chapter 6 Flashcards
current:
Water in motion
set
the direction toward which water in motion moves
drift
the speed of water in motion
The impact of ocean currents became less on modern shipping speeds, due to
continuous adjustment of course through electronic navigation.
a knowledge of ocean currents can be used in
cruise planning to reduce transit times, and ocean current models are an integral part of ship routing systems.
Currents may be classified according to their
forcing mechanism as either
- wind driven (drift currents)or
- thermohaline (gradient currents).
or according to their depth as
- surface
- intermediate
- deep
- bottom
The surface circulation of the world’s oceans is
mostly wind driven
Thermohaline currents are driven by
differences in heat and salt
Thermohaline currents are driven by differences in heat and salt and are associated with
the sinking of dense water at high latitudes
Thermohaline currents are driven by differences in heat and salt and are associated with the sinking of dense water at high latitudes; the currents driven by thermohaline forces are typically
subsurface
The primary generating forces are
wind and differences in water density caused by variations in heat and salinity
The primary generating forces are wind and differences in water density caused by
variations in heat and salinity.
Currents generated by these forces are modified by
such factors as
- depth of water,
- underwater topography including
- shape of the basin in which the current is running,
- extent and location of land, and
- deflection by the rotation of the Earth.
The wind is the primary agent which causes
the currents
the more consistent the wind the
steadier the current
how are ocean surface currents produced?
If winds blow constantly from the same direction on the ocean’s surface for long durations
Currents range in size from ……………… to ……………………
relatively small longshore currents near a beach, to currents that span ocean basins
Prevailing winds are examples of
prolonged winds that produce large‐scale ocean basin currents.
depth of surface currents
50 to 100 meters deep
Winds are described by
the direction from which they blow
water currents are described by
the direction toward which they flow
As a constant wind blows over the sea, it tends to
move the surface water along with it due to frictional drag.
theCoriolis force, which is due to
the Earth’s rotation, deflects this flow
define ekman spiral
the result of drag created from increasing ocean depth, when ocean water encounters wind at the surface
how is the flow deflected
- Near the surface the water is deflected at a 20 – 45 angle to the right in the NHS and this angle increases with depth.
- • Each successively deeper layer of water deflects further to the right of the wind in the Northern Hemisphere.
- • As depth increases the speed of each layer also decreases.
- • As current moves down the water column, some water flows in a direction opposite to the surface current
Averaging the movement of all of the layers of water affected by the Ekman spiral, water in a wind‐driven current moves about
90° to the right of the wind in the Northern Hemisphere
Ekman transport
This water movement (average deflection) in surface currents
if the wind blows from the south to the north, the current flows
90˚ to the right—directly east.
The actual depth of water affected by the wind depends upon
- the strength of the wind and
- the latitude.
what disrupt the Ekman spiral?
In the open ocean, turbulent mixing of surface water or surface waves often disrupt the Ekman spiral
ekman spiral stops working at
150 to 300m depth in deep water
gyres:
- Ocean surface currents tend to form ring‐like circulation systems
- circular ocean current formed by a combination of the prevailing winds, the rotation of the Earth, and landmasses
Continents interfere with the movement of
- surface winds
- currents
Gyres form in
both the northern and southern hemispheres
Gyres circulate ……………………. in north hemisphere and ……………………….. in the southern hemisphere
- clockwise in the northern Hemisphere and
- counter‐clockwise in the Southern Hemisphere.
When discussing currents in the open ocean, we measure the magnitude of currents in units of
sverdrups (Sv)
Each sverdrup is
one million cubic meters per second
The magnitude of the major ocean currents range from
a few sverdrup to over 100 sverdrup.
the largest river in the world, the Amazon, has a flow of
approximately 0.2 sverdrup
The Mississippi River has a flow of
approximately 0.014 sverdrup
Ocean surface currents organize into
Gyres that are characterized by circulation at the scale of the ocean basin.
The world’s open‐ocean currents are dominated by
- five subtropical gyres.
These include
- gyres in the north and south Pacific,
- northand south Atlantic, and
- Indian Oceans.
each subtropical gyre comprises
- a westward‐flowing equatorial current,
- poleward‐flowing western boundary current,
- eastward‐flowing midlatitude current, and
- an eastern boundary current that returns to the equatorial current.
Equatorial currents are found in
every ocean basin
Equatorial currents are found in every ocean basin. They form the
low‐latitude leg of the subtropical gyres
Equatorial currents are found in every ocean basin. They form the low‐latitude leg of the subtropical gyres and are driven to
the west by trade winds.
This simple view of equatorial currents is complicated by the presence of
eastward‐flowing equatorial countercurrents that split the westward‐flowing equatorial currents into northern and southern branches.
When equatorial currents encounter land on the west side of an ocean basin
warm water piles up, leading to a sea surface slope back down to the east.
what results in equatorial counter currents
Along the equator, between the northern and southern trades, winds become very light. Because there is no wind, the sea surface slope drives some water back to the east
Western boundary currents constitute the
most familiar ocean currents
Western boundary currents constitute themost familiar ocean currents. Because
they originate at low latitudes, they bring warmth to higher latitudes.
Western boundary currents constitute themost familiar ocean currents. Because they originate at low latitudes, they bring warmth to higher latitudes. For example,
heat from the Gulf Stream keeps northern Europe habitable.
Western boundary currents are among the
strongest currents
Western boundary currents are among the strongest currents. For example,
the Gulf Stream flows at approximately 26 Sv, second only to the West Wind Drift.
Western Boundary Currents are (charactaristics)
swift, narrow and deep relative to Eastern Boundary Currents, which are slower, broader and shallower than WBC’s.
Western Boundary Currents are swift, narrow and deep relative to Eastern Boundary Currents, which are slower, broader and shallower than WBC’s.
Western Boundary Currents tend to
carry heat from the equator poleward.
The high speed and large volume of western boundary currents results from
western intensification process
western intensification.:
- In response to the spin of the earth and friction along the coast, the center of the subtropical gyres is displaced westward.
- • In essence, water on the west side of a gyre must pass through a narrower, better‐defined channel. Meanwhile, return flow on the east side of the gyre is less constrained and much more diffuse.
western boundary currents examples:
- the gulf stream
- Kuroship current off japan
- the Agulhas current off southeast africa
- Brazil current
To the north and south of the equator, there are two
westward moving currents,
i.e., the North and the South Equatorial Currents
To the north and south of the equator, there are two westward moving currents, i.e., the North and the South Equatorial Currents. Between these two, there is the
counter equatorial current which moves from west to east
The South Equatorial Current bifurcates into
two branches near the Cape De Sao Roque in Brazil and its northern branch joins the North Equatorial Current
The South Equatorial Current bifurcates into two branches near the Cape De Sao
Roque in Brazil and its northern branch joins the North Equatorial Current.
a part
of this combined current enters the Caribbean sea and the Gulf of Mexico, while the remaining current passes along the eastern side of the West Indies as theAntilles Current.
A part of this combined current enters the Caribbean sea and the Gulf of Mexico,
while the remaining current passes along the eastern side of the West Indies as
theAntilles Current.
The part of the current which
enters the Gulf of Mexico comes out from the Florida Straight and joins the Antilles current.
The part of the current which enters the Gulf of Mexico comes out from the
Florida Straight and joins the Antilles current.
This combined current
moves along the south‐eastern coast of the U.S.A and is known as theFlorida Currentup to Cape of Hatteras.
This combined current moves along the south‐eastern coast of the U.S.A and is
known as theFlorida Currentup to Cape of Hatteras.
Beyond ………………… it is known as …..
Cape of Hatteras, it is known as theGulf Stream
Labrador Current
A cold current from the Arctic Ocean calledLabrador Current, which flows along the eastern coast of Canada, meets the warm Gulf Stream near the north‐east corner of U.S.A.
A cold current from the Arctic Ocean calledLabrador Current, which flows along
the eastern coast of Canada, meets the warm Gulf Stream near the north‐east
corner of U.S.A.
The confluence of these two currents,
one cold and the other warm, produce fog around the region and makes it the most important fishing ground in the world
The confluence of these two currents, one cold and the other warm, produce fog around the region and makes it the most important fishing ground in the world.
The ………………….. then ……………..
Gulf Stream then deflected eastward under the combined influence of the westerlies and the rotation of the earth.
The Gulf Stream then deflected eastward under the combined influence of the westerlies and the rotation of the earth. It then
crosses the Atlantic Ocean as the warm North Atlantic Drift.
The North Atlantic Drift bifurcates into
two branches on reaching the eastern part of the ocean.
The North Atlantic Drift bifurcates into two branches on reaching the eastern part
of the ocean.:
ThenorthernbranchcontinuesasNorth Atlantic Drift; reaches the British Isles from where it flows along the coast of Norway as the warm Norwegian Currentand enters the Arctic Ocean.
• The southern branch flows between Spain and Azores Island as the cold Canaries Current
The North Atlantic Drift bifurcates into two branches on reaching the eastern part of the ocean.
• ThenorthernbranchcontinuesasNorth Atlantic Drift; reaches the British Isles from where it flows along the coast of Norway as the warm Norwegian Currentand enters the Arctic Ocean.
• The southern branch flows between Spain and Azores Island as the cold Canaries Current.
The
Canaries Current finally joins the NE Current and completes the circuit.
The South Equatorial Current turns
south and flows along the eastern coast of South America as Brazil Current.
The South Equatorial Current turns south and flows along the eastern coast of South America as Brazil Current
at about …………………………, due to ……………………………………….. the current moves ……………..
At about 35 south latitude, due to the influence of westerlies and the rotation of the earth, the current moves eastward.
Falkland Current
A cold current called as theFalkland Currentwhich flows along the south‐eastern coast of South America from south to north joins with the current at this time
South Atlantic Current.
The Brazil Current moves eastward and crosses the Atlantic Ocean asSouth Atlantic Current.
The Brazil Current moves eastward and crosses the Atlantic Ocean as South Atlantic Current
………………………………………………. with the South Atlantic Current while crossing the Atlantic.
A part of the west wind drift or the Antarctic Circumpolar Current merges
A part of the west wind drift or the Antarctic Circumpolar Current merges with the South Atlantic Current while crossing the Atlantic.
near ……………………………. the ………………………………. is diverted ………………………… as …………..
Near the Cape of Good Hope, the South Atlantic Current is diverted northward as the Cold Benguela Current.
Near the Cape of Good Hope, the South Atlantic Current is diverted northward as the ColdBenguela Current.
Benguela Current finally
joins with theSouth Equatorial Current and completes the circuit
Pacific Ocean: Northern Hemisphere
The
North Equatorial Current turns northward and flows along the Philippines Islands, Taiwan, and Japan to from the warm Kuro Shiocurrent
The North Equatorial Current turns northward and flows along the Philippines Islands, Taiwan, and Japan to from the warmKuro Shiocurrent.
Later,
a cold current calledOya Shio,which flows along the eastern coast of the Kamchatka Peninsula merges with the Kuro Shio Current (Okhotsk Current is a cold current which merges with the Oya Shio before it confluence with Kuro Shio).
Later, a cold current calledOya Shio,which flows along the eastern coast of the Kamchatka Peninsula merges with the Kuro Shio Current (Okhotsk Current is a cold current which merges with the Oya Shio before it confluence with Kuro Shio).
From
south‐east coast of Japan, the Kuro Shio current comes under the influence of westerlies and flow right across the ocean as theNorth Pacific Current.
From south‐east coast of Japan, the Kuro Shio current comes under the influence of westerlies and flow right across the ocean as theNorth Pacific Current.
After reaching
the west coast of North America, it bifurcates into two branches: the northern branch flows anti‐clockwise along the coast of Alaska as warm Alaska Currentand the southern branch moves southward along the coast of California as the coldCalifornia Current.
After reaching the west coast of North America, it bifurcates into two branches: the northern branch flows anti‐clockwise along the coast of Alaska as warm Alaska Currentand the southern branch moves southward along the coast of California as the coldCalifornia Current.
California Current eventually
joins with the North Equatorial Current and completes the circuit
pacific ocean: southern hemisphere
in the
South Pacific Ocean, the South Equatorial Current flows towards the west and turns southward as the East Australian Curren
In the South Pacific Ocean, the South Equatorial Current flows towards the west and turns southward as theEast Australian Current.
From
Tasmania, it flows as the coldSouth Pacific Currentfrom west to east and crosses the Pacific Ocean along with the West Wind Drift.
In the South Pacific Ocean, the South Equatorial Current flows towards the west and turns southward as theEast Australian Current.
From Tasmania, it flows as the coldSouth Pacific Currentfrom west to east and crosses the Pacific Ocean along with the West Wind Drift.
On
reaching the south‐western coast of South America, it turns northward and flows as the cold Peru Current or Humboldt Current.
On reaching the south‐western coast of South America, it turns northward and flows as the cold Peru Current or Humboldt Current.
The
cold waters of the Peru Current are partially responsible for making the coast of the northern Chile and western Peru with very scanty rainfall
The cold waters of the Peru Current are partially responsible for making the coast of the northern Chile and western Peru with very scanty rainfall.
Peru Current eventually
joins with the South Equatorial Current and completes the circuit
The pattern of circulation of ocean currents in the Indian Ocean
differs from the general pattern of circulation in the Atlantic and the Pacific Oceans.
The pattern of circulation of ocean currents in the Indian Ocean differs from the general pattern of circulation in the Atlantic and the Pacific Oceans. This is because
the Indian Ocean is blocked by the continental masses in the north.
The general pattern of circulation in the southern hemisphere of the Indian Ocean is
anti‐clockwise as that of the other oceans
The general pattern of circulation in the southern hemisphere of the Indian Ocean is anti‐clockwise as that of the other oceans.
In the northern hemisphere
there is a clear reversal of currents in the winter and summer seasons, which are completely under the influence of the seasonal changes of monsoon winds
indian ocean: northern hemisphere during winter
during
During winter, Sri Lanka divides the currents of the Arabian Sea from those of the
Bay of Bengal.
TheNorth East Monsoon Driftflows
westward just south of Sri Lanka with a counter current flow between it and the South Equatorial Current.
TheNorth East Monsoon Drift flows westward just south of Sri Lanka with a counter current flow between it and the South Equatorial Current.
During the winter season
in the northern section, the Bay of Bengal and the Arabian Sea are under the influence of North East Monsoon Winds.
During the winter season, in the northern section, the Bay of Bengal and the Arabian Sea are under the influence of North East Monsoon Winds
These North East Monsoon winds drive
the waters of the Bay of Bengal and the Arabian Sea westward to circulate in an anti‐clockwise direction.
indian ocean: northern hemisphere during summer
In summer, the northern part comes under the influence of the South West Monsoon.
indian ocean: northern hemisphere during summer
in summer, the northern part comes under the influence of the south west moonsoon
it resilts in
an easterly movement of water in the Bay of Bengal and the Arabian Sea in a clockwise direction. This current is called asSouth West Monsoon Drift
In the Indian Ocean, the summer currents are
more regular than those of the winter
indian ocean: southern hemisphere:
In the southern part, the South Equatorial Current which flows from east to west is strengthened by its corresponding current of the Pacific Ocean.
In the southern part, the South Equatorial Current which flows from east to west is strengthened by its corresponding current of the Pacific Ocean.
It then
turns southward along the coast of Mozambique in Africa
indian ocean: southern hemisphere
In the southern part, the South Equatorial Current which flows from east to west is strengthened by its corresponding current of the Pacific Ocean.
It then turns southward along the coast of Mozambique in Africa.
A part of this current moving in between the African mainland and the Mozambique is called as the warm Mozambique Current
It then turns southward along the coast of Mozambique in Africa. Indian Ocean: Southern Hemisphere
A part of this current moving in between the African mainland and the Mozambique is called as the warmMozambique Current.
After the confluence of these two parts, the current is called as
Agulhas Current
After the confluence of these two parts, the current is called asAgulhas Current.
Agulhas Current
merges with the West Wind Drift when it crosses the Indian Ocean
After the confluence of these two parts, the current is called asAgulhas Current. Agulhas Current merges with the West Wind Drift when it crosses the Indian Ocean.
A branch of this merged current flows along the western coast of the Australia as cold West Australian Current.
A branch of this merged current flows along the western coast of the Australia as cold West Australian Current.
It later
joins with the South Equatorial Current to complete the circuit
general surface circulation
subtropical gyres
equatorial currents
western boundary currents
major ocean currents
- atlantic ocean
- northern hemisphere
- southern hemisphere
- pacific ocean
- NH
- SH
- indian ocea
- NH during winter
- SH during summer
