Upwelling is an oceanographic process where deep, cold, and nutrient-rich water rises to the ocean surface. This phenomenon contrasts with the warmer, nutrient-depleted surface waters it displaces. Upwelling plays a role in marine ecosystems globally.
Understanding Upwelling
The water that rises during upwelling is characterized by its lower temperature and higher density compared to the surface waters. This deep water is also abundant in dissolved nutrients, such as nitrates, phosphates, and silicates. These nutrients accumulate in deeper waters from the decomposition of organic matter that sinks from the surface.
This process commonly takes place along coastlines, particularly on the western sides of continents, and in equatorial regions. It can also occur in the open ocean where surface currents diverge. The influx of these dissolved nutrients to the sunlit upper layers of the ocean supports marine life.
The Driving Forces of Upwelling
The primary forces driving upwelling involve a combination of wind, the Coriolis effect, and Ekman transport. Winds blowing across the ocean surface are a main instigator, pushing surface water away from certain areas. When winds blow parallel to a coastline, for instance, they can push surface water offshore.
The Coriolis effect, a force resulting from Earth’s rotation, deflects moving water. In the Northern Hemisphere, this deflection is to the right of the wind’s direction, while in the Southern Hemisphere, it is to the left. This deflection, combined with wind, leads to Ekman transport, which is the net movement of surface water at about a 90-degree angle to the wind direction. When Ekman transport moves surface water away from a coast or causes surface waters to diverge in the open ocean, deeper water rises to fill the void.
Coastal upwelling occurs when winds cause Ekman transport to move surface waters away from the shore, such as along the west coasts of continents like California or Peru. Equatorial upwelling happens near the equator where trade winds and the Coriolis force cause surface waters to diverge. Upwelling can also be influenced by underwater topography, where deep currents encountering features like seamounts are forced upwards.
Ecological and Economic Significance
Upwelling systems are highly productive marine areas due to the continuous supply of nutrients from the deep ocean to the surface. These nutrients act as fertilizers for microscopic marine plants called phytoplankton. Phytoplankton form the base of the marine food web, converting sunlight and carbon dioxide into organic matter through photosynthesis.
The enhanced growth of phytoplankton, often visible as blooms, supports a wide array of marine organisms. Zooplankton feed on phytoplankton, and in turn, become food for various fish, marine mammals, and seabirds. This robust food chain makes upwelling zones some of the most biologically diverse and productive ecosystems globally.
These productive waters are also economically significant, particularly for global fisheries. Upwelling regions, despite accounting for only about one percent of the ocean surface, contribute approximately 50 percent of the world’s fisheries landings. For example, the coastal waters off Peru, influenced by the Humboldt Current, constitute the world’s largest upwelling system and support the immense anchoveta fishery. Similarly, the California Current System, another upwelling region, supports commercial fisheries valued at billions of dollars annually.
Upwelling also contributes to the global carbon cycle. While upwelling can bring carbon dioxide-rich water from the deep ocean to the surface, potentially releasing it into the atmosphere, it also fuels phytoplankton growth. These phytoplankton absorb carbon dioxide during photosynthesis, which can help to sequester carbon. The balance between these processes determines the net effect on atmospheric carbon dioxide levels.