Transcript Ocean Currents

Ocean Currents
Why is Ocean Circulation
Important?
• Transport ~ 20% of
latitudinal heat
– Equator to poles
• Transport nutrients and
organisms
• Influences weather and
climate
• Influences commerce
Non-rotating Earth
Convection
cell model
Add rotation and add landmasses
unequal heating and cooling of the Earth
Physical properties of the
atmosphere: Density
• Warm, low density
air rises
• Cool, high density
air sinks
• Creates circularmoving loop of air
(convection cell)
Physical properties of the
atmosphere: Water vapor
• Cool air cannot hold much water
vapor, so is typically dry
• Warm air can hold more water
vapor, so is typically moist
• Water vapor decreases the density
of air
Physical properties of the
atmosphere: Pressure
90o
60o
High pressure, dry climate
Low pressure, wet climate
High pressure, dry climate
30o
0o
30o
60o
90o
ITCZ intertropical
convergence
zone= doldrums
Low pressure,
wet climate
The Coriolis effect
• The Coriolis effect
– Is a result of Earth’s rotation
– Causes moving objects to follow
curved paths:
• In Northern Hemisphere, curvature
is to right
• In Southern Hemisphere, curvature
is to left
– Changes with latitude:
• No Coriolis effect at Equator
• Maximum Coriolis effect at poles
The Coriolis effect on Earth
• As Earth rotates,
different latitudes
travel at different
speeds
• The change in
speed with
latitude causes
the Coriolis effect
North Pole
Buffalo moves 783 mph
Quito moves 1036 mph
Buffalo
15o
N
Quito
Buffalo
79oW
Quito
South Pole
A)
B)
Idealized winds generated by pressure gradient and Coriolis Force.
Actual wind patterns owing to land mass distribution..
Ocean Currents
Surface Currents
The upper 400 meters of the ocean
(10%).
Deep Water Currents
Thermal currents (90%)
Surface Currents
Forces
1. Solar Heating (temp, density)
2. Winds
3. Coriolis
Wind-driven surface currents
Wind-Driven and Density-Driven
Currents
• Wind-driven currents occur in the
uppermost 100 m or less
• Density differences causes by
salinity and temperature produce
very slow flows in deeper waters.
Sailors have know about ocean
currents for centuries
Sailors have know that “rivers” flow in the
seas since ancient times. They used
them to shorten voyages, or were
delayed by trying to stem them.
If navigators do not correct to deflection
by currents, they may be far away from
where they think they are and meet
disaster.
Ben Franklin and the Gulf Stream
Matthew Fontaine Maury
The first systematic study
of currents was done
by Maury based on
logbooks in the US
Navy’s Depot of Charts
and Instruments.
His charts and “Physical
Geography of the Sea”
assisted navigators
worldwide.
http://www.npg.si.edu/exh/brady/gallery/97gal.html
Winds and surface water
• Wind blowing over the ocean can move
it due to frictional drag.
• Waves create necessary roughness for
wind to couple with water.
• One “rule of thumb” holds that wind
blowing for 12 hrs at 100 cm per sec will
produce a 2 cm per sec current (about
2% of the wind speed)
Top-down drag
• Wind acts only on the surface water layer.
• This layer will also drag the underlying water,
but with less force.
• Consequently, there is a diminution of speed
downward.
• Direction of movement is also influenced by
the Coriolis Effect and Ekman Spiral
Ekman spiral
Ekman spiral describes
the speed and
direction of flow of
surface waters at
various depths
• Factors:
– Wind
– Coriolis effect
Ekman transport
Ekman transport is
the overall water
movement due to
Ekman spiral
• Ideal transport is
90º from the wind
• Transport direction
depends on the
hemisphere
Ekman Transport
Water flow in the Northern hemisphere- 90o to the right of the
wind direction
Depth is important
Currents in the “Real” Ocean
Currents rarely behave exactly as
predicted by these theoretical
explanations due to factors such as
• Depth—shallow water does not permit
full development of the Ekman spiral
• Density—deeper currents moving in
different directions influence the
overlying surface movement
Geostrophic Flow
Surface currents generally mirror average
planetary atmospheric circulation patterns
Current Gyres
Gyres are large circular-moving loops of water
Five main gyres (one in each ocean
basin):
• North Pacific
• South Pacific
• North Atlantic
• South Atlantic
• Indian
• Generally 4 currents in each gyre
• Centered about 30o north or south
latitude
Geostrophic flow and western
intensification
• Geostrophic flow
causes a hill to form
in subtropical gyres
• The center of the
gyre is shifted to the
west because of
Earth’s rotation
• Western boundary
currents are
intensified
Figure 7-7
Western intensification of
subtropical gyres
• The western boundary currents of all
subtropical gyres are:
– Fast
– Narrow
– Deep
• Western boundary currents are also warm
• Eastern boundary currents of subtropical gyres
have opposite characteristics
Boundary Currents in the Northern Hemisphere
Type of Current
General Features
Speed
Special Features
Western boundary Currents
Gulf Stream, Kuroshio
warm
narrow
deep
swift
sharp boundary
w/coastal circulation,
little coastal upwelling
Eastern Boundary Currents
California, Canary
cold
broad
shallow
slow
diffuse boundaries
separating from coastal
currents, coastal
upwelling common
Pacific Ocean surface currents
“Hills and Valleys” in the
Ocean
• A balance between the
Ekman transport and
Coriolis effect
produces “hills” in the
center of the gyres and
“valleys” elsewhere
• Gravitational effects
from sea floor features
also produce
variations in sea
surface topography
http://earth.usc.edu/~stott/Catalina/Oceans.html
What do Nike shoes,
rubber ducks, and
hockey gloves have to
do with currents?
Lost at Sea
Duckie Progress
•January 1992 - shipwrecked in the Pacific Ocean, off
the coast of China
•November 1992 - half had drifted north to the Bering
Sea and Alaska; the other half went south to
Indonesia and Australia
•1995 to 2000 - spent five years in the Arctic ice floes,
slowly working their way through the glaciers
2001 - the duckies bobbed over the place where the
Titanic had sunk
•2003 - they were predicted to begin washing up
onshore in New England, but only one was spotted in
Maine
•2007 - a couple duckies and frogs were found on the
beaches of Scotland and southwest England.
2004-2007
Barber’s Point
North Pacific Subtropical Gyre
• “Great Pacific Garbage Patch”
• Estimate: 46,000 pieces of floating garbage/mi2.
North Pacific Subtropical Gyre
135° to 155°W and 35° to 42°N
North Pacific Subtropical Gyre
Great Pacific Garbage Patch- Good Morning America 2010
http://www.youtube.com/watch?v=uLrVCI4N67M&feature=player_embedded
http://marinedebris.noaa.gov/info/patch.html#6
Eddy
A circular movement of water formed along the edge
of a permanent current
In an average year, 10-15 rings are formed
150-300 km in diameter
Speed 1 m/sec
Warm core ring
1. Rotates clockwise
2. Found on the landward side of the current
Cold core ring (cyclonic eddy)
1. Rotates counterclockwise
2. Forms on the ocean side of the current
Sargasso Sea
Upwelling and downwelling
Vertical movement of water ()
– Upwelling = movement of deep water to
surface
• Hoists cold, nutrient-rich water to surface
• Produces high productivities and abundant
marine life
– Downwelling = movement of surface water
down
• Moves warm, nutrient-depleted surface water
down
• Not associated with high productivities or
abundant marine life
upwelling
downwelling
Langmuir Circulation
Satellite Observations
• TOPEX/Poseidon, Jason 1, and other
satellites have observed patterns of
change over the past few years
• Animation of seasonal and climaticallyinfluence shifts available at
http://seawifs.gsfc.nasa.gov/OCEAN_PLANET/MOVIES
/Topex_Dynamic_Ocean_Topography.mpg
El Niño-Southern Oscillation
(ENSO)
• El Niño = warm surface current in
equatorial eastern Pacific that occurs
periodically around Christmastime
• Southern Oscillation = change in
atmospheric pressure over Pacific
Ocean accompanying El Niño
• ENSO describes a combined oceanicatmospheric disturbance
El Niño
• Oceanic and atmospheric
phenomenon in the Pacific Ocean
• Occurs during December
• 2 to 7 year cycle
Sea Surface Temperature
Atmospheric Winds
Upwelling
Normal conditions in the
Pacific Ocean
El Niño conditions (ENSO
warm phase)
La Niña conditions (ENSO
cool phase; opposite of El
Niño)
Non El Niño
El Niño
1997
Non El Niño
upwelling
El Niño
thermocline
El Niño events over the last 55 years
El Niño warmings (red) and La Niña coolings (blue) since
1950. Source: NOAA Climate Diagnostics Center
World Wide Effects of El Niño
• Weather patterns
• Marine Life
• Economic resources
El Nino Animation
http://esminfo.prenhall.com/science/geoanimations/ani
mations/26_NinoNina.html
Effects of severe El Niños
Surface and Deep-Sea Current
Interactions
Unifying concept: “Global Ocean Conveyor Belt”
http://seis.natsci.csulb.edu/rbehl/ConvBelt.htm
Heat Transport by Currents
• Surface currents play significant roles in
transport heat energy from equatorial
waters towards the poles
• May serve as “heat sources” to cooler
overlying air, “heat sinks” from warmer
• Evaporation and condensation
participate in latent heat exchanges
Matter Transport and
Surface Currents
• Currents also involved with gas
exchanges, especially O2 and CO2
• Nutrient exchanges important within
surface waters (including outflow from
continents) and deeper waters
(upwelling and downwelling)
• Pollution dispersal
• Impact on fisheries and other resources
Global ocean circulation that is driven by differences in
the density of the sea water which is controlled by
temperature and salinity.
White sections represent warm surface currents.
Purple sections represent deep cold currents
What effect does global
warming play in
thermohaline circulation?
http://www.youtube.com/v/MZbsMlr9WRI?version=3
1
CO2 fossil fuel
combustion
2
Atmospheric and
ocean temp
3
4
Subtropical
evaporation
High latitude
precipitation & runoff
North Atlantic
regional cooling
Deep water formation
& thermohaline
circulation
6
Global climate
interconnections
Nordic seas
salinity & deep
convection
5
Potential feedback
of increased
tropical salinity
Inquiry
1. What is a convection cell?
2. Which direction do currents get deflected in
the Southern Hemisphere?
3. What depth should the water be for an Ekman
spiral to occur?
4. How are surface currents created?
5. What is a gyre?
6. How can an El Nino impact upwelling?
7. Coriolis Effect is strongest near the _____?