Biological Oceanography Lecture   Waves and Tides

Wind-generated waves

Wind cannot only generate currents, but also waves.

Capillary waves form as wind friction stretches the surface.  Wind deflected upward creates a partial vacuum and atmospheric pressure push down on the wave from behind.

In deep water the circular motion of waves is virtually friction-free.  What does this mean for the distance that swells can travel? Continuation of wind energy can result in the formation of seas.  Mature waves sort themselves into  smooth undulations called swells.

Growth of wind generated waves is determined by:
• wind duration
• wind speed
• fetch

Is the forward speed of a wave crest equal to the forward speed of the mass of water contained in that wave (i.e. does wave speed = current speed)?  What is a 'wave' in physics?

Path of water in wind-generated waves:
.   .

Waves are sustained because restoring forces (gravity in wind-generated waves) continue past the water level of calm seas resulting in oscillation.

The speed of waves, wavelength,  period and height are inter-related (e.g. for deep-water waves, speed = gT/2Ï€).
Are longer wavelength waves faster?

Water in waves maintain a circular path only when water depth from surface to bottom is greater than one-half the wave length (deep-water waves).

A wave at any given location in the ocean is likely to be the result of several independent wave trains arriving at that point at that time.  Interaction of wave trains can lead to destructive and constructive interferenceHow can rogue waves be explained? (more pics)

Other types of waves
Waves not generated by wind include tides, internal waves, storm surges, seiches, and tsunamis

Tides

The Equilbrium Theory of Tides

What is the main force generating tides?

How many high tides would this generate per day?

Why along the Georgia coast are there two low and two high tides (diurnal), evenly space over ~24 hours?

Why are high (and low) tide ~50 minutes later every day?

What pattern in tidal range becomes apparent when examining tidal ranges over 28 days?

Why does tidal range change at this frequency and not just over 28 days? Hint: what other astronomical body influences gravitational and centrifugal forces?

If the moon is smaller than the sun, why does it have a greater effect on tidal bulging than the sun?

Does it matter in terms of tidal strength whether the moon and sun are on opposite sides of the Earth or on the same side?

http://co-ops.nos.noaa.gov/restles3.html

So when is tide the highest, spring tide or neap tide?  When is tide the lowest?

The Dynamic Theory of Tides and other complicating factors

Can you tell when it is high tide just by looking to see if the moon is overhead?  (Hint: recall from lab the times at which high tide occurred along the East Coast; where they synchronous?).

Waves are not effected by friction with the ocean bottom when the ocean depth is deep relative to the wavelength.  What is the wave length of the tidal bulges created by the moon?

What else effects with the tidal bulge as it travels around the Earth?

As the moon passes over a continent, what happens to the water that has 'piled' up on the eastern side of the contintent

The point of  equilibrium (the node) of the slosh is toward the center of the basin.  Movement of the tidal crest creates a sloshing in the basin, and the Coriolis effect spins the water about the basin.  This creates an amphidromic point at its center, where the tidal range is zero.

Does water slosh at the same rate in containers of different sizes?
In smaller basins, the natural sloshing of the basin can be the same as the tidal cycle.  What will tidal range be like in this case? (i.e. what happens when the peaks of two different wave trains coincide?)  What will happen in a basin the frequency of sloshing is twice that of the tidal frequency?
Interference also results as a result of coastal and bottom morphology.

In the funnel-shaped Bay of Fundy the tidal range is a 2-meters at the mouth of the bay and  11-meters at the head of the bay.
Tide bores can form as crests move up rivers and estuaries.

Tidal prediction is further complicated because the moon's orbit around the Earth is elliptical and the Earth's axis of rotation is tilted with respect to the moon's orbit (and similarly the Earth's orbit about the sun).

What factor cannot be accounted for in predictions made in tide tables?