Where Will My Ship Go? Ocean Currents

1. Where Will My Ship Go? Ocean Currents Dr. Michael J. Passow White Plains (NY) Middle School Science Teachers Association of New York State michael@earth2class.org Prepared for STAO 2003

2. Wind-Driven and Density-Driven Currents • Wind-driven currents occur in the uppermost 100 m or less • Density differences causes by salinity and temperature produce very slow flows in deeper waters. • This workshop will focus on wind-driven currents, but will also discuss interactions between both circulation systems

3. Sailors have know about ocean currents for centuries Sailors have know that “rivers” flow in the seas since ancient times. They used them to shorten voyages, or were delayed by trying to stem them. If navigators do not correct to deflection by currents, they may be far away from where they think they are and meet disaster.

4. Ben Franklin and the Gulf Stream In the 1750s when Postmaster for the American Colonies, Ben Franklin and Capt. Timothy Folger created the first map of the Gulf Stream to help speed up delivery of mail to and from GB http://www.oceansonline.com/ben_franklin.htm

5. Matthew Fontaine Maury The first systematic study of currents was done by Maury based on logbooks in the US Navy’s Depot of Charts and Instruments. His charts and “Physical Geography of the Sea” assisted navigators worldwide. http://www.npg.si.edu/exh/brady/gallery/97gal.html

6. Winds and surface water • Wind blowing over the ocean can move it due to frictional drag. • Waves create necessary roughness for wind to couple with water. • One “rule of thumb” holds that wind blowing for 12 hrs at 100 cm per sec will produce a 2 cm per sec current (about 2% of the wind speed)

7. 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 influenced by the Coriolis Effect and Ekman Spiral

8. Coriolis Effect • The French scientist, Gaspard Coriolis, first explained the deflection of objects moving over the surface due to Earth’s rotation. http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/crls.rxml

9. Ekman Spiral Nansen and others exploring the Arctic noticed that ice and surface currents move at an angle to the wind direction. Ekman first explained the mathematics of this phenomena, and why it decreases with depth to produce the spiral pattern. http://www.humboldt.edu/~gdg1/Spiral.html

10. Currents in the “Real” Ocean Currents rarely behave exactly as predicted by these theoretical explanations due to factors such as • Depth—shallow water does not permit full development of the Ekman spiral • Density—deeper currents moving in different directions influence the overlying surface movement

11. Geostrophic Flow Surface currents generally mirror average planetary atmospheric circulation patterns

12. General Surface Current Patterns http://earth.usc.edu/~stott/Catalina/Oceans.html

13. Both atmospheric and surface currents patterns are dominated by nearly circular gyres Prevailing Westerlies and Trade Winds strongly influence these flows Clockwise rotation in the Northern Hemisphere, counterclockwise in the Southern Gyres http://earth.usc.edu/~stott/Catalina/Oceans.html

14. Currents with the Gyres • Some currents transport warmer waters, others cooler waters • Western “boundary currents” (Gulf Stream, Kuroshio, Brazil) flow poleward in narrow (50 – 75 km), swift (3 – 4 km/hr), relatively deep (to 400 m) movements • Eastern boundary currents (Canary, California) are hundreds of km wide and flow about 1 km/hr

15. Antarctic Circumpolar Current • Only current flowing unobstructed around the globe • Also known as West Wind Drift • Southern edge of South Atlantic, South Pacific, and South Indian Ocean gyres • Significant influence on climate and climate changes

16. 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 by MONSOON shifts in winds during rainy and dry seasons

17. Sub-Polar Gyres • High latitudes in North Hemisphere • Counterclockwise pattern due to influence of Aleutian Low and Icelandic Low • Nutrient-rich waters upwell into photic zone, increasing biological productivity

18. Polar Currents • Southward flowing Labrador Current and northward flowing West Greenland Current • Currents within Arctic Ocean constantly moving ice

19. Equatorial Currents and Countercurrents • Trade winds set up east-to-west Equatorial Currents north and south of Equator in both hemispheres • Equatorial Countercurrents flow west-to-east as part of mass balance • Shift seasonally north and south • Flow through Indonesian islands very complex, tied into climate patterns

20. “Hills and Valleys” in the Ocean • A balance between the Ekman transport and Coriolis effect produces “hills” in the center of the gyres and “valleys” elsewhere • Gravitational effects from sea floor features also produce variations in sea surface topography http://earth.usc.edu/~stott/Catalina/Oceans.html

21. Satellite Observations • TOPEX/Poseidon, Jason 1, and other satellites have observed patterns of change over the past few years • Animation of seasonal and climatically-influence shifts available at http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES/Topex_Dynamic_Ocean_Topography.mpg

22. Surface and Deep-Sea Current Interactions • Unifying concept: “Global Ocean Conveyor Belt” http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm

23. Impact of Global Warming? • Considerable concern about how global warming might impact ocean circulation • Wallace Broecker of LDEO, who first developed conveyor belt model, recently described potential effects http://faculty.washington.edu/wcalvin/teaching/Broecker99.html

24. Heat Transport by Currents • Surface currents play significant roles in transport heat energy from equatorial waters towards the poles • May serve as “heat sources” to cooler overlying air, “heat sinks” from warmer • Evaporation and condensation participate in latent heat exchanges

25. Matter Transport and Surface Currents • Currents also involved with gas exchanges, especially O2 and CO2 • Nutrient exchanges important within surface waters (including outflow from continents) and deeper waters (upwelling and downwelling) • Pollution dispersal • Impact on fisheries and other resources

26. Conclusions • Surface currents are those parts of the ocean most directly involved with Earth System processes • Transport heat globally • Supplies water vapor to atmosphere • Dissolves and transports salts, nutrients, and dissolved gases • Supports fisheries

27. Conclusions (cont’d.) • Significant influence on day-to-day weather and short-term climate variability • Reference: “DataStreme Ocean” M. Grant Gross and Elizabeth Gross American Meteorological Society Preview version, 2003

28. Activities about Surface Currents The American Meteorological Society has developed several activities to train teachers and students about ocean currents, which will be presented in modified versions • The Maury Project—“Wind-Driven Ocean Circulation” • DataStreme Ocean–Internet-based investigations