NAME SURFACE CURRENTS AND TIDES I. Origin of surface currents Surface currents arise due to the interaction of the prevailing wis a the ocean surface. Hence the surface wi pattern (Figure 1) plays a key role in establishing the surface currents. Once the water is set in motion it is acted upon by the Coriolis force. This force causes moving water in the Northern Hemisphere to deflect to the right a moving water in the Southern Hemisphere to deflect to the left. These deflections set up the large scale gyres that characterize the surface circulation pattern (Figure 2). Because the earth rotates from east to west, the centers of the oceanic gyres are offset to the west. Thus currents on the western side of ocean basins are narrower then currents on the eastern side. What this means is that in order for the volume of water moving through the gyres to remain constant currents must flow faster on the western side of the ocean basin. This is Figure 1. Major wi belts of the earth a zones of high a low pressure. Note that the meteorological o equator is 5-10 north of the geographical equator. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 98. Figure 2. Major surface currents of the world s oceans. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 98. -1-
referred to as the westward intensification of oceanic currents. Also note that the surface currents move warm water from lower latitudes to higher latitudes, a conversely move colder water from higher latitudes to lower latitudes. 1. Why is the meteorological equator north of the geographic equator? 2. With reference to Figure 2, describe the following currents as either warm or cold, a fast or slow. Current Relative temperature Relative speed Peru Kuroshio California Gulf Stream Agulhas Canary West Wi Drift 3. Briefly describe the function of ocean currents in the distribution of heat on the earth. 4. Capetown at the tip of South Africa has a cool mild climate, whereas Durban a few hured miles to the east is very hot a humid. Why? -2-
II. Dynamic topography a geostrophic flow Variations in density in the ocean lead to horizontal pressure gradients. The pressure exerted by a column of water can be determined from the hydrostatic equation, p = ρgh, where ρ is the density, g is the acceleration due to gravity a h is the height of the water column. If we assume that there is some level in the ocean at which the pressure is the same, then water columns of different mean densities must have different heights. This leads to dynamic topography, the variation in height of the ocean surface due to density differences. The dynamic topography can be used to calculate current direction a velocity. Because of the Coriolis force currents will parallel the height contours (see discussion below on geostrophic flow). The spacing between the contours will determine the velocity of the flow, the closer the contours the steeper the slope a the faster the flow. The dynamic topography arises because of the Ekman spiral a the Coriolis force. As discussed in a o previous laboratory, the Ekman spiral leads to a net deflection of water at 90 to the wi. This deflection o is to the right in the NH a to the left in the SH. With reference to Figure 3, a NH example at 30, the Westerlies a Trade Wis set up a gyre which rotates clockwise. Because of the Ekman spiral water is transported towards the center of the gyre forming a mou. Water will flow down this mou, but once it starts moving it will be acted upon by the Coriolis force (Figure 4). The Coriolis force causes the water to Figure 3. Development of dynamic topography due to prevailing wi directions a Ekman transport. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 101. Figure 4. Geostrophic flow. When the water parcel moves parallel to the contours the Coriolis force, C, a the gravity force, G, are in balance. Numerals represent the height in dynamic meters. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 102 deflect to the right. With reference to Figure 4, at some point the Coriolis force becomes equal a opposite to the gravity force that is causing the water to move downhill. At this point the velocity of the water will remain constant (this is called the geostrophic velocity a the process is referred to as geostrophic flow, i.e. non-accelerated flow). Note that when geostrophic flow is achieved the water is moving parallel to the height contours, hence the observation in the preceding paragraph that the currents would flow parallel to the contours. 5. The dynamic topography for the ocean off the coast of California is shown on Figure 5. a. Using the scales on the map determine the maximum a minimum velocities off Point Conception. Use the scale closest to the speed to be determined. Report the maximum a minimum velocities in -1-1 cm sec a m sec. -3-
Figure 5. Dynamic topography relative to an arbitrary level at depth. Contour intervals are 0.02 dynamic meter. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 103. -4-
-1-1 -1 b. Given that 1 m sec = 3.6 km hr, convert the maximum a minimum velocities to km hr. -1 c. Compare these results with the Kuroshio Current where speeds of 2 m sec are common. 6. In what ways, a why do the Kuroshio (Figure 2) a California currents (Figure 5) differ? III. Tides Tides are caused by the gravitational attraction of the moon on the earth a the centrifugal force generated by the earth s rotation on its axis. In what is referred to as the equilibrium theory of tide formation it is assumed that the earth is completely covered with an ocean of uniform depth. On the side towards the moon the water of the ocean is attracted more strongly by the moon than the floor of the ocean basin thus forming a high tide. On the opposite side of the earth centrifugal force, a the smaller gravitational attraction by the moon on the water in the ocean basin, results in another tidal bulge. In between these two bulges there are nodes of low water level. If the moon is directly over the equator there will be two high a two low tides each day, a the highs a lows will be of equal Figure 6. The arrows represent the magnitude a direction of the horizontal tide-generating forces. The force towards the moon is gravitational attraction a the force away from the moon is the centrifugal. (a) When the moon is in the plane of the earth s equator the forces are equal in magnitude on the same parallel of latitude on opposite sides of the earth. (b) When the moon is at north or south declination the forces are unequal a thus te to cause an inequality in the two high a two low waters of a tidal day. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 112. -5-
magnitude (Figure 6a). However, the moon does not stay directly over the equator. Because of the tilt of its axis of revolution, with time the moon s position moves north or south of the equator. The result is that at many latitudes there two high a two low tides of unequal magnitude (Figure 6b), a at some latitudes there is only one high a one low tide per tidal day. Thus three major types of tides can be distinguished. In the case of diurnal tides there is one high a one low tide per tidal day. Note that the tidal day is actually 24 hours a 50 minutes long. This is because the moon advances 50 minutes each day in its orbit arou the earth. Semidiurnal tides occur twice daily, that is there are two high a two low tides per tidal day a the highs a lows are of about equal magnitude. Mixed tides show two highs a two lows per day, but the magnitudes of the two highs a two lows are very different. Tidal curves for the three types of tides are shown in Figure 7. Because of these variations in tidal height, the tide level is usually measured in reference to a local base level, or datum plane, which is an average of many years observations. A number of datum planes are possible (Figure 8). From the stapoint of navigation, where the most important factor is the depth of water uer the boat, mean lower low water (MLLW) would be an appropriate datum plane. Figure 8. Depths for various tidal datum planes. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 113. Figure 7. Types of tides from the Atlantic a Pacific ocean basins: (a) diurnal tide; (b) semidiurnal tide, (c) mixed tide. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 113. There is a monthly variation in the tidal range. Approximately 1/3 of the tide raising force is due to the sun a the other 2/3 is due to the moon. The variation in tidal range is due to the relative positions of the earth, sun a moon (Figure 9). When the earth, sun a moon are aligned (the time of new or full moon), the tide raising forces are at their maximum, a we have the greatest tidal range. When the earth, sun a -6-
moon are at right angles (first or third quarter moon), the sun partially cancels the lunar tide raising force, a we have the smallest tidal range. Spring tides occur when the tidal range is at its maximum a neap tides occur when tidal range is at its minimum. In some places extreme tidal ranges are observed. These occur when the natural oscillation of the basin correspos to the tidal cycle, i.e. 12 hours a 25 minutes. One such example is the Bay of Fuy where daily tidal range can be as great as 16 meters. Storm surges occur when tidal maximums correspo with onshore wis during a storm. The wis drive water shoreward a increase the height of the tidal maximum. Storm surges can be extremely destructive of property in coastal areas. Tsunamis are sometimes referred to as tidal waves. However these long wavelength waves are not related to the astronomical tides. They are caused by earthquakes, submarine laslides, or other disturbances of the sea floor. In the open oceans these waves are only a meter or so high a can travel at speeds in excess of 600 km per hour. When the waves enter shallow water they slow down a steepen. Such waves can be very destructive in coastal areas. 7. Table 1 lists tidal data for a 4 day period. Use these data to answer the following questions. a. Plot the data on the graph on the next page. Connect the data points with a straight line to produce a tide curve. Figure 9. The relative positions of the earth, sun a moon during the lunar month. The highest high a lowest low tides (spring tides) occur at new a full moons. The lowest high a highest low tides (neap tides) occur a first- a third-quarter moons. From Pipkin et al., 1987. Laboratory Exercises in Oceanography, 2 Ed. New York: Freeman, p. 114. b. During which days do the tide curves exhibit the following characteristics: Semidiurnal Diurnal Mixed c. What is the smallest tidal range a on what day does it occur? Range feet Day -7-
Time Height Day 24-hour day (feet) 1 0115 1.0 0730 4.1 1600 1.0 2015 4.0 2 0200 1.0 0830 6.5 1415 2.0 2100 5.0 3 0415 0.0 1600 7.0 4 0430 1.0 1000 3.0 1600 1.0 2200 2.9-8-
d. What is the elevation of mean high water for the 4 days? feet Of mean low water? feet What is the mean range? feet e. On what days a at what times could a person sail a boat that draws 5 feet through a passage uerlain by a reef that is exposed at mean lower low water? Assume that the datum for the graph is MLLW. -9-