OCEAN CIRCULATION
I.
INTRODUCTION
A. Ocean currents are masses of ocean water that flow from one place to another (water masses
in motion)
B. Wind belts transfer about 2/3 of the heat from the tropics to the poles, while ocean currents
transfer the other 1/3
C. Surface current patterns are driven by the sun and therefore, are similar to the wind belts
D. Surface currents affect the climates of coastal regions
II.
MEASURING OCEAN CURRENTS
A. WIND-DRIVEN
i. Major wind belts of the world set wind-driven ocean currents in motion
ii. These are called surface currents
iii. Winds are parallel to the surface
B. DENSITY-DRIVEN
i. This movement is vertical
ii. Mixing of deep masses of ocean water
iii. Temperature and salinity lead to density-driven circulation
iv. These are called deep currents
C. MEASURING SURFACE CURRENTS (WIND-DRIVEN)
i. They go in all sorts of directions, so they are hard to measure, but there is an overall
pattern
ii. DIRECT MEASUREMENT
a. Floating device is released into the current and tracked through time
i. Drift current meter
b. From a fixed point (pier) where a current-measuring device is lowered into the
water
i. Propeller flow meter
iii. INDIRECT MEASUREMENT
a. If we can determine density and the pressure, we can determine its velocity &
volume
i. Physics equations that relate them
b. Radar altimeters
i. Aboard the TOPEX/Poseidon satellite
ii. Measures shape of the surface & sea floor
c. Doppler Flow Meter
i. Compares the differences between sounds emitted and those
backscattered by water particles
iv. OTHER MEASUREMENTS
a. http://www.seos-project.eu/modules/oceancurrents/oceancurrents-c06p01.html
III.
SURFACE CURRENTS
A. INTRODUCTION
i. About 2% of wind’s energy is transferred to ocean
a. 50 knot wind creates 1 knot current
ii. Continents influence direction and nature of surface currents
a. p.234 – Trade Winds + Prevailing Westerlies = Loop of water(gyre)
iii. They are affected by
a. Gravity
b. Friction
c. Coriolis Effect
iv. Occur above the pycnocline to about 1 km
v. Affect only about 10% of the world’s ocean water
B. GENERAL DEFINITIONS – IDEAL CONDITIONS
i. EQUATORIAL CURRENTS
a. The NE & SE trade winds converge at the equator, creating a major wind
movement from East to West
b. These are called Equatorial currents
c. They are warm water since they are at the equator
ii. WESTERN BOUNDARY CURRENTS
a. Since the equator goes through land masses, currents hit continents and change
direction
b. Coriolis effect makes them away from the equator, going North or South
c. They travel along the Eastern side of a continent, which is the Western side of
the ocean basin
d. Therefore, they are called Western Boundary Currents
e. These bring warm water north toward the poles
iii. EASTERN BOUNDARY CURRENTS
a. The prevailing westerlies pull these surface currents away at an easterly angle
from the equator (NE or SE)
b. Coriolis affect brings them back toward the equator
c. This pull downward is along the western side of a continent, but the eastern side
of the ocean basin
d. Therefore, they are called Eastern Boundary Currents
e. They carry cool water away from the poles toward the equator
iv. GYRES
a. This entire process create a loop of surface currents, a gyre
b. There are 5 basic subtropical gyres
i. North Atlantic (clockwise)
ii. South Atlantic (counterclockwise)
iii. North Pacific (clockwise)
iv. South Pacific (counterclockwise)
v. Indian Ocean (counterclockwise)
c. There are about 5 basic subpolar gyres that operate similarly
i. North Atlantic gyre (counterclockwise)
ii. Alaskan gyre (counterclockwise)
iii. Weddell Gyre (clockwise)
iv. Ross Gyre (clockwise)
v. 1 unnamed (clockwise)
v. MAP OF SURFACE CURRENTS
a. Additional sheet
b. These need to be memorized!
c. http://www.youtube.com/watch?v=xusdWPuWAoU
C. FORMATION OF SURFACE CURRENTS (FOUR MAIN METHODS)
i. EKMAN SPIRAL & EKMAN TRANSPORT
a. EKMAN SPIRAL
i. 45o to the right due to Coriolis
ii. There are different “thin” layers of water, affected by the friction
of the layer above
iii. Energy is passed down and decreases with depth
iv. At about 300 ft down, there is no more Ekman spiral – depth of
frictional influence
b. EKMAN TRANSPORT
i. If you look at all of the layers, there is a general trend of 90o to
the right of the wind
ii. This average movement is called Ekman transport
iii. This follows along with the Coriolis effect
ii. CONVERGENCE
a. Clockwise rotation in the Northern hemisphere would deflect water to the right
b. This would mean that surface currents would converge in the middle of the gyre
c. This can be as much as 2 meters
iii. VORTICITY (CONSERVATION OF)
a. Vorticity is analgous to angular velocity
i. Angles/Rotations per time
b. As you head away from the equator, vorticity increases/decreases
iv. GEOSTROPHIC BALANCE
a. The Coriolis Effect opposes gravity, deflecting water to the right
b. Gravity brings down water back into the hill
c. This creates a geostrophic current
D. WESTERN INTENSIFICATION
i. The balance of wind stress, Coriolis Effect and friction is not perfectly symmetrical in the
middle of a gyre
ii. Also, Earth rotates toward the East, pushing water to the West a bit
iii. In fact, the apex of a hill tends to be closer to the western boundary of a gyre
iv. This is known as western intensification
v. The western boundary currents of all subtropical gyres are western intensified, even in
the Southern Hemisphere
E. EQUATORIAL COUNTERCURRENTS
i. The westward currents flow until they hit the continents
ii. This water will go under and back, counter to and between, the equatorial currents
F. OCEAN CURRENTS AND CLIMATE
i. Ocean Currents effect the climate
ii. Continental margins with warm currents off the coast typically have a humid climate
iii. Conversely, continental margins with cold currents off the coast typically have an arid
climate
IV.
UPWELLING AND DOWNWELLING
A. DEFINITIONS
i. Upwelling is the vertical movement of cold, deep, nutrient-rich water to the surface
a. Since the water is cold, a lot of dissolved nutrients are there
b. This adds to productivity
ii. Downwelling is the vertical movement of warm, surface water to deeper parts of the
ocean
B. DIVERGING SURFACE WATER
i. The currents are diverging – pushing away from each other
ii. Some water must replace the movement
iii. Hence, you have equatorial upwelling
C. CONVERGING SURFACE WATER
i. The currents are converging – moving toward each other
ii. Water stacks up and can only go downward
iii. Hence, you have downwelling
D. COASTAL UPWELLING & DOWNWELLING
i. Southern Hemisphere, winds coming from south, eastern boundary current, along
western side of continent, waters would pull away (to the left) from the coast
a. Water rises to replace
b. You have coastal upwelling
ii. Southern Hemisphere, winds coming from north, eastern boundary current, along
western side of continent, waters would push toward (to the left) from the coast
a. Water piles up
b. You have coastal downwelling
E. OTHER UPWELLING
i. Offshore winds, pushing water out
a. Land breeze
ii. Sea Floor obstruction
a. Tablemount
iii. Sharp Bend in Coastline
iv. No pycnocline
a. Both upwelling and downwelling can occur
V.
SURFACE CURRENTS OF THE OCEANS
A. ANTARCTIC OCEAN
i. ANTARCTIC CIRCUMPOLAR CURRENT
a. Aka West Wind Drift
b. Flows west to east
c. 40-65o south latitude
d. Subtropical convergence represents northern boundary of current
e. Strong winds created
i. Roaring Forties
ii. Furious Fifties
iii. Screaming Sixties
f. Only current to completely circumscribe Earth
g. Transports more water than any other surface current
ii. ANTARCTIC CONVERGENCE
a. At around 50o latitude south latitude
b. Marks the northern boundary of the Southern Ocean
c. Downwelling since it’s a convergence
iii. EAST WIND DRIFT
a. A surface current propelled by the polar easterlies
b. Moves west to east
c. Opposite that of the Antarctic Circumpolar current
iv. ANTARCTIC DIVERGENCE
a. Since the East Wind Drift and the West Wind Drift flow in opposite directions, a
divergence is created
b. Upwelling occurs
B.
C.
D.
E.
c. Very productive
ATLANTIC OCEAN
i. NORTH & SOUTH ATLANTIC GYRES
a. North Atlantic goes clockwise
b. South Atlantic goes counterclockwise
c. Atlantic Equatorial Current flows between them
d. SOUTH GYRE
i. South Equatorial Current splits into two around Brazil
ii. West Wind Drift brings it to the Benguela current, completing the
gyre
iii. Falkland current is in there (aka Malvinas)
e. NORTH GYRE
i. Gulf Stream is most studied
ii. North Equatorial current moves parallel to Equator
iii. Flow splits into Antilles & Caribbean Current
iv. Reconverge as Florida current
v. Gulf Stream is western intensified
vi. Sargasso Sea is east of Gulf Stream
vii. Meanders disconnect from Gulf Stream and create eddies or rings
1. Warm core (clockwise)
2. Cold core (counterclockwise)
3. Last for months
4. 15 per active year
5. http://kingfish.coastal.edu/gulfstream/p5.htm
PACIFIC OCEAN
i. North Pacific Gyre
a. Kuroshio Current
i. aka Japan Current
b. North Pacific Current
c. California Current
d. Alaskan Current
ii. South Pacific Gyre
a. East Australian Current
b. Peru Current
PACIFIC OCEAN - WALKER CIRCULATION CELLS (& UNDER NORMAL CONDITIONS)
i. High Pressure & Low Pressure regions
ii. Strong southeast trade winds blow across to low pressure
iii. Warm air rises, creating precipitation
iv. Upwelling along western coast of South America
v. ANIMATION!!
PACIFIC OCEAN - EL-NINO SOUTHERN OSCILLATION (ENSO)
i. EL NINO (WARM PHASE)
a. The high pressure area weakens
b. Trade winds diminish
i. May actually reverse!
c. Band of warm water stretches across Pacific
i. Kelvin wave
d. Typically starts to move in September of an El Nino year and reaches South
America by December/January
e. Thermal expansion
f. Affects weather & productivity & organisms
g. Thermocline stays horizontal, therefore upwelling isn’t bringing nutrient-rich
water to surface
h. Sometimes, downwelling actually occurs
i. Cause drought in Indonesia & Australia
ii. LA NINA (COOL PHASE)
a. Typically occur after an EL NINO
b. The high pressure area strengthens
c. Trade winds strengthen
d. Band of cold water stretches across Pacific
i. Does not impede warm water though
e. Way more upwelling
f. Thermocline rises and shrinks
iii. OCCURANCE
a. El Nino occurs on average every 2-10 years
b. Typically last 12-18 months
c. However, it’s very irregular
F. INDIAN OCEAN
i. Winds have a seasonal pattern, a monsoon
ii. During winter, land rapidly cools, creating winds toward the south – Northeast monsoon
iii. During summer, land rapidly warms, creating winds toward the north – Southwest
monsoon
iv. Somali current
v. Indian Ocean Gyre
vi. Agulhas Current
vii. West Australian Current
VI.
DEEP OCEAN CURRENTS
A. THERMOHALINE CIRCULATION ORIGIN
i. Increase in density means decrease in temperature and increase in salinity
ii. It starts at high latitudes at the surface
iii. Salinity increases as sea ice forms
iv. It sinks, initiating deep-ocean currents
v. At the same time, less dense, deep-water masses are rising to the surface
B. SOURCES OF DEEP WATER / DEEP WATER FORMATION
i. ANTARCTICA (beneath ice along continent)
a. Rapid winter freezing produces very cold, highly-dense water
b. Sinks down slope of Antarctica
c. Near Weddell Sea
d. This creates Antarctic Bottom Water (AABW), which is deep water
e. Returns to the surface about 1000 years later
ii. NORWEIGAN SEA / MARGINS OF IRMINGER SEA / LABRADOR SEA / GREENLAND SEA
a. Flows as a subsurface current into the North Altantic
b. This creates North Atlantic Deep Water (NADW)
iii. ANTARCTIC CONVERGENCE
a. Sinking because of convergence
b. Not cold enough or salty enough to reach sea floor
c. This creates Antarctic Intermediate Water (AAIW)
C. PACIFIC & INDIAN OCEANS
i. No source of Northern Hemisphere deep water
ii. They lack a deep water mass
iii. In Northern Pacific, low salinity surface waters prevents sinking
iv. In Northern Indian, surface waters are too warm to sink
v. When AABW & NADW mix, it creates Oceanic Common Water
a. Lines bottoms of both basins
b. aka circumpolar water
D. CONVEYER BELT CIRCULATION
i. Beginning in the North Atlantic, surface water carries heat to high latitudes via the Gulf
Stream
ii. Movement of Oxygen