Transcript No Slide Title

Introduction to
Oceanography
Dynamic Oceanography:
Currents and Water masses
6-2
Surface Ocean Currents
Geostrophic flow allows currents to flow long
distances with no apparent Coriolis deflection.
• Coriolis deflects water into the center of the gyres, forming a
low mound.
• As height of the mound increases, the pressure gradient
steepens pushing the water outward in an attempt to level the
mound.
• When the pressure gradient equals coriolis deflection, the
current flows parallel to the wind around the mound as a
geostrophic current and this is called geostrophic flow.
• Gyres in the northern hemisphere rotate clockwise and in the
southern hemispheres counterclockwise.
6-2
Surface Ocean Currents
• The current flow pattern in gyres is asymmetrical with narrow,
deep and swift currents along the basin’s western edge and
broad, shallow slower currents along the basin’s eastern edge.
• The geostrophic mound is deflected to the western part of the
ocean basin because of the eastward rotation of the Earth on
its axis.
• The Sargasso Sea is a large lens of warm water encircled by the
North Atlantic gyre and separated from cold waters below and
laterally by a strong thermocline.
• Western boundary currents, such as the Gulf Stream, form a
meandering boundary separating coastal waters from warmer
waters in the gyre’s center.
Currents Within Gyres
Water flow in the Gulf Stream and the Canary Current, parts of
the North Atlantic gyre.
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The Gulf Stream
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Geostrophic Gyres
Geostrophic gyres are gyres in balance between the pressure
gradient and the Coriolis effect. Of the six great currents in the world’s
ocean, five are geostrophic gyres.
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Currents Within Gyres
What currents are found within gyres?
Western boundary currents -These are narrow, deep, fast currents
found at the western boundaries of ocean basins.
 the Gulf Stream
 the Japan Current
 the Brazil Current
 the Agulhas Current
 the Eastern Australian Current
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Currents Within Gyres
What currents are found within gyres?
Eastern boundary currents –These currents are cold, shallow
and broad, and their boundaries are not well defined.
 the Canary Current
 the Benguela Current
 the California Current
 the West Australian Current
 the Peru Current
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Currents Within Gyres
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Countercurrents and Undercurrents
Countercurrents flow on the surface in the opposite direction from the
main currents.
Undercurrents are countercurrents that flow beneath the surface current.
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Upwelling And Downwelling
Wind induced vertical circulation is vertical movement induced by winddriven horizontal movement of water.
Upwelling is the upward motion of water. This motion brings cold, nutrient
rich water towards the surface.
Downwelling is downward motion of water. It supplies the deeper ocean
with dissolved gases.
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6-2
Surface Ocean Currents
Langmuir circulation is a complex horizontal helical
(spiral) motion that extends parallel to the wind.
• Adjacent helices rotate in opposite directions creating
alternating zones of convergence and divergence.
• Material floating on the surface becomes concentrated in the
zones of convergence and form sea stripes which parallel the
wind direction.
Effects of Surface Currents on Climate
Surface currents distribute tropical heat. The warm ocean
currents are shown in red and cold currents in blue.
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El Niño
An El Niño Year
A Non-El Niño Year
In an El Niño year, the trade winds diminish and reverse. Storms
over land may increase in these years.
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6-3
Deep-Ocean Circulation
Thermohaline circulation is a density driven flow of
water generated by differences in salinity or
temperature.
• Water at the surface is exposed to more rapid changes in salinity
through evaporation or precipitation and in temperature through
cooling or heating.
• Once water is isolated from the atmospheric influences, salinity and
temperature are largely set for an extended period of time.
• Based upon depth, surface water masses can be broadly classified as
Central waters (from 0 to 1 km), Intermediate waters (from 1 to 2
km), and Deep and bottom waters (greater than 2 km).
• Most deep and bottom water originated at the surface where cooling
and increased salinity raised their density until they sank.
• Ocean basins interconnect and exchange water with each other and
with the surface. Inter-ocean basin circulation and exchange between
surface and deep water appears largely driven by waters of the North
Atlantic.
The Temperature-Salinity Diagram
A general temperature-salinity (T-S) diagram.
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6-3
Thermocline Circulation
The movement of water due to different densities is thermohaline
circulation. Remember that the ocean is density stratified, with the
densest water at the bottom. There are five common water masses:
 Surface water
 Central water
 Intermediate water
 Deep water
 Bottom water
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.
Water Masses:
AABW
AADW
AAIW
CoW
ECW
MIW
NABW
NACW
NADW
NPCW
NPIW
PSW
RSIW
SACW
SICW
SPCW
Antarctic Bottom Water
Antarctic Deep Water
Antarctic Intermediate Water
Common Water
Equatorial Central Water
Mediterranean Intermediate Water
North Atlantic Bottom Water
North Atlantic Central Water
North Atlantic Deep Water
North Pacific Central Water
North Pacific Intermediate Water
Pacific Subarctic Water
Red Sea Intermediate Water
South Atlantic Central Water
South Indian Central Water
South Pacific Central Water
Underwater waterfalls!
= height of Angel Falls, Venezuela
5x106 m3.sec-1 of water flow over the Denmark Strait
(cf. Guairá Falls, 0.013x106 m3.sec-1)
6-3
Deep-Ocean Circulation
The major thermohaline currents appear to flow
mainly equatorward, but this is because they
originate in the polar regions and their outward flow
is confined between the continents.
• Warmer water (>10oC) is confined between 45o north and
south latitude.
• Poleward of 45o, density of water increases because of
declining temperature and increased salinity because of
evaporation or ice formation.
• The water sinks to a density-appropriate level and then
slowly flows outward in all directions across the basin until
they are blocked by a continent.
6-3
Deep-Ocean Circulation
• Deep water gradually mixes with other water masses and
eventually rises to the surface.
• The Atlantic Ocean has the most complex ocean
stratification containing the following layers: Antarctic
Bottom Water, Antarctic Deep Water, North Atlantic Deep
Water, Arctic Intermediate Water, and Mediterranean
Intermediate Water
• The Pacific Ocean has a less complex stratification, is
weakly layered, displays sluggish circulation and is
remarkably uniform below 2000m.
• The Indian Ocean has the simplest stratification consisting
of Common Water, Antarctic Intermediate Water, and Red
Sea Intermediate Water.
Thermohaline Circulation Patterns
A model of thermocline circulation caused by heating in lower latitudes
and cooling in higher latitudes.
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6-4
Water Flow in Semi-enclosed
Seaways
Most seas are indentations into continents, partially
isolated from the ocean and strongly influenced by
continental climate and river drainage.
• As Atlantic Ocean water flows through the Straits of Gibraltar
into the Mediterranean Sea at the surface, warm, highly saline
Mediterranean Sea water flows out through the Straits at the
bottom.
• In the Black Sea the surface water is brackish because of
excess precipitation and river inflow.
Studying Currents
How can scientists study currents?
Float methods depend on the movement of a free-floating object.
Flow methods measure the current as it flows past a fixed object
CODAR is a groundwave radar detection system that fixes on small
wave movements. The Gulf of Maine is one of the first experimental
sites for a large scale test of CODAR
© 2002 Brooks/Cole, a division of Thomson Learning, Inc.