This is a page to present six animations of the various models or theories of the oceans' tides. The animations are click-driven: you have to click the "Next Slide" button to advance the animation. That's a primitive but effective way to let you watch a series of images in which you control the rate of advance (or reversal). Each animation begins and ends with the opportunity to return to this page. The last animation (the dynamic model of the tides) is the main point of this page, if you want to short-circuit to it.
Because the tide waves stay in the ocean basins rather than sweeping across the continents, we clearly need to improve on the previous model. The next animation retains the concept of gravitational imbalance but uses it to move the water in one ocean basin from side to side, a vision closer to reality. In this model too, any one location in or around the ocean sees two high tides and two low tides every 24 hours. We recommend that you watch the animation twice, first to focus on the dark blue ocean sloshing back and forth, and then to focus on the tide changing at the marked red point. You're again watching from directly over the North Pole. See Animation III!
The only problem with the model above is that it implies that the tide would slosh back-and-forth from east-to-west and west-to-east across an ocean basin. The Coriolis effect will convert any seemingly straight motion across the earth surface into a curve. Thus in the northern hemisphere, as the tide turns to its right around an ocean basin, it takes a counter-clockwise circular path. (Imagine entering a room and walking along the walls of the room, always trying to turn to the right as you move forward. You'll take a counter-clockwise path around the room.) In the southern hemisphere, as the tide turns to its left around an ocean basin, it takes a clockwise circular path.
That leads us to Animation IV of tides in a simple basin on a simple planet, where you're watching from above the equator, with the moon behind you.
If you're having trouble envisioning this idea of a rotating tide, there's a one-jpeg single image and, once you've seen that, there's Animation IVa for the Northern Hemisphere
The result is that the tides sweep around the ocean basins, counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere. As an example, the next and final animation shows the progression of the tides in the Atlantic Ocean. You're watching from above the equator, with the moon behind you, or over your right shoulder. The light blue and dark blue fields indicate the areas covered by high and low tide in an hour, so wider bands indicate faster movement of the tide. The bands curve and swell or thin in response to the depth of the ocean - the tide wave moves more slowly in shallower water and faster in deeper water.
If time and your web connection allow, we recommend watching the animation three times: first to watch the world go round and to get used to that, second to watch the tide go around the North Atlantic, and third to watch the tide go around the South Atlantic. See Animation V!
The result of all this is that, in the Northern Hemisphere, tides sweep south down east sides of continents (like the U.S. East Coast) and north up the west sides of Continents (like the U.S. West Coast). As examples, you can look at maps of times of high tide for the North Atlantic and northeast Pacific.
Many people who have heard about the clockwise motion of currents in the Northern Hemisphere gyre and the counterclockwise rotation of the tides sense a contradiction. The answer is that the current is moving water, whereas the tide is a wave passing over that water, but not moving the water significantly. A portrayal of this, and more, is in Animation 6, which is in a Powerpoint file.
Sources and comments: The base maps for Animation V were generated using Online Map Creation at Das Leibniz-Institut für Meereswissenschaften an der Universität Kiel (IFM-GEOMAR). The tide positions in Animation V are from Dietrich et al., 1980, General Oceanography: New York, John Wiley & Sons, 626 p. Animations I to IVa are unabashedly northern-hemisphere centric, but persons in the southern hemisphere can just run Animations I to III and IVa backwards.
e-mail to Bruce Railsback (firstname.lastname@example.org)
Railsback's GEOL 3030 (Elementary Oceanography) course web page
Railsback's main web page
UGA Geology Department web page