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Unlock This CourseImagine the ocean as a giant bathtub that's constantly moving. The water rises and falls twice a day and massive rivers of seawater flow around the globe like underwater highways. These movements are tides and currents - two of the most important forces shaping our marine environment.
Tides and currents control where marine animals live, how they feed and when they breed. They also affect human activities like fishing, shipping and coastal development. Understanding these powerful forces helps us predict weather patterns, plan coastal defences and protect marine ecosystems.
Key Definitions:
Tides happen because of gravity - mainly from the moon, but also from the sun. The moon pulls on Earth's water, creating a bulge of water on the side facing the moon. There's also a bulge on the opposite side because Earth itself is being pulled towards the moon, leaving the water behind. As Earth spins, different places experience these bulges as high tides.
Not all tides are the same. The height and timing of tides change depending on the positions of the moon and sun relative to Earth.
Every month, we see a pattern of stronger and weaker tides based on the moon's phases.
Occur during new and full moons when the sun and moon align. These create the highest high tides and lowest low tides - the biggest tidal range.
Happen during quarter moons when the sun and moon are at right angles. These create smaller tidal ranges with less extreme highs and lows.
Most places have two high tides and two low tides each day (semi-diurnal), but some areas have only one of each (diurnal).
The Bay of Fundy has the world's highest tides, with a tidal range reaching up to 16 metres! The funnel shape of the bay amplifies the tidal effect. At low tide, you can walk on the seafloor, but six hours later, that same spot is underwater. Local wildlife like seabirds time their feeding to these extreme tides and tourists come from around the world to witness this natural phenomenon.
While tides move water up and down, currents move water horizontally across ocean basins. These underwater rivers transport heat, nutrients and marine life around the globe.
Several forces work together to create ocean currents, making them much more complex than tides.
Surface winds push the top layer of seawater, creating currents. Trade winds near the equator drive major current systems like the Gulf Stream. These currents typically affect the top 100 metres of the ocean.
Cold water is denser than warm water, so it sinks. This creates deep currents as cold polar water flows towards the equator along the ocean floor.
Salty water is denser than fresh water. When seawater evaporates or freezes, it leaves salt behind, making the remaining water denser and causing it to sink.
Earth's rotation deflects moving water to the right in the northern hemisphere and left in the southern hemisphere, creating circular current patterns called gyres.
Ocean currents can be classified in different ways depending on their depth, temperature and what drives them.
The ocean has different layers of currents, like floors in a building, each moving at different speeds and directions.
These affect the top 10% of the ocean and are mainly driven by wind. They're faster-moving and carry warm water from the equator towards the poles. Examples include the Gulf Stream and Kuroshio Current.
The Gulf Stream is like a river within the ocean, carrying warm water from the Caribbean towards Europe. It's about 100 kilometres wide and moves at speeds up to 2 metres per second. This current keeps Western Europe much warmer than it would be otherwise - London is at the same latitude as Labrador, Canada, but has a much milder climate thanks to the Gulf Stream's warm water.
Tides and currents are like the heartbeat of the ocean - they control when and where marine life feeds, breeds and migrates.
Many marine animals have evolved to take advantage of tidal and current patterns for survival.
Many shore animals like crabs and wading birds time their feeding to low tide when food is exposed. Fish often feed more actively during tidal changes when currents bring fresh nutrients.
Sea turtles, whales and fish use currents like underwater highways for long-distance travel. Young sea turtles ride currents for thousands of kilometres during their first years of life.
Many marine animals time their reproduction to tidal cycles. Some fish spawn during spring tides when strong currents help disperse their eggs over wider areas.
Humans have learned to work with tides and currents for thousands of years, but we also face challenges from these powerful forces.
Understanding tides and currents helps humans in many practical ways.
Ships use currents to save fuel and time. The shipping industry plans routes to take advantage of favourable currents and avoid opposing ones. Tidal information is crucial for ships entering shallow harbours.
Tidal power stations generate electricity from the movement of tides. Countries like the UK are developing tidal energy as a clean, predictable power source.
Fishermen use tidal charts to plan when to fish. Many fish are more active during certain tidal states and some fishing methods only work at specific tide levels.
Engineers must consider tides and currents when building coastal defences, bridges and harbours. Storm surges combined with high tides can cause severe flooding.
London's Thames Barrier is one of the world's largest movable flood barriers. It protects the city from storm surges that could combine with high tides to flood central London. The barrier has 10 steel gates that can be raised when dangerous tide conditions are predicted. Since opening in 1984, it has been closed over 200 times, preventing billions of pounds of flood damage.
As our climate changes, tides and currents are also changing, with important consequences for marine ecosystems and human communities.
Global warming is affecting ocean circulation and tidal patterns in several ways.
As ice caps melt and seawater expands due to warming, sea levels are rising. This means higher high tides and more frequent coastal flooding, even during normal weather conditions.
Scientists are also concerned that climate change could disrupt major current systems like the Gulf Stream. If this happened, it could dramatically alter weather patterns across Europe and North America, affecting everything from agriculture to marine ecosystems.
Understanding tides and currents is more important than ever as we face these challenges. By studying these systems, we can better predict future changes and develop strategies to protect both marine life and human communities.