Introduction to Oceanography Midterm 2
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Introduction to Oceanography Lecture 17, Current NOAA Ocean-Atmosphere Sea Surface Temperature Model, Public Domain, http://www.gfdl.noaa.gov/visualizations-oceans 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, http://commons.wikimedia.org/wiki/File:Sea_foam.JPG 1 Santa Ana Winds Winter: Canadian cold air pushes down into Southwestern US High pressure pushes dry desert air 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 NASA image, Public Domain, http://photojournal.jpl.nasa.gov/catalog/PIA03892 Santa Ana Winds ~ 30 mph 2 Santa Ana Winds Santa Ana Winds: dry & warm, Encourage 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 UCSD GOES-10/NASA, Public Domain, http://meteora.ucsd.edu/cap/images/junegloom_16jun2004.gif Questions? 3 Currents The Gulf Stream, Winslow Homer (1899), Metropolitan Museum of Art (NY). Public Domain, https://commons.wikimedia.org/wiki/File:Winslow_Homer_004.jpg Portrait of Ben Franklin, 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 Gulf Stream, Public domain. 4 Currents in the Ocean • Two Types: Surface and Deep • Driving Forces – Surface Currents: Wind-driven – Deep Currents: Density-driven American Meteorological Society, http://oceanmotion.org/images/oc ean-vertical- structure_clip_image002.jpg Surface Currents Caused by: Wind Stress Pressure Gradients CorioLis Effects Friction Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif 5 Wind-Driven Currents • Wind acceLerates ocean currents – FrictionaL Drag Atmosphere Wind Drag Wind Current Current Ocean Map View SIDEVIEW TOPVIEW Figures, UCLA Pressure-Driven Currents • Wind drives ocean currents • Currents run into continents and can’t continue – UNLIKE ATMOSPHERIC FLOWS • Water piLes up ---Pressure Gradients form Atmosphere L Wind Drag H Wind H Current Current Ocean L SIDEVIEW TOPVIEW Figures, UCLA 6 North AtLantic Surface Currents SimpLe picture – winds push surface currents, water piLes 60ºN up where wind bLows onto coastLines (e.g., AtLantic coast of S. 30ºN America). BUT: CorioLis acts on currents! Equator Background image: U.S. Government, extracted from PDF version of the Vol 26-4 2004 DISAM Journal (direct PDF URL [1]), Public Domain, http://commons.wikimedia.org/wiki/File:Globe_ Atlantic.svg CorioLis & currents • Currents are affected by Earth’s ROTATION! – This is caLLed the “Ekman” response Atmosphere Wind Drag Wind Currents Currents Ocean SIDEVIEW TOPVIEW Figures, UCLA 7 Ekman SpiraL 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: Transport 90º to the right of the wind direction in the northern hemisphere Left of wind in the southern hemisphere NOAA image, Public Domain, http://www.windows.ucar.edu/tour/link=/earth/Water/ekman.ht ml&edu=elem Winds ~60ºN Horse Latitudes ~ 30o N Equator Figures, UCLA 8 Drag from the wind on surface water Horse Latitudes ~ 30o N Figures, UCLA Wind-dragged & Basin-Edge Currents L H Horse Latitudes ~ 30o N H L Figures, UCLA 9 FinaL resuLt: a Loop of current, a Gyre Gene Paull, UT Brownsville, Public Domain(?), http://upload.wiki media.org/wikipe dia/commons/0/0 6/Corrientes- N. AtLantic oceanicas.gif Gyre One subtropicaL gyre is found in each hemisphere in each ocean. Gyres rotate clockwise in the N. Hemisphere, counterclockwise in S. Hemisphere. CorioLis “Geostrophic” Response L H Horse LatitudesH ~ 30o N H L Figures, UCLA 10 CorioLis “Geostrophic” Response 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. Why isn’t the high pressure in the center of the ocean? QUESTIONS? Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif 11 Major Current Systems 1. North AtLantic gyre 2. South AtLantic gyre 3. North Pacific gyre 4. South Pacific gyre 5. Indian Ocean gyre 6. Antarctic CircumpoLar Current • Not a gyre! Major Current Systems N. AtLantic N. Pacific S. Pacific Indian S. AtLantic Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif 12 Currents on each edge of a Gyre have names. Transverse Current Transverse Current Cropped from Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif Transverse Currents • Driven primariLy by Wind Stress • Antarctic CircumpoLar Current is the Largest of these Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif 13 Antarctic CircumpoLar Current Not a gyre! Southern Westerlies drive largest volume current on Earth – 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 Equatorial Currents • Found on either side of the equator • FLow to the west Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes-oceanicas.gif 14 Western Boundary Currents Cold, cold Northern Canada Temperate Northern Gulf Stream Europe British National Centre for Ocean Forecasting, Public Domain(?), http://www.nerc- essc.ac.uk/ncof/merse a/css- gif/Mapwithrectangle.g Redhttp://www.itrd.gov/pubs/blue96/images/temp.atlantic.gif -- Warm surface waterBlue -- CoLd surface water if North AtLantic Gyre Boundary Currents 100 50 Sea Surface 0 Height (cm) 0 500 1000 1500 2000 2500 Distance (km) 15 CorioLis “Geostrophic” Response Centers of gyre “hills”: Sargasso Sea, W. Pacific, Madagascar, etc. Animation from the Naval Research Laboratory, Public Domain, http://www7320.nrlssc.navy.mil/modas2d/anims/gbl/httot_gbl_12mon.fli Current fLow rate in the GuLf Stream, in units of 106 m3/sec (roughLy: miLLions of tons/sec) 106 m3/sec = 60 Mississippi Rivers! Image from Sverdrup, Johnson, and Fleming, Sverdrup H.U., Johnson M.W., Fleming R.H. The Oceans.. their physics, chemistry, and general biology (1942), http://oceanworld.tamu.edu/resources/ocng_textbook/chapter11/Images/Fig11-7.htm 16 Gulf Stream time-Lapse Surface Temp. U. Miami /CIMAS, Public Domain. http://oceancurrents.rsmas.miami.edu/atlantic/img_rrsl/sst-composit.avi 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); Kuroshio Current (N. Pacific); BraziL Current (S. AtLantic); AguLhas Current (Indian); East Australian Current (S. Pacific) • Gulf Stream is king of them aLL! 2 m/s ≈ 200 km/day 55x106 m3/sec = 55 Sverdrups transported > 3000 Mississippi Rivers! 17 Eastern Boundary Currents • ShaLLow, broad, slow currents – Return flow to low latitudes – Transport coLd water to the equator – RoughLy 1/10 the speed of WBCs • Canary Current (N. AtLantic); California Current (N. Pacific); Benguela Current (S. AtLantic); West AustraLian Current (Indian); Peru Current (S. Pacific) Boundary Currents Current Width (km) Depth (km) Flow Rate (km/day) Western < 100 km 1-2 km ~ 100 Boundary km/day Eastern > 1000 km < 0.5 km ~ 10 Boundary km/day 18.