Why Spinning Matters – The Coriolis Effect and the World’s Oceans

Photo Credit: NASA – Earth Observatory

Getting Started

Objectives

• What Do You Know?

Take 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?

• 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?Descriptions of Earth’s Convection

Edmond Halley1656-1742

• 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?Descriptions of Earth’s Convection

George Hadley1685-1768

• 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 Contribution

Vagn 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

Work CitedCastro, Peter & Michael E. Huber. Marine Biology. 5th. New York: The McGraw-Hill Companies, 2005.“Ekman_spirale.svg” Wikimedia commons. 5 Jan 2008. Chabacano. 6 Mar 2008

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

19 Dec 2005 <http://www.cpc.noaa.gov/products/analysis_monitoring/ensocycle/ensocycle.shtml>Gore, Pamela. "Wind and global wind systems." 01 Apr 2005. Georgia Perimeter College. 5 Mar 2008

<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>.

Madl, Pierre. The El Nino (ENSO) Phenomenon. 01 Dec 2000. Environmental Physics , 437-503 retrieved 07 Mar 2008 from <http://www.sbg.ac.at/ipk/avstudio/pierofun/atmo/elnino.htm>

"Pressure and winds." 5 Mar 2008 <http://atschool.eduweb.co.uk/kingworc/departments/geography/nottingham/atmosphere/pages/pressureandwindsalevel.html>.

Srinivasan, Margaret. "Ocean surface topography from space." Overview - climate. NASA - Jet Propulsion Laboratory - California Institute of technology . 4 Mar 2008 <http://sealevel.jpl.nasa.gov/overview/climate-earth.html>.

"Traveling on a rotating sphere." Ocean motion and surface currents. NASA. 4 Mar 2008 <http://www.oceanmotion.org/guides/cf_2/cf_student_2.htm>.

TidesTides

NOAA Ocean Service Education

November 30, 2010

(http://oceanservice.noaa.gov)

What are Tides?What are Tides?• A tide is defined as a periodic rise and fall of

the sea surface– very-long period waves noticeable only at the

shoreline– originate in the open ocean

• High tide – wave crest reaches the shoreline• Low tide – wave trough reaches the shoreline

Tidal Range & Tidal Currents• Tidal range - the difference in water height

between high and low tides– varies from a few cm to up to 48 feet (14 m) at the

Bay of Fundy in Canada

• Tidal current – horizontal movement of water that accompanies the rising and falling tide– incoming current is a flood current– outgoing is an ebb current– strongest during high or low tides, weakest in between

the two – Weak currents in the open ocean, strongest at estuary

entrances, straights and inlets – Animation– Blue planet video segment

Tidal Range & Tidal Current

What Causes Tides?• Result from the

gravitational pull of the moon and sun on the Earth.

• Newton’s law of universal gravitation

– More massive an object, greater its gravitational pull

– Varies inversely as the square of the distance between objects

• Tide generating force– vary inversely as the cube of

the distance from the tide generating object.

• Therefore the moon has a greater effect on the tides due to its proximity

Gravity, Inertia, and the Two BulgesGravity, Inertia, and the Two Bulges

• Two tidal bulges (high tides) form

• Bulge on the moon side– caused by gravitational attraction of

moon and Earth exerted on the fluid (oceans)

– largest bulge– overcomes inertia of water

• Bulge on the “far side” of the Earth opposite the moon– caused by inertia – the tendency of

moving objects to continue moving in a straight line

– inertia exceeds gravitational force here

Earth

Moon

Gravitational force

Inertia

Gravity, Inertia, & the Two Bulges

Changing Angles and Changing Tides

Changing Angles and Changing Tides

• Moon revolves around the Earth– Its declination - angle relative to the equator - increases

and decreases– Varies the height and intensity of tides– Monthly variation

• Animation

Changing Angles and Changing Tides

• Because of Earth’s tilted axis, the sun’s relative position (declination) to the equator changes throughout the year– Minimum (spring/fall equinoxes)– Maximum (summer/winter solstices)

Frequency of Tides – The Lunar Day

Frequency of Tides – The Lunar Day

• Lunar day – time it takes for a specific site on Earth to rotate from an exact point under the moon to the same point under the moon

• Lunar day – 24 hours, 50 minutes

• The lunar day is 50 minutes longer than a solar day because the moon revolves around the Earth in the same direction that the Earth rotates around its axis. So, it takes the Earth an extra 50 minutes to “catch up” to the moon

• Lunar Tides Animation

Lunar Orbit – 29.5 days

The Frequency of Tides

• Because the Earth rotates through two tidal “bulges” every lunar day, coastal areas experience two high and two low tides every 24 hours and 50 minutes. High tides occur 12 hours and 25 minutes apart. It takes six hours and 12.5 minutes for the water at the shore to go from high to low, or from low to high.

Tidal Variations• Solar tide (1/2 as large as lunar one)• When both in alignment spring tides

occur. When moon not in alignment, neap tides.

• Solar Tide Animation

Tidal Variation due to Variation in Moon and Earth Orbits.

Continents in the Way .. Again

Continents in the Way .. Again

Land masses can create three different tide patterns

• Diurnal - only one high and one low tide each day

• Semidiurnal - two highs and two lows, about the same height

• Mixed semidiurnal - high and low tides differ in height

Tidal Cycles of the World

Other factors affecting tides…

• Shoreline and coastline topography– Continental margins– Islands

• Shape of bays and estuaries– Funnel Shaped especially– Narrow, shallow waters dissipate the

water– Tidal rivers lessen the effect of incoming

water

• Local wind and weather patterns

Tide ResourcesMcNish, Larry. "RASC Calgary Centre - A Complete Guide to." 17 Dec 2007. 29

Oct 2008 <http://calgary.rasc.ca/radecl.htm>. "Tides and Water Levels." NOAA Ocean Service Education. 25 Mar 2008.

National Oceanic and Atmospheric Association, U.S. Department of Commerce. 29 Oct 2008 <http://www.oceanservice.noaa.gov/education/kits/tides/welcome.html>.