Why Spinning Matters – Why Spinning Matters – The Coriolis Effect and the World’s The Coriolis Effect and the World’s OceansOceansOctober 17-21, 2008October 17-21, 2008

Photo Credit: NASA – Earth Observatory

Getting StartedGetting Started

ObjectivesObjectives

• What Do You Know?

Take the QUIZTake the QUIZ• To demonstrate an understanding

of convection.• To demonstrate an understanding

of how a rotating sphere affects the speed of fluids at different locations on its surface.

• To demonstrate an understanding of how motion appears to be affected by the rotating motion of a sphere.

• To demonstrate an understanding of the Coriolis force and how it affects the trade winds

• To demonstrate an understanding of how the Coriolis force varies with latitude.

What is Convection?What is Convection?

• Movement that results when heat is transferred in a fluid

• First, warmed fluids (like water or air) become less dense and will rise opposite to the force of gravity.

• Next, cooler fluid will move to replace the rising warm fluid and it will be warmed itself.

• This cycle repeats to mix the fluid.

• Convection model

• Julius Sumner Miller on Convection - a riot!

What on Earth?What on Earth?Descriptions of Earth’s ConvectionDescriptions of Earth’s Convection

Edmond HalleyEdmond Halley1656-17421656-1742

• reasoned that intense solar radiation heated the air near the Equator and caused it to expand and rise up.

• This rising air is replaced by cooler air converging on the Equator from the northern and southern hemispheres.

• Circulation of the air is driven by a pressure-gradient force, which causes high-pressure (cooler, more dense) air to move into regions of low-pressure (warmer, less dense) air.

• predicted a flow of air from the poles to the Equator where the air masses converge.

Another View of Convection

Note that it is the sinking of cold, dense air NOT the rising of warm air that drives the circulation pattern.

Image credit: NASA

What on Earth?What on Earth?Descriptions of Earth’s ConvectionDescriptions of Earth’s Convection

George HadleyGeorge Hadley1685-17681685-1768

• English lawyer and amateur meteorologist

• First to describe the reason the equatorial trade winds preferentially blow westward.

• Recognized that Earth is a rotating sphere and that sites on its surface travel with different speeds (travel different distances in equal times).

• Model of Earth’s convection termed the ‘Hadley cell’ in his honor.

The Coriolis Effect• French mathematician, mechanical

engineer, and scientist• Determined simple rules for the

direction of moving objects on the surface of a rotating sphere, now known as the Coriolis effect:– The apparent (Coriolis) force is

perpendicular to the velocity of the object and the rotation axis.

– A balance of forces causes objects traveling in the Northern Hemisphere to curve to the right.

– A balance of forces causes objects traveling in the Southern Hemisphere to curve to the left.

Gustave Gaspard de Coriolis 1792 - 1843

Visualizing the Coriolis Effect• Earth rotates at different speeds at different latitudes.• v = d/t

– Rotating earth visualization– The Coriolis Model

A

B

C

Major Wind Belts

Prevailing Wind Belts of Earth

The earth is encircled by several broad prevailing wind belts, which are separated by narrower regions of either subsidence or ascent. The direction and location of these wind belts are determined by solar radiation and the rotation of the earth. The three primary circulation cells are known as the: Hadley cell; Ferrel cell; and Polar cell.

A Horizontal View

Another Look at the Wind Belts

Significance of Wind Belts?

•Guide weather and storms• Jet Stream

–100 mph–Between 30-60º–Above friction zone

• Influences Sailing & Navigation•Deserts @ 30º

Ekman’s ContributionVagn Walfrid Ekman (1874-1954)• Swedish oceanographer• First to describe the Ekman spiral – the

movement of ocean currents in response to the rotation of the Earth (Coriolis effect)1. direction of wind2. Effective force of wind on the sea surface3. Effective direction of the current flow (Ekman

transport)4. Coriolis force (effect)

• Based on observations by Norwegian Fridtjof Nansen (1861-1930) on the Fram expedition.

• Extends to a depth of 100-150 meters– Limited by surface turbulence, diurnal cycles– Depth of effect is termed Ekman layer

The Ekman Layer

• Direction of Ekman transport varies with ocean depth (15º in shallow waters)

• Pycnocline can be a boundary

• Can pile up water in the ocean causing pressure gradients.

Background - Ekman Transport

Geostrophic FlowEkman transport causes surface waters to move toward the central region of a subtropical gyre. – A gyre is a large, nearly circular

system of wind-driven surface currents that center around latitude 30º in both hemispheres.

– produces a broad mound of water– Steepening gyre causes a

horizontal pressure gradient. Water flows “downhill”

– The Coriolis effect acts on the “downhill” sliding parcels of water

– When the outward-directed pressure gradient force balances the apparent force due to the Coriolis effect then the water parcels flow around the gyre along contours of sea surface elevation.

Ocean Basin Model

The horizontal movement of surface water arising from a balance between the pressure gradient force and the Coriolis force is known as geostrophic flow.

World Subtropical Gyres

Worldwide Surface Currents• Equatorial currents

– East to west under influence of tradewinds

• Western Boundary currents– South to north (Northern

Hemisphere – opposite in the Southern Hemisphere)

– Coriolis effect piles up water on the Western sides of oceans

– Moves very fast (25 -75 mi/day)

– Deeper– Can be influenced by

continental margins– Transports large amounts of

heat.

Pictured above is the East Coast of the United States, in grey, with the Gulf Stream, in yellow and orange, revealed through Sea Surface Temperature data (SST), made from the MODIS instrument on the Terra satellite. In this image, blue represents the coldest temperatures (between 1-10 °C) and orange and yellow represent the warmest temperatures (between 19-30°C). The Gulf Stream is readily visible as the warmest water in the image.

Up and Down in the Oceans

• Coastal upwelling occurs where Ekman transport moves surface waters away from the coast; surface waters are replaced by water that wells up from below.

– Brings nutrient rich waters to the surface

– Increases biological productivity– Influences weather patterns

• i.e California ‘s summer fogs

• Coastal downwelling occurs where Ekman transport moves surface waters toward the coast, the water piles up and sinks.

– Sends oxygen-rich waters to the deep sea

– Decreases biological productivity

El Niño – Southern Oscillation

• El Nino – Southern Oscillation – periodic event that reduces or reverses the Pacific Equatorial Current

El NiñoUpwelling and Downwelling

Southern OscillationThe Atmospheric Component

El Niño

El Nino Animation – from NOAA – Earth System Research Laboratory

El NiñoEffects on Global Weather Patterns

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<http://commons.wikimedia.org/wiki/Image:Ekman_spirale.svg>“El Nino and La Nina Ocean Temperature Patterns” National Weather Service Climate Prediction Center.

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<http://gpc.edu/~pgore/Earth&Space/GPS/wind.html>."Hadley cell circulation and the trade winds." 5 Mar 2008

<http://www.newmediastudio.org/DataDiscovery/Hurr_ED_Center/Easterly_Waves/Trade_Winds/Trade_Winds.html>.

"In the zone." Getting the global picture. 2003. University of Wisconsin, Board of Regents. 4 Mar 2008 <http://whyfiles.org/174earth_observe/4.html>.

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