Coastal Upwelling and Shelf Dynamics

Along certain windswept coasts, surface water is driven offshore and replaced by cold, nutrient-rich water from below, fueling some of the most productive marine ecosystems and fisheries on Earth.

Definition

Coastal upwelling is the wind-driven rise of cold, nutrient-rich deep water to the surface near coasts; shelf dynamics is the study of the currents, mixing, and exchange processes that govern circulation over continental shelves.

Scope

This topic covers the wind-driven Ekman dynamics that produce coastal upwelling, the major eastern boundary upwelling systems, the circulation and fronts of continental shelves, the seasonality and variability of upwelling, and the high biological productivity and fisheries that upwelling supports.

Core questions

• How do alongshore winds and the Coriolis effect drive coastal upwelling?
• Where and when do the major upwelling systems occur?
• What governs the circulation, fronts, and exchange of continental shelves?
• Why does upwelling make some coasts extraordinarily productive?

Key theories

Ekman-driven coastal upwelling

Alongshore wind drives offshore Ekman transport in the surface layer, and conservation of mass draws cold, nutrient-rich water up to replace it near the coast.

Bottom-up control of upwelling productivity

The nutrients supplied by upwelling fuel intense primary production that propagates up short food chains, explaining why eastern boundary upwelling systems sustain the world's largest fisheries.

Mechanisms

Winds blowing along a coast drive surface water offshore through Ekman transport; to conserve mass, deeper water rises along the coast, bringing cold, nutrient-laden water into the sunlit zone. These nutrients fuel phytoplankton blooms that support abundant fish, while shelf currents, fronts, and topography organize the resulting circulation and exchange with the open ocean.

Clinical relevance

Upwelling systems such as those off Peru, California, northwest Africa, and Benguela produce a large share of global fish catch, so their variability driven by El Nino and climate change has major economic consequences; upwelling can also bring low-oxygen, acidified water onto shelves, stressing ecosystems.

History

Ekman's theory of wind-driven transport (1905) and Sverdrup's work on the California Current laid the foundation for understanding coastal upwelling; later programs quantified the world's eastern boundary upwelling systems and, through work such as Bakun's, examined how changing winds might intensify upwelling under climate change.

Key figures

• Vagn Walfrid Ekman
• Harald Sverdrup
• Andrew Bakun

Seminal works

• talley2011
• mann2006

Frequently asked questions

Why does wind blowing along the coast cause cold water to rise?

The Coriolis effect deflects wind-driven surface water away from the coast, so deeper, colder water must rise to take its place, a process called coastal upwelling.

How does El Nino affect coastal upwelling?

During El Nino the trade winds weaken and the thermocline deepens off the eastern Pacific, so upwelling brings up warmer, nutrient-poor water, sharply reducing productivity and harming fisheries such as the Peruvian anchovy.