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[Note: All of the images in this Web article are linked to full-size versions. Click on any image to view it full-size.]
Research Project - Introduction
In the spring of 2007, six midshipmen at the United States Naval Academy undertook a unique assignment – a supervised research project using Ocean Color Giovanni as their primary investigative tool. This LOCUS Web article summarizes how the research project was presented and conducted, and demonstrates the results that were obtained by the midshipmen students.
The first step of the process was to introduce the midshipmen to Giovanni. James Acker of the GES DISC met the class, instructed by Cmdr. Emil Petruncio, on February 20. Prior to the demonstration, the class members took a short “knowledge survey” to assess their level of oceanographic knowledge. Many of the questions on the survey could be answered through the use of Giovanni. The demonstration was then provided (using the same Powerpoint presentation given at the National Marine Educators Association Annual Meeting). Following this presentation, the class members were then given a chance to try out Giovanni prior to the end of the class session.
Subsequent to the demonstration, Cmdr. Petruncio provided the class with their research project assignment. The area of interest assigned for the investigation was the northern South China Sea, in the vicinity of the island of Taiwan, the Luzon Strait, and the Taiwan Strait.
Oceanographic Setting
Though less well-known than the monsoonal weather pattern of the Arabian Sea, the South China Sea is also subject to a monsoonal pattern, with markedly different wind and air mass characteristics in the winter and summer. The summer monsoonal flow here, occurring primarily from May to September, is characterized by southwesterly winds which bring warm, very humid air northward over Taiwan and southeastern mainland China. The winter monsoonal flow reverses this pattern; the winds are northeasterly, and these winds are colder, drier, and stronger than the summer monsoon pattern. The figures below show the two characteristic patterns of the “Asian Monsoon”. (These two figures are not linked to larger versions.)
For the research project, the six members of the class were paired into three groups, and each group was assigned a small region in the study area to investigate with Giovanni. Cmdr. Petruncio investigated a fourth region as an example.
The study regions are depicted below.
The East Box was selected as the example. For each of the boxes, the students were instructed to utilize Ocean Color Giovanni MODIS-Aqua Monthly SST data and the Multi-dataset Intercomparison Interface (MDII) to generate time-series plots of sea surface temperature, chlorophyll a concentration (chl a) , the diffuse attenuation coefficient at 490 nm (K490), and the normalized water-leaving radiance at 551 nm, nLw(551).
An interesting aspect of this study is the difference between the three parameters chl a, K490, and nLw(551). Chl a estimates are possible due to the absorption of light by the chlorophyll contained in phytoplankton, which is strongest at 443 nm. K490 is a measure of how the intensity of light is attenuated (reduced) with depth. When there are more substances or particles present in the water, K490 will be smaller, because less light can penetrate deeper into the ocean. In clearer waters, K490 is larger. Thus, when there is a phytoplankton bloom, K490 will decrease because the absorption and scattering of light by the phytoplankton cells reduce the amount of light penetrating to deeper waters. So K490 will frequently (but not always) co-vary with chl a. K490 will also be reduced by the presence of suspended sediments, which also reduce the amount of light that can penetrate to deeper waters.
On the other hand, nLw(551) is a measure of the intensity of light leaving the ocean surface (“water-leaving radiance) at 551 nm. There are no absorption effects at this wavelength, so nLw(551) will increase when there is an increasing amount of light reflected or scattered by non-absorbing particles, which are usually suspended sediments. Chl a and nLw(551) can therefore help to distinguish between phytoplankton blooms and sediments.
Seasonal Variability of Sea Surface Temperature
The first time-series results show the seasonal SST variability in each of the study boxes. In the North Box (top), the temperature ranged between 14 - 28° C, and in the West Box (left), the range was similar.. In the East Box (right), the range was about 24-30° C, and in the South Box (bottom) the range was approximately 24-29° C.
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Clearly, SST seasonal variability is much greater in the North and West boxes closer to the Chinese mainland, where the oceanic regions are more influenced by the Asian Monsoon, than the SST seasonal variability in the East and South boxes.
The next step in the project was to generate time-series plots of SST, chl a, K490, and nLw(551) in the four boxes using MDII. The results are depicted on each of the next pages. Each page describes the results in one of the four study boxes; a short summary appears following the South Box result discussion.
Introduction -- East -- North -- West -- South
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Atmos Composition
