# Notes on “Ocean Currents”

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Notes on “Ocean Currents”
Currents make circular patterns called gyres. The gyres in the Northern hemisphere run clockwise, and the gyres in the Southern hemisphere run counter-clockwise. Earth/Space

Explain how winds and the Coriolis effect influence surface currents
S.W.B.A.T. Explain how winds and the Coriolis effect influence surface currents Discuss the temperatures of coastal waters Describe density currents and how they affect Earth “Students Will Be Able To…” - another way to state the lesson objectives.

Ocean currents are like rivers within the oceans
Surface Currents Ocean currents are like rivers within the oceans Surface Currents - water that moves horizontally or parallel to Earth’s surface by wind The water of the ocean surface moves in a regular pattern called surface ocean currents. In this map, warm currents are shown in red and cold currents are shown in blue.

Only a few hundred meters of surface water is moved
Surface Currents Only a few hundred meters of surface water is moved Some seeds and plants are carried between continents by surface currents Diagram shows how the surface current affects the water less and less as depth increases. 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.

Major Surface Currents of Earth’s Oceans
The Robinson projection map shows the main surface currents of the world. A Robinson projection is a “compromise” map showing the poles as lines rather than points and more accurately portraying high latitude lands and water to land ratio. These currents are part of the seventeen major surface currents found in the world’s oceans.

Because Earth rotates west to east surface winds causes water to shift
Surface Currents Because Earth rotates west to east surface winds causes water to shift Coriolis Effect - shifting of winds & waters due to Earth’s rotation The effect was discovered by the nineteenth century French engineer Gaspard C. Coriolis. He used mathematical formulas to explain that the path of any object set in motion above a rotating surface will curve in relation to objects on that surface.

Surface Currents Wind and surface currents will deflect to the right (or “clockwise”) in the northern hemisphere and to the left (counter-clockwise) in the southern hemisphere “Right” and “left” in this case is an observer who is looking down at Earth’s surface from a sufficiently high altitude. To sum up ‘what is the Coriolis effect’- it is an important meteorological force that is used to predict the path of storms; currents and explains why a projectile will not hit a target at a great distance if the Earth’s rotation is not accounted for.

Coriolis Effect or “Force”
Video (2:12) “Coriolis Effect Explained” The short video gives a very good overview of the Coriolis Effect.

The Gulf Stream is the world’s most powerful ocean surface current
Surface Currents Gulf Stream – flows up the U.S. east coast and keeps Britain & Northern Europe’s climate mild The Gulf Stream is the world’s most powerful ocean surface current The Gulf Stream is bigger than the combined flow of the Mississippi, the Nile, the Congo, the Amazon, the Volga, the Yangtze and many other major rivers of the world. This great mass of flowing water, or energy, has no beginning or ending, for its waters flow continuously northward along our east coast then east across the North Atlantic to the coasts of Europe and the United Kingdom, where it turns south and flows along western Europe and Africa, before again turning westward across the South Atlantic to the Caribbean basin. Only the western part of this giant circulation is known as the Gulf Stream.

Satellites can track currents based on ocean temperature
Surface Currents Satellites can track currents based on ocean temperature Surface currents can also be tracked by using messages in bottles and other flotation devices Surface currents also impact navigation, search and rescue activities and maritime operations. The ability to track surface currents and to accurately predict trajectories is critical in managing coastal ecosystems, mitigating environmental hazards, and facilitating safe and efficient marine operations.

Ocean Surface Temperatures - Satellite Data
Ocean surface currents that are warmer than the surrounding water they may have a thermal signature, that can be seen in (Advanced Very High Resolution Radiometer (AVHRR) imagery.  Depending on their thermal contrast (the temperature difference between the two water masses) ocean surface currents can have a strong or weak thermal signature.

This distributes heat into the atmosphere and influences climate
Surface Currents Warm currents (red) originate near the equator and flow towards the poles This distributes heat into the atmosphere and influences climate Cold currents (blue) flow towards the equator & heats up creating a cycle Both the atmosphere and the oceans transport heat from low latitudes near the equator to high latitudes near the poles. Cool air and water currents make the return trip from high latitudes back towards the equator.  For the atmosphere, this circulation that globally redistributes heat is accomplished by winds; for the ocean, it is accomplished by currents.

Upwelling occurs when deep, cold ocean water is brought to the surface
Winds along some coasts will move warm water away from the surface This form of upwelling is called coastal upwelling. There are several types of upwelling including open-ocean; southern ocean (around Antarctica) and equatorial.

Cold bottom water will rise up to take the place of the warmer water
Upwelling Cold bottom water will rise up to take the place of the warmer water This water contains organisms that have died, sank to the bottom and decayed Nutrients from upwelling attract fish The increased availability in upwelling regions results in high levels of primary productivity and thus fishery production. Approximately 25% of the total global marine fish catches come from five upwellings that occupy only 5% of the total ocean area. Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient-enriched waters and global fishery yields.

Areas of Upwelling (in Red) and Major World Fisheries
Commercial fishing in Antarctica (not shown in the diagram) began in the 1960s, mostly for fish but also for krill. The Patagonian toothfish, a prized delicacy in Japan, has already been over exploited by the fishing industry. In 1998 it was estimated that the total catch of this species was some 10-fold over the legal catch. limit.

Density Currents Density currents – when a mass of seawater circulates because of density differences Density increases with higher salinity Density also increases with a decrease in temperature Also known as “thermohaline” currents (“thermo” = temperature; “haline” = salinity) Differences in water density affect vertical ocean currents – the movement of surface ocean water to the bottom of the ocean and movement of deep ocean water to the surface.

These currents move very slowly and can take many years to circulate
Density Currents Changes in temperature and salinity work together to create density currents These currents move very slowly and can take many years to circulate Variations in water density are caused by variations in salinity (the amount of salt in water) and temperature. Saltier water is more dense than less salty water; cold water is more dense than warm water. Denser water tends to sink, while less dense water tends to rise. Cold-water currents occur as the cold water at the poles sinks and slowly moves toward the equator. Warm-water currents travel out from the equator along the surface, flowing toward the poles to replace the sinking cold water.

Seawater freezes leaving behind high-salinity water that sinks
Density Currents Global Ocean Conveyor Belt – starts in the winter in the arctic regions Seawater freezes leaving behind high-salinity water that sinks This deep-ocean conveyor belt takes about 1000 years to circulate the Earth once! As these currents mix the oceans surface and deep waters, they help replenish the oxygen in the water.

Deep-Water Global Ocean Conveyor Belt
Video (2:40) “Ocean Odyssey: Density Current” The thicker blue (cold) and pink (warm) lines are representative of the Global Ocean Conveyor Belt; the dashed lines represent surface currents.

Density Currents Mediterranean water is very high in salinity due to a warm climate and evaporation This causes an intermediate layer to form on top of the cold water in the Atlantic The Mediterranean Sea is connected to the North Atlantic through the narrow Strait of Gibraltar. Flow is into the Mediterranean at the sea surface in the Strait. Within the Mediterranean there is large evaporation and cooling and production of dense water. This flows out into the North Atlantic at the bottom of the Strait and increases the oxygen levels.

These currents also help in regulating global climate
Density Currents Thermohaline (density) currents are very important in replenishing nutrients from warmer surface waters These currents also help in regulating global climate Global warming may affect density currents causing further climate change There is an intense interaction between Earth’s 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.

A river flows into the ocean
Class Activity A river flows into the ocean Predict what will happen to this layer of fresh-water Explain your prediction Since we know that freshwater is less dense than seawater students may conclude that the freshwater will form a layer that is on top of the seawater. Also – students may ask about the temperature of the freshwater river as we know that cold water is denser than warm water.