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CZCS and SeaWiFS

The world's oceans (and its freshwater bodies, as well) teem with life. Central to the marine foodchain is phytoplankton - microscopic plants that photosynthesize chemicals in sea water. This process depends on the plankton content of chlorophyll a, a pigment that strongly absorbs red and blue light. As plankton concentrations increase, there is a corresponding rise in spectral radiances, peaking in the green. Upwelling masses of water (usually associated with thermal convection) containing phytoplankton take on green hues in contrast to the deep blues of ocean water with few nutrients.

The Coastal Zone Color Scanner (CZCS) is a sensor specifically developed to study ocean color properties. It launched in October 1978, as part of Nimbus-7's instrument complement and continued to operate until late 1986. It sensed colors in the visible region in four bands, each 0.02 µm in bandwidth, centered at 0.44 (1), 0.52 (2), 0.57 (3), and 0.67 (4) µm. A fifth band between 0.7 and 0.8 µ monitored surface vegetation and band six, at 10.5-12.5 µ sensed sea surface temperatures. In monitoring ocean color, band 1 (blue) measured chlorophyll absorption; band 2 (green), tracked chlorophyll concentration; band 3 (yellow), was sensitive to yellow pigments ("gelbstoff"); and band 4 (red), reacted to aerosol absorption. With data from those bands, we calculate the chlorophyll variation, which correlates closely with relative abundance of marine phytoplankton, as the ratio of band 1 to band 2 (for phytoplankton concentrations less than 1.5 mg/m3) and band 2 to band 3 (>1.5 mg/m3)

The next pair of images are CZCS color composites: the left one emphasizes chlorophyll-enrichment (in reds) in the Georges Bank off the New England coast (BGR = 3,2,1) whereas the right one simulates natural ocean color (BGR = 1,2,3)

Color composite CZCS image pair showng chlorophyll-enrichment and natural ocean color in the Georges Bank off the New England coast.

Using data from all relevant bands, we show below, the chlorophyll concentrations spread over the Fall to Spring season in 1978-79 in the Atlantic Ocean:

Colorized CZCS image showing chlorophyll concentrations in the Atlantic Ocean, Fall to Spring 1978-79.Color key to previous image.

The distribution of chlorophyll on a global scale averaged between 1978 and 1986 appears here:

Colorized CZCS image showing the ditribution of chlorophyll on a global scale averaged between 1978 and 1986.

14-33: These chlorophyll distribution maps show concentration gradients that seem to run counter to logic (or intuition), in that the lowest amounts of chlorophyll (hence plankton) are in the equatorial and subtropical zones. Those zones, being warmer, should support greater quantities of plant life (as do the tropics on land). What gives here? ANSWER

We can correlate thermal data from CZCS with chlorophyll, as demonstrated in this plot:

Colorized CZCS thermal image of chlorophyll along the East Coast of the U.S.

In the color coding, blues correspond to warmest and reds to coolest temperatures. Note the eddies or rings. Phytoplankton tends to concentrate along the edges of warm core rings (which rotate clockwise) but concentrate centrally in cold core rings (counterclockwise motion). The motions in these rings are analogous to circulation around atmospheric high- and low-pressure systems. Warm core rings can extend over several hundred kilometers, as seen in this HCMM, Night-IR, thermal image (below, top), which shows such "gyres" in varying stages of development and coherence in Atlantic waters near the Canary Islands off the African coast. Cold core rings may be less well shaped, but with many individual rings, as displayed in this June 19, 1976, Landsat-1 band 4 image (bottom) of waters off the southwest coast of Iceland, concentrations of phytoplankton are very clear (the lighter tones may be "murkiness" due to large concentrations of phytoplankton).

B/W HCMM Night-IR thermal image of the Atlantic waters around the Canary Islands. B/W Landsat-1 Band 4 image of waters off the southwest coast of Iceland.

14-34: Which way do the eddies seem to be rotating in the above two images? ANSWER

These types of observations - ocean color and thermal patterns - aid in locating conditions where large schools of fish are likely to live, so commercial and sport fishermen actively apply them to locate the best current fisheries.

The follow-on to the CZCS is NASA Goddard's SeaWiFS (Sea-viewing Wide Field-of-View Sensor), launched successfully on August 1, 1997. Once again, the sensor system monitors ocean color variations, especially those caused by concentrations of plankton and other sealife that strongly moderates chlorophyll response, detectable spectrally. Thus, the prime objectives are: 1) to quantify ocean plankton production; 2) to determine observable couplings of physical/biological processes; and 3) to characterize estuarine and coastal ecosystems. We show the prime sensor here:

Color photograph of the SeaWiFS sensor.

The sensor consists of eight channels at: 412, 443, 490, 510, 555, 670, 765, and 865 nm (nanometers: 1µm = 1,000 nm), each with bandwidths of 20 or 40 nm. The instrument can swing ±58° off nadir. From an orbital altitude of 705 km (438 mi), spatial resolution in the Local Area Coverage (LAC) mode is about 1.1 km (0.68 mi) (the optimal resolution is 0.6 km at nadir), and in the Global Area Coverage (GAC) mode, it is 4 km (2.5 mi). Swath widths are: LAC = 2,801 km, and GAC = 1,502 km.

Here is a characteristic regional map, showing SeaWiFS-derived chlorophyll content for the globe using data recovered from September 4 through November 20, 1997, using the same color coding as CZCS:

Colorized SeaWiFS global map showing chlorophyll content, September 4 through November 20 1997.

The next two images show (top) chlorophyll mapped off the eastern U.S. coast on September 30, 1997, and (bottom) ocean color off southern Florida on September 25, with greens denoting high phytoplankton concentrations.

Colorized SeaWiFS image showing chlorophyll content off of the East Coast of the U.S., September 30 1997.

Colorized SeaWiFS image showing phytoplankton concentrations and ocean color off of southern Florida, September 25 1997.

We can also use the sensor data from SeaWiFS to map land surfaces on a local to global scale. We showed a global, true-color map of Earth near the end of the Introduction Section. Further information about SeaWiFS is available online at:

14-35: Imagine you are a commercial fisherman. How would you use satellite imagery to find the best fishing grounds? ANSWER

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Primary Author: Nicholas M. Short, Sr. email:

Collaborators: Code 935 NASA GSFC, GST, USAF Academy
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