Five Year Mean Sea-surface Salinity in the Northern Gulf of Mexico for 2005 through 2009

These images were created by combining the mean sea-surface salinity values to produce seasonal representations for winter, spring, summer and fall. Winter includes the months of January, February and March; spring includes the months of April, May and June; summer includes the months of July, August and September; fall includes the months of October, November and December. Sea-surface salinity values were originally obtained from the Naval Research Laboratory at the Stennis Space Center (NRL/SSC). NRL/SSC processed MODIS-Aqua satellite imagery covering the Gulf of Mexico, for a 5-year time period from January 2005 through December 2009. The Gulf of Mexico was divided into three regions, western, central and eastern. For purposes of this image, the three regions were joined in ArcMAP 10. Sea-surface salinity was estimated from the ocean color imagery, based on an empirical relationship between colored dissolved organic matter (CDOM) absorption and salinity. Researchers at NRL have exploited this relationship to empirically estimate salinity from ocean color satellite imagery, since the absorption slope difference between 412 nm and 443 nm can be used as a proxy to estimate the CDOM absorption coefficient: Salinity = 36.208 - 46.488x + 27.683x2 - 8.338x3 + 0.965x4 In the above equation, x = difference between the absorption coefficients at 412 and 443 nm (a412-a443). The algorithm was developed using in situ data from a wide variety of locations and has been validated with independent ship and mooring data (Ladner et al. 2006; 2008). Currently the algorithm has an accuracy of about 1 PSU and is valid in coastal waters where salinity is strongly impacted by freshwater discharge (out to approximately mid-shelf). The algorithm is based on the difference in total absorption at 412 and 443 nm. NRL produced salinity images from the MODIS satellite ocean color data at both 250 m and 1 km spatial resolution in the Gulf of Mexico using this relationship. The salinity product derived from the ocean color imagery is still experimental and requires further validation. It is expected that different mixing regimes will affect the conservative nature of the CDOM/salinity proxy relationship, resulting in deviations from the above empirical relationship. These values were then reclassified into salinity ranges in accordance with the Coastal and Marine Ecological Classification Standard (CMECS). There are five CMECS categories presented: oligohaline (0 - < 5, shown in red), mesohaline (5 - < 18, shown in green), lower polyhaline (18 - < 25, shown in purple), upper polyhaline (25 - < 30, shown in yellow), and euhaline (> 30, shown in blue).