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
This module provides introductory information about upwelling and phytoplankton productivity. This module will also introduce or enhance understanding of the importance of nutrient concentration and variability to phytoplankton productivity, and emphasize the role of physical pocesses in the maintenance of biologically productive areas.
B. Summary of Topic
Upwelling describes the process in whichcold, nutrient-rich subsurface deep water ascends to the surface of the ocean. Upwelling is caused by favorable persistent ocean surface winds, which induce a movement of water from nearshore to offshore. This flow of water causes deep waters to flow upward to replace the surface water which is moving offshore. Upwelling can be easily seen in SST data as cold coastal waters very close to the coast. The resultingenhancement of primary productivity, due to the supply of nutrients to surface waters, can commonly be observed in chlorophyll concentration data.
Phytoplankton productivity results from two primary input factors: sunlight and nutrients. Upwelling provides nutrients, primarily nitrate and phosphate, that phytoplankton utilize for growth. Productivity refers to the amount of organic carbon that the phytoplankton produce by the process of photosynthesis. Actively growing phytoplankton frequently divide into new cells, and because each of the phytoplankton cells contains chlorophyll, oceanic areas with high phytoplankton productivity usually also have high concentrations of chlorophyll which can be observed with remote sensing.
C. Research Setting
The research setting for this module is the Benguela Upwelling Zone along the western coast of South Africa. The Benguela Upwelling Zone is one of the ocean's permanent upwelling zones -- an area where the oceanic setting and meteorological conditions create a continuous system that constantly brings cold, nutrient-rich water to the ocean surface, fostering intense phytoplankton growth regardless of the season. As noted in the previous module, another permanent upwelling zone is located in the Peru Current on the western coast of South America, though the intensity of Peru Current upwelling is dramatically affected by El Nino events. The Benguela Upwelling Zone is rarely affected by oceanic variability, making it one of the most productive areas of the world ocean and one of the most prominent in remotely-sensed chlorophyll concentration data.
D. Data Description
For this module, SeaWiFS monthly global 9km products (chlorophyll a concentration) and MODIS SST data, both available in Giovanni, were utilized.
E. Stepwise Investigational Instructions
The same area which was used to define the Benguela Current and the Benguela Upwelling Zone in Module 2 was used. The latitude and longitude coordinates for this area are:
- North Latitude: 15.0 South
- South Laitude: 40.0 South
- West Longitude: 0.0 E (the Prime or Greenwich Meridian)
- East Longitude: 27.0 E
The year 2003 was chosen so that MODIS-Aqua SST and chlorophyll concentration data could be examined side-by-side.
F. Presentation of Data Analyses
The Benguela Upwelling Zone -- Annual Area Average Plots
In the SST image, the location of the coldest upwelled water is observed to be very close to the coast. In the chlorophyll image, the dramatic effect of the nutrients provided by the upwelling process can be seen. Also note the much warmer water of the Agulhas Current south of the African continent, and the distribution of chlorophyll corresponding tothe Agulhas Current.
Benguela Upwelling Zone -- January Area Average Plots
January is summer in the Southern Hemisphere, and the waters offshore of Namibia and South Africa are very warm, except in the Benguela Upwellng Zone. There is nearly a 10 degree C difference between the upwelled waters and the warmest waters offshore. Chlorophyll concentrations are high, as expected.
Benguela Upwelling Zone -- July Area Average Plots
July is wintertime in the Southern Hemisphere, and sea surface temperatures in the South Atlantic are less than in summer. The upwelled waters appear to be a degree or so colder, and the upwelled area appears larger. It is hard to discern a difference in the chlorophyll concentration data, as this region is one with high constant productivity.
Benguela Upwelling Zone -- Hovmoller Plots of SST and chlorophyll concentration
One question that can be examined in an upwelling zone with constant high productivity is how much the productivity in a small region of the entire zone varies over a year. Using a Hovmoller plot, the monthly variability is easier to examine. Below are two Hovmoller plots for a small area, between 18 degrees E and 12 degrees E longitude, and between 30 degrees South and 26 degrees South latitude. The white area on the right marks the location of the coast.
In the image above, the intensification of upwelling in the winter is obvious. Now compare this image to the chlorophyll concentration data:
Though the chlorophyll concentration remains high, there does appear to be higher concentrations in the spring and autumn. With one slight change to Giovanni, the "dynamic" color range can be selected instead of the "Pre-defined" color bar above. The dynamic color range uses only the minimum and maximum data values in the selected area and time period:
Now it is clearer that the periods when chlorophyll concentrations are highest do not occur at the same time as when the upwelled waters are at their coldest. So even in an area where productivity is consistently high, seasonal variability can be observed.
G. Interpretation of Data Analyses
The images of SST and chlorophyll concentration data for the Benguela Upwelling Zone demonstrate the basic processes which create this productive oceanic environment -- a constant supply of cold water from the ocean depths which provides nutrients that maintain high phytoplankton productivity. The images also show, despite the general appearance of high productivity throughout the year, that there appears to be a seasonal cycle of variability in this region.
H. Discussion and Statement of Conclusions
The Benguela Upwelling Zone is an example of a permanent upwelling zone, and there are only a few locations in the world where this situation occurs. There are two other types of upwelling observed in the oceans -- seasonal upwelling and episodic upwelling. Seasonal upwelling occurs when wind direction is consistent in a direction favorable to upwelling in a certain season. The most familiar example of this is the Arabian Sea monsoon, which is discussed in Module 5. Other areas where seasonal upwelling occurs are the coast of Portugal and along the equatorial coast of western Africa.
Episodic upwelling is triggered by upwelling favorable conditions that may last only a few days or a few weeks. Episodic upwelling can occur in just about any coastal area of the world, but in most places the probability of upwelling is dependent on the season. Episodic upwelling frequently occurs along the east coast of the United States in the spring and summer, sometimes surprising beachgoers with abnormally cold water conditions when they expected warm water to wade and play and swim in.
CZCS Classic Scenes: Chapter 2: The Benguela Upwelling Zone
A Bloom By Any Other Name... Might Never Be a Bloom At All