Detecting Gulf of Mexico Oceanographic Features in Summer Using AVHRR Channel 3

Abstract

Efforts to monitor the Gulf of Mexico Loop Current and mesoscale ocean features using IR satellite imagery in the summertime have been significantly hindered by 1) strong surface heating that masks surface frontal gradients and 2) extremely high atmospheric water vapor attenuation that lowers effective satellite brightness-temperature values. These problems can now be addressed, provided high-quality multichannel infrared data are available during nighttime satellite passes. The National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) consists of three IR channels that include channels 3 (3.55?3.93 µm), 4 (10.3?11.3 µm), and 5 (11.5?12.5 µm). Of these, channel 3 is least affected by water vapor attenuation, making it better suited for viewing the ocean through a humid atmosphere. All satellites prior to NOAA-11, however, experienced substantial noise in channel 3 soon after launch, rendering the channel relatively useless for long-term oceanographic monitoring. NOAA-11, with a high-quality 3.7-µm channel, has enabled us to detect Loop Current and eddy features throughout the typical worst summertime conditions. A three-channel cross-product sea surface temperature (CPSST) algorithm was applied to nighttime images in August and September 1990 to monitor the Loop Current a major warm-core eddy (Quiet Eddy), and a minor warm-core eddy (Quiet Eddy II). Feature locations are verified using drifting data buoys. This capability demonstrates the importance of a low-noise AVHRR channel 3, and will increase our knowledge about Loop Current dynamics and ring periodicity during periods previously unfavorable for IR images.