The restless ocean
The ocean is always moving. Waves roll onto beaches, and huge currents carry water around the planet like rivers in the sea. These movements shape coastlines, control climate and carry nutrients and living things across the world.
Ocean waves
An ocean wave is energy moving through water. Most waves are made by wind blowing across the surface.
Parts of a wave
| Part | Meaning |
|---|---|
| Crest | The top of the wave |
| Trough | The bottom between waves |
| Wave height | Distance from trough to crest |
| Wavelength | Distance from crest to crest |
| Period | Time between two crests passing a point |
Water moves in circles
As a wave passes, water particles move in small circles. They don’t travel along with the wave. That’s why a floating duck bobs up and down rather than riding toward shore. The circles get smaller with depth; about half a wavelength down, the water barely moves.
What makes big waves?
Wave size depends on three things:
- Wind speed: faster winds make bigger waves.
- Wind duration: longer-blowing winds make bigger waves.
- Fetch: the distance of open water the wind blows across.
Storm waves can reach over 15 m. The largest wave ever reliably measured in the open ocean was about 29 m, recorded near Scotland in 2000. The Grand Banks off Newfoundland are known for rough seas.
Breaking waves
As waves approach shore, the bottom of each wave drags on the sea floor and slows down. The top keeps going, gets taller and steeper, and eventually tips over: the wave breaks. This is surf.
Waves shape coasts
| Effect | Landforms |
|---|---|
| Erosion | Sea cliffs, sea caves, sea arches, sea stacks (like the Hopewell Rocks in New Brunswick) |
| Deposition | Beaches, sandbars, spits |
Ocean currents
An ocean current is a continuous flow of seawater in one direction.
Surface currents
Surface currents flow in the top few hundred metres of the ocean. They’re driven mainly by wind and are steered by Earth’s rotation (the Coriolis effect) and the shapes of continents.
Surface currents form huge loops called gyres. In the Northern Hemisphere, gyres turn clockwise; in the Southern Hemisphere, counterclockwise.
| Current | Type | Effect |
|---|---|---|
| Gulf Stream | Warm | Carries tropical water north along North America and toward Europe, keeping western Europe mild |
| Labrador Current | Cold | Flows south past Labrador and Newfoundland, carrying icebergs |
| California Current | Cold | Cools the U.S. west coast |
| Kuroshio | Warm | Warms Japan |
Where the warm Gulf Stream meets the cold Labrador Current off Newfoundland, thick fog forms, and the mixing waters create rich fishing grounds.
Deep currents
Deep currents flow slowly far below the surface. They’re driven by differences in water density, which depend on temperature and salt content (salinity). This is called thermohaline circulation.
- Near Greenland and Antarctica, seawater becomes very cold and salty (when sea ice forms, salt is left behind).
- Cold, salty water is dense, so it sinks.
- It flows slowly along the ocean floor toward the equator.
- Eventually, it rises and returns as surface currents.
This "global ocean conveyor belt" moves heat around the planet. A single loop takes about 1000 years.
Upwelling
In some places, winds push surface water away from the coast, and cold, nutrient-rich water rises from below. This upwelling supports huge populations of plankton and fish, like off Peru and the BC coast.
Why currents matter
- Climate: currents move heat from the tropics toward the poles.
- Weather: El Niño, a change in Pacific currents, affects weather worldwide, often bringing Canada milder winters.
- Navigation: sailors have used currents for centuries.
- Marine life: currents carry nutrients, plankton, eggs and young animals.
- Pollution: currents gather plastic into "garbage patches" in the middle of gyres.
Common mistakes
- Thinking water travels with waves across the ocean. Energy travels; water moves in circles.
- Confusing tides, waves and currents. Waves come from wind; tides from the Moon’s gravity; currents from wind and density differences.
Try it yourself
- Float a cork in a tub and make waves. Watch how the cork moves.
- Fill a clear container with room-temperature water. Gently add blue-coloured ice water at one end and red-coloured warm water at the other. Watch the layers form.
- On a map, trace the Gulf Stream and the Labrador Current. Explain why fog is common near Newfoundland.
Quick recap
- Waves carry energy through water; water moves in circles.
- Wind speed, duration and fetch control wave size.
- Surface currents are driven by wind and form gyres.
- Deep currents are driven by temperature and salinity differences.
- Currents move heat around Earth and shape climate, weather and marine life.