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Long Waves in Ocean Circulation
• Wave response of ocean to transient
conditions such as changes in wind forcing
• Two dominant wave types
– Rossby waves - westward propagating
– Kelvin waves - travel along boundaries
• Space scales of 100’s km & time scales of
a few months to a few years
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More Long Waves
• How ocean “communicates” within a
basin
• Kelvin wave pulse is the precursor of an
ENSO event
• Hard to detect from direct field obs due
to the scales involved
• Satellite altimetry though is useful
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More Long Waves
• Can be barotropic or baroclinic in nature
Barotropic waves propagate fast (many m/s)
Baroclinic waves are slower (several cm/s)
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Kelvin Waves
• Needs boundary or “wave guide” to pile
against
– Coastal boundary
– The equator or “double” Kelvin wave
• Can be wave of depression or elevation
• How ENSO pulse is transmitted in ocean
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Kelvin Waves
Wave of
depression
Greatest onshore
Propagates CCW
in a NH basin (CW in SH)
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Kelvin Waves
• Simple geostrophic balance (HPF =
CF)
• Propagates into the page
• Wave of depression would go in
same direction
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Coastal Kelvin Waves
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Coastal Kelvin Waves
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Coastal Kelvin Waves
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Coastal Kelvin Waves
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Coastal Kelvin Waves
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Coastal Kelvin Waves
c ~ 200 km/d
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Equatorial Kelvin Waves
• Simple geostrophic balance (HPF =
CF)
• Two Kelvin waves “rest” against each
other on equator - double Kelvin
waves
• Propagates to the east
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Equatorial Kelvin Waves
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Equatorial Kelvin Waves
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Equatorial Kelvin Waves
• Satellite altimetry from
TOPEX/Poseidon
• Scenes are 10 days apart
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Kelvin Waves
• NH CCW propagation
• SH CW propagation
• Way of building time
cycles into ocean
(climate oscillators)
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Equatorial Kelvin Waves
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TOGA-TAO Array
• Equatorial array of buoys
• U.S., Japan & French partnership
• http://www.pmel.noaa.gov/tao/
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Moorings
• Measure met &
ocean properties
• Thermocline focus
• Real-time data
transmission
• Used in weather &
climate forecasts
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Propagates 13,000 km in 4 months - 1.3 m/s
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Kelvin Waves
• Needs boundary or “wave guide” to pile
against
– Coastal boundary
– Equatorial boundary or “double” Kelvin
wave
• Can be wave of depression or elevation
• Baroclinic waves travel at 1-2 m/s
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Rossby Waves
• Propagate zonally from east to west
• Results from conservation of potential
vorticity, PV = ( + f)/D
• Propagation speeds are slow (< 5 cm/s)
poleward of 30o
• Wave speeds increase dramatically
towards equator
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Rossby Waves
movie
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Rossby Waves
• Propagate zonally from east to west
• Results from PV conservation
• Propagation speeds are slow (< 5
cm/s) poleward of 30o
• Wave speeds increase dramatically
towards equator
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Interactions Among Rossby & Kelvin
Waves
• Kelvin waves can excite Rossby
waves
• Occurs in equatorial & coastal wave
guides
• Important for progression of El Nino
events
Page 34
Equatorial Kelvin & Rossby Waves
Page 35
El Niño Conditions
Page 37
Interactions Among Rossby & Kelvin
Waves
• Kelvin waves can excite Rossby
waves
• Occurs in equatorial & coastal wave
guides
• Important for progression of El Nino
events