Why do whirlpools form in the middle of the sea?
― A "height gap" between two seas creates a current like a river
The sea looks flat. And yet in narrow straits, water roars through and whirlpools appear and vanish, one after another. In fact, the tide rises at different times on the right and left sides of a strait. That gap creates a difference in sea level, and forms a "river" inside the sea itself.
You're looking down from a bridge over a narrow strait. The sea should be an endless, flat sheet of water.
But right below you, the water is racing past in white streaks. Beside it, a funnel-shaped hollow opens up, then unwinds and vanishes within a few dozen seconds. Almost at once, a new whirl appears a short distance away.
The wind isn't strong. It isn't a river. So why does the water move so fast, and why does it spin?
There are two main reasons
The tidal wave travels around the shape of the coastline as it moves. Because of that, the timing of high and low tide differs on the two sides of a strait — while one side is at high tide, the other may still be at low tide. This creates a difference in sea level, and water flows from the higher side to the lower one.
The middle of a strait is deep, so the current runs fast there. Near the shore the water is shallow, and friction with the bottom slows it down. When fast water and slow water sit side by side, the boundary between them twists, just as it would if you stirred it with a stick, and curls up into a whirl.
These two are linked in sequence. Step 1 creates a "river within the sea," and step 2 makes the edges of that river curl into whirls. Let's look at each in turn.
First, a "slope" forms on the sea surface
Tides rise and fall because the sea surface bulges under the pull of the Moon and Sun. But that bulge travels while bumping against the coastline, so it arrives at different times in different places.
Say two seas are connected by a narrow strait. The tidal bulge reaches one sea quickly, by a short path, and reaches the other only after a longer detour. Compare the two at the same moment, and one sea surface is higher, the other lower.
Water flows from high ground to low ground. The strait is the narrow passage linking the two seas. Because a height difference across the whole open sea has to be evened out through one narrow channel, the current inside that channel becomes very fast. At Japan's Naruto Strait, the tidal current is said to reach speeds of around 5 metres per second — far faster than a brisk walking pace.
Next, the edges of the current curl into whirls
If a fast current flowed uniformly, no whirls would form. What creates a whirl is a "difference." Look at Figure 1. It shows the strait from above: the current is fastest in the middle and slower near the shore.
Picture a particle of water sitting right at the boundary. One side of it is dragged by the fast water, the other held back by the slow water. When something is pushed harder on one side than the other, it starts to rotate — the same reason a car turns when only one wheel is spun faster.
The small rotation that forms at the boundary then draws in the surrounding water and grows larger. Because it forms at the shores on both sides of the fast central current, the whirls above and below spin in opposite directions. Once a whirl has grown, it eventually breaks apart, and a new one forms. That's why whirlpools appear "again and again, one after another."
The current is fastest when the tidal height difference is largest. Conversely, as the tide turns, the difference shrinks, and both the current and the whirls calm down. Sightseeing-boat timetables vary from day to day because the timing of "peak difference" shifts along with the Moon's motion.
Around the full moon and new moon, when the Moon and Sun line up in the same direction or opposite directions, the tidal range widens. The difference in sea level grows too, so the current speeds up and whirlpools tend to grow larger. It varies from year to year, but large whirlpools are said to be more common around the spring and autumn spring tides.
Summary
A whirlpool forms because the tide arrives at different times on either side of a strait, creating a difference in sea level; a fast current forms to even that gap out; and the edges of that current curl into whirls. It's a sight that only appears when three things line up together: the Moon's motion, the shape of the coast, and the depth of the seabed.
In the moment the sea stops being flat,
water shows us that moment in the shape of a whirl.
The mechanism behind tides themselves is explained in Why does the tide rise and fall twice a day?. The way a boundary between currents twists and reshapes a channel of water is also covered in Why do rivers meander instead of flowing straight?. For the side of ocean currents that can carry people away, see also Why do people suddenly find themselves swept out to sea?.
- Pour about 5cm of water into a basin or large bowl. Sprinkle a pinch of powder (pepper or powdered stock, for example) on the surface as a marker.
- Put your palm just below the surface and move it slowly in a straight line from the edge of the bowl toward the centre, in one direction only. The trick is to move it in a straight line, not to stir.
- Watch both sides of the trail your hand leaves. A handful of small whirls appear side by side at the boundary between the fast water and the still water. The faster you move your hand, the smaller and more numerous the whirls become.
This is the same mechanism as Figure 1. In a strait, it's the difference in tidal height, not a hand, that pushes the water.
Want to know more? ― Terms, formulas, and how this connects to the curriculumWe've marked which level each part belongs to, from junior-high science to university specialist courses
- JHSCovered in junior high school science
- HSCovered in high school "Basic Physics / Basic Earth Science"
- HS+High school advanced content, or textbook sidebar material
- UnivNot covered in high school — university specialist subjects (fluid dynamics, physical oceanography)
- ResearchNot yet settled as textbook fact even at university — an active research topic
JHSTerms: this phenomenon has names
- Tidal current: the horizontal flow of seawater that accompanies the rise and fall of the tide. The vertical motion is "tidal height change"; the horizontal motion is "tidal current."
- Water-level difference: the difference in sea-surface height, measured at the same moment, between two neighbouring sea areas. This is the driving force behind the current.
- Whirl: a mass of water moving while rotating. Here it refers specifically to the kind that forms at the boundary between currents of different speeds.
JHSHSCheck with a formula: estimating current speed from a height difference
We'll use the simplest way to estimate current speed from a water-level difference: imagine that water sitting at a height sets off with the speed it would gain by falling that same height. In this formula, the height difference is measured in metres, the strength of gravity in metres per second squared, and the speed in metres per second.
| In symbols | v ＝ √( 2 × g × h ) |
| In words | current speed ＝ the square root of (2 × strength of gravity × sea-level height difference) |
| Where it comes from | Conservation of energy. The potential energy of the water on the higher side is thought of as converting into kinetic energy as it flows toward the lower side (this is Bernoulli's principle applied to a frictionless flow) |
| Sea-level height difference across the strait | taken as 1.5 metres |
| Approximate strength of gravity | about 9.8 |
| Fixed factor for converting m/s to km/h | 3.6 |
| First, double the strength of gravity | 2 × 9.8 ＝ 19.6 |
| Multiply that by the height difference | 19.6 × 1.5 ＝ 29.4 |
| Find a number that, squared, comes close to 29.4 | 5.4 × 5.4 ＝ 29.16 |
| Convert that speed in m/s to km/h | 5.4 × 3.6 ＝ 19.44 |
With a height difference of 1.5 metres, the current comes out at roughly 5.4 metres per second — about 19 kilometres per hour, or roughly cycling speed. In real straits, bottom friction and the length of the channel slow this down. Even so, it lands in about the same range as tidal currents actually measured.
HSHS+Why does "narrowness" matter?
HSWhen the cross-section of a channel narrows, the current has to speed up to carry the same amount of water through. It's the same reason a wide river speeds up sharply when it enters a narrow channel. The cross-sectional area and the flow speed are related so that their product stays roughly constant. A strait is exactly this kind of "narrow channel."
HS+A boundary between currents of different speeds is known to become unstable at even the slightest disturbance, rippling and then curling up. The same phenomenon shows up in cloud shapes and at the boundaries between layers of the atmosphere. The whirls in a whirlpool are this same instability, made visible at the scale of the sea.
UnivA whirl can be counted as an "amount of rotation"
In university-level fluid dynamics, the twist in a flow field is treated as an amount of rotation defined at each point. The degree to which speed varies from place to place is exactly this quantity. In a flow that's fast in the middle and slow near the shore, this quantity becomes large at the boundary, and that's where whirls are born. This quantity is called "vorticity." Physical oceanography adds the effects of seabed topography and the Earth's rotation to this calculation, to predict tidal currents strait by strait.
📖 For the derivation of the formula and further reading: Bernoulli's principle (Wikipedia, Japanese) ／ Vorticity (Wikipedia, Japanese)
ResearchWhat's still not fully understood
- How long a whirl lives, and how large it grows How much a given whirl grows, and when it breaks apart, depends on that day's water-level difference and the fine detail of the seabed shape. Predicting an individual whirl in advance is still said to be difficult.
- Its shape below the surface How far down a surface whirl extends underwater is hard to observe, and records are sparse. Sonar-based observation has been advancing in recent years.
- Links to marine life Strong tidal currents are thought to stir up oxygen and nutrients, creating rich fishing grounds. But how to quantify that effect is still an active area of research.
In other words, this article too describes things only "as far as they're currently understood." The behaviour of individual whirls, in particular, may well be revised in the future.
Links to the curriculum, by level
| Level | Subject / unit | Where in this article |
|---|---|---|
| JHS | Science: how forces act; motion of the Moon and Earth | How a height difference pushes water; tides |
| HS | Basic Physics: energy transformation / Basic Earth Science: oceans | Estimating speed from a height difference |
| HS+ | Physics: relation between flow and cross-section | Why the current speeds up in narrow places |
| Univ | Fluid dynamics / physical oceanography | Counting whirls as an amount of rotation |
| Research | Coastal oceanography observation and modelling | Whirl lifespan, shape below the surface |
| ― | Everyday connections | Sightseeing-boat timetables, shipping lanes, fishing-ground richness |
- Japan Coast Guard, Hydrographic and Oceanographic Department (海上保安庁 海洋情報部) — explanatory pages on "Tides and Tidal Currents"
- Japan Meteorological Agency (気象庁) — explanatory pages on "Knowledge of Tides and Sea-Level"
- Sanae Unoki (宇野木早苗), Coastal Physical Oceanography (沿岸の海洋物理学), Tokai University Press
- Tetsuo Yanagi (柳哲雄), The Science of the Sea: An Introduction to Oceanography (海の科学 ― 海洋学入門), Kōseisha Kōseikaku
※This article is a general-audience science explainer. The figures given are approximate, meant to help illustrate the mechanism. In areas with fast tidal currents, approaching by small boat or from the shore can be dangerous. Please follow local signage and the guidance of the Japan Coast Guard and local authorities.