Explain daily, spring and neap tides, deep and shallow currents including rip and longshore currents, waves, and the link between thermal energy and water density, and evaluate energy from the ocean

Learning goal: Explain what causes daily tides and why spring tides and neap tides differ; explain how wind drives shallow (surface) currents and how differences in water density drive deep currents; explain how thermal energy and salinity set the density of seawater and layer the ocean; describe how waves move energy, change in shallow water and create longshore and rip currents; trace an El Niño or La Niña event; evaluate designs that use waves, tides or currents to produce energy; and explain how engineering on the Mississippi and Colorado rivers has changed their deltas where they meet the tides.

Where this lesson sits. Ohio's PG.IMS.4 (Ocean) lists eight topics, and this site teaches them in two lessons meant to be studied together. This lesson is part 1 of 2: tides (daily, spring and neap), currents (deep and shallow, rip and longshore), thermal energy and water density, and waves, with El Niño, energy from the ocean and two river deltas as applications. Part 2, the lesson on the ocean floor, continental margins, sea level and streams, teaches ocean features, passive and active continental margins, transgressing and regressing sea levels, and streams. Neither part alone covers PG.IMS.4.

Daily tides. The Moon's gravity pulls on every part of Earth, but more strongly on the side nearer the Moon than on the far side, because gravity weakens with distance. The difference between these pulls, not the pull itself, raises the ocean into two bulges: one facing the Moon, where water is pulled toward the Moon more than Earth's center is, and one on the opposite side, where Earth's center is pulled toward the Moon more than the far water is. As Earth rotates beneath the two bulges, most coasts pass through two high tides and two low tides a day, a semidiurnal tide. The pattern repeats about every 24 hours 50 minutes, not 24 hours, because the Moon moves along its orbit while Earth turns, so each high tide comes roughly 50 minutes later than on the previous day. Some coasts, where the shape of the ocean basin lets only one bulge dominate, have one high and one low tide a day (a diurnal tide); others have two highs of unequal height (a mixed tide). A common misconception is that the far-side bulge happens because the Moon "pushes" water away; there is no push, only the difference in pull.

Spring and neap tides. The Sun also raises tides, but less than half as strongly as the Moon, because its tide-raising effect depends on how much its pull differs across Earth's width, and the Sun is so distant that the difference is small despite its great mass. At new moon and full moon, the Sun, Earth and Moon are lined up and the two tidal effects add: high tides are higher and low tides lower. These are spring tides, which have nothing to do with the season. At first-quarter and third-quarter moon, the Sun and Moon pull at right angles and partly cancel: the tidal range, the difference between high and low water, is smallest. These are neap tides. Spring and neap tides each come about twice a month, roughly every 15 days. For coasts, the danger lies in coincidences: a storm surge, the rise of water pushed ashore by storm winds and low air pressure, arriving at a spring high tide floods land that the same storm at a neap low tide would not reach.

Thermal energy and water density. Seawater contains on average about 35 grams of dissolved salts per kilogram, and its density is about 1.02 to 1.03 g/cm³. Density rises as temperature falls (colder water molecules pack more closely) and as salinity rises (more dissolved mass in the same volume). Unlike fresh water, which is densest at about 4 °C, typical seawater keeps getting denser all the way down to its freezing point near −2 °C. Sunlight warms only the upper layer, so the ocean is layered: a warm, well-mixed surface layer; a thermocline, a zone where temperature falls rapidly with depth; and cold, dense deep water, only a few degrees above freezing even in the tropics. Because dense water lies beneath light water, the layers resist mixing, which matters for currents and for life, since nutrients from below reach the sunlit surface only where water rises.

Shallow and deep currents. Shallow (surface) currents are driven by wind blowing over the water. Earth's rotation deflects moving water to the right in the Northern Hemisphere and to the left in the Southern (the Coriolis effect), and the continents block flow, so surface currents form huge loops called gyres. There are five major subtropical gyres, in the North and South Atlantic, the North and South Pacific and the Indian Ocean; they turn clockwise in the Northern Hemisphere and counterclockwise in the Southern. Currents on the western sides of ocean basins carry warm water poleward (the Gulf Stream); those on the eastern sides carry cool water toward the equator (the California Current). Deep currents are driven by density, so the circulation is called thermohaline (heat and salt). In the far North Atlantic and around Antarctica, surface water is chilled and made saltier when sea ice forms and leaves salt behind; this dense water sinks and spreads slowly along the ocean floor, taking on the order of a thousand years to travel through the ocean basins before rising again. Deep and shallow currents connect into one global circulation that carries thermal energy from the tropics toward the poles.

Thermal energy shifts: El Niño and La Niña. Normally, trade winds blow west across the equatorial Pacific, piling warm surface water toward Indonesia and Australia and letting cold, nutrient-rich water rise (upwell) off Peru, which supports large fisheries. During an El Niño, the trade winds weaken, warm water spreads back east, upwelling off Peru is suppressed and fish catches fall. The shift of thermal energy moves the heavy tropical rain eastward too: Peru and Ecuador may flood while Indonesia and Australia face drought, and weather patterns shift in North America, often wetter across the southern United States in winter. During La Niña, the trade winds strengthen and the pattern is roughly reversed. Effects far from the Pacific, including in Ohio, are weaker and less consistent from one event to the next.

Waves. Most ocean waves are made by wind, and their height depends on the wind's speed, how long it blows and the fetch, the distance of open water it blows across. A wave moves energy much farther than it moves water: as a wave passes, water near the surface moves in a nearly circular path and returns almost to where it started, with only a small forward drift, which is why a floating ball bobs rather than traveling with the wave. The circles shrink with depth and become negligible at a depth of about half the wavelength, the wave base. When a wave enters water shallower than its wave base, the circles drag on the bottom, the wave slows, its wavelength shortens and its height grows until it becomes too steep and breaks. Here is a model worked through: waves with a wavelength of 60 m begin to feel the bottom where the water is about 60 ÷ 2 = 30 m deep, and from there inward they slow and steepen. Where they break depends on their height: a wave breaks when it becomes too steep for the depth, so larger waves break in deeper water, and a sandbar about 3 m deep hundreds of meters offshore can make the larger waves break there, far from the beach, while small waves pass over it.

Longshore and rip currents. Waves usually arrive at a slight angle to the beach. The water they push up the beach runs back straight down the slope, so sand zig-zags along the shore, and a longshore current flows parallel to the beach inside the breaking waves, moving large volumes of sand (longshore drift); structures that block it, such as jetties, trap sand on one side and starve the beach on the other. A rip current is a narrow, fast flow of water straight out to sea, where water piled against the beach by waves drains back through a gap in a sandbar. A swimmer caught in one should not fight it: swim parallel to the shore until out of it if able; otherwise float or tread water, let the current weaken and wave or call for help.

Where rivers meet the tides: the Mississippi and Colorado deltas. A delta is built where a river's sediment outpaces the waves and tides that rework it, so engineering upstream changes the coast. The Mississippi has switched its lower course many times over thousands of years, each time the channel grew long and a shorter, steeper route to the Gulf opened; in the twentieth century the Atchafalaya River threatened the next switch, and since 1963 the Old River Control Structure has held the flow split so that most of the water stays in the main channel. The benefits are real: New Orleans, Baton Rouge and their ports keep a reliable river, and levees protect towns from floods. The costs are also real: levees keep floodwater, and the sediment it carries, from spreading over the wetlands, so most of the sediment is carried to the river's mouths, where it settles on the delta front and in the deep water beyond, while the delta's soft sediment goes on compacting and sinking. The US Geological Survey estimates that coastal Louisiana as a whole (not the delta alone) lost about 4,800 km² of land between 1932 and 2016, roughly a quarter of its 1932 land area; starving the wetlands of sediment is one cause, along with subsidence, canals cut through the marsh, storms and rising sea level. With less marsh to slow it, a hurricane storm surge now reaches farther inland than it once did, and a surge that arrives at spring high tide rides on the highest water of the month. The Colorado River shows the opposite extreme. Dams such as Hoover and Glen Canyon store its water, and cities and farms in seven US states and in Mexico divert it; the shares promised by the 1922 Colorado River Compact and a 1944 treaty with Mexico add up to more than the river's long-term average flow, and reservoirs lose more by evaporation. Since the 1960s the river has rarely reached the Gulf of California (the Sea of Cortez), apart from some wet years in the 1980s and a planned release in 2014. Without fresh water and sediment, most of its once-vast delta wetlands dried out, and the large tides of the narrow upper Gulf now push salt water up the old channel and rework the delta's edge instead of being balanced by the river.

Energy from the ocean. Tides, waves and currents carry large, predictable amounts of energy. A tidal barrage traps water behind a dam across an estuary at high tide and releases it through turbines; the La Rance station in France has operated since 1966. Tidal-stream turbines sit in fast tidal currents like underwater wind turbines. Wave converters use the rise and fall of the surface or the compression of air in a chamber. The trade-offs are cost, corrosion and storm damage, and effects on sediment movement, fish and navigation.

Worked example, tide timing: High tide at a hypothetical harbor is at 06:10 on Monday. With a semidiurnal tide the next high comes about 12 hours 25 minutes later, at 18:35. On Tuesday the morning high is about 50 minutes later than Monday's: about 07:00. After 7 days it is about 7 × 50 = 350 minutes, 5 hours 50 minutes, later: near midday. After about 15 days the Moon has moved from, say, new to full, and spring tides return.

Worked example, spring and neap ranges: A hypothetical harbor records a high of +1.5 m and a low of −1.5 m relative to mean sea level at spring tide, a range of 3.0 m, and +0.6 m and −0.6 m at neap tide, a range of 1.2 m. A storm surge of 1.2 m arriving at spring high tide brings water to 1.5 + 1.2 = 2.7 m, overtopping a 2.5 m sea wall; the same surge at neap high tide brings water to 0.6 + 1.2 = 1.8 m, below the wall. The timing, not only the storm, decides the damage.

Worked example, energy from a tidal barrage: A hypothetical basin of area 1 km² (1,000,000 m²) has a tidal range of 5 m. The water trapped at high tide has mass ρAh (density × area × height), and its center of mass falls through half the range, h/2, as it drains, so the energy per emptying is E = ½ρgAh² = 0.5 × 1,025 × 9.8 × 1,000,000 × 5² ≈ 1.26 × 10¹¹ J. With about two emptyings a day, 2.5 × 10¹¹ J ÷ 86,400 s ≈ 2.9 MW on average, before turbine losses. Because energy scales with h², doubling the range to 10 m gives four times as much, which is why barrages are built only where tidal ranges are large.

Worked example, speed of a tsunami: A tsunami's wavelength, often over 100 km, is so long that even the deep ocean is shallow compared with its wave base, so it travels at the shallow-water speed v = √(g × depth). Over ocean 4,000 m deep, v = √(9.8 × 4,000) = √39,200 ≈ 198 m/s, about 710 km/h. Over a 50 m shelf, v = √(9.8 × 50) ≈ 22 m/s: the wave slows by a factor of 9 and its height grows as the energy crowds together.

Investigation, density currents: Materials: a clear plastic storage box about 30 cm long, a removable cardboard divider sealed with clay, tap water at room temperature, water chilled with ice and salted (about 35 g of salt per liter), food coloring, a thermometer and a ruler. Fill one side with colored cold salty water and the other with clear room-temperature fresh water to the same level. Measure and record the temperature of each side. Lift the divider and record, every 10 seconds, how far the colored front has traveled along the bottom and the top. Repeat with cold fresh water, and with warm salty water. Result that supports the claim: the colder, saltier water slides along the bottom beneath the lighter water as a deep current, and the speed of the front increases with the density difference. What the model cannot show: Earth's rotation, which steers real deep currents, and the thousand-year time scale of the global circulation. Wipe up spills at once.

Representation task: On a blank world map draw the five major subtropical gyres with arrows, color warm currents red and cool currents blue, and label the Gulf Stream and the California Current. On a second sketch, draw the Sun, Earth and Moon positions for spring tides and neap tides and label which gives the larger range.

Independent performance, an ocean-energy proposal: Choose a coastal site (real or hypothetical) and propose a tidal barrage, tidal-stream turbines or wave converters. Success criteria: the proposal states the tidal range or wave conditions the design needs; calculates an energy or power estimate with units and states its assumptions; explains how spring and neap tides would change the output; names at least three trade-offs (cost, storms, sediment, wildlife, navigation); and ends with a recommendation supported by that evidence.