Ch.9-12
Science of Forecasting Waves
GNM 1136
Chapter 9 – Surfing in the Storm
Challenge
• When does NJ typically get the most
windswell events?
• How about groundswell events?
• How can you tell the difference between the
two types of swell?
Buoy Models
• Buoyweather.com
• Weatherunderground.com
SMB Theory
• H. Sverdrup, W. Munk, C. Bretschneider
developed the SMB Model in the Mid 1940s.
• Calculates wave height from values of wind
strength, fetch and duration.
• Currently not in practice, but used
academically.
Life of a Windswell
• Gutless Upper Trestles, CA
• Windswells are swells that are generated by local winds
within a few hundred miles of the coast. As a result, the
swell periods are short (four to 10 seconds between crests)
and the waves tend to stack up on each other with very
consistent sets.
• Also, a windswell's energy doesn't extend very deep -maybe only 100 feet or so -- so it can only travel a few
hundred miles before dissipating because most of the wave
energy is concentrated closer to the ocean surface.
• Swellinfo.com
Groundswell
• Groundswells are swells that are usually generated by winds much
farther away. These swells are created by strong winds over long
distances -- we refer to this distance as the "fetch" -- and over
longer periods of time. As a result, more energy is transferred into
the water during the generation of the waves, which create longer
swell periods. Waves with longer swell periods can travel great
distances without losing the swell energy like windswells do.
• Furthermore, swell energy in groundswells can also extend down to
around 1,000 feet deep. This allows greater interaction with the
ocean floor when the groundswells move into shallow water. For
example, this process of refraction will allow groundswells to wrap
into a spot -- up to 180 degrees or more -- while windswells wrap
very little if at all.
Windswells of the Mediterranean
• http://www.metacafe.com/watch/fu1556072136/on_surfari_italy/
• http://www.youtube.com/watch?v=7I95V7Df
9CY&feature=related
What happens when the wind stops?
• Fig. 9.3
• Gradual drop in size after storm dissipates.
Selective Refraction
• Different wavelengths are refracted to
different degrees.
• Longer wavelengths bend more than shorter
ones.
• What type of swell is better for flat
bathymetric beaches? Peaky or long?
Chapter 10 - Local Winds on the Coast
Challenge
• What time of day does the Sea Breeze occur
along our coast in the summer? Why?
• How does the Sea Breeze affect surfing
conditions?
Local Winds
• How to read a weather map – NOAA
• Surface Observations
• Sailflow.com
Sea breeze Info
• Rutgers Data – Sea Breeze & Coastal Upwelling
• Rutgers COOL Room
The “Sea breeze”
Opposing sea breezes meeting over
Cuba forming a line of cumulus clouds.
Can this happen at night?
Ch.11 Water Temperatures
Challenge
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What controls the ocean water temperatures?
What major currents control the ocean temperature off NJ?
What is the range of ocean water temp in NJ?
What causes coastal upwelling?
Ocean Currents
What patterns do you notice?
Explain what happens to the water off
of NJ.
• Labrador Current
• Gulf Stream
• NOAA – Water Temps
Layers of the Ocean
Thermocline
Coastal Upwelling
Coastal Upwelling
• Biogeochemical impact of summertime
coastal upwelling on the New Jersey Shelf
• http://www.coolclassroom.org/cool_projects/
tutorials/upwellingtutorial.html
Is there Upwelling?
Storm Surge
• NWS – Storm Surge
ENSO
• El Nino Southern Oscillation (ENSO)
Ch. 12 - Tides
Challenge
• What controls the tides?
• How important are tides when surfing?
• Are the tides the same all over the world?
Explain.
Tides
• http://www.srh.noaa.gov/jetstream/ocean/tid
es.htm
• NJ Tide Chart
• Nor’East Tides
• SaltwaterTides.com
Tide Apps for Smartphones
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Tide Graph
Shralp Tide
Mobile Geographics
tideApp
TIDES AND THE FORCES THAT GENERATE
THEM
• Tides are periodic, short-term (6 hour)
changes in the height of the ocean surface
caused by the gravitational force of the moon
and sun, and the motion of the earth
The Moon and Tractive Forces
• A balance between gravitational forces pulling
earth/moon together and inertial forces that
hold them apart
– Inertia is sometimes referred to as centrifugal
force
– Earth-moon system rotate around the earth-moon
center of mass
– Earth is 81 times more massive than moon
The Moon and Tractive Forces
• High tides are the crests of really large 10,000 km wavelength
stationary waves
– Low tides are the trough of really large 10,000 km wavelength
stationary waves
– The earth rotates beneath the wave which causes apparent motion
– Lunar tides complete their cycle in a tidal day (24 hours 50 minutes)
– Moon rises 50 minutes later each day
– Moon doesn't orbit the earth in the plane of the equator
– Causes bulges (tides) to follow the actual position of the moon into
northern and southern hemisphere
The Sun's Role
• Sun is 27 million times more massive than
moon
• Sun is 387 times further way from earth than
moon
• Net effect is that the sun's influence on tides is
a bit over 1/3 that of the moon
• This is called solar tides
Sun and Moon Together
• Spring tides (very high tides) are
when the alignment is either
moon-earth-sun (full moon) or
earth-moon-sun (new moon)
• Neap tides (hardly disturbed
tides) are when there is a 90°
angle between the moon-earthsun system
– Tides are pulled in four
directions
Tidal Patterns, Amphidromic Points
• Ocean depth isn't the only effect; continents also get
in the way of tides
• Tides are somewhat similar to seiches in that water
sloshes around in ocean basins
• Semidiurnal tides are twice a lunar day tides
• Diurnal tides are once a day tides
• Mixed tides if there is significantly different heights
between tides
Tidal Datum
• Tidal datum is the reference point that tidal
height is compared
– Zero point on tide graphs
• Not always mean sea level
– Average ocean surface over several years time
Tides in Confined Basins
• Tidal range is the difference between highwater and low water height
– Largest tidal ranges are in largest oceans
– Especially in bays or inlets that concentrate tidal
energy
Tidal Currents
• Tidal currents form when water flow into or
out of bays and harbors
– Morro Bay
– Golden Gate can reach 3 m/sec or 7 mph
• Flood Tide is a rise in sea level as a tide crest
approaches
• Slack Tide is when there is no current
• EBB Tide is when water is exiting the water
body (EBB = Exiting Barnegat Bay)
Predicting Tides
• About 140 tide-generating and tide-altering
forces and factors
• Makes predicting tides extremely difficult
• Most tide chars rely on past records and don't
really predict tides
Datum Plane
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MLLW most typical
MLW
MSL
MHW
MHHW
Tides Around the World
• Semidiurnal Tides
• Semidiurnal Mixed
• Diurnal
• Predicting the Tides!
Why are there different tidal patterns?
Extreme tidal fluctuations
• The highest tides on
planet Earth occur near
Wolfville, in Nova
Scotia's Minas Basin.
The water level at high
tide can be as much as
16 metres (45 feet)
higher than at low tide!
Tidal Bore
• A tidal bore (or just bore, or
eagre) is a tidal
phenomenon in which the
leading edge of the
incoming tide forms a wave
(or waves) of water that
travel up a river or narrow
bay against the direction of
the current. As such, it is a
true tidal wave (not to be
confused with a tsunami
Rogue Waves
• NWS – Rogue Waves