where will my ship go
TRANSCRIPT
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Where Will My Ship Go?Ocean Currents
Dr. Michael J. Passow
White Plains (NY) Middle SchoolScience Teachers Association of
New York State
[email protected] for STAO 2003
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Wind-Driven and Density-Driven
CurrentsWind-driven currents occur in theuppermost 100 m or lessDensity differences causes by salinity andtemperature produce very slow flows indeeper waters.
This workshop will focus on wind-drivencurrents, but will also discuss interactionsbetween both circulation systems
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Sailors have know about oceancurrents for centuries
Sailors have know that rivers flow in theseas since ancient times. They used them
to shorten voyages, or were delayed bytrying to stem them.If navigators do not correct to deflection by
currents, they may be far away from wherethey think they are and meet disaster.
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B en Franklin and the Gulf Stream
In the 1750s whenPostmaster for the
American Colonies,B en Franklin andCapt. Timothy Folger created the first mapof the Gulf Stream tohelp speed updelivery of mail to andfrom G B
http://www.oceansonline.com/ben_franklin.htm
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Matthew Fontaine Maury
The first systematic studyof currents was doneby Maury based onlogbooks in the USNavys Depot of Chartsand Instruments.
His charts and PhysicalGeography of the Seaassisted navigatorsworldwide.
http://www.npg.si.edu/exh/brady/gallery/97gal.html
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Winds and surface water
Wind blowing over the ocean can move itdue to frictional drag.
Waves create necessary roughness for wind to couple with water.One rule of thumb holds that windblowing for 12 hrs at 100 cm per sec willproduce a 2 cm per sec current (about 2%of the wind speed)
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Top-down drag
Wind acts only on the surface water layer.This layer will also drag the underlying
water, but with less force.Consequently, there is a diminution of speed downward.
Direction of movement is also influencedby the Coriolis Effect and Ekman Spiral
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Coriolis Effect
The French scientist,Gaspard Coriolis, firstexplained thedeflection of objectsmoving over thesurface due to Earthsrotation.
http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml
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Ekman SpiralNansen and othersexploring the Arcticnoticed that ice andsurface currentsmove at an angle tothe wind direction.Ekman first explainedthe mathematics of this phenomena, andwhy it decreases withdepth to produce thespiral pattern.
http://www.humboldt.edu/~gdg1/Spiral.html
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Currents in the Real Ocean
Currents rarely behave exactly aspredicted by these theoretical explanationsdue to factors such asDepthshallow water does not permit fulldevelopment of the Ekman spiralDensitydeeper currents moving indifferent directions influence the overlyingsurface movement
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Geostrophic FlowSurface currents generally mirror average
planetary atmospheric circulation patterns
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currents Examine global surface currents.
General Surface Current Patterns
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GyresB oth atmospheric andsurface currentspatterns are dominatedby nearly circular gyresPrevailing Westerliesand Trade Windsstrongly influence theseflowsClockwise rotation inthe NorthernHemisphere,counterclockwise in theSouthern
http://earth.usc.edu/~stott/Catalina/Oceans.html
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Currents with the Gyres
Some currents transport warmer waters,others cooler watersWestern boundary currents (Gulf Stream,Kuroshio, B razil) flow poleward in narrow(50 75 km), swift (3 4 km/hr), relativelydeep (to 400 m) movements
Eastern boundary currents (Canary,California) are hundreds of km wide andflow about 1 km/hr
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Antarctic Circumpolar Current
Only current flowing unobstructed aroundthe globe
Also known as West Wind DriftSouthern edge of South Atlantic, SouthPacific, and South Indian Ocean gyres
Significant influence on climate andclimate changes
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Indian Ocean Currents
South Indian Ocean similar to South Atlantic and Pacific
North Indian Ocean does not extend far above Equator Seasonal changes in direction caused byMONSOON shifts in winds during rainyand dry seasons
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Sub-Polar Gyres
High latitudes in North HemisphereCounterclockwise pattern due to influence
of Aleutian Low and Icelandic LowNutrient-rich waters upwell into photiczone, increasing biological productivity
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Polar Currents
Southward flowing Labrador Current andnorthward flowing West Greenland Current
Currents within Arctic Ocean constantlymoving ice
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Equatorial Currents and
CountercurrentsTrade winds set up east-to-west EquatorialCurrents north and south of Equator inboth hemispheresEquatorial Countercurrents flow west-to-east as part of mass balanceShift seasonally north and southFlow through Indonesian islands verycomplex, tied into climate patterns
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Hills and Valleys in the Ocean
A balance between theEkman transport andCoriolis effect
produces hills in thecenter of the gyres andvalleys elsewhereGravitational effectsfrom sea floor featuresalso producevariations in seasurface topography
http://earth.usc.edu/~stott/Catalina/Oceans.html
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Satellite Observations
TOPEX/Poseidon, Jason 1, and other satellites have observed patterns of change over the past few yearsAnimation of seasonal and climatically-influence shifts available at
http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES/To
pex_Dynamic_Ocean_Topography.mpg
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Surface and Deep-Sea CurrentInteractions
Unifying concept: Global Ocean Conveyor B elt
http://seis.natsci.csulb.edu/rbehl/Conv B elt.htm
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Impact of Global Warming?
Considerable concern about how globalwarming might impact ocean circulation
WallaceB
roecker of LDEO, who firstdeveloped conveyor belt model, recentlydescribed potential effectshttp://faculty.washington.edu/wcalvin/teaching/ B roecker99.
html
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Heat Transport by Currents
Surface currents play significant roles intransport heat energy from equatorialwaters towards the polesMay serve as heat sources to cooler overlying air, heat sinks from warmer Evaporation and condensation participatein latent heat exchanges
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Matter Transport andSurface Currents
Currents also involved with gasexchanges, especially O 2 and CO 2
Nutrient exchanges important withinsurface waters (including outflow fromcontinents) and deeper waters (upwellingand downwelling)Pollution dispersalImpact on fisheries and other resources
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Conclusions
Surface currents are those parts of theocean most directly involved with EarthSystem processesTransport heat globallySupplies water vapor to atmosphere
Dissolves and transports salts, nutrients,and dissolved gasesSupports fisheries
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Conclusions (contd.)
Significant influence on day-to-dayweather and short-term climate variability
Reference: DataStreme OceanM. Grant Gross and Elizabeth Gross
American Meteorological SocietyPreview version, 2003
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Activities about Surface Currents
The American Meteorological Societyhas developed several activities to trainteachers and students about oceancurrents, which will be presented inmodified versions
The Maury ProjectWind-Driven OceanCirculationDataStreme OceanInternet-basedinvestigations