currents and el nino
TRANSCRIPT
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There are two type of Ocean Currents:
1. Surface Currents--Surface Circulation (the upper 400
meters of the ocean).
2. Deep Water Currents--Thermohaline Circulation (move
around the ocean basins by density driven forces and gravity).
The density difference is a function of different temperatures
and salinity.
These deep waters sink into the deep ocean basins at high
latitudes where the temperatures are cold enough to cause
the density to increase.
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ocean conveyor belt
http://localhost/var/www/apps/conversion/tmp/scratch_8//upload.wikimedia.org/wikipedia/commons/4/4c/Thermohaline_Circulation_2.png -
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Surface - Wind Driven Currents
Wind driven currents are produced when wind blows
over a water surface Momentum is transferred from the wind to the water at
the air-sea boundary
When water in an open ocean starts to move the Coriolis
force deviates movement to the right in the Northernhemisphere and the left in the Southern hemisphere
the full strength of the current is not reached until the
wind has been blowing fairly steadily for 24 hours or
more.
after the wind has ceased or changed direction, the
current due to the original wind may persist for a time
gradually weakening.
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Gyres
The currents cross the oceans from east to west in lower latitudesand from west to east in higher latitudes.
Near continental coasts they are forced to follow the coastlines
forming East and West Boundary currents.
The dominant trend of the general circulation in the upper layers of
the oceans is in the form of anti-cyclonic or cyclonic gyres.
In most oceans there is a gyre at 30
produced by the trade windsand westerlies and a gyre at 60produced by the westerlies and
easterlies.
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the gyres are not symmetrical around the centre of the
ocean, but are displaced towards the west.
the reason: the rotation of the earth exerts a torque on the
ocean as a whole and shifts the centre of rotation
westwards; this causes a western intensification of currents
currents on the western side flow more quickly whilecurrents on the eastern side are sluggish and more diffuse.
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Gyres
Solar heating cause water to expand. Near the equator thewater is about 8 cm higher than in middle latitudes. This
causes a very slight slope and water wants to flow down the
slope.
Winds blowing on the surface of the ocean push the water.
Friction is the coupling between the wind and the water's
surface.
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Western intensification of currents
60S
0
30S
30N
60N
45N
45S
Prevailing Westerlies
Prevailing Westerlies
Trade Winds
Western intensification of currents
60S
0
30S
30N
60N
45N
45S
Prevailing Westerlies
Prevailing Westerlies
Trade Winds
Coriolis is stronger in higher latitudes, eastward flowing
waters are strongly deflected back toward the equator
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Currents of the
Atlantic Ocean
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Sargasso Sea
North Atlantic Gyre.
All the currents deposit the marine
plants and refuse they carry into thissea.
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Ekman effect
Theory based on an assumption that:
- The wind blows at a constant velocity
- The wind blows over a deep, infinite and homogenous
ocean
- Water movement not impeded by continental barriers
- Coefficient of viscosity does not depend on the depth
- Frictional coupling between adjacent layers of water is the
only means by which water beneath the surface is set in
motion.
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Surface water
Net transport
45
Surface current
45(20-45) tothe right of wind
in deep water.
Net water
transport is 90tothe right of wind
direction.
Ekman Spiral
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THEORY OF EASTERN OCEAN BOUNDARY CURRENTS
N
Wind
Surface
current
Mean Transportation
Consider wind blowing parallel to the coast in the northern
hemisphere.
The wind produces a surface current which flows at 45to the
wind direction. It also moves the water in the Ekman layer, the
mean movement of this water is at 90to the wind direction.
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Cross section
Water piles up off-shore so that the sea surface slopes
downwards towards the coast. Movement of the surface water
away from the shore leads to upwellingof cold water from
lower layers.
Mean Transportation
W E
Upwelling
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The gradient current tries to flow down the slope and is
deflected by Coriolis effect to flow south along the coast in the
northern hemisphere.
Thus, the resultant surface current is the combination of the
initial surface current and the gradient current.
N
Gradient
current
Surface
current
Mean Transportation
Down slope
Resultant surface
current
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Gradient Currents
caused by pressure gradients in the water.
they occur:- whenever the water surface develops a slope, whether under
the action of the wind,
-or through a juxtaposition of waters of differing temperaturesand/or salinity.
The initial water movement is down slope but the effect of the
earths rotation is to deflect the movement through 90 to the
right in the northern hemisphere (left in the southern
hemisphere).
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The Benguela Current (Southern Hemisphere)
Resultant WNW
current flow
Mean
Transportation
30 (Coriolis)
Movement of water away from the coast allows cold
dense water to upwell, to produce a cold current
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The Peru Current (Southern Hemisphere)
ANDES
Wind
Resultant
Gradient
current
Mean
Transportation
Downward slope
30
Winds over the
coastal watersof Peru and
northern Chile
tend to be
deflected by theAndes so that
they blow
predominantly
parallel to the
coast.
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Currents of the
Atlantic Ocean
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The California Current (Northern Hemisphere)
Direct Surface
Mean
Transportation
30
a continuation of
the AleutianCurrent
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Currents of the Pacific Ocean
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The Canary Current (Northern Hemisphere)
NE Trade Wind
Direct Surface
Mean
Transportation
30
AFRICA
Resultant
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Currents of the
Indian Ocean
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Currents of the Pacific Ocean
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Starts because the easterly trade windsweaken and allow the warm waters in theWestern Pacific to move east towardSouth America
This changes where the convectioncurrent occurs.
Causing rain where it usually doesn'toccur and drought where it usually rains
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The economic effects of this upwelling are dramatic.
The area is a major anchovy breeding ground and millions of seabirds thrive here.
Periodically upwelling fails and the warm El Nino current flows.
It occurs in December/January and hence its name of Holy
Child.
El Nino flows in response to northerly winds, surface water is
driven shorewards and there is mass mortality fish and birds.
El Nino is associated with the southward movement of the
tropical rain belt causing damaging floods.
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Colder years often follow these periods ofabnormally warm temperatures.
These cold periods have been called La Nia,or the girl.
The La Nia will cause a strengthening of thetrade winds with a decrease in precipitation onthe eastern side of the basin and acorresponding increase on the western side.
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