E-Letter responses to:

editorial: Keith Alverson and D. James Baker Taking the Pulse of the Oceans Science 2006; 314: 1657 [Summary] [PDF]

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Vision for Ocean Observations

John Marra, Arnold L. Gordon, Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964   (23 March 2007)

Vision for Ocean Observations 23 March 2007
John Marra Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964, Arnold L. Gordon, Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964 Respond to this E-Letter: Re: Vision for Ocean Observations	We enjoyed reading the Editorial by Drs. Alverson and Baker (15 Dec. 2006, p. 1657). We fully agree that an international commitment for global ocean observations is needed. And we congratulate the authors and Science for an Editorial giving greater visibility for ocean science. We take issue, however, with one aspect of the Editorial, regarding viewing the ocean from ships and from space. Seafarers can see the moon, but they have for years also seen beneath the ocean’s surface using a fundamental observational tool: the Secchi disk. Developed by Pietro Angelo Secchi in the mid–1800s, the Secchi disk is used to measure the ocean’s transparency to visible wavelengths of light. The depth to which the disk can be seen from the deck of a ship ranges from less than 2 m to greater than 40 m. The ocean is certainly not “opaque to all wavelengths of electromagnetic radiation.” Satellite sensors can also see beneath the ocean’s surface. There are a whole series of Earth-orbiting spectrometers, notably the Sea-viewing Wide-Field-of-view Sensor (SeaWiFS), and the two Moderate resolution Imaging Spectrometers (MODIS) aboard NASA’s Aqua and Terra satellites that measure the ocean’s color over the top several meters (the first optical depth). Compared with an average ocean depth of 4000 m, such images might seem a minor contribution to understanding the oceanwide distributions of temperature and chemical properties. Yet it is in this top illuminated layer where wind and solar heating drive ocean circulation and where photosynthesis supports the entire ocean’s food web. Additionally, satellite altimetry observes sea level variability, which depends on tides and on the density stratification within the ocean. Synthetic aperture radars indirectly image internal waves and often reveal the three-dimensional circulation associated with ocean mesoscale fronts. Satellites can view the surface ocean and, by inference, aspects of the sub-surface ocean in its entirety in a few days. Satellite-based observations represent an effective way to fill the inevitable spatial and temporal gaps of an in situ observational array, and hence should be well integrated into the GEOSS design of the in situ array.