Upwelling is an oceanographic phenomenon that involves wind-driven motion of dense, cooler and usually nutrient-rich water towards the ocean surface, replacing the warmer, usually nutrient-depleted surface water. The upwelling in the specific regions results in high levels of primary productivity and thus, fishery production. About 1/4th of the total global marine fish catches come from the five upwellings. The sea area occupied by these upwellings could be just about 5% of the total ocean area.
The various types of upwellings are:
- Coastal upwelling.
- Large-scale wind-driven upwelling in the ocean interior.
- Upwelling associated with eddies.
- Topographically-associated upwelling.
- Broad-diffusive upwelling in the ocean interior.
1. Coastal Upwelling
Coastal upwelling is a well known type of upwelling and is closely related to the most productive fisheries in the world. Currents in northern hemisphere are diverted to the right in the northern hemisphere and to the left in the southern hemisphere due to the Coriolis Effect. The result is a net movement of the surface water at right angles to the direction of the wind, known as the Ekman Transport. When the Ekman Transport takes the water mass away from the coast, as it happens on the south western coast of American & African continent, the water body moving away are replaced by deeper, cooler and denser waters.
Actually, worldwide, there are five major coastal currents associated with upwelling areas: the Canary Current off northwest Africa, the Benguaela Current off sothern Africa, the Californean Current off Californea and Oregeon, the Humbolt Current off Peru & Chile, and the Somali Current off Somalia and Oman. Deep waters near the coast are rich in nutrients including nitrates and phosphates. This is the result of decomposition of sinking organic matter (dead / detrital plankton) from surface waters. When brought to the surface, these nutrients are utilized by phytoplankton along with dissolved CO2 (carbon dioxide) and light energy from the sun to produce organic compounds through the process of photosynthesis. Upwelling regions therefore result in very high levels of primary production which in turn propagates up the food chain.
2. Large-scale Wind-driven Upwelling in the Ocean Interior
Strong westerly winds blow round Antarctica, driving a significant flow of water northwards (Coriolis Effect). This is the reason that large-scale upwelling is found in the southern ocean. This would be called coastal upwelling but actually there are no continents on the right hence some of this water is drawn up from great depths. The Southern Ocean upwelling represents the primary means by which deep dense water is brought to the surface. In some regions of Antarctica, wind-driven upwelling near the coast pulls relatively warm circumpolar deep water onto the continental shelf, where it can enhance ice shelf melt and ice sheet stability.
3. Large Scale Equator Upwelling
Upwelling at the equator is associated with the Inter-tropical Convergence Zone (ITCZ), which actually moves and consequently often located north or south of the equator. Easterly winds blowing along the ITCZ in both the Pacific and Atlantic Basins drive water to the right (northwards) in the northern hemisphere and to the left (southwards) in the southern hemisphere. This results in a divergence with denser, nutrient-rich water being upwelled from below. The equatorial region in the Pacific can be detected from space as a broad line of high phytoplankton concentration.
4. Topographically-Associated Upwelling
Local and intermittent upwellings may occur when offshore islands, ridges or seamounts cause a deflection of deep currents, providing a nutrient rich area in the otherwise low productivity ocean areas. Examples include upwellings around the Galapagos Islands and the Seychelles Islands which have major pelagic fisheries.
Upwelling Associated With Tropical Cyclone
Upwelling can also occur when a tropical cyclone transits an area, usually when moving at speeds of less than 5 mph (8 km/h). The churning of a cyclone eventually draws up cooler water from lower layers of the ocean.
5. Artificial Upwelling
Artificial upwelling is produced by devices that use ocean wave energy or ocean thermal energy conversion to pump water to the surface. Such devices have been shown to produce plankton blooms.
Global Upwelling phenomenon
A large scale vertical movement of ocean water due to the difference in density is worth understanding at this point.
The term Thermohaline Circulation (THC) refers to the part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes. The adjective thermohaline derives from thermo referring to temperature and haline referring to salt content. These factors together determine the density of sea water. Wind-driven surface currents (such as the Gulf Stream) head polewards from the equatorial Atlantic Ocean, cooling during the passage up north and eventually sinking at high latitudes (forming North Atlantic Deep Water). This dense water then flows into the ocean basins.
The thermocline is the transition layer between the mixed layer at the surface and the deep water layer. The terminology of these layers are based on temperature. The mixed layer is near the surface where the temperature is roughly that of surface water. In the thermocline, the temperature decreases rapidly from the mixed layer temperature to the much cooler deep water temperature.
A deeper thermocline (often observed during El Nino years) controls the amount of nutrients brought to shallower depths by upwelling processes, greatly impacting the year’s fish crop.
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