A Voyage of Optical Oceanography


From: Kevin Arrigo, Stanford University





Stan Hooker (Photo by Kathryn Hansen)



“NOW there will be a boat brief on the bridge in 10 minutes.”


That’s the cue for Stan Hooker of NASA’s Goddard Space Flight Center in Greenbelt, Md., and his research associate, Joaquin Chaves, that it’s about time to leave the relatively comfortable confines of the USCGC Healy and embark on a short but important voyage of their own aboard the Arctic Survey Boat (ASB). This small gray metal vessel will carry the team of optical oceanographers less than a mile away — far enough to avoid the enormous shadow cast by the Healy. The ASB also allows them to maneuver in and around the loose pack ice to make their light measurements under a variety of different conditions.


Stan wants to understand what happens to sunlight between the time it enters the ocean and is reflected back out to space, eventually to be measured by our suite of Earth-observing satellites.  In some sense, he is interested in answering a variation of the age-old question, “Why is the ocean blue?” In his case, it’s more like, “Why is the ocean green?” Or greenish-blue? Or brown? The color of the ocean can tell us a lot about what’s in the water and because we can measure the color of water from space, we can use satellites to tell us what the surface ocean contains.


But we need a translator. A sort of optical Rosetta Stone. That’s where Stan’s work comes in. He makes detailed measurements of the color of the ocean — actually, he measures absorption and scattering at many different wavelengths of visible light. He does this by lowering a number of different light sensors over the side of the ASB and letting them sink slowly below the surface, making measurements all along the way. This gives him an idea about how the total amount and the color of light changes as it penetrates the upper ocean.


He also collects samples of water and brings them back to the lab to measure what in the sample is giving it color.  By linking the color of the ocean to what’s in the water, Stan constructs a sort of optical translator. Then, when a satellite sees a piece of ocean of the same color, we can be pretty sure about what lurks below its surface. Green water screams phytoplankton— large numbers of those little green algae. Blue water indicates a virtual biological desert. Yellow-brown water means lots of river runoff from the land.


Because the Arctic Ocean see-saws between open water and ice cover, and is surrounded by land strewn with big rivers dumping a huge amount of material into its shallow waters, it is a complex place to make these kinds optical measurements. Stan sees this as a challenge. If he can decipher the meaning of the varied colors of the Arctic Ocean, understanding other simpler parts of the world ocean should be a snap.





The ASB comes back aboard after a full station’s work. (Photo by Haley Smith Kingsland)





Joaquin Chaves (Photo by Karen Romano Young)