MAY 2011
Oceanography Ch # 7
OCEAN CIRCULATION
Ocean Currents
Masses of water that flow from one place to another (small or large, surface or deep),
Wind or density driven. These masses follow a pattern of a wind belt, transfer heat, influence
life distribution and climate. Density is a function of salinity or temp changes.
7 – 1. How are currents measured?
Surface – A floating device, tracked thru time,
Propeller flow meter, lowered into the water from a fixed location
Propeller device, towed by a ship.
Indirect methods:
A radar altimeter, which determines the sea floor topography.
A Doppler flow meter, which transmits a low frequency sound wave.
Deep Current measurement
Under water floats within the current. (Argo uses a drifting device that moves vertically
and measures temp, salinity and other characteristics (restricted to the upper 2000 meters of
the water column)
Chemical tracers or radioactive isotopes of Tritium.
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7 – 2. How are Ocean Currents Organized?
Sfc currents occupy space above the layer of rapidly changing density. Max depth 1 km.
Origin of the surface currents. Wind , Without the presence of continents, the currents
would follow the wind patterns. Their nature and direction are influenced by Wind, distribution
of the Continents, gravity, friction and the Coriolis effect.
The main components of the Ocean surface circulation are:
Subtropical Gyres – large circular moving loops of water driven by winds. They
rotate clockwise in the northern hemisphere. The average drift time for the smaller gyres is 3
years, for the larger it is 6 years. Each gyre has 4 main currents the flow progressively into one
another. There are 5 total gyres.
These main currents are:
Equatorial Currents . The Trade winds set up their motion (Equatorial Boundary
Currents)
Western Boundary Currents. When the equatorial currents reach the continents,
they are deflected away from the equator. Take water to higher latitudes.
Northern and Southern Boundary Currents. Generated by the Westerlies.
Eastern Boundary Currents. The Coriolis effect and presence of land turn these
currents towards the equator. Takes cool water to the warm tropical areas.
Equatorial Countercurrents. Water piles up on the western margin of the ocean,
then flows back down hill due to gravity
Sub-Polar Gyres. The Boundary currents resulting from the westerlies move into
the sub polar latitudes to produce sub polar gyres.
Other factors affecting Ocean Surface Circulation.
Ekman Spiral
Ekman Transport. The layers combine to create a net movement that is at right angles to
(from) the wind direction.
Geostrophic Currents – the Ekman transport deflects surface water, so a rotation
develops and produce a sub-tropical convergence in the middle of the gyre. This causes the
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water to pile up within the gyre. Result is a hill of water up to 6 ft. high. The Coriolis effect
directs the water on to the hill once more and the net effect is a geostrophic current.
Western Intensification of Sub-Tropical Gyres. Because the apex of the water hill is
closer to the western boundary of the gyre, the western currents are faster, narrower and
deeper than the eastern boundary counterpart. This phenomenon is called the Western
Intensification .
Ocean Currents and Climate
Surface currents influence the climate of adjacent land mass. Warm air cause higher
humidity and ppt. Cold air cools off the continental margins.
7 – 3. Upwelling and Downwelling Currents
Vertical movement of deep cold, nutrient rich waters (high productivity) – Upwelling.
Downwelling is the opposite; carries dissolved O2 and causes mixing.
Diverging surface waters are caused by the Coriolis effect and Ekman transport., which
in turn causes upwelling.
Converging Surface waters. When surface waters turn (move) toward each other, water
piles up and slowly sinks – Downwelling (low productivity)
Coastal Up and Downwelling. Winds can cause up or downwelling. Winds blowing from
North to South along the west coast. The Ekman transport will take the water away and cause
upwelling. Winds from the South and the Ekman transport will take the water into the coast,
where it piles up and sinks slowly. (downwelling)
Other causes of upwelling.
Subsea obstructions
Absence of a rapidly changing density layer allows upwelling when land breezes
take the surface water away from the land.
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7 – 4. The Main Surface Circulation Patterns
Depends on the ocean basin geometry, wind belts, seasonal factors and other periodic
changes.
Antarctic Circulation. A circumpolar current, also called West Wind Drift. It is
driven by the Westerly wind belt (Roaring 40s, Furious 50s, or Screaming 60s)
Antarctic Convergence and Divergence. Where cold dense Antarctic waters meet
with and sink below warmer , less dense sub Antarctic waters. Marks the boundary of the
Southern Ocean.
The East wind Drift driven by the Polar Easterlies, moves from east to west
around the margin of Antarctica.
A surface water divergence is created between the Antarctic Polar current and
the East wind Drift – an area of upwelling
Atlantic Ocean Circulation. North Atlantic clockwise moving gyre, and the South
Atlantic gyre rotates counterclockwise.. The Atlantic Equatorial Countercurrent moves between
them.
The Gulf stream moderates winter climate along the east coast of NA. and
Northern Europe. Made up of several currents
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North Equatorial
South Equatorial
Antilles current
Caribbean current
Florida current.
Approx 1.5 km deep, 50 to 75 km wide, moves at the rate of 3 to 10 km/hr. Fastest moving
current . Made up of warm and cold core rings, meanders
The Sargasso Sea – name is derived from the floating algae Sargassum..
The Gulf Stream breaks up into several branches. Labrador Current, Norwegian Current, North
Atlantic Current and the Canary Current. Climatic effects of the North Atlantic currents. Temp
due to heat transfer
Indian Ocean Circulation. Mostly located in the Southern Hemisphere. Equatorial current flows
east to west and the countercurrent goes from west to east.
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Monsoons: During winter, the air over the land cools rapidly and flows from the southwest Asia
– no ppt. In summer the winds are reversed. Produce the Somali current along the coast of
Africa.
Pacific Ocean Circulation. There are two large gyres. The Equatorial countercurrent is much
better developed. Off Peru, is the world’s best fishing ground.
Walker Circulation Cell. Sinking cool air dominates the coastal region of SA. On the western
boundary, it is the opposite. Low pressure, rising warm air produces cloudy, rainy conditions.
As a consequence of atmospheric conditions, a warm wedge of water collects on the western
side of the Pacific.
El Nino Southern Oscillation Conditions ENSO. La Nina is the cold phase
7 – 5. Deep Ocean Currents
Deep Currents influence 90% of all ocean waters, Created by density differences. Move slower
than sfc currents
These currents originate at the surface in the Polar regions. Major source is the Antarctic.
7 – 6. Currents have the potential as a source of energy.
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