An empirical approach to derive MODIS ocean color patterns under severe sun glint

Abstract

Oil tracking in the Gulf of Mexico in response to the Deepwater Horizon accident requires timely and accurate observations of major circulation patterns such as the Loop Current and LC eddies. When the eastern GOM becomes nearly isothermal at the surface and the use of sea surface temperature imagery is limited, MODIS ocean color data can be used instead. However, frequent and extensive sun glint prevents such an application when glint reflectance, L g, is >0.01 sr -1. Here, an empirical approach is developed to remove sun glint and clouds based on band ratios between the Rayleigh-corrected reflectance (R rc) at 469, 555, 645, 859, and 1240-nm. To minimize the effect of residual errors due to variable aerosols and imperfect glint correction, a color index (CI) is derived to represent the color patterns. Comparison between results from adjacent days with different glint and aerosol patterns suggests that the approach is able to derive consistent color patterns under severe sun glint (L g < 0.15 sr -1). Tests of the approach over the Tropical Atlantic, East China Sea, and ocean waters off South Africa further validate the approach's general applicability. The color index (CI) also shows significant correlation with MODIS band-ratio Chl (<1 mg m -3) for each case examined. The simple design of the approach makes it straightforward to implement for other subtropical and tropical regions when a qualitative MODIS CI is desired to infer circulation patterns and to trace eddies under severe sun glint. Copyright © 2011 by the American Geophysical Union.