Chapter 2 · Try it yourself

Ekman Hill

The middle of the North Pacific is literally uphill. Winds on both sides of the gyre shove surface water toward the center, and Earth’s spin decides which way it goes. Build the hill, then take the spin away.

100% of average

Seen from above

Top view of the gyre. Westerlies blow east (about 45°N) Trade winds blow west (about 15°N) +1.0 m North is up
  • Wind
  • Ekman transport of surface water (90° right of the wind)
  • Flow circling the hill
  • Floating debris
  • Hill contours; in the slice, the flat sea level

Sliced from south to north

Cross-section of the gyre. South: trade winds North: westerlies Sea surface: +1.0 m in the middle Warm, light surface water Thermocline pushed down Cold, dense deep water

Heights hugely exaggerated. A 1 m bump spread over about 3,000 km is far too gentle to see from a ship; the thermocline dips hundreds of meters.

Height of the hill in the middle
≈ 1.0 m

  • Surface water is pushedtoward the middle
  • Flow around the hillclockwise

Why the water circles the hill instead of sliding off

A parcel of water on the side of the hill. Hilltop Lower sea surface Pressure Coriolis Flow North is up

Step 1 of 4

Simplified model. Ekman transport moves the top layer of water (roughly the top 100 m) 90° to the right of the wind in the Northern Hemisphere, so both wind belts push water toward the middle. Hill height here scales directly with wind strength and is set to about 1 m at average winds, the order of the real bulge (WHOI: up to about 1 m above the surrounding sea). In the real ocean the hill sits closer to the western side of the basin and builds over years, not instantly. The thermocline depth is drawn illustratively.