Currents and El Nino

8/11/2019 Currents and El Nino

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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


Trade Winds

Western intensification of currents

60S

0

30S

30N

60N

45N

45S



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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Currents and El Nino