Tides Tides, or the rising and falling of the sea surface level, have a dominant effect on marine life on nearshore and coastal areas. The two factors that create tides are gravitational and centrifugal forces… Lets see how they work… Based on this information, one would expect tides to have a 24 hour cycle… that is not however exactly the case…

Tides Since the moon moves while the earth rotates, the full tidal cycle takes 50 minutes longer than the expected 24 hours.

Tides In addition to the moon, the sun’s gravity also influences tides… Although the sun is much larger/heavier than the moon (and therefore has a higher gravitation force), it only exerts a fraction of the gravitational effect of the moon on tides. Why do you think this is?

Tides If the earth was covered by water, then we would expect two daily high and two daily low tides of equal size. The earth is not however covered entirely by water and tides are affected by the continents, shape of various land masses, and the sea floor… “Real world” tides can take different forms:

Tides

Over different regions of the earth, not only do the cycle of tides vary, but so do the observed magnitude of the tides. The magnitude of the tides can be influenced both by 1. shape of the land mass where the tides are being observed, and 2. the location of the tidal observation relative to amphidromic points in the ocean (no-tide points). 1. Effect of shape… in Bay of Fundy for example, tidal range can be 50 feet, with water rising as much as 10 feet in one hour during the peak of the flood!

Tides 2. Location relative to Amphidromic points… Note that tides in some ways act like standing waves, where there are nodes that have no change in relative height. The further away one goes from these nodes, the greater the change in the relative height of the tides.

Tides Amphidromic points

Tides A few general terms for tides: Tidal current – the current observed as water rushes in or out bays and harbors during the tidal cycle. Flood current – water rushing into an enclosed area as a result of tides. Ebb current – water rushing out of an enclosed area as a result of tides. Slack water – a time of no-current when there is a shift in direction of the flow during high or low tide. Tidal range – high-water to low-water range. Intertidal zone – coastal region between the highest high tide and the lowest low tide. Mean sea level - the height of the low water level averaged over several years. Tidal friction – the frictional force of tides on the earth’s rotation… tides are slowing down the earth!! 350 million years ago, a day was only about 22 hours long!! That would allow for about days per year.

Waves and Seawater Next lecture…