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 transportDeep 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

Gulf Stream

California Current

Surface

150 meter depth

Temperature

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 Filamentsand Phytoplankton

Hawaiian Island Wakes

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

Some important differences between ocean and atmosphere

How is the energy of the winds transferred to the ocean?

Ekman Theory …

How does wind force propagate in the ocean?

surface

100 meter depth

balance between friction and rotation

1

2

Ekman Theory …and vertical advection in the ocean

COASTAL UPWELLING and DOWNWELLING

OPEN OCEAN EKMAN PUMPING

Effects of Ekman Currents

Atmosphere

Ocean30

60

Fpressure

FCoriolis

1) Particle will have the Coriolis effect 90 degrees to the right2) Particles will tend to move along line of constant pressure3) Particles will have the high pressure on their right (same as

Coriolis)

Some practical rules to remember:

High Pressure

Low Pressure

Applies to the Ocean same as Atmosphere!

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

Arctic sea ice

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

Evidence of Global Warming in the Climate System:

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:

CO2

Temperature

70 ppm

A paleo prospective

5 C

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_GRA

PHIC.html#

Arctic warming• Sea ice melting

– Key feedback! (animation)Reduces albedo (reflectivity) of earth,

allowing more radiation to be absorbedhttp://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

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)

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

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 timescalesVertical circulation of the ocean MOVIE:

The Day After Tomorrow

Biological Productivity in the Ocean