Ocean Currents Lesson: Wind, Density & Coriolis Effect

Lesson Overview

• What Are Ocean Currents?
• Surface Currents: Driven by Wind and Earth's Rotation
• Deep Ocean Currents and Thermohaline Circulation
• The Coriolis Effect – Why Currents Curve

Ocean water is not static – it's constantly on the move in patterns called currents. These currents can carry warm water from the equator toward the poles and send cold water back toward the equator, shaping climates around the world. 

If you've ever heard of a message in a bottle traveling across an ocean, that's all because of the currents! Let's understand what ocean currents are, what causes them, and why they're important.

What Are Ocean Currents?

An ocean current is a continuous flow of ocean water along a path. Unlike waves (which move up and down) or tides (the rise and fall of sea level), currents involve water actually moving from one place to another. 

For example, the Gulf Stream is a current that carries warm water across the Atlantic Ocean. Currents occur both at the surface and in the deep ocean:

• Surface currents – currents that flow in the upper layer of the ocean (often the top few hundred meters).
• Deep ocean currents – currents that flow deeper below the surface, sometimes across ocean basins.

It's important to note the difference between waves, tides, and currents. 

Take This Quiz:

Ocean Currents Trivia Quiz Questions!

Surface Currents: Driven by Wind and Earth's Rotation

Let's summarize the main factors that influence surface currents and how they work:

By combining these factors, surface currents create the "ocean highways" that transport water and heat across the globe. For instance, the Gulf Stream (driven by wind and deflection) carries warm water from the Gulf of Mexico across the Atlantic, warming the climate of Western Europe. In fact, because of the Gulf Stream, places like the UK are warmer than other regions at the same latitude.

Take This Quiz:

Trivia Quiz: How much do you know about Ocean Currents and Climate?

Deep Ocean Currents and Thermohaline Circulation

Deep beneath the surface, another system of currents is at work. Deep ocean currents are driven by differences in water density. The term thermohaline circulation describes this process – "thermo" for temperature and "haline" for salt. 

Both temperature and salinity (saltiness) affect water density: colder or saltier water = higher density, and warmer or less salty water = lower density.

Here's how it works:

• In polar regions (near the North and South Poles), ocean water gets very cold. When sea water freezes into ice, it leaves salt behind in the unfrozen water, making that water extra salty. Cold + salty means the water becomes very dense.
• This cold, dense water sinks down into the deep ocean (because denser fluids sink below less dense fluids). As it sinks, it pushes deep water outward and begins flowing along the ocean floor.
• Meanwhile, at the surface, water that is warmer (and less salty) is pushed away and eventually makes its way toward the poles to replace the sinking water. As that warm water travels poleward at the surface, it cools down (and can get saltier if evaporation or ice forming increases its salinity), eventually also sinking when it becomes dense enough.

This creates a global conveyor belt of water movement: cold deep currents flow from the poles toward the equator, and warmer surface currents flow from the equator toward the poles, in a continuous loop. 

For example, in the North Atlantic near Greenland, very cold, salty water sinks and flows southward deep below, while warm surface water from the tropics flows northward to take its place. All together, these connected deep and surface flows are called the global ocean conveyor belt.

The Coriolis Effect – Why Currents Curve

The Coriolis effect is an important concept when studying ocean currents (and winds). It often confuses students, but it's simpler than it sounds: it's the apparent curving of the path of moving objects (like air or water currents) due to Earth's rotation. 

Imagine throwing a ball straight north from the equator; because the Earth rotates under it, the ball will land slightly to the east of where you aimed. Similarly, as water flows, the Earth is turning, so the water's path curves relative to the Earth. This effect causes the big rotating gyres we learned about above. The Coriolis effect itself does not cause currents, but it steers them.

Take This Quiz:

How well do you know about Wind and Ocean Currents? Trivia Quiz