The process that brings deep, cold ocean currents up to the surface is known as upwelling, an oceanographic phenomenon defined by the vertical movement of water. This movement transports cooler water from the depths toward the sunlit surface layer. Upwelling is fundamentally important for sustaining marine life globally, acting as a natural mechanism to replenish the surface ocean with essential materials.
The Origin of Deep Ocean Water
The deep water that rises during upwelling originates from a slow, global circulation system driven primarily by density differences, often called the thermohaline circulation or the Global Ocean Conveyor Belt. This circulation begins in the Earth’s polar regions, where cold temperatures cause surface water to cool and sink. The formation of sea ice extracts freshwater, leaving the remaining seawater saltier and denser. This dense, cold, and highly saline water sinks to the ocean floor, forming deep water masses that travel slowly along the ocean basins. During this transit, organic matter from the surface sinks and decomposes, releasing high concentrations of dissolved nutrients, including nitrates, phosphates, and silicates, which accumulate in the deep water.
The Mechanics of Water Transport
The lifting of this deep, nutrient-rich water is accomplished by physical forces acting on the surface ocean. The primary catalyst for upwelling is the stress of wind blowing across the ocean surface. As the wind pushes the surface water, Earth’s rotation introduces the Coriolis effect, which deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The total movement of the upper layer of water, known as Ekman Transport, is deflected approximately 90 degrees from the direction of the wind. When surface water is continuously moved away from a region, the resulting suction draws water up from the deeper layers to replace the displaced surface water. This vertical movement occurs most efficiently along continental shelves where the deeper water is close to the surface.
Where Upwelling Occurs
Upwelling is concentrated in two primary geographic contexts: along coastlines and near the equator. Coastal upwelling occurs when alongshore winds cause Ekman Transport to move surface water away from the coast. Along the western coasts of continents, such as California and Peru, winds push surface water offshore, leading to cold, deep water rising along the shorelines and resulting in significantly cooler sea surface temperatures. Equatorial upwelling happens in the open ocean where the Coriolis effect causes a divergence of surface water. As trade winds push surface water westward, the Coriolis effect deflects water slightly north and south, creating a continuous band of surface water removal. The resulting suction draws deep water up to the surface to fill the gap, creating a narrow, highly productive zone.
Ecological and Climatic Importance
The ecological consequence of upwelling is the creation of the most biologically productive marine environments on Earth. The deep water brought to the surface is a rich source of dissolved nutrients, acting as fertilizer for phytoplankton, the microscopic plant life at the base of the marine food web. This influx triggers massive phytoplankton blooms, which support zooplankton, small fish, and large populations of marine mammals and commercially valuable fish species. Upwelling zones account for only about 1% of the ocean surface area but are responsible for a disproportionately large percentage of the global fish catch. The cold water rising to the surface also significantly cools the overlying atmosphere, contributing to the formation of coastal fog. Upwelling also plays a role in the global carbon cycle by bringing water that has stored carbon dioxide at depth back into contact with the atmosphere.