The Ocean General Circulation (satellite)
Biological Productivity in the Ocean
Ocean Circulation
Learning objectives:
Mass flow of ocean water is driven by two
forces - wind (10%) and gravity (90%)
Surface currents are driven by wind –
Coriolis effect; Ekman transport
Deep ocean, slow and density driven
currents - Thermohaline circulation;
Waves (transmit energy not water mass)
and Tides (the longest of all ocean waves)
http://www.oceanmotion.org/
http://www.oceanmotion.org/html/resources/etopo.htm
Mean Circulation in
the Ocean
Gulf Stream
Mean Circulation in
the Ocean
California Current
Gulf Stream
Temperature
Surface
150 meter depth
Annual mean Salinity Map
Major oceanic circulation systems
Gulf Stream, Plankton Bloom (SeaWiFS, AVHRR)
Gulf Stream Spiral Eddies
Mediterranean Sea, Shear Wall Spiral Eddies
Tropical Atlantic, Spiral Eddy
Greek Island, Spiral Eddies and Wakes
Strait of Gibraltar
Kelvin Waves, from Ships
Coastal Dynamics
California Filaments
and Phytoplankton
Hawaiian Island Wakes
Some important differences between ocean
and atmosphere
Ocean is heated from above
Feels both
Mechanical forcing by the winds
&
Thermal forcing from the sun
Boundaries and complex geometry associated with
continents and bottom topography, and bathymetry
Ocean is denser than atmosphere
Tides
Salinity
Atmosphere has clouds and moisture
How is the energy of the winds
transferred to the ocean?
Ekman Theory …
How does wind force
propagate in the ocean?
surface
balance between
friction and rotation
100 meter
depth
1
2
Ekman Theory …and vertical advection in
the ocean
COASTAL UPWELLING and DOWNWELLING
OPEN OCEAN EKMAN PUMPING
Effects of
Ekman Currents
Atmosphere
60
30
Ocean
Some practical rules to remember:
Applies to the Ocean same as Atmosphere!
F
pressure
Low Pressure
High Pressure
F
Coriolis
1) Particle will have the Coriolis effect 90 degrees to the right
2) Particles will tend to move along line of constant pressure
3) Particles will have the high pressure on their right (same as
Coriolis)
Arctic sea ice
http://maps.grida.no/go/graphic/arctic-sea-ice-minimum-extent-in-september-1982-2005-and-2007
http://maps.grida.no/go/collection/global-outlook-for-ice-and-snow
Evidence of Global Warming in
the Climate System:
Glacier melting
• Kilimanjaro: ice caps are 80% gone since early
1900’s
• All glaciers in tropics are melting rapidly
• Impacts: water supply, power generation,
tourism, local climate and ecology
To understand how climate has changed
in the past, we need to use records of
climate preserved in ice cores, ancient
tree rings, coral bands, and other
“paleoclimatic” sources:
A paleo prospective
CO2
Temperature
70 ppm
5C
Arctic - the most sensitive ecosystem?
• Arctic sea ice has shrunk by 1 million sq km and thinned from
3.1m to 1.8m average
– More freshwater, reduced ability to travel over ice
– All summer ice gone in this century
• Ecological consequences huge!
• http://www.nytimes.com/interactive/2007/10/01/science/20071002_ARCTIC_
GRAPHIC.html#
Arctic warming
• Sea ice melting
– Key feedback! (animation)
Reduces albedo (reflectivity) of earth,
allowing more radiation to be absorbed
http://www.gsfc.nasa.gov/topstory/2003/1023esuice.html
• Warming temperature
– Melts permafrost
– Ice sheet stability? Key sea level
question!
• Rate of warming 8x faster in last 20
years than in last 100
1979
2003
Arctic ecosystems impacts
• Reduced ice: less algal production
under ice = undermines base of food
chain
• Seal pups emerge just when ice is
melting - earlier melt means they
are exposed before ready to thrive
• Caribou need ice to island-hop;
they are falling through as ice thins
• Polar bears hunt on ice in winter,
retreat to land in summer. Less ice
forces them onto land earlier
Higher sea level…
•
As water warms, it expands (“thermal expansion”).
• Glaciers are melting
• Observation: 3mm/yr in past few decades
• Prediction: ~0.5m rise by the end of this century, 2-4m in 500 years
This will have a major impact on
•Developed coastal regions
•Low-lying island nations
•Intensity of coastal flooding during
storm surges
•Coastal ecosystems (e.g. mangroves,
estuaries)
More intense
storms…
Hurricanes get their energy
and staying power from warm
water in the tropical oceans.
As waters get warmer, we
expect that hurricanes will
become more intense.
Significant change not yet
observed.
Warmer temperatures…
Prediction: Doubled
CO2 will warm the
planet by 1.8-5.8°C
(before 2100)
Some areas warm up
more than others.
Continents warm
faster than oceans.
Higher latitudes
warm more than low.
Map of predicted temperature change for a
doubling of atmospheric CO2
Oceanography and Climate - millennial timescales
Vertical circulation of the ocean
MOVIE:
The Day After Tomorrow
Biological Productivity in the Ocean