EAE 04 Ocean water movement 8

Question 1

What 2 factors can create pressure differences in the ocean?

Answer 1

Both horizontal differences in height and density can create pressure differences

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Question 2

Is a calm ocean flat?

Answer 2

“Even a calm ocean does not have a truly flat surface.”

This means that even a calm ocean does not have a truly flat surface. The ocean surface undulates i.e., there are hills and valleys (beyond variations due to sea floor).

The ocean sits a little higher in some places and a little lower in others (these aren’t huge values –mm to cm).

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Question 3

What impact does wind force have on the ocean?

Answer 3

• Wind tries to push water (or boats) in the same direction as the wind.
• The frictional force at the sea surface (i.e., the wind stress), while slowing the winds, also acts to transfers momentum to the sea surface.
• Stronger wind means stronger force on the water and the faster it will accelerate.

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Question 4

How deep is the wind’s interaction with the ocean?

Answer 4

Wind only directly affects water in the top 10-100m

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Question 5

What impact does friction have on the ocean?

Answer 5

Outside of the surface ocean, where momentum is largely sourced, friction is important

• Near the ocean boundaries
• Near the bottom
• Where currents of different strength or direction flow next to each other

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Question 6

What is Ekman Trnasport?

Answer 6

Wind forced near-surface motion

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Question 7

What direction is net Ekman Transport relative to the wind?

Answer 7

Net flow of water is 90° to direction of wind

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Question 8

How is wind stress transmitted through the water column?

Answer 8

Effects of a wind stress on the ocean surface are transmitted down through the water column by the action of

• Turbulent eddies that are themselves generated by the wind
• Breaking waves
• Boundary shear stresses

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Question 9

What is the Ekman layer thickness?

Answer 9

The depth to which the effects of wind are felt is called the Ekman layer thickness or Ekman depth (Hₑ), which is inversely proportional to the Coriolis force magnitude (deeper near the equator and shallower away towards the poles).

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Question 10

How does Ekman Transport work?

Answer 10

• Wind force acts in the direction of the wind
• Coriolis pushes a moving object to the left (SH) or right (NH)
• As you move away from the surface the layer of ocean above the layer of interest becomes the driver of motion rather than the surface wind.
• Integrating over the Ekman layer, Ekman transport is ~90° to the left (right) in the Southern (Northern) Hemisphere.

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Question 11

What are Ocean gyres?

Answer 11

In oceanography, a gyre is any large system of circulating ocean currents, particularly those involved with large wind movements.

Gyres are caused by the Coriolis effect; planetary vorticity, horizontal friction and vertical friction determine the circulatory patterns from the wind stress curl.

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Question 12

What drives the ocean gyre currents?

Answer 12

• Ekman transport creates region of high pressure (i.e., in the central gyre regions)
• Pressure gradient force acts to push the water back outwards again (i.e., from high pressure to low pressure).
• Coriolis force always tries to push a moving object to the left (SH) or right (NH)
• This results in Geostrophic flow in ocean, which is analogous to that of the atmosphere
• Results in an anticyclonic (cyclonic) rotational circulation in each SH (NH) ocean basin. This circulation can reach down to 1-2km.

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Question 13

What is Upwelling?

Answer 13

Upwelling is the upward motion of water. This motion brings cold, nutrient rich water towards the surface.

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Question 14

What is downwelling?

Answer 14

Downwelling is downward motion of water. It supplies the deeper ocean with dissolved gases. Sometimes referred to as ventilation

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Question 15

What drives the oceanic upwelling?

Answer 15

• Consider a Northerly Wind (i.e. from the north) blowing along the NSW coast
• Ekman transport (in top few 10s of meters) 90° to the left of the wind, i.e. to the EAST
• Offshore flow of warm near surface waters
• Water must be replaced from somewhere
• Water comes from the cold deep water
• UPWELLING

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Question 16

What drives the oceanic downwelling

Answer 16

• Now consider a Southerly Wind (i.e. from the south) blowing along the same section of NSW coast
• Ekman transport (in top few 10s of meters) 90° to the left of the wind, i.e. to the EAST
• Onshore surface flow
• Water pushed downwards
• DOWNWELLING
• Warm swimming conditions

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Question 17

Where else does Ekman downwelling occur?

Answer 17

Centre of subtropical gyres

Ekman downwelling (convergence) also occurs in centre of subtropical gyres (between trade wind easterlies and mid-latitude westerlies) as the trade wind cause the accumulation of warm near-surface water.

As such, these regions are also typically considered regions of low biological productivity.

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Question 18

What is equatorial upwelling

Answer 18

• Trade winds winds blow from west to east so Ekman transport leads to equatorial upwelling (Ekman divergence) in the east Pacific and east Atlantic.
• This creates off-equatorial Ekman transport and upwelling of cool nutrient rich water occurs to replace the offshore movement of warm near-surface water .
• These upwelling regions are typically productive fishing regions due to the upwelling of cool nutrient rich waters.

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