The incorporation of the recently released 10-year climatology of ocean color data from NASA’s Moderate Resolution Imaging Spectroradiometer on the Aqua satellite (MODIS-Aqua) allowed the Giovanni data system to provide anomaly analysis for this important ocean data set.
The Earth Observing System Aqua satellite was launched in early 2002, and the onboard MODIS instrument began collecting data on July 4, 2002. Thus, at the end of June 2012, there were 10 years of monthly averaged MODIS data for each month of the year. The Ocean Biology Processing Group (OBPG) at Goddard Space Flight Center updates their monthly climatology files as each new month is completed. In July 2012, the NASA Goddard Earth Sciences Data and Information Services Center (GES DISC) acquired these monthly climatology files to create a 10-year climatology base period for the Giovanni data system.
With the climatology files in Giovanni, researchers could then examine these climatological averages, or use the new data for anomaly analysis. The latter enhances the detection of unusual events in any long-term data set, by comparing data from a selected period to the average of the data for a much longer period (i.e., “base period”). For ocean color data in Giovanni, anomaly analysis has been available for the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) for several years, but the SeaWiFS mission ended in December 2010. For MODIS-Aqua ocean color climatology data, the mission is still continuing. Furthermore, MODIS-Aqua data are available at 4 km spatial resolution, rather than the 9 km spatial resolution of SeaWiFS data. Several new data products are available from MODIS-Aqua, including normalized fluorescence line height (nFLH), colored dissolved organic matter (CDOM) Index, absorption coefficients for dissolved and detrital matter (acdm) and phytoplankton (aph), backscattering coefficient for particulate matter (bbp), and euphotic depth (Zeu). The MODIS-Aqua climatology data also provide remote-sensing reflectance data at several more wavelengths than were available from SeaWiFS.
Giovanni’s anomaly analysis capability allows users to examine monthly anomalies, and any other time period up to a year in duration. So, for example, it is possible to average data for a season (such as the spring months of March, April, and May of a particular year) and compare this three-month average to the climatological average for March through May.
The higher spatial resolution of the MODIS-Aqua data makes patterns in estuaries and large lakes more apparent. As an example, Figures 1a and 1b show euphotic depth anomalies in the Great Lakes for May 2004 and May 2007, respectively. A considerable difference in the optical clarity of the water can be seen, particularly in Lakes Michigan and Huron. Euphotic depth is defined as the depth in the water column at which the light intensity is one percent of the light intensity at the surface. (The current euphotic depth algorithm was developed by Zhongping Lee, University of Massachusetts.) The dark blue and purple areas visible in Figure 1a indicate where the water clarity was significantly less than the average water clarity for the month of May, whereas the green and yellow colors visible over most of the lakes in Figure 1b indicates near-average or slightly greater than average water clarity.
[Potential causes of the decreased water clarity in May 2004 could be phytoplankton blooms or sediments stirred up by spring winds. Because reduced euphotic depth anomalies extend over most of Lake Huron, a phytoplankton bloom seems the more likely cause; but, in Lake Michigan, spring winds are known to stir up sediments in the southern part of the lake. Because these sediment suspension events usually occur earlier in the spring, the reduced euphotic depth in Lake Michigan in May 2004 might be due to the passage of one or two weather systems.]
Figure 1. Euphotic depth (Zeu) anomaly for the Great Lakes in (a) May 2004 and (b) May 2007. Distinct differences are seen in the Zeu anomaly between the two months , particularly in Lake Michigan and Lake Huron. (Click on either image to view it full-size.)
The availability of climatologies and, thus, anomaly analysis, adds to the diagnostic capability of Giovanni regarding events occurring in coastal waters. Figure 2 (a-e) shows an example of this capability for the Mississippi River delta region in the northern Gulf of Mexico in May 2011, during a period of major flooding in the lower Mississippi River. The chlorophyll a anomaly (Figure 2a) shows a large area of red and orange, indicating elevated values of chlorophyll a over the climatological monthly average. However, chlorophyll a estimates can be inaccurate due to the presence of dissolved organic matter or sediments in coastal waters. Figure 2b shows the anomaly for the absorption coefficient of dissolved and detrital matter (adg) that is significantly elevated to the east of the delta but less so to the south and west.
In Figure 2c, the anomaly for the backscattering coefficient of particulate matter (bbp) is highest very close to the shores of the delta – which is expected because sediment is deposited when the river flow slows as it enters the Gulf of Mexico – while water carrying dissolved constituents is transported farther. The anomaly of the normalized fluorescence line height (nFLH; Figure 2d) shows increased values at considerable distance to the west of the delta. The latter area is of particular interest, because it coincides with the no-oxygen (anoxic) ‘dead zone’ on the Gulf of Mexico seafloor. An elevated nFLH anomaly is a good indicator for increased growth of phytoplankton, which could lead to the expansion of the ‘dead zone,’ because, when the phytoplankton die and sink to the bottom, respiration of their organic matter by bacteria uses up oxygen. The euphotic depth (Zeu) anomaly (Figure 2e) shows that an area of reduced water clarity extended well to the east and west of the Mississippi River delta region.
Figure 2. MODIS-Aqua climatological anomaly data for the Mississippi River delta region, May 2011, when flood waters were entering the Gulf of Mexico. (a) Map of the chl a anomaly. (b) Map of the absorption coefficient for dissolved and detrital material (adg) anomaly. (c) Map of the backscattering coefficient for suspended matter (bbp) anomaly. (d) Map of the normalized fluorescence line height (nFLH) anomaly. (e) Map of the euphotic depth (Zeu) anomaly. (Click on any image to view it full-size.)
These examples demonstrate that MODIS-Aqua climatology data and anomaly analysis capability in Giovanni are likely to be of considerable use to scientists, water quality managers, local, state, and federal governments, and even the general public interested in the condition of nearby coastal or lake waters. The influences of natural phenomena and human activities on water quality in these important regions can now be observed over a decade of change, using data from NASA’s satellite sensors, produced by the dedicated NASA staff who analyze and process these data.
Article by James Acker. Editing by Bill Teng. Web formatting by James Acker.
Funding for support of ocean color radiometry data in Giovanni is provided through the "Water Quality for Coastal and Inland Waters Project," NNX09AV57G, from the National Aeronautics and Space Administration (NASA). Zhongping Lee of the University of Massachusetts – Boston is the Principal Investigator on this project.
The GES DISC is a NASA earth science data center, part of the NASA Earth Science Data and Information System (ESDIS) Project.