Chem 1103: Lecture 8: Tides: Chap. 8

 Why are tides important o Mixing in the ocean o Transport sediments o Erosion o Navigation  Tides are waves o Caused by gravitational pull of Moon and Sun o Have frequencies related to rotation of Earth o Tide tables  Tidal Characteristics o Tides have a wave form but differ from other waves because they caused by gravitational attraction between the ocean and the Sun and Moon . Crest of the wave form is high tide and trough is low tide . The vertical difference between high tide and low tide is the tidal range . Tidal period is the time between consecutive high or low tides and varies between 12 hrs 25 min to 24 hrs 50 min . There are three basic types of daily tides defined by their period and regularity  Diurnal tides have one high and one low tide each day and tidal periods of 24 hrs or 24 hrs 50 min  Semidiurnal tides have two high and two log tides of similar tidal range each day and a tidal period of 12 hrs 25 min or 12 hrs  Mixed tides have two high and two low tides of dissimilar tidal range each day  Show how diurnal and semidiurnal add to get these three . Over a month the daily tidal ranges vary systematically with the cycle of the moon  Spring tides are the two tides each month with the largest tidal range  Neap tides are the two tides each month with the smallest tidal range  Tidal range is also altered by the shape of a basin and sea floor configuration  Origin of the tides o The tides result from gravitational attraction and centrifugal effect . Gravity varies directly with mass, but inversely with distance . Although much smaller, the moon exerts twice the gravitational attraction and tide-generating force as the Sun because it is closer . Gravitational attraction pulls the ocean towards the Moon and Sun, crating two gravitational tidal bulges in the ocean (high tides) . Centrifugal effect is the push outward from the center of rotation

1  As the Earth orbits the Sun, centrifugal effect raises a tidal bulge on the side of Earth facing away from the Sun  Earth and Moon revolve around a point between them. Centrifugal effect raises a tidal bulge on the side of the Earth facing away from the Moon . Latitude of the tidal bulges is determined by the declination, the angle between Earth’s axis and the lunar and solar orbital plane  As Earth rotates on its axis a point on Earth’s surface alternately passes through and between tidal bulges, experiencing high and low tides respectively . In two weeks, the tidal range varies from maximum to minimum to maximum  Spring tides occur when the Earth, Sun an Moon are aligned in a straight line and the tidal bulges display constructive interference, producing very high high tides and very low low tides o Spring tides coincide with full and new moons  Neap tides occur when the Earth Moon and Sun are aligned forming a right angle and tidal bulges display destructive interference, producing low high tides and high low tides o Neap tides coincide with the first and last quarter moon o Equilibrium model of tides . Earth’s surface is covered by seawater of infinite depth . Waves progressive . Water in equilibrium with tide generating force . Two bulges . Gravity and Coriolis BREAK o Dynamic model of tides . Movement of tides across the ocean basins is deflected by Coriolis, block by continental landmasses and forms a rotary wave, which each day completes two cycles around the basin if the tide is semidiurnal or one cycle if it is diurnal . High tide at the ocean basin’s western edge creates a pressure gradient sloping downward towards the east . As water flows down the gradient, Coriolis deflects water towards the equator, where it accumulates and establishes a pressure gradient sloping downward towards the pole . Water flowing down this gradient is deflected eastward, forming a pressure gradient sloping downward to the west . Westward flow along this gradient is diverted poleward forming a pressure gradient sloping downward toward the equator . Finally, the flow toward the equator is deflected westward completing the cycle

2 o A rotary wave is part of an amphidromic system (rotary standing wave) in which the wave progresses about a node (no vertical displacement) with the antinode (maximum vertical displacement) rotating about the basin’s edges . Cotidal lines connect points on the rotary wave that experience high tide at the same time  Cotidal lines are not evenly spaced because tides are shallow water waves and their speed depends upon water depth . Corange circles are lines connecting points which experience the same tidal range  The lines form irregular circles which are concentric about the node  Tidal range increases outward from the noted . Amphidromic systems rotate clockwise in the southern hemisphere and counterclockwise in the northern hemisphere because of the difference in the direction of Coriolis deflection . Irregular coastlines distort the rotary motion . Actual tide expressed at any location is a composite of 65 tidal components – book wrong – 180 tidal constituents  Tides in small and elongated basins o In long and narrow basins tides cannot rotate . Currents in these basins simply reverse direction between high and low tide, flowing in with the high tide and out with the low tide . Cotidal and corange lines are nearly parallel to each other . Tidal ranges increase if a bay tapers landward because water is funneled towards the basin’s narrow end . Tidal resonance occurs if the period of the basin is similar to the tidal period  Resonance = period of wave matches natural period of basin  Resonance can greatly enhance the tidal range . A tidal bore is a wall of water that surges upriver with the advancing high tide  A tidal bore occurs if: o Tidal range is larger than 5 m o The river valley tapers headward o Water depth systematically decreases upstream  As the tide flows in, it is constricted on the bottom and sides, forcing to flow faster and the front to steepen into a collapsing wave  Tidal currents o The movement of water towards and away from land with the high and low tides, respectively, generates tidal currents . Flood current is the flow of water towards the land with the approaching high tide . Ebb current is the flow of water away from the land with the approaching low tide

3 . Far off shore the tidal currents inscribe a circular path over a complete tidal cycle . Near shore the tidal currents produce simple landward and then seaward currents  Antarctic tides o Ice shelves o Amphidromic points o Tidal currents o Internal tides o Mixing and why it is important

If extra time– then discuss homework and/or word problems

Lab 1. Types of tides 2. Tsunamis 3. Storm surge

Bring 1. Tide tables 2. Tide calendar 3. Tanks 4. Sugar water