Classic CZCS Scenes

Classic CZCS Scenes Chapter 2: Southwest Africa - The Benguela Upwelling Zone In the first chapter, an image of the island of Tasmania illustrated the complex interactions that can result between the currents in the ocean and the phytoplankton living in the ocean. The CZCS image of Tasmania might make it seem impossible to figure out how such complex patterns are generated. In many regions of the ocean, however, the patterns are much simpler. There are some basic processes that cause the patterns found in many different areas of the ocean, all resulting from the same combination of winds, ocean currents, and the essential elements that allow life to exist in the ocean. Surface Currents of the South Atlantic The flow of most surface currents in the ocean is induced by the wind, and these currents are therefore called wind-driven currents. The northward direction of the Benguela current is part of the geostrophic flow of the world ocean, the currents which flow due to the Coriolis Effect. One result of the interaction between the prevailing winds and the Coriolis Effect is the large-scale rotational circulation in major ocean basins. These large rotational patterns are called gyres, and they result from the necessary balance between the Coriolis Effect and pressure gradients in the ocean produced by winds. In the South Atlantic, the north-flowing Benguela is part of a counterclockwise gyre. This diagram illustrates the location of the Benguela Current as well as its relationship to other currents affecting the South African coastlines. The following link provides an animation showing the flow of surface ocean currents using sea surface temperature as a tool for following that motion. Note that the first 17 seconds of the animation represent one and a half years of data for the Benguela Current. Click here for the link. Coastal Zone Productivity Over the eight years that the CZCS acquired images, very clear patterns of basic productivity emerged. For example, it became quite obvious that the continents are surrounded by areas of higher productivity. While some of these areas can be very broad and others very thin, it is a general observation that there is more productivity near the coast. There is a reason that coastal areas usually have higher productivity than the open ocean. Besides sunlight and carbon dioxide from the atmosphere, plants also require certain elements, called nutrients, for growth. The supply of these nutrients is generally greater near the coast. The most important nutrients in the ocean are nitrate (NO3) and phosphate (PO4), though there are other necessary nutrients. Just as plants on land receive fertilizer with nitrogen and phosphorus to enhance growth, when phytoplankton receive more nitrate and phosphate, their growth rate will increase. Upwelling and Increased Coastal Productivity Along the western coast of South Africa, an oceanographic process known as upwelling results in an area of very high primary productivity. Known as the Benguela Upwelling Zone, this region is easily seen by the large areas of red and yellow in a false color image. CZCS image of the southwest African coast, South showing the high productivity of the Benguela Africa upwelling zone and associated features of the Benguela current. The Benguela upwelling zone is located where winds and currents combine to bring cold ocean waters rich with nutrients from the ocean depths to the surface. Upwelled waters have high nutrient concentrations because when organisms living in the lighted surface waters die, the organic matter that is in their cells sinks into the deep ocean. Very slowly, the cells are decomposed by bacteria. This process enriches these deep waters with nutrients. For an animation illustrating the process of coastal upwelling, click here. Note that the arrows indicate wind flow parallel to the coast. Note that this animation example shows upwelling in a Northern Hemisphere coastal setting. The Benguela Upwelling Zone The Benguela Upwelling Zone is produced due to an interaction of the wind flowing parallel to the coast and the resulting Ekman Transport of surface water away from the coast. As shown in the previous animation, the movement of surface water results in the upwelling of nutrient-rich waters from greater depths. These nutrient-rich waters support greater productivity than one would typically see in a near shore environment. In the case of the Benguela Upwelling Zone, the prevailing winds are from the southeast and the resulting Ekman Transport of surface water is to the west (remembering that Coriolis Effect in the Southern Hemisphere is to the left), bringing in the steady supply of nutrients that phytoplankton require for growth. Surface water moves offshore due to Ekman Transport Plankton bloom Upwelling Southeast wind Nutrient-rich water The image above depicts the interaction of the winds, ocean upwelling, and nutrients that cause the Benguela Upwelling Zone to be one of the most productive oceanic regions in the world. .

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