Introduction to Oceanography Lecture 12, Current 2
NOAA Ocean-Atmosphere Sea Surface Temperature Model, Public Domain, http://www.gfdl.noaa.gov/visualizations-oceans
Coriolis “Geostrophic” Response L H
WESTERLIES
Horse LatitudesH ~ 30o N
TRADE WINDS
H L
Figures, UCLA
1 Coriolis “Geostrophic” Response
Mean 1992-2002 dynamic ocean topography, Nikolai Maximenko (IPRC) and Peter Niiler (SIO), Public Domain, http:// apdrc.soest.hawaii.edu/ 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
2 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
3 Currents on each edge of a Gyre have names. Transverse Current
Eastern
Boundary 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
4 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
5 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/mersea/css-gif/ Mapwithrectangle.gif
Redhttp://www.itrd.gov/pubs/blue96/images/temp.atlantic.gif -- Warm surface water Blue -- Cold surface water
North Atlantic Gyre Boundary Currents
100
50
Sea Surface 0 Height (cm) 0 500 1000 1500 2000 2500 -50 Distance (km)
6 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
Gulf Stream
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
7 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!
8 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 km/ Boundary day
Eastern > 1000 km < 0.5 km ~ 10 km/ Boundary day
9 Important non-gyre currents: Equatorial Counter Currents
• Right between the two Equatorial currents • No Coriolis at equator: only wind stress vs. pressure • Response of water to constantly being pushed / piled up on the west side of the basin (up to 50 cm high) • Tends to flow back towards East.
QUESTIONS?
Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes- oceanicas.gif
10 Upwelling of Deep Water
British National Centre for Ocean Forecasting, Public Domain(?), http:// www.nerc-essc.ac.uk/ ncof/mersea/css-gif/ Mapwithrectangle.gif Red -- Warm surface water Blue -- Cold surface water
Sites & Causes of Upwelling • Equatorial Divergence: Opposite Coriolis bending on each side of the equator causes pull surface water away. Cold deep water rises to replace it.
CORIOLIS
N. Equ. Current
Equator UPWELLING S. Equ. Current
CORIOLIS
Figures, UCLA
11 Equatorial Pacific Upwelling
UPWELLING
Chris Henze, NASA Ames, Public Domain, http:// people.nas.nasa.g ov/~chenze/ECCO/ 93-02.T_1.raw.mpg
Blue-Yellow - Cold water Red-Magenta - Warm water
Coastal Upwelling
• Coastal Upwellings: Ekman transport away from shoreline forces upwellings
CORIOLIS UPWELLING
Figures, UCLA
12 Coastal upwelling, California
California
NOAA image, Public Domain, http://oceanexplorer.noaa.gov/explorations/02quest/background/upwelling/media/ Fig1_cartoon.html
Upwelling and Downwelling Flows • Antarctic Divergence:
– Two causes: Dense sinking waters (vertical mixing) and Ekman transport (upwelling) away from Antarctic Circumpolar Current – Results in intense upwelling around Antarctica – Very high biological Chlorophyll a productivity Concentration
NASA image, Public Domain, http://earthobservatory.nasa.gov/Study/Polynyas/
13 Liusen Xie, UBC Climate Prediction Group, http://www.ocgy.ubc.ca/projects/clim.pred/Upwell/annualL.jpg
QUESTIONS?
Gene Paull, UT Brownsville, Public Domain(?), http://upload.wikimedia.org/wikipedia/commons/0/06/Corrientes- oceanicas.gif
14 Deep Currents in the Ocean • Two Types – Surface Currents • Mixed layer (0-300 m), most surface currents here • Pycnocline (to 1000m) – Sub-surface (deep) Currents • Deep water
American Meteorological Society, http://oceanmotion.org/images/ ocean-vertical- structure_clip_image002.jpg
Where does deep water come from? Labrador Sea
Weddell Sea
Adapted from figure by Helen Hill(?), MIT, http://puddle.mit.edu/~helen/
15 Deep Water Formation
Deep currents – sinking of dense water near the poles
Figure from Matthew England, Climate Change Research Centre (CCRC) University of New South Wales, http://web.maths.unsw.edu.au/~matthew/southern_ocean_variability.htm
Water Mass Classifications
• Deep Waters: – North Atlantic Deep Water (NADW) • Vertical mixing & incorporation of salty N. Atlantic waters near Greenland • Bottom Waters: – Antarctic Bottom Water (AABW) • Forms dominantly in Weddell Sea in regions of active sea ice formation (polynyas) • Cold, saline sinking water • Densest water mass in the oceans
16 Water Mass Classifications Intermediate Waters: Water between cold, deep polar water and surface Example: Mediterranean outflow: Warm and highly saline at ~1000 m Med. outflow figure by G. P. King, http://www.eng.warwick.ac.uk/ staff/gpk/Teaching-undergrad/es427/rice.glacier.edu-oceans/ GLACIER%20Oceans-%20--%20Densitydriven.htm Sea-surface salinity map by Rosarinagazos, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Wiki_plot_04.png
Strait of Gibraltar
%
• • • • • • • • •
Atlantic Deep Circulation
UCAR/NOAA/NASA, Public Domain, http:// www.meted.ucar.edu/ tropical/met_topics/ media/graphics/ moc_atlantic_salinity. jpg MIW – Mediterranean Intermediate Water
Antarctic Intermediate Water MIW
North Atlantic Deep Water Antarctic Bottom Water
17 Pacific
0 0 0 0 0 0
UCAR/NOAA/NASA figure, Public Domain, http://www.meted.ucar.edu/oceans/currents/media/ graphics/pacific_salinity_section.jpg
Global deep water ‘conveyor’
Robert Simmon/Robert Rohde, NASA/Wikimedia, Public Domain, http://en.wikipedia.org/wiki/ File:Thermohaline_Circulation_2.png
18 Hannes Grobe, Wikimedia Commons, Creative Commons A S-A 2.5, http://upload.wikimedia.org/wikipedia/ commons/2/2c/Antarctic_bottom_water_hg.png
QUESTIONS?
Excellent vintage 1960’s fluid dynamics movies: National Committee for Waves Fluid Mechanics Films http://web.mit.edu/hml/ncfmf.html
Hurricane wavemaker, Hinsdale Wave Research Laboratory, Oregon State University, http://oregonstate.edu/ media/twvwz-hiq
19 Waves • Traveling disturbances of the water column • Waves move but there is little or no net transport of the water – If you put dye in the water, a wave can pass through and the dye will basically remain in place • Most familiar are wind waves - produced on the ocean surface by effects of time-varying winds
Animation courtesy Dr. Dan Russell, Kettering University, http:// paws.kettering.edu/ ~drussell/Demos/waves/ wavemotion.html
The Anatomy of a Wave
Remember These!
Adapted from figure by Kraaiennest, Wikimedia Commons, Creative Commons A S-A 3.0, http:// commons.wikimedia.org/wiki/File:Sine_wave_amplitude.svg
20 The Dynamics of a Wave Wave Period – time between crests Wave Frequency – number of crests per second Wave Speed – rate crests move (meters/second)
Animation courtesy Dr. Dan Russell, Kettering University, http:// paws.kettering.edu/ ~drussell/Demos/wave-x-t/ wave-x-t.html
Period, frequency, speed and wavelength are related!
Period = 1/frequency Speed = wavelength / period = wavelength x frequency
Oscillation • As waves travels through, water locally moves in circular orbits • Like a seagull bobbing on water surface • In most cases the water, like the seagull, ends up back where it started (little or no net transport).
Movie from National Weather Service/NOAA, Movie by Patricia E. Videtich and Erik J. Crooks Public Domain, http://www.srh.noaa.gov/ http://faculty.gvsu.edu/videticp/waves.htm jetstream//ocean/wave_max.htm
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