Upwelling

[4] Upwellings that are driven by coastal currents or diverging open ocean have the greatest impact on nutrient-enriched waters and global fishery yields.

[2][6] The major upwellings in the ocean are associated with the divergence of currents that bring deeper, colder, nutrient rich waters to the surface.

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 (the amount of carbon fixed by phytoplankton) in comparison to other areas of the ocean.

[13] The Benguela Current is the eastern boundary of the South Atlantic subtropical gyre and can be divided into a northern and southern sub-system with upwelling occurring in both areas.

Here, strong westerly (eastward) winds blow around Antarctica, driving a significant flow of water northwards.

Since there are no continents in a band of open latitudes between South America and the tip of the Antarctic Peninsula, some of this water is drawn up from great depths.

In many numerical models and observational syntheses, the Southern Ocean upwelling represents the primary means by which deep dense water is brought to the surface.

Some models of the ocean circulation suggest that broad-scale upwelling occurs in the tropics, as pressure driven flows converge water toward the low latitudes where it is diffusively warmed from above.

Upwelling intensity depends on wind strength and seasonal variability, as well as the vertical structure of the water, variations in the bottom bathymetry, and instabilities in the currents.

In some areas, upwelling is a seasonal event leading to periodic bursts of productivity similar to spring blooms in coastal waters.

The Peruvian upwelling, for instance, occurs throughout most of the year, resulting in one of the world's largest marine fisheries for sardines and anchovies.

[5] In anomalous years when the trade winds weaken or reverse, the water that is upwelled is much warmer and low in nutrients, resulting in a sharp reduction in the biomass and phytoplankton productivity.

The Peruvian upwelling system is particularly vulnerable to ENSO events, and can cause extreme interannual variability in productivity.

Since upwelling regions are the most productive and species rich areas in the world, they attract a high number of commercial fishers and fisheries.

On one hand, this is another benefit of the upwelling process as it serves as a viable source of food and income for so many people and nations besides marine animals.

The animals higher in the trophic levels may not completely starve to death and die off, but the decreased food supply could still hurt the populations.

This can lead to a decreasing population, especially in species that do not breed often under normal circumstances or become reproductively mature late in life.

During the normal period and La Niña events, the easterly trade winds are still strong, which continues to drive the process of upwelling.

However, during El Niño events, trade winds are weaker, causing decreased upwelling in the equatorial regions as the divergence of water north and south of the equator is not as strong or as prevalent.

This also inhibits the formation of higher altitude clouds, showers and thunderstorms and results in rainfall over the ocean leaving the land dry.

[19][20] In year-round upwelling systems (like that of the western coasts of Southern Africa and South America), temperatures are generally cooler and precipitation scarce.

Some worldwide cities affected by strong upwelling regimes include: San Francisco, Antofagasta, Sines, Essaouira, Walvis Bay, Curaçao among others.