The oceans are vast not only in size, but also in their ability to store and release energy.

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1 WHAT ENERGIES ARE ASSOCIATED WITH EARTH S OCEANS? The oceans are vast not only in size, but also in their ability to store and release energy. Scientists recognize various forms of energy in nature. Many types of energy affect the dynamics of our oceans. For example, solar heating produces winds which drive our surface ocean currents. Thermal energy is transferred from the atmosphere and drives ocean convection and deep currents. A minor source of thermal energy in our oceans is hydrothermal venting. Another common energy-driven ocean feature is waves. Ocean waves, whose motion strikes a balance between kinetic and potential energy, are formed by wind, sudden displacement of the ocean bottom (usually by earthquake-triggered landslides or volcanic eruption), and changes in gravity s pull in the Sun-Earth-Moon system (i.e., tides). This theme (Oceans - Energy ) describes the physical nature of energy and the role of the oceans in storing, releasing, and redistributing energy. Related Themes: The effect of tides on Earth s oceans is examined in Climate - Scale and Structure. The influence of Coriolis effect on wind-driven surface ocean currents is covered in Climate - Systems and Interactions. How ocean energy might be harvested for industrial use is presented in Climate - Human Interactions. The variation in heat capacity between oceans and land is included in Climate - Systems & Interactions. The exchange of energy between the oceans and atmosphere is addressed in Climate- Energy. Interesting and exotic life forms associated with hydrothermal vents are described in Life - Energy. Related Activities: Making and Using a Wave Machine Ocean Wave Characteristics INTRODUCTION Earth s oceans are not only reservoirs of water but are reservoirs of energy as well. Energy is defined as the ability to perform work. Various forms of energy exist in nature, and one form of energy can often be converted into another. A car s engine, for example, converts thermal energy from burning gasoline into kinetic energy (the energy of motion). In the context of ocean science, the most important form of energy is thermal energy, that is, energy stored in the form of heat. Although less significant, the energy imparted to the oceans by earthquakes, submarine landslides, and hydrothermal vents is described here. The effect of gravity on ocean dynamics, namely as tides, is addressed in Oceans - Systems and Interactions. ENERGY & OCEAN CURRENTS The oceans receive almost all of their energy from the Sun. Sunlight heats ocean water near the surface and some of the heat diffuses down a few hundred meters in depth. 1

2 Figure 1. Incidence of solar radiation on Earth. Note that the angle of sunlight striking Earth is steeper (i.e., more nearly perpendicular) near the equator than near the poles. The intensity of sunlight per unit area (called insolation) is therefore greater near the equator. Compared to other common substances, water is relatively slow to heat up and cool down. Raising the temperature of a sample of water by one degree Celsius requires more heat than raising the temperature of the same air mass by one degree Celsius. The amount of heat required to raise the temperature of one gram of water by one degree Celsius is, by definition, one calorie. The calorie is a standard unit of heat energy. (It should be noted that in the context of food consumption, the unit Calories is equal to 1,000 metric system calories.) Light from the Sun, on average, strikes Earth more directly near the equator than near the poles [Fig. 1]. The degree or intensity of sunlight at a given location is called insolation. The greater the insolation, the more heat is being transferred from the Sun to that point on Earth. But because of continuous ocean currents that travel the globe, some heat from the equatorial regions is redistributed to the temperate and polar regions. If it were not for this redistribution of heat, many places on Earth would be much colder (on average) than they are now. Figure 2. The Coriolis effect. As Earth turns toward the east, currents in the northern hemisphere are deflected from their initial path to the right, currents in the southern hemisphere are deflected to the left. 2

3 When sunlight is absorbed by Earth s oceans, it warms the water, which evaporates and increases the water vapor in the atmosphere. When the water vapor condenses to form rain, heat is released, warming the atmosphere. Differential heating causes winds to blow across the surface of the ocean, transferring kinetic energy (energy of motion) to surface water through friction. The energy of motion of the wind is converted into ocean currents. As winds blow across the surface of the ocean, they push on the underlying water, setting it into motion. Thus it would seem reasonable that ocean currents closely follow prevailing winds, but the dynamics of currents are not so simple. Part of the reason is that continents and islands divert ocean currents. However, the most important influence on the direction of ocean currents is Earth s rotation. Because ocean currents move on a rotating Earth, they tend to be deflected from their initial path. Winds and ocean currents will deflect to the right in the northern hemisphere as Earth rotates beneath them; in the southern hemisphere, winds and currents are diverted to the left [Fig. 2]. On a merry-go-round [Movie 1], the velocity of the ball is constant as it moves from the outside to the inside of the merry-goround. However, the velocity of the platform beneath the ball is changing as it moves from the outside of the merry-go-round, to Movie 1. The Coriolis effect on a merry-go-round. This demonstration shows from above a spinning merry-go-round the path the ball travels appears to be straight. However, from the perspective on the merry-go-round the ball appears to curve to the left. Likewise, to a person sitting on the rotating Earth the path of moving objects appears to be deflected. Movie 2. How the Coriolis effect changes wind and ocean current directions. The Coriolis effect deflects winds and currents until they are parallel to isobars (lines of equal pressure), then the pressure gradient balances the Coriolis force. 3

4 the inside, and back outside again. Overall, this makes the ball s path appear to deflect along its path. This tendency of moving objects to deflect in a rotating reference frame is known as the Coriolis effect. In Earth s atmosphere and oceans, the Coriolis effect causes wind and currents to follow approximately circular paths. This is because their flow balances both the pressure gradient force and the Coriolis force. The overall effect is that winds and currents flow along paths of constant pressure [Movie 2]. OCEAN WAVES A wave is one of the most important and most basic forms of movement in nature. A plucked guitar string and a vibrating atom are both examples of wave motion. Similar to the wind process that drives ocean currents, waves are created by the transfer of kinetic energy by winds blowing across the surface of the ocean. Ocean waves possess both kinetic energy and potential energy. As a wave moves through water, there is a continuous exchange of potential energy and kinetic energy. Shallow- Versus Deep-Water Waves The familiar sight of parallel wave fronts approaching the coast [Fig. 3] is unique to the nearshore shallow-water waves, whose shapes and speeds are controlled by bottom topography. In this theme, the focus is on waves that are unaffected by the ocean bottom, known as deep-water waves. Wind-Driven Waves In the open ocean, wind drives not only surface ocean currents but creates waves as well. Waves form when wind begins to blow. The faster and the longer the winds blow, the bigger the waves. At different locations in the open ocean, waves are influenced by local winds, nearby land masses, and distant storms. Thus open ocean waves are often choppy and rough because they are product of numerous wave-generating forces Figure 3. Parallel wave fronts approaching a coast near Cape throughout each ocean basin. The Mendocino. direct relationship between ocean wind and waves is shown in Figure 4. Water in an individual surface wave does not move an appreciable distance due to the wave. Water particles actually move in a circular, or orbital pattern [Fig. 5]. This is easily demonstrated 4

5 Figure 4. TOP: Wind speeds over Earth s oceans, BOTTOM: Wave heights in Earth s oceans. Data from the TOPEX/Poseidon satellite show the high correlation between ocean wind speed (top) and wave height (bottom) during the month of October In this image, the strongest winds and highest waves are found in the Southern Ocean. Note that the areas of strong ocean winds and high ocean waves change seasonally. 5

6 Figure 5. Orbital motion and displacement of water in a surface wave. Water particles move in circular orbits with the passage of deep-water waves. Deeper down in the ocean, the orbital diameters become smaller. Wave crests, wavelength, and wave height are shown. Deep-water waves occur in water with a depth greater than one-half the wavelength. by placing a cork in the ocean and observing how it bobs up and down on passing waves rather than moving along with a particular wave. The wave form moves, carrying with it a considerable amount of kinetic energy. Individual water molecules in surface waves travel as fast as the wave form itself but in a circle, forward at the crest and backward at the trough. The speed of deepwater waves is related to the wavelength and wave period (the time it takes for two successive wave crests to pass a fixed point). Ocean Floor Displacement-Triggered Waves Seismic sea waves, called tsunamis, are ocean waves generated by submarine earthquakes, landslides or volcanic eruptions. Tsunamis have wave periods, or time between successive crests, of about 10 minutes. Their typical wavelength is 200 kilometers (about 120 miles). Because shallow-water waves are defined as occurring at depths less than 1/20th the wavelength, tsunamis behave as shallow water waves [Fig. 6], even when they travel through the deep ocean. 6

7 Figure 6. Characteristics of shallow-water waves. Particle motion in shallow water waves is a flat, ellipse-shaped orbit. This motion can be almost horizontal in very shallow waters (shown as line with arrows at both ends). Note that shallow water waves occur where the depth is less than 1/20th of their wavelength. Thus, shallow water waves can feel and affect the ocean floor. As a tsunami approaches a coastline, interaction with the ocean bottom results in a rapid, often dangerous, increase in wave height. For example, waves from the 1946 Aleutian earthquake reached heights of more than 10 meters (40 feet) when they arrived at the Hawaiian Islands. More than 150 people were killed during this event. Development of early warning systems for the coastal approach of tsunamis has had significant positive impact [Fig. 7]. For example, a 1957 tsunami hit Hawaii with no causalities even though it generated larger waves than the 1946 Aleutian event. Ocean waves possess both kinetic energy and potential energy. Wave s kinetic energy is associated with their motion and their potential energy is dependent on the wave s position relative to the still-water level. Waves transmit energy gained from the forces that caused them. Thus the total wave energy and wave period are linked to the wave-forming forces [Fig. 8]. Overall, the greatest amount of energy is associated with wind-driven waves. Other forces such as earth- 7

8 Figure 7. Tsunami warning system in the Pacific Ocean. The tsunami warning system is used to monitor Pacific Ocean events. The concentric lines show travel time in hours for a tsunami to reach Hawaii. quakes, storms, and volcanic eruptions create waves with periods between 5 seconds and 10 minutes. Sun and Moon-induced tides have periods that peak at 12 and 24 hours. ENERGY FROM HYDROTHERMAL VENTS Until recently, a locally important energy source was unknown to ocean scientists, hydrothermal (hot-water) vents. Although the heat from these submarine springs is only a small fraction of the thermal energy provided to the oceans by the Sun, they supply significant energy in localized settings. They are part of a unique community in which bacteria fix carbon dioxide, methane, and hydrogen sulfide through chemosynthesis, rather than photosynthesis. Remarkably, black smoker vents spew plumes of high-temperature (300 to 400 C) water from up to 10-8

9 Figure 8. Distribution of energy in ocean waves and wave-forming forces. This figure shows that most of the energy possessed by ocean waves is in wind-generated waves with periods shorter than 5 minutes. meter tall chimney-shaped mounds of silica, sulfur, and sulfur-bearing minerals [Fig. 9]. Hydrothermal vents are the surface expression of the larger hot-water circulation system found where tectonic plates spread away from a central area. At these spreading centers, hydrothermal circulation occurs when seawater penetrates into the ocean crust, becomes heated, reacts with the crustal rock, and rises to the seafloor [Fig. 10]. Some chemical tracers (especially helium) can be mapped thousands of kilometers from their hydrothermal sources, and Figure 9. Black smoker hydrothermal vent. 9

10 Figure 10. Hydrothermal vent circulation. This cross-sectional view shows idealized twin peaks that can develop where new crust forms at centers of seafloor spreading. These peaks are composed of cooled lava (basalt) that is derived from hot magma found deep within Earth s crust. The cloud-like feature above the spreading center is called the hydrothermal plume. Seafloor hydrothermal systems have a major local impact on the chemistry of the ocean that can be measured in hydrothermal plumes. Because hydrothermal circulation removes some compounds from seawater and adds many others, it is an important process in governing the composition of seawater. can be used to better understand deep ocean circulation. The total flow of water through oceanic hydrothermal circulation systems is significant. In fact, the entire volume of the ocean has flowed through such hydrothermal plumbing systems many hundreds of times over Earth s history. CONCLUSION Thermal, kinetic, and potential energy are key drivers in motion of the oceans. Heat energy from seawater evaporation drives the atmosphere. Kinetic energy from the atmosphere, in turn, forces ocean currents to flow. The movement of both the atmosphere and ocean currents are 10

11 important for transferring heat around the globe and moderating Earth s climate. Ocean waves, which are mostly caused by winds, possess both kinetic and potential energy. A recently discovered source of ocean thermal energy is hydrothermal venting. The heat input by hydrothermal convection systems is significant near discrete centers of seafloor spreading. VOCABULARY black smoker calorie chemosynthesis convection Coriolis effect dynamics gradient hydrothermal hydrothermal circulation insolation kinetic energy potential energy reference frame tectonic plates thermal energy tide tsunami wave crest wave form wave height wavelength wave period 11