Ch 8 - Ocean Circulation
8(1)
Types circulation, Driving forces; Gyres
I. 2 types circulation in ocean:
A.Surtace currents
-10% of ocean involved
-Occur from surf to base pycnocline
-Current defined (informally) as water flowing
horizontally, like a river
-After "okeanus", coined by Pytheas of Massilia 4th con
BC for Canary current in NE Atlantic ...thought it was
immense river
B.Thermohaline circulation
-90% of ocean involved
-Occurs below pycnocline
II- Driving Forces of circulation
A- Surf currents: driven by 2 types of forces
B- 1.Primary driving forces (function to start mvmt &
determine velocity)~
-Wind friction over surf water
-Thermal expansion/contraction
-Density diffs
2.Secondary driving forces (function to influence direc
& nature of flow)
-Coriolis effect
-Gravity
-Friction
-Shape of ocean basin
B.Thermohaline circulation
Driving force is density diffs Density diffs due to :
-Temp ("thermo-")
-Salinity ("haline.")
1st focus on Surface currents:
8(2)
1. Primary driving forces:
*Thermal expansion/contraction
-Uneven solar heating causes tropical water to expand
-Sea level 3" higher in tropics than poles
-Water moves downhill, toward poles
•Wind - major force responsible for surface currents
-Trade winds & westerlies exert a drag on ocean
surface via irregular capillary waves
-Water responds, starts flowing as surface current
-Coriolis effect causes clockwise rotation in N Hemi,
counterclockwise in S Hemi
-Circular flow path around periphery of ocean basin
known as "gyre"develops
Flow w/in a gyre-subdivide into separate currents
Example - N. Atlantic
-N Equatorial current to W
-Gulf Stream to N
-N Atlantic current to E
-Canary current to S
Question: Why doesn't water just tighten up in a clockwise spiral
due to Coriolis effect, and spin toward _center of Atlantic, just like water
going down a drain??
8(3)
Why currents ride around periphery of basins:
Water particles at are deflected from wind direction by Coriolis
effect - note N Hem, deflec is right
Ekman spiral - As energy transferred to deeper layers, each
successive layer is deflected right relative to one above it by Coriolis
effect
Ekman transport- theoretical diagram
Net motion in upper 100m = sum of vectors
notice it's 90o to direction of original driving wind force
(In actuality, Ekman transport^< 45° from wind direc) why??
Look at water particle
Assume a small dome 5' tall exists in center of N Atlantic due to
water spiraling in & "bunching up"
Look at forces at work:
-Force of gravity pulls particle to south (downhill)
-Trade winds push particle to s-west
-Coriolis force pulls particle to north
Result:
Coriolis force & gravitational force cancel each other out..
The dominant force that's left continues to push particle to west
Gyres in balance between gravity & Coriolis effect are
Geostrophic gyres
Of 6 great ocean currents, 5 are geostrophic gyres
8(4)
I. SURFACE CURRENTS WITHIN GYRES
3 MAIN TYPES
A. Western Boundary
B. Eastern Boundary
C. Transverse
D-G. Other types of currents
A. WESTERN BOUNDARY CURRENTS
-Fastest
-Deepest
-Move warm water poleward
-All on W. side of ocean basins
5 large currents
Gulf stream (N Atlantic)
-Brazil (S. Atlantic)
-Japan (N. Pacific)
-E. Australian (S. Pacific)
-Agulhas (Indian Ocean)
Gulf Stream largest -Discharge (volume transported) -- 55
Sverdrups (How big is a sverdrup?)
-Relatively fast - 5 mph
-Wide - Approx. avg. 70 km = 43 mi.
-Well-defined western edge
-In current - blue, clear, warm, not much life
-Outside current-green, cold, "teeming with life'
8(5)
Aberrations: -Eddy currents
-can be 1000 km diam
-can last for 3 years or more
B.EASTERN Boundary Currents
-move cold water tropicward
-Slow
-Shallow
-Broad (can be greater than = 1000 km wide)
-Discharge not as high as western currents
5 large currents
-Canary (N. Atlantic)
-Banquets (S. Atlantic)
-California (N. Pacific)
-Peru (Humboldt) (S. Pacific)
-W Australia (Indian Ocean)
-Explanation of differences
Western vs eastern boundary currents
I.
Converging Tradewinds push water westward along
equator, then it turns poleward at the western edge of
ocean basin. Westerlies do not converge, so water is
not swept along a line of convergence - not as much
push
II.
Rotation of earth creates "hills" that force converging
water around them on western edges of basins
8(6)
C TRANSVERSE CURRENTS -flow east to west and west to
east, linking the western and eastern boundary currents
-Equatorial Currents in Pacific and Atlantic driven by
tradewinds from east to west discharge 30 sv -Mid-latitude
currents driven by westerlies- flow from west to east
-Notable exception to other transverse currents
-West Wind Drift
aka Antarctic Gircumpolar Current
-Largest volume of discharged of any current -- 100
sverdrups
-Rotates freely eastward around Antarctica and
S. Pole, hence "circumpolar" (Around)
D. EQUATORIAL COUNTER CURRENTS
- on surface
- -opposite direction from main currents
-equatorial
-REASON "backward" flow occurs because there's no "push"
westward in area of doldrums right over the equator
-also possible that earth's rotation "drags" countercurrent
eastward.
E UNDERCURRENTS
Counter to, but under surface currents,
usually at depth of 100-200 meters
8(7)
Reason-not all water deflected to N. or S at western sides of
basins; Instead, some water flows downward and then back
under itself; then Coriolis effect keeps it in line as it moves
eastward
F. SURFACE CURRENT EFFECTS ON CLIMATE
-Distribute tropical heat worldwide
Example-winter at let 51o N at 2 places
-1 in Canada, faraway from Gulf Stream
-very cold, sanctuary for polar bears
-1 in British Isles (Edinburgh) near N Atlantic current which
inherited warm water from Gulf Stream
This area much milder-
8(8)
I VERTICAL CIRCULATION
A. Wind Induced Vertical Circulation
B. Thermohaline vertical Circulation
II MISC STUFF
--------------------------------A WIND INDUCED VERTICAL CIRCULATION
Definitions: Upward Movement = UPWELLING
Downward Movement =DOWNWELLING
UPWELLING -Equatorial upwelling caused by surface water divergence;
water moves to N and 5 as it moves west
- divergence allows room for nutrient rich deeper water to rise to surface
-Coastal upwelling
-caused by wind blowing parallel to shore or offshore
-cool deeper water rises, bringing nutrients
DOWNWELLING
-caused by water being driven toward coastline and
forced downward, returning seaward like equatorial
undercurrent
'
B. THERMOHALINE CIRCULATION
-Below the pycnoline (below 1000 m)
-Driven by density differences, no wind
-'thermo" (temp) + "Haline" (Salinity) - density
8(9)
- process responsible for most of vertical movement of
water in ocean.
1.
Water masses
-Individual temperature and salinity characteristics
-density stratification most pronounced at tropics and
temperate latitudes, where temperatures range between
surface and sea floor greatest
5 common masses
a surface water (down to top thermocline at 200 m)
b. Central water (top thermocline to bottom thermocline)
c. Intermediate water (bottom thermocline to 1500 m)
d. Deep water (1500 to 4000 m)
e. Bottom water (in contact with sea floor)
(Surface currents located in surface and central layers)
Important point- no matter what depth mass is now, its
characteristics were determined at ocean surface -when mass
was formed:
-heaviest, deepest masses formed in very cold and salty
conditions (poles)
-near-surface masses somewhat warm, less saline (may have
formed in warm areas where there was quite a bit of precipitation)
2.
T-S diagrams
-show that each ocean has its own masses with their own
characteristics
8(10)
3. Formation and downwelling of deep water
-Antarctic Bottom Water
-extremely cold (0.5° C)
-extremely dense (1.0279) Denser than northern waters
-forms during winter, sinks, flows north very slowly
- Can take IOOO yrs to move to equator -identified as far north as 40° N.
- -some dense water forms in Arctic, but most of it stays far north, doesn't
move south
- -thus, most ocean bottom water originated in Antarctica
4.
Thermohaline Circulation Patterns
-dense water sinking displaces water, forces it
to rise elsewhere
-upwelling = I cm (I/2 inch) per day
-usually residence time for deep water approx 200-300
years before it rises
Some localized bottom currents can be generated, called
contour currents because they flow around the contours of
bed forms
Some possible vertical currents too, due to hot water
rising from vents on spreading centers
8(11)
II MISC STUFF
A.
Studying currents
-Traditional current measuring methods fall into 2 broad
categories
-Name them
What are some of the devices used in each category
-Newer methods:
-computer technology
-radioactive isotopes (tritium)
-satellite imagery
-acoustical tomography
(like low frequency sonar)
B.
Human understanding of currents through time
-Pytheas
-Columbus/Magellan
-tat to develop and publish theory David Bernoulli
-1751
-Benjamm Franklin's cousin Timothy Folger told Ben
about "Gulph Stream" in 1753-fastway t0 Europe. Ben
published chart of G. S. in 1769 (was the route using the
G. S. immediately embraced by all those who saw the
chart?
-M Maury-1840's, and 1850's compiled charts,
developed sense of worldwide pattern of
winds/currents
-Ferrel-1858- actually applied work of Coriolis to real
world