Ekman Transport and Coastal Upwelling
Wind does not drag the whole ocean with it. Friction and Earth’s rotation turn the flow. The result is Ekman transport. Water in the surface layer moves to the right of the wind in the Northern Hemisphere. It moves to the left in the Southern Hemisphere. That sideways transport can pull deep water up the continental slope. The rise is coastal upwelling.
Vagn Walfrid Ekman explained the spiral more than a century ago. Each deeper layer feels a weaker push from the layer above. Coriolis force then turns that layer a little more. Speed falls with depth. Direction keeps rotating. The net transport of the spiral is ninety degrees from the wind.
Along a west coast in the Northern Hemisphere, a wind that blows toward the equator drives surface water offshore. Deep water then climbs to replace it. The climbing water is cold and rich in nutrients. Phytoplankton bloom. Fisheries often follow. California, Peru, and parts of the Arabian Sea show this pattern.
The opposite wind piles water against the shore. Surface water sinks. That is downwelling. Nutrients stay below. The coastal sea can look clearer and less productive.
The Indian west coast feels a seasonal version. During the summer monsoon, winds along the Arabian Sea favour upwelling off Kerala, Karnataka, and parts of Gujarat. Sea-surface temperature falls. Chlorophyll rises. Anchovy and other small fish respond. When the monsoon wind fades, the pump weakens.
Upwelling is not only a surface story. The depth of the thermocline matters. A shallow thermocline lets nutrient-rich water reach the light. A deep thermocline can starve the bloom even if the wind is right. Remote waves and large climate events, including El Niño, can push that thermocline down and shut the system.
Satellites now map the cold tongue and the green plume. Moorings and gliders measure the offshore flow. Models then test how strong the wind must be to lift a given water mass.
Ekman transport is a small deflection with a large ecological cost. Turn the wind along a coast, and the ocean either feeds the shelf or seals it. That is why a lecture on rotating fluids ends at the fishing ground.