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1. What is the composition and physical structure (layering) of the ocean? 2. How and why do ocean waters circulate? 3. What role does the ocean play in climate change? What we wish to learn Today:

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What we wish to learn Today:. The Blue Planet. What is the composition and physical structure (layering) of the ocean? How and why do ocean waters circulate? What role does the ocean play in climate change?. Causes of Climate change. A. Tectonic. B. Orbital. - PowerPoint PPT Presentation

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Page 1: What we wish to learn Today:

1. What is the composition and physical structure (layering) of the ocean?

2. How and why do ocean waters circulate?

3. What role does the ocean play in climate change?

What we wish to learn Today:

Page 2: What we wish to learn Today:

Causes of Climate change

A. Tectonic C. ?? B. Orbital D. ??

Page 3: What we wish to learn Today:

1. Composition of the Oceans

Density – weight of the water. Density increases with more salt and decreases with warmer temperatures

Fresh water has ~0.02% dissolved salt (density = 1 g/cm3)

Sea water has ~3.5% dissolved salt (density = 1.024-1.028 g/cm3)

Salinity

Number of grams of dissolved salt in a kilogram of sea water. Ranges from 33–38 g/kg in open oceans

Ocean Salt composition

Page 4: What we wish to learn Today:

Ocean Structure - The ocean is “stratified” into

layers• Stratification is based on water density – light water floats on top of denser, heavier water and creates a stable structure• The surface layer is separated from the bottom layer by a transition zone

The transition is called a pycnocline (for density)

or a thermocline (temperature) or a halocline (for salt)

Page 5: What we wish to learn Today:

www.ewoce.org

The tropics have higher heat input from the sun, and temperatures there are warmer than at the

poles

Atlantic Ocean TemperatureTemperature oC

Page 6: What we wish to learn Today:

The tropics have more heat to drive evaporation, concentrating the ocean salts. The Gulf Stream moves this

salt northward. The poles also have greater freshwater input from ice caps and glaciers. www.ewoce.org

Salinity (ppt)

(pss 78)

Atlantic Ocean Salinity

Page 7: What we wish to learn Today:

2. Ocean Circulation• Ultimately driven by solar energy

– Distribution of solar energy drives global winds

• Surface currents– Affect surface water above the pycnocline (10-

15% of ocean water)– Driven by major wind belts of the world

• Deep currents– Affect deep water below the pycnocline (up to

90% of ocean water)– Driven by density differences (not by winds)– Larger and slower than surface currents

Page 8: What we wish to learn Today:

The Gulf Stream is a major surface current

Red is warmer water

Page 9: What we wish to learn Today:

Ocean Surface Currents

Surface currents are driven by wind and follow the circulation pattern of the

atmosphere

Page 10: What we wish to learn Today:

Coriolis “Force”Not a true “force”, but an apparent movement or deflection to account for being on a rotating body instead of fixed in space

The Coriolis effect deflects moving fluids (atmosphere and ocean) to the right in the Northern Hemisphere, and to the left in the Southern Hemisphere

Page 11: What we wish to learn Today:

The Eckman “Spiral”

Surface currents are deflected due to the Coriolis effect, and flow at ~45° to the surface wind direction

Currents “spiral” to the right at depth, so the net “Eckman Transport” of water will be 90o to the right of the wind (N. Hemisphere)

Along coastal areas, Ekman transport can cause upwelling and downwelling of water

Page 12: What we wish to learn Today:

Coastal Upwelling Water movement offshore (due to Coriolis effect and Eckman

transport) results in upwelling as cold water rises to replace

surface water

Northern Hemisphere

Page 13: What we wish to learn Today:

Ekman transport either moves surface water away from shore, producing upwelling, or moves water toward the shore, producing downwelling

DownwellingUpwelling

West CoastSouthern Hemisphere

West CoastSouthern Hemisphere

Page 14: What we wish to learn Today:

July 1992 AVHRR Ch 4

Illustration of Coastal

Upwelling off California. Blue and green are cooler waters

upwelling close to shore, while red represents warmer waters further offshore

Page 15: What we wish to learn Today:

El Niño-Southern Oscillation (ENSO)

• El Niño = warm surface current in equatorial eastern Pacific that occurs every 3-8 yrs around Christmas time

• Southern Oscillation = change in atmospheric pressure over Pacific Ocean accompanying El Niño

• La Niña = opposite of El Niño, cool phase with enhanced upwelling

Page 16: What we wish to learn Today:

El Nino

El-Nino Southern Oscillation

(ENSO)

Warm surface water blocks upwelling, fishery collapses

Normal

NOAA/PMEL/TAO

Upwelling of cool, nutrient-rich water supports productive fishery

Pacific Ocean

Page 17: What we wish to learn Today:

Illustration of the very strong 1997-1998 El Niño

See also: http://www.vets.ucar.edu/vg/ http://www.pmel.noaa.gov/tao/jsdisplay

Page 18: What we wish to learn Today:

El Niño impacts and recurrenceImpacts on global climate can be large - $8-10 Billion for the 1982-83 event

Temperature and PrecipitationTypical interval for El Niños is 3-8 years, which is too short to explain longer-term climate fluctuations

Page 19: What we wish to learn Today:

Deep Water Formation and Currents

• Warm saline water moves north by the Gulf Stream

• Water cools in Arctic• Sea ice forms and

increases salinity• Water becomes

dense and sinks

Sinking water produces a deep current that travels worldwide. Deep-water also formed near Antarctica.

This movement is called “Thermohaline Circulation” (heat and salt)

Page 20: What we wish to learn Today:

Global “Conveyer Belt” circulation

Flow in conveyor is ~ 30 times larger than all river flow

Page 21: What we wish to learn Today:

3. Ocean Circulation and Climate

• Heat re-distribution (keeps Europe warm)

• Monsoons, Cyclones, Hurricanes, El Nino

• Upwelling can create coastal deserts on land

• Great thermal buffer (high heat capacity)

• Oceans absorb ~1/3 of anthropogenic CO2

Page 22: What we wish to learn Today:

Sea-s

urf

ace

Tem

p C

1

8O

(o/o

o)

Can ocean circulation explain abrupt climate shifts?

Melting of the Ice Cap may have stopped or reduced the Conveyor Belt

Younger Dryas

Ice core data

Time (thousands of years B.P.)

Page 23: What we wish to learn Today:

The large Earth in the center shows the cold climate state prevailing during most of the Normal Ice Age

The bottom shows an advance of the Conveyor Belt into the Nordic Seas, resulting in a warm anomaly and Rapid Warming 

The upper globe shows climate when the Conveyor Belt collapses, resulting in Rapid Cooling

Abrupt climate change due to switches between 3 modes

Modes of Atlantic Thermohaline Modes of Atlantic Thermohaline CirculationCirculation

NormalIce age

RapidWarming

RapidCooling

Page 24: What we wish to learn Today:

Modeled changes in surface air temperature caused by a shutdown of the North Atlantic

Deep Water formation are large!

Page 25: What we wish to learn Today:

Water Freshening in the North Atlantic Ocean is

increasing due to climate warming

Melting of Greenland Ice-sheet

If the water gets too fresh, deep water can’t form (the surface freshwater can’t sink, no matter how cold it gets)

Salinity is decreasing (ocean is freshening)

Page 26: What we wish to learn Today:

Model projections of changes in ocean circulation cause disruptions of the carbon cycle in a warmer

world

• 50% slowdown in N. Atlantic circulation

• 20-30% reduction in oceanic CO2 uptakeSarmiento, 1998

Page 27: What we wish to learn Today:

Take Home Message:

Remember, Rome wasn’t built in a day,

It just burned down in one…

Page 28: What we wish to learn Today:

Summary:

• Solar heat inputs create surface winds, which in turn drive the surface ocean currents.

• Differences in water density allow deep-water to sink near the poles, which establishes a 3-dimensional thermohaline current that encircles the globe (the Conveyor Belt).

• Ocean circulation strongly controls climate on Earth through heat transport, upwelling, El Niño, etc.  

• Weakening of the deep-water formation and Conveyor Belt circulation probably affected paleoclimate over the last 100,000 years.

• Abrupt, rapid climate changes can be strong and can be caused by a breakdown of the atmosphere-ocean interactions that control climate.