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 PresentationTRANSCRIPT
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:
Causes of Climate change
A. Tectonic C. ?? B. Orbital D. ??
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
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)
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
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
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
The Gulf Stream is a major surface current
Red is warmer water
Ocean Surface Currents
Surface currents are driven by wind and follow the circulation pattern of the
atmosphere
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
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
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
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
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
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
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
Illustration of the very strong 1997-1998 El Niño
See also: http://www.vets.ucar.edu/vg/ http://www.pmel.noaa.gov/tao/jsdisplay
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
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)
Global “Conveyer Belt” circulation
Flow in conveyor is ~ 30 times larger than all river flow
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
Sea-s
urf
ace
Tem
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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.)
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
Modeled changes in surface air temperature caused by a shutdown of the North Atlantic
Deep Water formation are large!
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)
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
Take Home Message:
Remember, Rome wasn’t built in a day,
It just burned down in one…
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.