Highways in the SeaHighways in the Sea

Chapter 9Chapter 9

Causes of CurrentsCauses of Currents

• Three major factors drive ocean Three major factors drive ocean currentscurrents– WindWind– Changes in sea levelChanges in sea level

– Ocean circulation causes slopes to develop. The Ocean circulation causes slopes to develop. The steeper the mound of water the larger and faster steeper the mound of water the larger and faster the currentthe current

– Variations in water densityVariations in water density

What is a gyre?What is a gyre?

• Large persistent currents in the Large persistent currents in the ocean eventually run into land.ocean eventually run into land.

• The combination of the The combination of the westerlieswesterlies pushing water eastward, the pushing water eastward, the tradewindstradewinds pushing it westward near pushing it westward near the equator, and the the equator, and the Coriolis effectCoriolis effect results in a circular flow in each results in a circular flow in each ocean basin; a ocean basin; a gyregyre..

How many gyres are there, How many gyres are there, where are they?where are they?

• There are There are 55 major gyres, one in each major gyres, one in each major ocean basinmajor ocean basin

• For those living in the United States, For those living in the United States, the the North Atlantic GyreNorth Atlantic Gyre is perhaps is perhaps the most familiar as well as one of the most familiar as well as one of the most studiedthe most studied

What is the Ekman What is the Ekman transport?transport?• is the natural process by which wind is the natural process by which wind

causes movement of water near the ocean causes movement of water near the ocean surface. surface.

• Each layer of water in the ocean drags Each layer of water in the ocean drags with it the layer beneath. with it the layer beneath.

• Thus the movement of each layer of water Thus the movement of each layer of water is affected by the movement of the layer is affected by the movement of the layer above, or below in the case of a frictional above, or below in the case of a frictional bottom boundary layer bottom boundary layer

What are Geostrophic What are Geostrophic currentscurrents

• All five oceanic gyres behave like the All five oceanic gyres behave like the Atlantic Gyre.Atlantic Gyre.

• These gyres are called geostrophic These gyres are called geostrophic currentscurrents

• Geostrophic means related to the Geostrophic means related to the Earth’s rotation.Earth’s rotation.

The Gulf Stream (10,000 The Gulf Stream (10,000 niagra falls worth of water per niagra falls worth of water per second)second)• The narrowest, fastest, and deepest surface The narrowest, fastest, and deepest surface

currents are the western boundary currents. currents are the western boundary currents. (found on the east coast of continents)(found on the east coast of continents)

• The Gulf Stream (~4 mph / 7 kph) in the North The Gulf Stream (~4 mph / 7 kph) in the North Atlantic is a great example of a western Atlantic is a great example of a western boundary current.boundary current.

• Western boundary currents moves water from Western boundary currents moves water from the equator towards the poles.the equator towards the poles.

• Part of the global conveyer belt of heat.Part of the global conveyer belt of heat.• Day After Tomorrow…science or Day After Tomorrow…science or

psuedoscience?psuedoscience?

Global Conveyer Belt of Global Conveyer Belt of HeatHeat

The thermohaline The thermohaline circulationcirculation• The thermohaline circulation is The thermohaline circulation is

sometimes called the sometimes called the ocean conveyor ocean conveyor beltbelt, the , the great ocean conveyergreat ocean conveyer, the , the global conveyor beltglobal conveyor belt, or, most , or, most commonly, the commonly, the meridional overturning meridional overturning circulationcirculation (often abbreviated as (often abbreviated as MOCMOC).).

• The The thermohaline circulation (THC)thermohaline circulation (THC) is is the global density-driven circulation of the global density-driven circulation of the oceansthe oceans

What are Eddies?What are Eddies?

• Friction with adjacent water can cause swirling Friction with adjacent water can cause swirling currents, called eddies.currents, called eddies.

• Eddies can form large circular loops that break Eddies can form large circular loops that break away as cold core or warm core eddies.away as cold core or warm core eddies.

• Eddies are important because they can profoundly Eddies are important because they can profoundly affect local temperatures and water conditions.affect local temperatures and water conditions.

• Commercial fishing vessels use eddies to locate Commercial fishing vessels use eddies to locate fishfish

• These currents also affect shipping by influencing These currents also affect shipping by influencing ship speed.ship speed.

• 200 kilometers / 320 miles across200 kilometers / 320 miles across• Water’s inhabitants???Water’s inhabitants???

What role do currents play in What role do currents play in transporting heat? How do transporting heat? How do currents affect climate?currents affect climate?• Water’s high heat capacity is the Water’s high heat capacity is the

reason that oceans affect weather reason that oceans affect weather and climate.and climate.

• Currents redistribute about a third of Currents redistribute about a third of the heat in the tropicsthe heat in the tropics

• Without currents moving heat, the Without currents moving heat, the world’s climate would be more world’s climate would be more extremeextreme

To be cont.To be cont.