Introduction to Ocean Sciences, 4Th Edition, Chapter 10
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CHAPTER 10 Tides Introduction to Ocean Sciences Fourth Edition, Second digital edition ver 4.3 DOUGLAS A. SEGAR Contributing author Elaine Stamman Segar © 2018 by Douglas A. Segar This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-nd/3.0/ or send a letter to Creative Com- mons, 444 Castro Street, Suite 900, Mountain View, California, 94041, USA. What does this Creative Commons Licence mean? You are free to use the book except that you can not use the book or any part of it for any commercial purpose including personal financial gain without written permission from the author. Uses that require written permission include, but may not be limited to, sale for profit, advertising use, or use as an incentive to purchase any other product. If you transmit or transfer the book to anyone by any means, you must attribute the book by at minimum citing the author, title, and ISBN number. Also, you can not alter, transform, or build on the work. Most images and artwork contained in the book are copyrighted to the author (or others) and you may not use any of these in any way, except in your use of the book itself, without written permission from the copyright holder. Library of Congress Cataloging-in-Publication Data Segar, Douglas A. Introduction to ocean sciences / Douglas A. Segar with contributions from Elaine Stamman Segar p. cm. ISBN: 978-0-9857859-1-8 1.Oceanography. I. Title CHAPTER 10 Tides CRITICAL CONCEPTS USED IN THIS CHAPTER CC12 The Coriolis Effect These two sets of photographs were taken from the same location 6 h apart at the port of Anchorage, Alaska, at approximately high tide and low tide. The tidal range this day was almost 11 m. For scale, note the cargo containers on the stern of the ship. They are the type hauled on 18-wheel trucks or flatbed railcars. Most of us are familiar with the water movements that slowly waters and within harbors and estuaries. In estuaries, tidal cur- expose and then cover the seaward part of the shore during a day. rents reverse direction as the tide rises and falls. When the current This rise and fall of sea level is called the tide. If we observe tidal flows in from the sea, it is aflood current. When it flows out, it motions long enough at one location, we will see that they are is an ebb current. The reversals between flood and ebb currents periodic. At some locations, the tide rises and falls twice during a do not necessarily occur at high or low tide. However, where day; at others, it rises and falls only once. The times of high and only one tide occurs each day, there is one flood and one ebb; low tide vary predictably, and the height of the tide also changes and where two tides occur each day, there are two floods and two somewhat from day to day. Indeed, the tidal range is often very ebbs. different for the two tides on a single day. In addition, we may In the deep ocean tides produce generally weaker currents observe that the sea-level change between high and low tide is than in coastal waters. However, throughout the oceans depths large in some places, while in other places there appears to be no tidal motions are responsible for generating currents that interact tide at all. with density differences between water layers and with seafloor Tides are important to mariners because many harbors and topography to cause internal waves and turbulence that is a major channels are not deep enough for vessels to navigate at low tide. contributor to the vertical mixing of deep layer water upwards as Tides are an energy source that has been harnessed for electricity part of the MOC discussed in Chapter 8. generation in some parts of the world. They are also important to It has been known for more than 2000 years that tides are many marine creatures, especially species that live in the inter- related to the movements of the sun and moon. However, the tidal zone between the high-tide line and low-tide line and must relationship was not fully explained until 1686, when Sir Isaac cope with alternate periods of immersion in water and exposure Newton published his theory of gravity. Tides are a simple phe- to air. nomenon in physical principles, yet they can be extraordinarily Tides cause tidal currents that can be very swift in coastal complex. 236 CHAPTER 10: Tides TIDE-GENERATING FORCES Earth’s center. For example, gravitational attractions that the sun Tides continuously raise and lower the sea surface. The verti- and moon exert on any object at the Earth’s surface are approxi- cal movement of the water surface is as much as several meters, mately 0.06% and 0.0003% of the Earth’s gravitational attraction, twice a day in some areas. Clearly large amounts of energy are respectively. Gravitational attractions of other celestial bodies are necessary to move the water involved in this process. The energy much smaller. is supplied by the gravitational attraction between the water and Although they are almost negligible in comparison with the the Earth, moon, and sun—but how? Tides are cyclical, but they Earth’s gravitational pull, gravitational forces exerted by the do not come and go at the same times each day, and the height moon and sun on the Earth are the cause of the tides. To under- to which the water rises or recedes varies from day to day. Why stand how these forces cause the tides, we must first consider the does this happen? To answer these questions, we must first learn characteristics of orbits in which celestial bodies move. some basic information about gravity and the motions of the Orbital Motions and Centripetal Force moon around the Earth and the Earth around the sun. Any two bodies that orbit together in space, orbit around their Gravity common center of mass. This works like a seesaw. If the two Newton’s law of gravitation states that every particle of riders are of equal weight, the seesaw balances when the rid- mass in the universe attracts every other particle of mass with a ers are equal distances from the center. Similarly, two planetary force that is proportional to the product of the two masses and bodies of equal mass would orbit around a point exactly midway inversely proportional to the square of the distance between the between them. However, if one rider is heavier than the other, the two masses. The gravitation force (F) between two masses can be heavier rider must sit closer to the center of the seesaw to make expressed as it balance. Similarly, two planetary bodies of unequal masses orbit around a point that is closer to the more massive of the two M × M F = G × ————1 2 bodies. The Earth’s mass is 81 times that of the moon. Therefore, 2 D the common point of rotation is about 4680 km from the Earth’s where G is the gravitational constant (a constant definedby center, or approximately 1700 km below the Earth’s surface (Fig. -11 -1 10-1). Similarly, because the sun has far greater mass than the Newton’s law and equal to 6.673 ×10 m·kg ·s), M1 and M2 are the two masses, and D is the distance between them. Earth, the common point of rotation of the Earth and the sun is For approximately spherical objects, such as planets, the en- deep within the sun. tire mass can be considered to be at the object’s center of gravity, As two planetary bodies orbit each other, they must be con- and the distance D is measured between the centers of the two strained in their orbit by a centripetal force that prevents each masses. The gravitational force increases as either mass increases from flying off in a straight lineCC12 ( ). The centripetal force or the distance between the two objects decreases. is supplied by the gravitational attraction between them. The We are familiar with gravity because it is the force that at- centripetal force required is the same for all parts of each of the tracts all objects on the Earth’s surface, including ourselves, to two bodies. Because centripetal force varies with distance from the much greater mass of the Earth itself. In addition, the Earth the center of rotation (CC12), this may be difficult to understand. and every object on it are subject to gravitational attractions However, within either of the orbiting bodies, all particles of toward the sun, the moon, other planets, and every other celestial mass move at the same speed in circular paths of the same diam- body. However, these gravitational forces are extremely small in eter. We can understand this motion if we examine the motions of relation to the Earth’s gravitational pull. Although the sun, stars, the two objects as they orbit each other without considering other and some planets each have a mass much greater than the Earth’s motions, such as the Earth’s spin on its axis or orbits that involve mass, they are much farther away from us than we are from the a third body. 384,800 km FIGURE 10-1 Any two bodies in space or- D1 = bit around a common point of rotation. Just 4680 km D2 = 380,100 km as, on a seesaw, the heavier person must sit nearer the pivot point to provide balance, Earth Moon the common point of rotation for two bod- Pivot ies orbiting in space must be closer to the more massive body.