chapter 9: circulation of the ocean -...

Chapter 9:

Circulation of the Ocean

“Tropical” gardens on Britain’s

Scilly Isles. Only 48 kilometers

(30 miles) off the coast of

Cornwall at 50° N, these

scenic islands lie in the path of

the warm waters of the Gulf

Stream.

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Learning Goals:

Circulation of the Ocean

• Ocean circulation is driven by winds and by differences

in water density. Winds and ocean currents distribute

heat worldwide.

• Surface currents: driven by wind, they move in

circular patterns (gyres) around the edges of major

ocean basins. Surface = top 100 meters

• Upwelling supports the most productive fisheries.

• Thermohaline currents: slow, deep currents that affect

the bulk of seawater below the pycnocline.

• El Niño and La Niña affect ocean and atmosphere.

In El Niño years, the normal wind and current flow

changes near the coast of Peru (and influences the

whole world). 2

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Winds, driven by

uneven solar heating

and Earth’s spin,

drive the movement

of the ocean’s surface

currents.

Two types of winds

are most important to

ocean circulation:

1) powerful westerlies

and

2) persistent trade

winds (easterlies).

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Compare the two figures.

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Hurricanes move with trade winds then westerlies at higher latitudes.

Cannot form on equator (air is too still here).

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Another example of isobars…

http://www.youtube.com/watch?v=g3gw8DyVsco

The low pressure cell in the northeast is

Hurricane Sandy.

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Low pressure over ocean picks up evaporation from the ocean.

Hurricanes

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In Southern hemisphere,

winds spin opposite.

In the Northern

Hemisphere,

hurricanes

(tropical cyclones)

turn

counterclockwise,

as winds become

deflected to the

right.

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Two major things are needed for hurricane development:

1) High humidity

2) High temperatures (>79°F)

Red zones = where hurricanes develop 10

Compare the two figures.

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Winds, driven by

uneven solar heating

and Earth’s spin,

drive the movement

of the ocean’s surface

currents.

Two types of winds

are most important to

ocean circulation:

1) powerful westerlies

and

2) persistent trade

winds (easterlies).

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Northern

Hemisphere:

Coriolis Effect moves

surface waters to

right of winds.

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Gyres – surface

currents circulating

around ocean

basins

Four forces

cause gyres:

1.Surface

winds

2.Sun’s heat

3.Coriolis

effect

4.Gravity

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For example,

the North

Atlantic Gyre

includes four

currents.

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Earth has 2 northern gyres, 4 southern.

North Pacific Gyre

North

Atlantic Gyre

South Pacific GyreSouth

Atlantic Gyre

Indian

Ocean Gyre

Circum - Antarctic Gyre

Surface currents can be illustrated by

sea surface temperature.

Red: 25°–28°Celsius (77°–82°F).

Yellow: 20°–25°C (68°–77°F);

Green: 15°–20°C (59°–68°F);

Blue: 0°–15°C (32°–59°F).

Purple: below the freezing point of fresh water.

Note the distortion of the temperature patterns we might expect

from the effects of solar heating alone—the patterns twist

clockwise in the Northern Hemisphere, counterclockwise in the

Southern.

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Waters brought from equator – warm-water current

“ “ “ poles – cold-water current

Surface currents, summarized with

names and usual directions.

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Ekman spiral –

two examples of what happens

1. How water is transported in the North Atlantic

(as one example of an ocean gyre)

2. Upwelling vs. downwelling

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Coriolis Effects felt

increasingly smaller

below sea level.

Net Effect –

90⁰ motion of winds.

Ekman spiral and

Ekman transport:

how the process works

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Ekman Spiral – circular motion caused by geostrophic flow.

Hill pushes mixed layer deeper.

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Center of hill not at basin center as expected.21

If the Coriolis effect didn’t exist,

ocean gyres would look like this:

Because of the Coriolis effect,

ocean gyres are “curved”:

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Key points:

The Coriolis effect is strongest near the poles.

Red arrows: Water flowing eastward at high latitudes turns sooner

to the right (doesn’t make a square)

Green arrows: water flowing westward near the equator doesn’t

turn right until it “hits” a continent

If the Coriolis effect didn’t exist,

ocean gyres would look like this:

Because of the Coriolis effect,

ocean gyres are “curved”

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Western boundary currents are stronger than

eastern boundary currents.

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Sverdrup – 1 million m3 of flow per second.

Western part of gyre has stronger, narrower, deeper flow.

Eastern section has weaker, wider, shallower flow.

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Upwelling – Cold, nutrient-rich waters brought from below.

Downwelling – warm, nutrient-poor waters forced down.27

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29

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Slightly off equator with minimal Coriolis, upwelling caused by trade winds.

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Upwelling:

cold water brings nutrients to the surface.

Canary Current

There are four major upwelling zones, and they are eastern boundary currents:

Where are they?

Benguela

Current

California Current

Humboldt Current.

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Normally trade winds

bring upwelled nutrient-

rich waters to

Peru/Ecuador.

https://media.pearsoncmg.com/bc/bc_0media_geo/interactiveanimations/noqzs/026_ElNinoLaNina_HS_GG_Ins.html

El Niño and upwelling

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El Niño - When trade winds slow, upwelling stops.

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A non-El Niño year

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An El Niño year

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Deep bottom waters - Arctic caused by

cooling waters39

Thermohaline circulation

also known as

the global conveyor belt

https://www.youtube.com/watch?v=EafneRiy1ls

https://www.youtube.com/watch?v=UuGrBhK2c7U

Fundamental process:

Gulf Stream & conveyor belt:

https://www.youtube.com/watch?v=3niR_-Kv4SM

Conveyor belt animation (takes 1,000 years)

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Thermohaline circulation (temperature and

salinity) influence sub-surface circulations.

Distributes excess Earth’s heat. 43

Deep bottom waters – Antarctic caused by

freezing ice increasing salinity44