- Upwelling brings deep, nutrient-rich water to the ocean surface
- Four eastern boundary systems supply 25% of global fish catch
- Ekman transport deflects wind-driven surface water 90 degrees offshore
Upwelling is the wind-driven process that brings cold, nutrient-rich water from the deep ocean to the surface, fueling some of the most productive marine ecosystems on Earth.
Key figure
~25%
of the global marine fish catch comes from upwelling zones covering only 5% of ocean area
Why It Matters
The world's four major eastern boundary upwelling systems, the Humboldt Current off Peru and Chile, the Benguela Current off southwest Africa, the California Current, and the Canary Current off northwest Africa, together supply roughly 25 percent of the global marine fish catch while covering only about 5 percent of the ocean's total area.
Peru's anchovy fishery alone has yielded over 10 million metric tons in peak years, making it one of the largest single-species fisheries ever recorded.
Upwelling also shapes ocean chemistry. When deep water reaches the surface, it releases dissolved carbon dioxide into the atmosphere, making upwelling zones both carbon sources and, through the phytoplankton blooms they support, carbon sinks.
This dual role places upwelling at the center of marine carbon cycling, a process that influences global climate on timescales from decades to millennia. Changes in upwelling intensity have been linked to past productivity collapses, including during El Nino events when trade winds weaken and upwelling slows along the equatorial Pacific.
How Ekman Transport Drives Upwelling
The primary driver is Ekman transport, a mechanism first described by Swedish oceanographer Vagn Walfrid Ekman in 1902. When persistent winds blow parallel to a coastline, the Coriolis effect deflects the net movement of surface water 90 degrees to the right in the Northern Hemisphere and 90 degrees to the left in the Southern Hemisphere.
Along a west-facing coast with equatorward winds, this deflection pushes surface water offshore. Deep water rises to fill the gap.
Key figure
100-300 m
typical depth from which upwelled water rises, carrying accumulated nitrates, phosphates, and silicates
This replacement water typically comes from depths of 100 to 300 meters, where it has accumulated nitrates, phosphates, and silicates from the decomposition of sinking organic matter. Once at the sunlit surface, these nutrients feed massive phytoplankton blooms that form the base of a food web supporting zooplankton, fish, seabirds, and marine mammals.
Equatorial upwelling follows a related but distinct pattern. Trade winds blowing westward along the equator drive surface water away from the equator in both hemispheres simultaneously, creating a divergence zone. The equatorial Pacific's cold tongue, a band of cooler surface temperatures stretching from the South American coast toward the central Pacific, is the most visible result.
Key Context
Andrew Bakun, a fisheries oceanographer at the University of Miami, proposed in a 1990 Science paper that global warming would intensify coastal upwelling by steepening the temperature gradient between land and sea. Stronger gradients produce stronger alongshore winds, which drive more upwelling.
Research published in 2024 by the Copernicus Marine Service has broadly supported this hypothesis, finding statistically significant intensification of upwelling in several poleward regions of eastern boundary current systems. The ecological consequences, however, remain uncertain.
During the 1972 collapse of Peru's anchovy fishery, overfishing combined with a strong El Nino reduced catches from 10.2 million metric tons to under 2 million in a single year. The crisis devastated coastal communities and reshaped international fisheries management.
FAQ
What is the difference between coastal and equatorial upwelling?
Coastal upwelling occurs when winds blowing parallel to a shoreline push surface water offshore via Ekman transport, drawing deep water up near the coast. Equatorial upwelling happens when trade winds drive surface water away from the equator in both directions simultaneously, creating a divergence zone along the equatorial belt.
Does upwelling happen everywhere in the ocean?
No. Persistent upwelling requires specific wind patterns and geography. The strongest systems occur along the western coasts of continents, where equatorward winds and the Coriolis effect combine to push surface water offshore. Seasonal upwelling also occurs in regions like the Arabian Sea during monsoon shifts.
How does climate change affect upwelling zones?
Research suggests that warming may intensify coastal upwelling in some regions by steepening land-sea temperature gradients. However, increased ocean stratification from surface warming can also restrict the depth from which water is drawn, potentially reducing nutrient supply even as upwelling winds strengthen.
Related Reading
Sources
- Primary Sources:
- What is upwelling? (NOAA Ocean Service)
- Global Climate Change and Intensification of Coastal Ocean Upwelling (Bakun, Science, 1990)
- Additional Context:
- Intensified upwelling: normalized sea surface temperature trends (Copernicus Ocean Science, 2024)
- What is upwelling? (NOAA Ocean Exploration)
Fact Check: Claim-by-Claim Verification Verified
All major claims verified against NOAA, Bakun (1990), and Copernicus Ocean Science (2024). Ekman publication date corrected from 1905 to 1902 during editorial review. Fisheries statistics cross-referenced with FAO data.
Sources used for verification
- What is upwelling? - NOAA
- Bakun (1990) - Science
- Copernicus Ocean Science (2024) - os.copernicus.org
- What is upwelling? - NOAA Ocean Exploration
- Climate Change Impacts on EBUS - Annual Reviews
