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Tides Unit III: Real Tides (2 Pts) Section T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 1 Name: Tides Unit III: Real Tides (2 pts) Section: Real Tides Real tides can differ significantly from the predictions of the bulge theory of tides, because the bulge theory does not take into account a variety of factors like the presence of the continents, the depth of the ocean, weather conditions, and the shape of the shoreline. These factors can be strong enough to drastically alter the tides. For example, there are some places in the world with only 1 high tide and 1 low tide each day. This is the normal tidal pattern for these places. They are called diurnal tides. There are two kinds of tidal patterns in which there are 2 high tides per day and 2 low tides per day. Semidiurnal tides have roughly the same heights each day: Both high tides have about the same height, and both low tides have about the same height. Mixed semidiurnal tides have significantly different heights on most days: One high tide is much higher than the other high tide, and one low tide is much lower than the other low tide. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 2 Semidiurnal and mixed semidiurnal tides are the most common tidal patterns. Diurnal tides are rarer. On the west coast of the United States has mixed semidiurnal tides. The east coast of the United States has semidiurnal tides. The Gulf (southern) coast of the United States has diurnal tides. There are more than 2 tidal bulges and most move in circles Perhaps the most obvious reason that the bulge theory of tides fails to accurately predict the tides is that the continents get in the way of the bulges. As the Earth turns, the bulges try to stay underneath the Moon (and on the side away from the Moon). In doing so, the bulges move over the ocean floor, but they cannot travel over the continents. Instead, the bulges bounce off the continents, split up into smaller bulges. These bulges are trying to reach the positions predicted by the bulge theory of tides. Most of the small bulges travel around the ocean basins in large circles. The circular motion is partially related to the Coriolis effect, something discussed in more detail in Unit 8A-1 “The Atmosphere and Winds.” In essence, the Coriolis effect causes objects moving over the surface of the Earth (like the tidal bulges) to bend off course owing to the Earth’s rotation underneath them. In addition, drag from the bottom of the ocean also slows down the bulges, keeping them from remaining under the Moon. Tides “feel the bottom” at all times, because they have huge orbitals (due to their very large wavelength, the distance between the bulges), so rubbing against the bottom is always a factor. Only at the Poles (where there is less distance to cover) can the tidal bulges move fast enough to stay under the Moon. In other places, the bulges are often “ahead” of the Moon, because the Earth rotates faster than the Moon orbits the Earth. The bulges tend to be pushed out from under the Moon owing to friction with the bottom of the ocean. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 3 1. True or false? “Some places have only 1 high tide and 1 low tide each day.” 2. The “bulge” theory of tides does not accurately describe the motion of the tidal bulges over the surface of the Earth: they do not travel all the way around the Earth each day. Describe how the tidal bulges actually move over the surface of the Earth. 3. Why can’t the tidal bulges stay underneath (“keep up with”) the Moon? Give at least 1 reason. Storms and Tides Storms are low-pressure systems, which causes the sea surface to rise beneath them. Very large storms (like hurricanes) cause the sea surface to rise significantly (20 feet or more), and the resulting storm surge can flood the shoreline when they hit land, reaching miles inland. Recall that air pressure is caused by the weight of the air above pressing down. Outside the storm, air pressure is higher and therefore pushing down more strongly on the ocean surface. The ocean surface goes down beneath the high-pressure air, and rises up where the downward push is weakest, beneath the storm. (The water that is pushed down outside the storm has to go somewhere, so it rises in the place where the downward push is weakest, beneath the storm.) This is somewhat like a teeter-totter or see saw on a playground. If a heavy person sits on one end and a light person sits on the other, the end with the heavy person goes down and the end with the light person goes up. The downward pressure of the light person is simply not enough to keep it down. When the storm comes ashore, the increase in sea level grows even higher in places where the storms’ winds push the mound of water into coast. As the water comes ashore, it often gets even higher, because the water runs out of space as the ocean gets shallower. In addition, as the water meets resistance from vegetation and buildings and simply friction with the ground, the water in front slows while more water continues to be pushed in from behind by the storm winds. Since storm surge effectively raises sea level along the coast, storm surge will be worst (flood inland the farthest) if the storm comes ashore at high tide, especially during spring tide T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 4 conditions. This is when Superstorm Sandy hit New York in 2012, helping it do more damage. On the other hand, storm surge will be smallest (flood inland the least) if the storm comes ashore at low tide. In other words, high tides raise sea level and thus make storm surge higher and worse. Low tides push the ocean surface down so the make storm surge less high and less dangerous. 4. Do storms make high tides higher or lower (than they would otherwise be)? 5. Do storms make low tides higher or lower (than they would otherwise be)? Tides and Currents Tides play an important role in creating currents in many estuaries. As sea level rises towards high tide in the ocean, water flows into the estuary because sea level in the estuary is now lower than in the ocean: Water flows downhill, pulled down by gravity. When sea level sinks towards low tide in the ocean, water flows out of the estuary, because in the estuary sea level is now higher than in the ocean. Again, water is just flowing downhill. By the way, the tide flowing into estuaries and towards land is called the flood tide. The flood tide is when sea level is rising. The tide flowing out of estuaries and way from land is the ebb tide. The ebb tide is when sea level is sinking. In estuaries that are large and have narrow or shallow connections with the ocean, it can take a long time for enough water to flow in or out so that the level of the sea surface in the estuary is the same as the level of the sea surface in the ocean. As sea level changes more and more in the ocean, a bigger and bigger difference between sea level in the ocean and sea level in the estuary can develop. The bigger the difference in sea level, the faster water flows between the locations. T. James Noyes, El Camino College Tides Unit III: Real Tides (Topic 7A-3) – page 5 A big difference is sea level means that a great weight of water is being held back and “wants” to go through. (The Earth’s gravitational pull is stronger on a lot of water than on a little water.) In short, the larger the tidal range, the greater the size of the estuary, and the smaller the connection with the ocean, the faster the water moves into or out of an estuary. Tidal flows can be quite treacherous and are often an important cause of mixing in estuaries. Sailors must be aware of these flows, or their ship could get pushed into rocks or shallow water and run aground. In the days when all ships were powered by the wind via sails, ships could not leave port if the tidal currents were against them. There is an old saying: “time and tide wait for no man.” If members of a ship’s crew did not get back to the ship in time, the captain would have to decide whether to leave without them or to wait for them and the next tide causing water to flow in the right direction. 6. If sea level is rising in the ocean, does water flow into bays (estuaries) or out of them? 7. If sea level is falling in the ocean, does water flow into bays (estuaries) or out of them? 8. Do high tides in a bay (estuary) occur earlier or later than the high tides along the coast? 9. Do low tides in a bay (estuary) occur earlier or later than the low tides along the coast? 10. Is it safer for a ship to enter or leave an estuary near high tide or near low tide? (Hint: Think about how deep the water is.
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