A deep dive into upwelling physics: Ekman transport, the world's great upwelling systems, how scientists track them from space, and what climate change does.
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This article is stop 3 of 9 on Kelp Forests 101 — Meet the architects — and the grazers that can undo them.
Most of the ocean's food lives in the dark. Cold deep water holds enormous banks of nitrate, phosphate, and dissolved iron — but no sunlight, so phytoplankton cannot grow there. Sunlit surface water has light and warmth — but its nutrients were long ago consumed. Upwelling is the ocean's reunion mechanism: the few places on Earth where physics pushes that deep, nutrient-loaded water back into the light. This article goes deep on the how — the rotation-driven mechanics, the world's great upwelling provinces, how satellites watch them, and what a changing climate is doing to them. For the kelp-forest side of the story, our Point Reyes atlas guide walks a coast that upwelling built.
Upwelling is counter-intuitive because winds do not push water where you would expect. Here is the chain of reasoning, step by step.
A wind blowing across open water drags the surface layer along with it — plainly intuitive. In the northern hemisphere, though, Earth's rotation (the Coriolis effect) deflects moving water to the right of the wind's direction.
The deflected surface layer drags the layer beneath it, which is itself deflected a little further to the right, and so on downward: a spiralling column of motion called the Ekman spiral, weakening with depth. The net transport of the whole wind-driven layer — averaged through the spiral — points 90 degrees to the right of the wind (in the southern hemisphere, 90 degrees to the left). This net movement is Ekman transport, worked out by Swedish oceanographer Vagn Walfrid Ekman in 1905 after Nansen's arctic observations of drifting ice.
Now place that 90-degree rule along a coast. Along California or Peru, winds commonly blow equatorward — parallel to shore. Ekman transport pushes the surface layer offshore, away from the coast. Surface water cannot come from the land, so deep water rises to replace it: coastal upwelling. At the equator, the trades drive westward flow that is deflected northward just north of the line and southward just below it — the waters part — and deep water rises along the equator itself: equatorial upwelling.
A fourth setting matters to kelp people: wherever currents drag surface water away from a reef or headland, or deep currents strike a seamount, local "topographic" upwelling delivers nutrients to places no wind map would predict.
Deep water carries the accumulated remains of countless generations of sinking organic matter, remineralised into nitrate, phosphate, silicate, and dissolved iron. When that water reaches the sunlit euphotic zone, phytoplankton respond within days — the deep blue of oligotrophic sea turns milky green-turquoise, and the entire food web follows.
The numbers explain the world's seafood geography. Upwelling zones occupy roughly one percent of ocean area yet have historically supported a wildly disproportionate share of global fish catch — on the order of a fifth or more by many estimates. Anchoveta off Peru, sardines off Namibia, mackerel and squid — the great fisheries are upwelling fisheries. And kelp, the fastest-growing primary producer among large plants, is an upwelling specialist: giant kelp can add tens of centimetres per day precisely where nitrate-rich water meets sunlit rock.
Four systems dominate: the California Current (British Columbia to Baja), the Peru or Humboldt Current (Chile and Peru — the most productive of all), the Canary Current (Northwest Africa), and the Benguela Current (Namibia and South Africa). All sit on the eastern margins of ocean basins, where equatorward winds and low-pressure systems align to blow surface water seaward. Our guides to Hokkaido and Mendocino both show these dynamics shaping particular coasts.
Across the Pacific equator, diverging flow brings cool water to the surface in a band thousands of kilometres long — visible in any global SST map as the equatorial "cold tongue." It fertilises the open ocean from Ecuador nearly to the date line.
Other systems turn on and off: the Somali coast during the southwest monsoon becomes one of the most productive waters on Earth within weeks. Australia's Bonney coast upwells each summer. These seasonal windows set the timing for herbivores, fisheries, and kelp recruitment — and are increasingly the reference point against which change is measured.
Upwelling announces itself to instruments with unusual clarity:
Two confident statements and one genuinely open question:
Heatwaves can switch off the benefit. During the 2014–2016 Blob event, anomalously deep warm surface layers meant even upwelled water arrived without its usual nutrient cargo — "upwelling the wrong water" is how some oceanographers describe it. Our marine heatwaves explainer covers the mechanism.
Upwelled water is increasingly acidic. Deep water is naturally CO₂-rich, and as anthropogenic CO₂ accumulates, upwelling zones rise toward shell-damaging chemistry earlier and harder than open seas — see Ocean Acidification Explained. Hatcheries on upwelling coasts already monitor pH hour by hour.
Will upwelling intensify or weaken? Landmark theory work suggested that land heating outpacing ocean heating would strengthen alongshore winds — and upwelling — in some systems. Since then, observations have supported intensification in some regions (portions of the California and Benguela systems) but not others. Separating a long-term trend from noisy natural variability like ENSO and the Pacific Decadal Oscillation remains an active, honest scientific frontier.
If you restore kelp forests, your entire enterprise hangs on the nutrient tap staying open. Restoration planners — including our own team, using illustrative Tidewright siting methods — score candidate reefs by upwelling exposure: proximity to capes, annual cold-water return, modelled nutrient flux under climate scenarios. A forest planted where upwelling persists for decades is an investment in planetary plumbing; one planted where heatwaves regularly smother the nutrient supply is gambling on weather.
No — upwelling is a vertical movement of water, driven ultimately by horizontal winds and rotation. It often appears alongside strong horizontal currents (like the California Current) because both arise from the same wind field, but they are distinct motions.
Coastal upwelling typically lifts water from 50–300 metres — shallow compared with the full ocean depth, but deep enough to cross the nutricline and bring up water enriched by decades of remineralisation.
Deep water left the surface decades to centuries ago in cold regions, and water's high heat capacity keeps it cold as it rises. Temperature drops of 5–10°C across an upwelling event are typical — that's why fog pours onto coasts like Point Reyes in upwelling season.
Yes. Cyclones and strong wind events at sea can pump deep water upward along their tracks — hurricane upwelling is visible in post-storm satellite imagery as cold wakes. Seamounts and mid-ocean ridges also force water upward when deep currents flow against them.
It can. Upwelled water is naturally low in oxygen (it has been out of contact with the atmosphere for some time), and in some systems hypoxic events have reached shallow coastal waters, stressing fisheries. Monitoring this is a growing priority on upwelling coasts as oxygen minimum zones expand.
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