HL only: Topic 4.1 Water systems: Upwelling occurs when surface water moves away from an area and deeper water rises to replace it. It is commonly driven by wind, but water movement associated with density differences and underwater topography can also contribute.
How Upwelling Works
Upwelling is a vertical flow that transports deeper water toward the surface. Because deep water is generally colder and contains nutrients released through decomposition, upwelling often supplies surface ecosystems with cold, nutrient-rich water.
The mechanism begins when wind pushes surface water. In oceans, Earth’s rotation produces the Coriolis effect, causing the net movement of surface water, called Ekman transport, to occur at an angle to the wind. If surface water moves away from a coastline or diverges at the equator, deeper water rises to replace it.
| Setting | Main cause and mechanism | Ecological effect |
|---|---|---|
| Coastal ocean | Winds blowing parallel to a coast produce offshore Ekman transport | Cold, nutrient-rich water rises and increases phytoplankton productivity |
| Equatorial ocean | Surface currents are deflected in opposite directions on either side of the equator | Deep water rises through the resulting surface-water divergence |
| Freshwater lake | Persistent winds push surface water toward one shore, allowing deeper water to rise on the opposite side | Nutrients and colder water are transported upward |
| Underwater topography | Currents encounter continental slopes, ridges or other barriers and are redirected upward | Nutrients may reach the sunlit surface zone |
A named example is the Humboldt Current system off Peru and Chile. Strong coastal upwelling supports high phytoplankton productivity and major fisheries, linking the physical water system to food supply and economic activity.
A common misconception is that upwelling creates nutrients. It does not: it transfers nutrients already stored in deeper water to the surface.
Exam Technique
For an IB ESS explain question, identify the driver, describe surface-water displacement, and state that deeper water rises as replacement. Then connect the process to nutrient availability, primary productivity and a named example such as the Humboldt Current system.
