Table of Contents
- A. Statement of Topic
- B. Summary of Topic
- C. Research Setting (Spatial Region and Temporal Period)
- D. Data Description
- E. Stepwise Investigational Instructions
- F. Presentation of Data Analyses
- G. Interpretation of Data Analyses
- H. Discussion and Statement of Conclusion
A. Statement of Topic
In this module, images of several of the oceans' major boundary current systems are shown. The module also explains the definition of a boundary current and provides the location of these boundary currents. Similarities and differences between the systems will be noted. At the end of this module, readers should have a basic understanding of major boundary current system characteristics.
B. Summary of Topic
This module will examine six major boundary current systems. The current systems generally present similiar patterns in ocean color and SST data. Concurrent investigation with these data types will provide the first introduction to the dynamics of physical - biological interactions in the oceanic system.
C. Research Setting
The research settings for this module are the Kuroshio, Benguela, Agulhas, California, Peru, and Brazil -Malvinas current systems. These regions are in both the Atlantic and Pacific Ocean.
The Agulhas Current is the western boundary current of the southern Indian Ocean, and flows down the east coast of Africa. The Agulhas Current takes its name from the point of the cape, called Cabo das Agulhas (or Cape of Needles). There are views on why this name was chosen. The first one claims that the sharp rocks and reefs offshore were often described as needles. The second explanation contends that the name is derived from the discovery that at the tip of the Cape, the compass needle points due north with no deviation between true and magnetic. As one of the major currents in the Southern Hemisphere, the Agulhas Current system transports large volumes of water.
The Benguela Current is the eastern boundary of the South Atlantic subtropical gyre. The Benguela Current flows through a strong, biologically productive upwelling region, and advects cool waters to the tropics. This water is warm and is one of the source waters for the South Equatorial Current. The Benguela Current has a well – defined mean flow that is mostly confined near the continent and a more variable transient flow on its western side. The transient flow is dominated by large eddies shed from the Agulhas Retroflection.
The Brazil Current is the western boundary current of the South Atlantic subtropical gyre. The Brazil Current is considered small when compared to that of the Gulf Stream, its counterpart in the Northern Atlantic. The Brazil Current in the northern region is shallow and closely confined to the continental shelf. As it flows to the south, the Brazil Current splits in two directions. One component flows eastward, and the other component hugs the coast and flows toward the southwest, interacting with the colder Malvinas Current. The combined flow of the two currents causes a strong thermohaline frontal region, called the Brazil–Malvinas Confluence (BMC).
The Malvinas Current is a branch of the Southern Circumpolar Current, and flows northward along the continental shelf of Argentina until it interacts with the Brazil Current.
The California Current is a cold current originating in the northern Pacific Ocean and passing southward and then southwestward along the western coast of North America. The California Current's diverse coastal communities are the result of many different natural forces. Tectonic and volcanic activity, occurring over the past 250 million years, created coastal mountain ranges. The powerful, ceaseless waves of the Pacific also cut into the coast mountain ranges, carving vertical cliffs, terraces, and rock stacks.
The Kuroshio Current (also known as the Japan Current) is a northward flowing branch of the North Equatorial Current in the Pacific Ocean. The Tsushima Current separates from the main current and flows into the Sea of Japan. Dense fogs develop along the boundary between the Kuroshio and the cold Oyashio current, and colder air moving over the warm Kuroshio Current becomes more temperate and acts to moderate the climate of Taiwan and Japan. The Kuroshio Extension turns eastward from Japan into the North Pacific Ocean.
The Peru Current is a cold ocean current that flows north from the Antarctic along the west coast of South America tosouthern Ecuador, then west. Peru's coast is a bleak, often rocky, and mountainous desert that runs from Chile to Ecuador, punctuated by fifty-two small rivers that descard through steep, arid mountains and empty into the Pacific Ocean. The cold current helps cause dry conditions to persist continuously along the Peruvian littoral (coastal) zone, making the land strip between the Andes and the Pacific Ocean one of the most arid deserts in the world.
D. Data Description
For this module, SeaWiFS monthly global 9km products (chlorophyll a concentration) and MODIS SST data, which are available in Giovanni, were utilized.
E. Stepwise Investigational Instructions
Agulhas Current Brazil/Malvinas Current Benguela Current
California Current Peru Current Kuroshio Current and Extension
The methods used to create each image shown above were the same. After going to Giovanni, the exact coordinates for each area were entered. The exact latitude and longitude coordinates for each are:
SST or chlorophyll is chosen as the parameter of interest, and area plot of time- averaged parameter are chosen as plot type. The desired months and years are indicated for temporal selection, followed by a click on "Generate Plot".
F. Presentation of Data Analyses
Agulhas Current and Agulhas Retroflection
Brazil-Malvinas Current System
Benguela Current and Benguela Upwelling Zone
Peru Current and Peru Upwelling Zone
Kuroshio Current and Kuroshio Current Extension
G. Interpretation of Data Analyses
Agulhas Current: The Agulhas Current transports warm water poleward as it follows the western coast of South Africa. As it turns eastward, the current is deflected southward and then "retroflected" westward by the flow of the Southern Circumpolar Current.
Brazil-Malvinas Current System: The most noticeable expression of the Brazil-Malvinas Current System occurs during summer in the Southern Hemisphere. At this time, the collision of the two currents creates a convergence zone that is hundreds of miles long, with the warm Brazil Current flowing south along the coast of South America, and the cold Malvinas flowing north and then turning to the east. The temperature boundaries between the two water masses is clearly seen in the SST image, and the convergence zone can be seen as higher chlorophyll concentrations parallel to the coast.
Benguela current and Benguela Upwelling Zone: The southward-flowing Benguela Current creates the most productive upwelling zones in the world, clearly seen as cold temperatures along the South African and Namibian coast in the SST image, and high chlorophyll values in the same location. Also note the Agulhas Retroflection in the southwestern corner.
California Current: The flow of the cold California Current along the coast creates numerous jets and eddies as the current interacts with the coastal geography. Other jets and eddies can result from offshore winds or the flow from rivers. These jets and eddies create the higher chlorophyll zones seen in the chlorophyll image.
Peru Current: The Benguela Upwelling Zone may be the most productive, but the Peru Upwelling Zone is more famous, for two reasons: anchovies and El Nino. The Peru anchovy fishery has been subject to the vagaries of El Nino for centuries -- El Nino results in the suppression of upwelling and a strong reduction in the fish catch. Because the advent of El Nino usually coincided with the advent of Christmas, fishermen knew that rain before Christmas usually indicated a bad fishing season was just about to happen.
Kuroshio Current: The Kuroshio Current is unique, because it terminates essentially in the center of the north Pacific Ocean. This sea surface temperature image provides an excellent demonstration of the variability of the flow of the Kuroshio as it marks the boundary of the North Pacific Gyre. Similar to the Gulf Stream, the Kuroshio marks the boundary of warm low-productivity water and colder, higher-productivity water, clearly seen in the chlorophyll image.
H. Discussion and Statement of Conclusions
All six major boundary currents provide interesting and variable dynamics that can be investigated more fully with ocean color and SST data. There are some similarities, particularly between the Benguela and Peru Upwelling Zones, but the Benguela Upwelling Zone is not subject to the irregular recurrence of the El Nino phenomenon (or is it?).
CZCS Classic Scenes: The Benguela Upwelling Zone
A Clear Day Over the Agulhas Retroflection
Convergence Zones - Where the Action Is