17 Neap Tides Neap tides are especially weak tides. They occur when the gravitational forces of the Moon and the Sun are perpendicular to one another (with respect to the Earth). Neap tides occur during quarter moons.
19 Tide levels in the port of Brest (France) in Dec 2000. The illustration shows the alternating high and low tides and, over a month, the variations in amplitude due to the respective positions of the Moon and Sun.
27 Origin of waves Most waves wind-driven Moving energy along ocean/air interface –Wind main disturbing force –Boundary between and within fluids with different densities Air/ocean interface (ocean waves) Air/air interface (atmospheric waves) Water/water interface (internal waves)
28 Wind Generation of Waves The type of wave generated by wind is determined by: –Wind velocity (how fast) –Wind duration (how long) –Fetch (distance over which wind blows) Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.
29 Internal waves Fig. 8.1a Associated with pycnocline Associated with pycnocline Larger than surface waves Larger than surface waves Caused by tides, turbidity currents, winds, ships Caused by tides, turbidity currents, winds, ships Possible hazard for submarines Possible hazard for submarines
30 Internal waves form within the water column on the pycnocline.
37 Orbital waves Wave characteristics –Wave steepness = H/L If wave steepness > 1/7, wave breaks –Wave period (T) = time for one wavelength to pass fixed point –Wave frequency = inverse of period or 1/T
38 Circular orbital motion Water particles move in circle Movement up and down and Back and forth Fig. 8.4
39 Orbital motion Diameter of orbital motion decreases with depth of water Wave base = ½ L Hardly any motion below wave base due to wave activity Fig. 8.5