Skills & Environment — Topics

Tides Currents Waves & Rips Altitude Diving Search & Recovery / Night Diving Practice Questions
← Theory Hub

This section covers what you need to know about currents

Watch Will Welbourn explain the Coriolis effect and ocean gyre circulation, including a simple doodle method that lets you work out current direction along any coastline in seconds — one of the most commonly failed questions in the IDC.

What Causes Ocean Currents

The major ocean currents (gyres) are caused by the wind and the Earth's rotation — without both, they would not exist. Wind alone would not create circular ocean-wide circulation; it is the Earth's rotation that gives the wind its curve, and that curve is what drives the gyres. To understand why currents run the direction they do, you first need to understand what makes the wind curve — and that's the Coriolis effect.

Exam trap — wind alone is not the correct answer If a question asks what causes major ocean currents and lists "wind" as one option and "wind and the Earth's rotation" as another, the second answer is always correct. Wind is part of the picture, but without the Earth's rotation there would be no Coriolis effect and no circular gyre pattern.

The Coriolis Effect

Wind moves from high pressure to low pressure. The equator is generally low pressure; the poles are generally high pressure. So wind wants to travel in a straight line from the poles toward the equator. But the Earth is rotating — and that rotation causes the wind to curve.

  • In the northern hemisphere, wind curves to the right
  • In the southern hemisphere, wind curves to the left
Definition for your notes The Coriolis effect is the apparent curving of the path of moving objects (wind, water) due to the Earth's rotation. If the Earth were not rotating, wind would move in a straight line from high to low pressure.

How the Coriolis Effect Drives Ocean Circulation

The curving wind sets up ocean-wide circular current patterns — called gyres. The direction of that circulation flips between hemispheres:

Hemisphere Wind curves Ocean currents circulate
Northern To the right Clockwise
Southern To the left Counter-clockwise

Answering Coastline Current Questions in the Exam

PADI exams often take this one step further — instead of asking which direction the ocean circulates, they'll name a specific coastline and ask which direction the current runs along it. Here's a reliable method that works every time.

The doodle method

Draw a vertical line to represent the continent. Then draw a circle on each side of the line — one for the ocean on the west coast, one for the ocean on the east coast. Add arrowheads around each circle to show the direction of rotation: clockwise for the northern hemisphere, counter-clockwise for the southern. Now look at the side of each circle that is closest to the line (the coastline) — the arrowhead there tells you which direction the current runs along that coast.

NORTHERN HEMISPHERE Draw a clockwise circle on each side of the line N S WEST COAST EAST COAST OCEAN N→S OCEAN S→N SOUTHERN HEMISPHERE Draw a counter-clockwise circle on each side of the line N S WEST COAST EAST COAST OCEAN S→N OCEAN N→S

The highlighted arc on each circle is the side closest to the coastline. Read that arrowhead direction to get the current direction for that coast.

Hemisphere West coast current direction East coast current direction
Northern (e.g. North America, Europe) North → South South → North
Southern (e.g. South America, southern Africa) South → North North → South
Real-world examples to anchor this
  • West coast of North America (northern hemisphere, west coast) — current runs north to south, bringing cold polar water down past Los Angeles and San Diego. That's why the Pacific off California is cold despite the warm climate.
  • West coast of Europe (northern hemisphere, west coast) — current also runs broadly north to south, drawing cold water down from higher latitudes.
Exam trap — this is one of the most commonly answered questions wrong in IDCs Don't try to memorise specific coastlines. Draw the doodle every time. Whether the question says North America, South America, Africa, or Europe — it doesn't matter. What matters is: northern or southern hemisphere, and west coast or east coast. The doodle gives you the answer in seconds.

I really like this video to help you better understand the way the rotating earth causes a deflection of the wind and therefore the ocean gyres. Just imagine that ball is wind flowing from the poles to the equator.

Skills & Environment — Topics

Tides Currents Waves & Rips Altitude Diving Search & Recovery / Night Diving Practice Questions
← Theory Hub