coriolis “geostrophic” response questions?schauble/epss15_oceanography/lec12s17_cu… ·...

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Introduction to Oceanography

NOAA Ocean-Atmosphere Sea Surface Temperature Model, Public Domain, http://www.gfdl.noaa.gov/visualizations-oceans

Lecture 12, Current 2 Coriolis “Geostrophic” Response

WESTERLIES

Horse Latitudes ~ 30o N

TRADE WINDS

H

HL

L

H

Figures, UCLA

Coriolis “Geostrophic” Response

Average satellite-measured “hill” of water at western side of gyre. Why isn’t the high pressure in the center of the ocean?

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

QUESTIONS?

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

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. Pacific

S. Pacific S. Atlantic

N. Atlantic

Indian

2

Currents on each edge of a Gyre have names. W

este

rn

Bound

ary C

urre

nt

East

ern

Boun

dary

Cur

rent

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


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


Western Boundary Currents

http://www.itrd.gov/pubs/blue96/images/temp.atlantic.gif

Gulf Stream

TemperateNorthernEurope

Cold,cold Northern

Canada

Red -- Warm surface water Blue -- Cold surface water

British National Centre for Ocean

Forecasting, Public Domain(?), http://

www.nerc-essc.ac.uk/ncof/mersea/css-gif/Mapwithrectangle.gif

North Atlantic Gyre Boundary Currents

-50

0

50

100

0 500 1000 1500 2000 2500

Sea Surface Height (cm)

Distance (km)

3

Centers of gyre “hills”: Sargasso Sea, W. Pacific, Madagascar, etc.

Coriolis “Geostrophic” Response

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

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!

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 Boundary

< 100 km 1-2 km ~ 100 km/day

Eastern Boundary

> 1000 km < 0.5 km ~ 10 km/day

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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?


British National Centre for Ocean

Forecasting, Public Domain(?), http://

www.nerc-essc.ac.uk/ncof/mersea/css-gif/Mapwithrectangle.gif

Upwelling of Deep Water

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.

Equator

N. Equ. Current

S. Equ. Current

CORIOLIS

CORIOLIS

UPWELLING

Figures, UCLA

Equatorial Pacific Upwelling

UPWELLING

Blue-Yellow - Cold water Red-Magenta - Warm water

Chris Henze, NASA Ames, Public Domain, http://people.nas.nasa.gov/~chenze/ECCO/93-02.T_1.raw.mpg

Coastal Upwelling

•  Coastal Upwellings: Ekman transport away from shoreline forces upwellings

UPW

ELLI

NG

CORI

OLI

S

Figures, UCLA

5

California

Coastal upwelling, California

NOAA image, Public Domain, http://oceanexplorer.noaa.gov/explorations/02quest/background/upwelling/media/Fig1_cartoon.html

Upwelling and Downwelling Flows •  Antarctic Divergence:

NASA image, Public Domain, �http://earthobservatory.nasa.gov/Study/Polynyas/

Chlorophyll aConcentration

– 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 productivity

Liusen Xie, UBC Climate Prediction Group, http://www.ocgy.ubc.ca/projects/clim.pred/Upwell/annualL.jpg

QUESTIONS?


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

Labrador Sea

WeddellSea

Where does deep water come from?

Adapted from figure by Helen Hill(?), MIT, http://puddle.mit.edu/~helen/

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Deep Water Formation

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

Deep currents – sinking of dense water near the poles

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

Water Mass Classifications Intermediate Waters: Water between

cold, deep polar water and surface Example: Mediterranean outflow:

Warm and highly saline at ~1000 m

%

• • • • • • • • •

Sea-surface salinity map by Rosarinagazos, Creative Commons A S-A 3.0, http://commons.wikimedia.org/wiki/File:Wiki_plot_04.png

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

Strait of Gibraltar

Atlantic Deep Circulation

MIW

North Atlantic Deep Water

Antarctic Intermediate Water

Antarctic Bottom Water

MIW – Mediterranean Intermediate Water

UCAR/NOAA/NASA, Public Domain, http://www.meted.ucar.edu/tropical/met_topics/media/graphics/moc_atlantic_salinity.jpg

0 0 0 0 0 0

Pacific

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

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QUESTIONS?

Hannes Grobe, Wikimedia Commons, Creative Commons A S-A 2.5, http://upload.wikimedia.org/wikipedia/commons/2/2c/Antarctic_bottom_water_hg.png Waves

Hurricane wavemaker, Hinsdale Wave Research Laboratory, Oregon State University, http://oregonstate.edu/media/twvwz-hiq

Excellent vintage 1960’s fluid dynamics movies: National Committee for Fluid Mechanics Films http://web.mit.edu/hml/ncfmf.html

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

The Dynamics of a Wave

Wave Frequency –

number of crests per second

Wave Speed – rate crests move (meters/second)

Wave Period – time between

crests

Period, frequency, speed and wavelength are related! �

Period = 1/frequencySpeed = wavelength / period = wavelength x frequency

Animation courtesy Dr. Dan Russell, Kettering University, http://

paws.kettering.edu/~drussell/Demos/wave-x-t/

wave-x-t.html

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 by Patricia E. Videtich and Erik J. Crooks �http://faculty.gvsu.edu/videticp/waves.htm

Movie from National Weather Service/NOAA, Public Domain, http://www.srh.noaa.gov/

jetstream//ocean/wave_max.htm

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