Questions: Coastal Upwelling and Ekman Layer Dynamics

Question 1

Along the California coast, prevailing winds blow equatorward (from north to south). In the Northern Hemisphere, the depth-integrated net Ekman transport moves surface water in which direction?

• A. Southward, following the wind direction
• B. Northward, opposing the wind direction
• C. Westward (offshore), approximately 90° to the right of the wind
• D. Eastward (onshore), approximately 90° to the left of the wind

Answer: (C)

In the Northern Hemisphere, the Coriolis force deflects moving objects to the right of their motion. Each layer of the Ekman spiral is deflected further right than the layer above it. When you integrate all layers from the surface to the bottom of the Ekman layer, the net transport is approximately 90° to the right of the wind direction. Wind blowing southward → net transport 90° to the right → westward (offshore). This offshore transport pulls surface water away from the California coastline, drawing cold, nutrient-rich water up from depth to replace it — coastal upwelling.

Question 2

A student argues that since wind drags the ocean surface, the surface current should flow in the same direction as the wind. Where does this reasoning go wrong?

• A. The reasoning is correct — the surface current does flow in the wind direction
• B. Wind does drag the surface layer in roughly the wind direction, but Coriolis deflects even the surface current ~45° to the right; crucially, the net depth-integrated transport is 90° to the right, which is what drives upwelling
• C. Friction from the seafloor reverses the surface current to flow opposite to the wind
• D. The ocean surface is frictionless, so wind has no direct effect on surface currents

Answer: (B)

The surface current actually does move somewhat in the wind direction — it is deflected about 45° to the right in the NH, not 90°. The student's error is equating surface current direction with net Ekman transport. As depth increases, successive layers are deflected further right and slow down. The integrated effect of all layers (the net transport) is 90° to the right. The 45° surface deflection and 90° net transport are both real; the upwelling mechanism depends on the net transport pulling water away from the coast, not on the surface current direction alone.

Question 3

During El Niño events off the coast of Peru, trade winds weaken and warm water pools in the eastern Pacific. This reduces coastal upwelling and causes local fisheries to collapse.

• T. True
• F. False

Answer: True

Normally, strong equatorward trade winds drive net Ekman transport offshore along the Peruvian coast (Southern Hemisphere: Coriolis deflects to the left, so equatorward wind → offshore transport → upwelling). During El Niño, the trade winds weaken — sometimes reversing. Reduced offshore Ekman transport means less upwelling; warm surface water from the west pools in the eastern Pacific and deepens the thermocline, suppressing the cold, nutrient-rich deep water from reaching the surface. Without nutrient upwelling, phytoplankton blooms collapse, and the entire food web — anchovies, seabirds, sea lions — follows. El Niño's ecological impact on Peru is a direct consequence of disrupted Ekman dynamics.

Question 4

Eastern boundary upwelling systems (California, Peru, northwest Africa) are biologically unproductive because the cold water they bring to the surface is oxygen-depleted and can seldom support marine life.

• T. True
• F. False

Answer: False

This reverses the reality. Deep water brought up by coastal upwelling is cold and dense, but it is rich in dissolved nutrients (nitrates, phosphates, silicates) that have accumulated from the decomposition of sinking organic matter in the deep ocean. These nutrients are the limiting factor for phytoplankton growth in surface waters. Coastal upwelling systems — California Current, Humboldt Current, Canary Current, Benguela Current — are among the most biologically productive ocean regions on Earth, supporting massive fisheries despite being adjacent to some of the world's driest deserts. The same atmospheric circulation that drives the winds creates both the upwelling and the coastal aridity.

Question 5

Explain why the net Ekman transport is approximately 90° to the right of the wind in the Northern Hemisphere (rather than in the wind direction), and trace the causal chain from this transport to coastal upwelling.

Model answer: Wind drags the surface layer in approximately the wind direction, but the Coriolis force deflects it to the right. That surface layer drags the next layer down via friction, but by the time momentum transfers, Coriolis has deflected this deeper layer even further right — and so on. Each successive layer moves more slowly and at a greater angle from the wind. When all layers are summed (integrated through the Ekman layer depth), the net water movement is approximately 90° to the right of the wind. Along a coastline where winds blow parallel to the shore (e.g., equatorward along western continental margins in the NH), this 90° transport is directed offshore. Surface water moves seaward, and cold, nutrient-rich water from 100–300 m depth rises to fill the gap — coastal upwelling.

The 90° result is an elegant consequence of a spiral: the Ekman spiral's vector sum points 90° from the forcing. The physical mechanism is a balance between wind-driven momentum input, Coriolis deflection, and frictional dissipation with depth. The practical consequence — that winds blowing parallel to a coast drive upwelling rather than along-shore currents — is deeply counterintuitive and explains why the world's major upwelling systems are all on the eastern sides of ocean basins, adjacent to equatorward wind systems.