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Place 06 · The North Pacific Garbage Patch · Chapter 2 of 7
The Hill in the Middle of the Ocean
The sea surface in the middle of the North Pacific stands up to about a meter higher than at its edges, and the wind keeps pushing floating things up the hill.
About 15 minutes · 1 video · 1 live wind map · 2 quick activities
Start here
The wind turns right, again
Remember Monterey Bay: in the Northern Hemisphere, wind-driven surface water ends up moving about 90° to the right of the wind. Around the North Pacific, two great wind belts blow in opposite directions, the trade winds toward the west in the south and the westerlies toward the east in the north. Turn both of them 90° to the right and they point the same way: into the middle.
Watch this
See it move
SciencePrimer — Ocean Gyres and Geostrophic Flow. Four and a half minutes of tidy animations on how wind, a spinning planet and gravity combine to build a gyre. It is the physics of the garbage patch with none of the garbage.
While you watch, look for
- Which way the wind-driven surface water moves compared with the wind, and where it piles up
- The two pushes on the water in the hill that balance each other, and what flow they create
- Something you did not expect. Hold onto it.
The short version
Three things going on here
The winds
Two winds, one loop
In the southern part of the North Pacific the trade winds blow steadily toward the west; farther north, the westerlies blow toward the east. Between them sits a zone of high pressure and light winds, the subtropical high. Together they spin the whole ocean clockwise into the giant loop of currents you met in the Galápagos: a subtropical gyre.
The hill
Water piles up in the middle
Ekman transport pushes surface water toward the center of the gyre from every side, so it converges there and piles up into a hill of seawater up to about a meter high, spread over thousands of kilometers. The piled-up water slowly sinks, so nutrients from the deep never reach the sunlit surface, and the middle of the gyre is one of the least productive, clearest blue waters on Earth. Anything that floats is pushed in with the surface water and cannot sink out.
The swirl
Around the hill, not down it
Gravity tries to pull water back down the hill, but the Coriolis effect bends that flow to the right until it runs around the hill instead: geostrophic flow, clockwise in the Northern Hemisphere. Up close, strong steady winds also roll the surface into long spirals called Langmuir cells, which gather foam, seaweed and plastic into neat lines called windrows. Where warm and cooler surface waters meet north of Hawaii, the Subtropical Convergence Zone collects floating debris into another long line.
Photos: Jan van der Wolf, Engin Akyurt, Bruno Almeida / Pexels
Play with it
Find the winds that build the hill
This live map shows today’s winds over the North Pacific, with the moving particles showing which way the air is flowing and thin lines showing air pressure. Look for the trade winds blowing west in the south, the westerlies blowing east in the north, and the calm high-pressure zone in between, roughly over the garbage patch.
Live map: Windy.com, using the ECMWF forecast.
Two questions to answer while you are in there
- Find a patch of wind blowing toward the west and a patch blowing toward the east. Turn each one 90° to the right: which way would the surface water under each one move?
- Where are the winds weakest on the map today? How does that compare with where the garbage patch sits, around 32°N 145°W?
Try it yourself
Check yourself
Try it
From Wind to Hill
Drag eight words into place and follow the wind all the way to the trash.
Try it · interactive
Ekman Hill
Take a slice across the gyre: turn on the trade winds and the westerlies, watch Ekman transport push water toward the middle and build a hill about a meter high, see the flow turn to circle the hill in geostrophic flow, and watch the debris collect at the top.
Open Ekman Hill in its own window (opens in a new tab).
Words you will hear
Four terms, that is all
Ekman transport
Net water movement 90° to the wind
Convergence
Surface water meeting, piling up and sinking
Geostrophic flow
Current balanced between gravity and Coriolis
Subtropical gyre
Giant current loop around a subtropical high
Check your understanding
Practice the whole chapter
Everything in this chapter, in eight quick questions. Retry as often as you like. Read the feedback on every answer, right or wrong, because that is where the learning is.
Practice
Practice: The Hill in the Middle of the Ocean
Eight questions covering the video, the three cards, the four terms and the interactive above.
If you want more
Go deeper
- Webb 9.3 — The Ekman Spiral and Geostrophic Flow — LibreTexts · 8 minutes
- Webb 9.7 — Langmuir Circulation — LibreTexts · 4 minutes
- WHOI — Currents, gyres and eddies — Woods Hole Oceanographic Institution · 6 minutes
Photo credits (Pexels): JASWINDER SINGH, Richard REVEL, Jan van der Wolf, Engin Akyurt, Bruno Almeida.
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