Introduction to Oceanography Lecture 17, Current Midterm 2: Nov. 20 (Monday)

Review Session & Video Screenings TBA

Breaking wave and sea foam, Vero Beach, FL, Robert Lawton, Creative Commons A S-A 2.5, NOAA Ocean-Atmosphere Sea Surface Temperature Model, Public Domain, http://www.gfdl.noaa.gov/visualizations-oceans http://commons.wikimedia.org/wiki/File:Sea_foam.JPG

Santa Ana Winds NASA image, Public Domain, http://photojournal.jpl.nasa.gov/catalog/PIA03892 Winter: Canadian cold air Santa Ana Winds pushes down into Southwestern US High pressure pushes dry desert air ~ 30 mph downslope, to sea Compression of sinking air causes heating Heating lowers humidity Piotr Flatau, Wikimedia Commons, Public Domain, http://en.wikipedia.org/wiki/File:Santa_ana_wind1.jpg Wind Speeds: up to ~ 70mph ≈115 km/hr San Gabriel/Bernardino Mtns. Funneling effect through canyons Feeds dangerous brush fires Weaker in summer Los High Plateau Angeles Mojave Desert Adapted from N. Short Remote Sensing Tutorial/NASA, Public Domain, http://rst.gsfc.nasa.gov/Sect14/katabati c.jpg

Santa Ana Winds UCSD GOES-10/NASA, Public Domain, http://meteora.ucsd.edu/cap/images/junegloom_16jun2004.gif

Santa Ana Winds: dry & warm, Encourage Questions? destructive fires Fertilize ocean? Porter Ranch Fire, Oct. 14 2008, NASA image, Public Domain, http://www.nasa.gov/mission_pages/fires/main/usa/califires_20081014.html

1 Portrait of Ben Franklin, Currents Currents in the Ocean 1785, by Duplessis What is a current? Ocean currents transport water A current is a flow of material Wind is a current of air MASS IS TRANSPORTED Map by Ben Franklin, 1787

Ben Franklin, 1769, Map of the , Public domain. The Gulf Stream, Winslow Homer (1899), Metropolitan Museum of Art (NY). Public Domain, https://commons.wikimedia.org/wiki/File:Winslow_Homer_004.jpg

Currents in the Ocean Surface Currents • Two Types: Surface and Deep Caused by: Wind Stress Pressure Gradients • Driving Forces Coriolis Effects Friction – Surface Currents: Wind-driven – Deep Currents: Density-driven

American Meteorological Society, http://oceanmotion.org/images/oc ean-vertical- Gene Paull, UT Brownsville, Public Domain(?), structure_clip_image002.jpg http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif

Pressure-Driven Currents Wind-Driven Currents • Wind drives ocean currents • Currents run into continents and can’t continue • Wind accelerates ocean currents – UNLIKE ATMOSPHERIC FLOWS – Frictional Drag • Water piles up ---Pressure Gradients form

Atmosphere Atmosphere L Wind Wind

Drag Wind Drag H Wind H Current Current Current Current

Ocean Map View Ocean L SIDEVIEW TOPVIEW SIDEVIEW TOPVIEW Figures, UCLA Figures, UCLA

2 North Atlantic Surface Currents Coriolis & currents Simple picture – winds push surface currents, water piles • Currents are affected by Earth’s ROTATION! 60ºN up where wind blows – This is called the “Ekman” response onto coastlines (e.g., Atlantic coast of S.

30ºN America). Atmosphere BUT: Coriolis acts on Wind currents! Drag Equator Wind

Background image: U.S. Government, Currents Currents extracted from PDF version of the Vol 26-4 Ocean 2004 DISAM Journal (direct PDF URL [1]), Public Domain, SIDEVIEW TOPVIEW http://commons.wikimedia.org/wiki/File:Globe_ Figures, UCLA Atlantic.svg

Ekman Spiral Winds ~60ºN V. Walfrid Ekman (Sweden) • Surface deflection 45o right of wind (in N. Hemisphere) • Deflected further right with depth

• Net (i.e., average) effect of Ekman spiral: Horse Latitudes ~ 30o N Transport 90º to the right of the wind direction in the northern hemisphere Left of wind in the southern hemisphere

Equator NOAA image, Public Domain, http://www.windows.ucar.edu/tour/link=/earth/Water/ekman.ht ml&edu=elem Figures, UCLA

Drag from the wind on surface water Wind-dragged & Basin-Edge Currents

L H

Horse Latitudes ~ 30o N Horse Latitudes ~ 30o N

H L

Figures, UCLA Figures, UCLA

3 Final result: a loop of current, a Gyre Coriolis “Geostrophic” Response L H

Gene Paull, UT Brownsville, Public Domain(?), http://upload.wiki Horse Latitudes ~ 30o N media.org/wikipe dia/commons/0/0 H 6/Corrientes- N. Atlantic oceanicas.gif Gyre

H L One subtropical gyre is found in each hemisphere in each ocean. Gyres rotate clockwise in the N. Hemisphere, counterclockwise in S. Hemisphere. Figures, UCLA

Coriolis “Geostrophic” Response QUESTIONS?

Mean 1992-2002 dynamic ocean topography, Nikolai Maximenko (IPRC) and Peter Niiler (SIO), Public Domain, http://apdrc.soest.hawaii.e du/projects/DOT/1992- 2002MDOT.jpeg

Average satellite-measured “hill” of water at western side of gyre. Gene Paull, UT Brownsville, Public Domain(?), Why isn’t the high pressure in the center of the ocean? http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif

Major Current Systems Major Current Systems 1. 2.

3. N. Atlantic 4. N. Pacific 5. Indian 6. Antarctic Circumpolar Current S. Pacific Indian • Not a gyre! S. Atlantic

Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif

4 Currents on each edge of a Gyre have names. Transverse Currents Transverse Current • Driven primarily by Wind Stress • Antarctic Circumpolar Current is the largest of these

Transverse Current

Cropped from Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif

Antarctic Circumpolar Current Equatorial Currents • Found on either side of the equator Not a gyre! Southern Westerlies drive largest volume current on Earth • Flow to the west – 100 x 106 m3/s on average. – 600 times the flow of the Amazon!

– WHY? • Constant strong westerly winds • No continents to disrupt flow in southern ocean • Home of the most violent seas on Earth

Storm waves, Southern Ocean, R. Easther, Australian Antarctic Division, http://www.aad.gov.au/default.asp ?casid=2341

Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif

Western Boundary Currents North Atlantic Gyre Boundary Currents

Cold, cold Northern Canada Temperate Northern Gulf Stream Europe

British National Centre for Ocean Forecasting, Public 100 Domain(?), http://www.nerc- 50 essc.ac.uk/ncof/merse Sea Surface a/css- 0 gif/Mapwithrectangle.g Height (cm) 0 500 1000 1500 2000 2500 if Redhttp://www.itrd.gov/pubs/blue96/images/temp.atlantic.gif -- Warm surface waterBlue -- Cold surface water Distance (km)

5 Coriolis “Geostrophic” Response

Current flow rate in the Gulf Stream, in units of 106 m3/sec (roughly: millions of tons/sec) 106 m3/sec = 60 Mississippi Rivers! Centers of gyre “hills”: , W. Pacific, Madagascar, etc. Image from Sverdrup, Johnson, and Fleming, Sverdrup H.U., Johnson M.W., Fleming R.H. The Oceans.. their Animation from the Naval Research Laboratory, Public Domain, physics, chemistry, and general biology (1942), http://www7320.nrlssc.navy.mil/modas2d/anims/gbl/httot_gbl_12mon.fli http://oceanworld.tamu.edu/resources/ocng_textbook/chapter11/Images/Fig11-7.htm

Gulf Stream time-lapse Surface Temp. Western Boundary Currents

• Fastest, deepest gyre currents – Equatorial currents are deflected to high latitudes by continents – Transport warm water to high latitudes • Gulf Stream (N. Atlantic); (N. Pacific); (S. Atlantic); (Indian); (S. Pacific) • Gulf Stream is king of them all! 2 m/s ≈ 200 km/day 55x106 m3/sec = 55 Sverdrups transported > 3000 Mississippi Rivers!

U. Miami /CIMAS, Public Domain. http://oceancurrents.rsmas.miami.edu/atlantic/img_rrsl/sst-composit.avi

Eastern Boundary Currents Boundary Currents

• Shallow, broad, slow currents Current Width (km) Depth (km) Flow Rate – Return flow to low latitudes (km/day) – Transport cold water to the equator – Roughly 1/10 the speed of WBCs Western < 100 km 1-2 km ~ 100 • Canary Current (N. Atlantic); California Boundary km/day Current (N. Pacific); (S. Atlantic); Eastern > 1000 km < 0.5 km ~ 10 (Indian); Peru Current (S. Pacific) Boundary km/day

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