Upwelling is an oceanographic phenomenon which entails winds driven movement of cooler, dense and nutrient rich water to the surface of the ocean whereby it replaces the warm, less dense and nutrient-depleted water. The nutrient rich upwelled water acts as a good environment for the growth of phytoplankton which is an example of a primary producer. The phenomenon takes place in both open seas and along coastlines. Increased presence of upwelling in a region leads to increased levels of primary productivity which leads to increased fishery production. As such, studies have shown that 25% of the global fishing are sourced from only five upwellings which occupy just 5% of all the ocean area.
There are at least three geographic or oceanographic situations that can cause an upwelling. The first situation is the Ekman transport. In this case, the wind blows along a coastline causing the surface water to move perpendicularly to the wind direction and away from the coast. When the surface water moves away from the coast, cooler and nutrient rich water rises to take its place hence causing an upwelling. The second cause encompasses the collision of two water currents. In this case, when two water currents meet head-on, both ends’ frontal water sinks and the bottom nutrient rich water is welled up. Similarly, in a scenario whereby two water currents diverge, bottom water is welled up to fill the void and hence creates an upwelling (Penven et al, 2006). Lastly, upwelling is also caused by the blowing of very cold winds over a water mass. This makes the surface water to become denser and hence moves to the bottom of the ocean and replaces the less dense water leading to an upwelling.
Six major areas of strong upwelling the Gulf Stream, Atlantic’s (Canary Current and Benguela Current), Pacific’s (California Current and Humboldt Current), northwestern Indian Ocean upwelling system in the Somali-Arabian Sea, the waters off Peru and the south equatorial counter (Croll et al, 2005). These regions encompass the motion of ocean current which influence on the upwelling systems in the regions. It is also noted that some of these regions encompass strategic wind systems. These wind system influence on the motion of the surface ocean waters and hence lead to the formation of upwelling systems accordingly. As such, different factors or a combination of the factors act as causative agents for the formation of different upwelling systems in different regions of ocean bodies (Penven et al, 2006).
Lastly, Southern Oscillation/El Nino effects upwelling off of S. America. El Niños are periods of anomalous climatic conditions which take place approximately every three to seven years in the tropical Pacific. They tend to last for year when they occur. At the southern region, the normal low atmospheric pressure influences the flowing of winds from the nearby high pressure zones into the region. This phenomenon causes the surface water to be swept away hence cooler nutrient rich water from deep into the ocean rises to fill the void hence causing an upwelling. However, during El Niño periods, the normal atmospheric pressure is changed hence the region experiences a high atmospheric pressure. This leads to the surface water being warmer, a phenomenon which effectively closes the upwelling system in the region (Croll et al, 2005). As such, it can be ascertained that El Ninos causes the reversal of the normal upwelling system in the southern Pacific Ocean region.
Croll, D. A., Marinovic, B., Benson, S., Chavez, F. P., Black, N., Ternullo, R., & Tershy, B. R. (2005). From wind to whales: trophic links in a coastal upwelling system. Marine Ecology Progress Series, 289, 117-130. http://www.jstor.org/stable/24867995
Penven, P., Debreu, L., Marchesiello, P., & McWilliams, J. C. (2006). Evaluation and application of the ROMS 1-way embedding procedure to the central California upwelling system. Ocean Modelling, 12(1), 157-187. http://www.sciencedirect.com/science/article/pii/S1463500305000491