Ocean waves Slides

El Niño Southern Oscillation
Pressure systems over Darwin Australia and Tahiti Oscillate
Typically occurs every 4-7 years
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What is it? Normal
Conditions...
What is it? During El Niño….
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Local Effects of El Niño
Capitola Hotel, 1926
Venetian Courts (Capitola), 1983
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1997-1998 El Niño
February, 1998. Venetian Courts, Capitola.
1997-1998 El Niño
February, 1998. Ocean front property….
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• Mean weaning weights for Elephant Seal pups
during 1997-98 were at an all time low
• 75% first-year mortalities for marine mammals
(seals, sea lions) was reported
• Monterey was inundated by starving marine
mammals
El Niño Impacts on Coastal California
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Monterey Bay Whale
Abundance During El Niño
Courtesy of B. Marinovic
Monterey Bay as a Biological Refuge?
Distance from sore
Distance from shore
During El Niño events (1992-1993, 1997-1998), plant
production was reduced by approximately 50%
inside Monterey Bay, and >95% outside the Bay
Source: Chavez et al. Prog. Oceanog. 2002
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The Pacific Decadal Oscillation
The Pacific Decadal Oscillation
• Oscillates between cool-wet and warm-dry
conditions
• Averages about 20-30 years
• Cool-wet PDOs make La Niñas more intense
• Warm-dry PDOs make El Niños more
intense
• The PDO seems to have shifted between
1998-1999, amplifying both the El Niño and
La Niña!
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North
NorthAtlantic
Atlantic
Oscillation
Oscillation
Arctic
North
Atlantic
Oscillation
Oscillation
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90-day (ending 4 Feb. 2007) % of average
precipitation
In any given year, the global patterns of climate anomalies
and events are a combination of random chance, short- and
long-term oscillations, and (potentially) global warming.
Because the Earth is not homogenous (not all water, for
example), it’s very difficult to tease apart the factors….
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Santa Cruz temperatures 1880-2005
Waves
• General Considerations
• Definitions
- Mathematical
- Classification
• Generating Waves
-Wind generated
- Storm surge
- Whitecaps
- Tsunamis
- Swell
- Internal Waves
• Wave-Shore Interactions
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Waves
• A wave is the transmission of
energy through a
medium…this is true for all
types of waves!
• The particles of the medium
stay in the same general area
(there are no “sound particles”)
Wave Types
• 3 Types of waves:
– Transverse (side to side)
– Longitudinal (up-down)
– Orbital (circular movement)
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For Simplicity, we will
assume that ocean waves
are Sine Waves (this isn’t
actually true!)
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• Net motion is up and down
(bobbing)
• NOT a sine wave, so a slight
forward motion as well
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Mathematical Description
Height (H) = crest to trough distance
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Height (H) = crest to trough distance
Wavelength (L) = crest to crest distance
Height (H) = crest to trough distance
Wavelength (L) = crest to crest distance
Steepness = H/L
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Period (T) = time for one wavelength
Speed (C) = L/T
Frequency (f) = 1/T, or C = L x f
Wave Classification
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Classification
• Deep-Water Waves
Water depth (d) Deeper than L>2
C = L/T, but it’s hard to measure L
C = gT / 2π, --> C = 156 x T (m/s)
Particles
move in a
circular
pattern
Classification
• Shallow-Water Waves (long waves)
Water depth (d) < 1/20 of L
C = ( g x d )1/2 --> 3.1 x (d) 1/2
Therefore, as depth shallows, wave slows
Particles move elliptically, almost horizontally
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Classification
• Transitional waves
– Length is > 2x but less than 20x depth (d)
– Properties are somewhere between deepwater and shallow-water waves
Wave Relations
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Wind-Driven Waves
• Capillary Waves --> Gravity Waves (Chop)
--> White Caps --> Swell
• Can be Deep or Shallow waves
Restoring Force: the source of energy
dispersion that destroys a wave
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Wind-Wave Formation
Waves increase energy by :
1) Wind Speed
2) Duration
3) Fetch
More
Definitions…
• Sea: local waves formed by wind
events
• Confused Sea: local irregular waves of
many periods and from many directions
• Fully developed Sea: the waves that
form when fetch, wind speed, and
duration are maximal
• Wave Train: waves that have left the
windy region and are sorted by period
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Wave Trains
• Sorted by
wavelength (and
therefore speed)…
this is wave
dispersion
• These become swell
• Individual waves
appear to be moving
faster (2x) than the
group velocity
Interference Patterns
• All waves can be combined algebraically
• They combine in 3 patterns:
– Constructive (phases match up)
– Destructive (phases are exactly out of
alignment)
– Mixed (most common…somewhere in
between)
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Rogue
Waves
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Rogue
Waves
• Can exceed 100 ft
• “ship-killers”
Wave Trains
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Storm Surge
• Not really a wave
(we can’t apply
our mathematical
descriptions to it)
• Hurricanes “pile
up” water in the
right front
quadrant
Tsunamis: “Harbor Wave”
• Caused by a seismic
disturbance
• Most common in the Pacific
• Harmless until they hit the
coast
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Internal Waves
• Caused by changes in pycnocline
• Doesn’t require much energy to get
them going
The Surf Zone…
• Primary source of energy dissipation for
swell
• When deep-water waves reach the
shore, they form breakers as H/L > 1/ 7
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Refraction, Diffraction,
Reflection
• Refraction: waves “feel bottom” and will turn
(or get dragged) towards headlands
Refraction, Diffraction,
Reflection
• Diffraction: waves
can move around
obstacles because
energy is propagated
in all directions
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Refraction, Diffraction,
Reflection
Reflection: two waves of same wavelength, but
moving in opposite directions, interact to form a
standing wave. There is NO circular motion!
Seiches (Standing Waves)
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Second Node Standing Waves
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