The ocean as a system
0 of 5 hands-on
01 Introduction
The sea is never still. Water is moving at every scale, from ripples on the surface to currents that cross entire oceans. Divers meet this motion as surge, current, tide and temperature layers, and it decides how a dive goes. This lesson gives you the big picture before we look at each part in turn.
Objectives
- Describe the ocean as a connected system driven by the Sun, wind and the Moon
- Explain how temperature and salinity set the density of seawater
- Name the main forces that move water: wind, tides and density differences
02 Watch
A moving ocean
4 min · Video in production
- 1
A view of Earth showing the great ocean currents as flowing arrows, warm in red and cold in blue.
- 2
A zoom into one coast, showing wind driving waves, the Moon pulling the tide and a river meeting the sea.
- 3
A cross-section showing warm water floating over cold water, with a clear boundary between them.
- 4
A diver drifting along a reef wall, with the current shown as a gentle arrow.
03 Learn
Three engines
Almost all the movement of the sea comes from three sources of energy. The Sun heats the water unevenly and drives the wind. The wind pushes the surface and makes waves and currents. The Moon, and to a lesser extent the Sun, pulls on the water and creates tides.
| Driver | What it produces | Time scale |
|---|---|---|
| Wind | Waves, swell and surface currents | Hours to days for waves, seasons for large currents |
| Moon and Sun | Tides and tidal streams | About 12 hours between high tides |
| Differences in density | Deep currents and layers in the water | Days to centuries |
| Rivers and rain | Fresh water that floats over the sea and changes salinity | Days to seasons |
Temperature, salinity and density
Seawater is about 3.5% dissolved salt on average. Salinity measures how salty water is. Cold water is denser than warm water, and salty water is denser than fresh water. Water of different density does not mix easily. Heavier water sinks beneath lighter water and forms layers, much as oil floats on vinegar.
Try different temperatures and salinities. Which has the stronger effect on density in the water you dive in?
Density and lift
Net force
+5.2 kg
positive: rising
Lead for neutral
5.7 kg
approximate
A simplified model. Real weighting depends on your suit, cylinder and body, and must be checked in the water with an instructor.
Surface currents and the great circulation
Winds that blow steadily push surface water along, and the rotation of the Earth bends the flow. Together these create giant circular systems called gyres in each ocean. Warm currents carry heat from the tropics towards the poles, and cold currents flow back towards the equator, which is why some coasts are much warmer or colder than their latitude suggests.
The deep conveyor
In cold regions, surface water becomes cold and salty enough to sink. It spreads slowly through the deep ocean and eventually rises again elsewhere. This slow, density-driven circulation is sometimes called the global conveyor. A single trip around it can take hundreds of years.
Predict first
Which of these would make surface seawater less dense?
Upwelling brings the deep to the surface
Where winds push surface water away from a coast, cold, nutrient-rich water rises to replace it. This is upwelling. It feeds plankton, which feeds fish and larger animals, so upwelling zones are some of the richest waters in the world. They are also often cold and cloudy, and sometimes have strong currents.
Take a guess first
A river floods after heavy rain and spreads a layer of fresh water over a bay. The layer is about 1 m thick. A diver descends through it into salt water. About how much denser, in percent, is the salt water than the fresh water above it?
How divers read the system
- Wind tells you about waves, surface chop and likely entry and exit conditions.
- The tide tells you when currents will be strong or slack.
- Recent rain and runoff tell you about visibility and fresh water layers.
- Season and water temperature tell you about layers and what thermal protection you need.
- Local knowledge from dive guides is often the quickest way to combine all of these.
04 Explore
Try it yourself
Match each driver with what it produces.
Match each driver with what it produces.
- Wind
- Moon and Sun
- Differences in density
- Fresh water from rivers
- Upwelling
05 Apply
Scenario
A diver visits a bay a day after very heavy rain. The surface water looks brown, and the guide says that it is much colder and cloudier than normal at the top, but clearer below.
What is the most likely explanation?
06 Check
07 Summary
Key points
- Wind, the Moon and Sun, and differences in density move the sea.
- Cold, salty water is dense. Warm, fresh water is light. Different layers do not mix easily.
- Surface winds and the Earth's rotation create large circular currents.
- Upwelling brings nutrients to the surface, and often cold, cloudy water.
Before you apply this in the water
- Have I been trained by a qualified instructor for this?
- Am I medically fit to dive?
- Do I have a buddy and a plan?
- Am I diving within my training and limits?
Answer every question in the Check section to mark this lesson complete.