Evidence That an Optical Tail in the Gulf of Mexico After Tropical Cyclone Isaac was the Result of Offshore Advection of Coastal Water

This study investigates the hypothesis of Acker's Web report in 2013 that an optical tail of high chlorophyll a, observed in the open Gulf of Mexico (GoM) approximately 2 weeks after tropical storm Isaac made landfall in coastal Louisiana, was due to advection of outflowing Mississippi River related with the mesoscale eddy field in the open GoM. By using available in situ data and data from multiple satellites, strong evidence was found to support Acker's hypothesis. Drifting buoy, remotely sensed sea surface salinity, and surface geostrophic current data were used to show that low-salinity water (LSW) was indeed associated with the optical tail. Remotely sensed colored dissolved organic matter indicated that the LSW was of coastal origin, and satellite-observed rain rate indicated that this LSW in the optical tail was not due to local precipitation. The path of freshwater from the Mississippi River Delta to the region offshore in the optical tail was shown to be similar to a simulated trajectory estimated by surface geostrophic currents; likewise, the drifting buoys deployed near the shelf break offshore of the Mississippi River Delta prior to the peak in discharge.