEVELOPS INTO A CLOSED **
**CIRCULATION OF EITHER COLD **
**OR WARM WATER - A WHIRLING **
**EDDY.**

## WARM EDDY

**Notes:**
Warm eddies are those that have their warmest temperatures at the center, surrounded by cooler water. They occur on the north side of a warm water current such as the gulf stream and generally rotate counterclockwise. Theyare generally shallow in depth, rarely extending below 100 meters, due to the warmer water being less dense. They generally are large in areal area and last only  short periods of time. Warm eddies are good for submarines to hide in due to the fact that sound travels upward toward the surface in warm water. This allows the submarine to “hide” in the warm eddie because it’s sound does not eminate outside the eddy.  

## COLD EDDIES

**Notes:**
Cold eddies are those with the coldest temperatures at their center surrounded by warmer water. They normally form on the southern side of warm water currents and rotate clockwise. They generally form near the surface but extend to very deep depths due to the denseness of the water. They last for long periods of time, but may or may not be reflected at the surface due to sinking. Cold eddies are prime areas to place sonar sensors to detect submarines due to coupling with the Deep Sound Channel.  

## EDDY STRUCTURE

- EDDY

**Notes:**
This slide depicts the vertical structure of a cold core eddy. Note that the eddy does not reflect at the surface. 

## LAGRANGIAN DRIFTERS

**These drifters measure the location, sea surface temperature, upwelling radiance, and downwelling irradiance. **
** ****Measurements are made every 90 seconds and averaged over a one hour period.  The most**
** ****recent set of hourly data is then transmitted to the user, via the ARGOS satellite system, every 90**
** ****seconds for an hour.   The typical lifetime for these drifters was 3 - 7 months, although some**
** ****failed immediately upon or shortly after deployment, and one lasted for 10 months.**

**LAGRANGIAN DRIFTERS**

## Slide 42