Where Will My Ship Go

8/7/2019 Where Will My Ship Go

1/28

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


2/28

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


3/28

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.


4/28

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


5/28

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


6/28

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)


7/28

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


8/28

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


9/28

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


10/28

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


11/28

Geostrophic FlowSurface currents generally mirror average

planetary atmospheric circulation patterns


12/28

currents Examine global surface currents.

General Surface Current Patterns


13/28

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


14/28

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


15/28

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


16/28

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


17/28

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


18/28

Polar Currents

Southward flowing Labrador Current andnorthward flowing West Greenland Current

Currents within Arctic Ocean constantlymoving ice


19/28

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


20/28

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


21/28

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


22/28

Surface and Deep-Sea CurrentInteractions

Unifying concept: Global Ocean Conveyor B elt

http://seis.natsci.csulb.edu/rbehl/Conv B elt.htm


23/28

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


24/28

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


25/28

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


26/28

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


27/28

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


28/28

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

Documents

Where Will My Ship Go