On the processes that influence the transport and fate of Mississippi waters under flooding outflow conditions
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The Mississippi River (MR) freshwater outflow is a major circulation forcing mechanism for the Northern Gulf of Mexico. We investigate the transport and fate of the brackish waters under flood conditions. The largest outflow in history (45,000 m3/s in 2011) is compared with the second largest outflow in the last 8 years (41,000 m3/s in 2008). Realistically forced simulations reveal the synergistic effect of enhanced discharge, winds, stratification of ambient shelf waters, and offshore circulation over the transport of plume waters. The strongest impact is attributed to the evolution of the Loop Current (LC) and associated frontal cyclonic eddies and anticyclonic rings, which exhibited distinctly different influence during the two study periods. The northward LC intrusion in the summer of 2011 weakened and blocked the buoyancy-driven downstream (westward) transport of brackish waters. The 2011 flood was thus characterized by upstream (eastward) flow and an extensive coverage of the Mississippi–Alabama–Florida shelf. An immediate response between the LC and the brackish offshore eastward spreading is computed during and after this historic event. The absence of a LC northward intrusion during the 2008 flood, in combination with wind effects, promotes downstream advection of MR waters towards the Louisiana–Texas shelf; large amounts of buoyant waters are also retained near the Delta, subject to local offshore advection under the synergistic action of LC-associated counter-rotating eddies.
KeywordsMississippi River Buoyant plume River flood Northern Gulf of Mexico Numerical modeling HYCOM
This research was made possible in part by a grant from BP/The Gulf of Mexico Research Initiative and in part by the National Science Foundation (NSF OCE-0929651). The MODIS/Aqua satellite images were provided by the Optical Oceanography Lab of USF/CMS (University of South Florida/College of Marine Science). We are grateful to HeeSook Kang (University of Miami) for performing the majority of the NGoM-HYCOM simulations and to P. Hogan and O.M. Smedstad (Naval Research Lab/SSC) for providing the regional GoM-HYCOM model fields. The manuscript greatly benefited from discussions with Rafael Schiller (MARINTEK do Brasil). We thank Nicolas Lopez (Florida State University) for providing us with in situ data for the NGoM domain from the Shipboard Automated Meteorological and Oceanographic System (SAMOS) and Richard Patchen (NOAA/NOS) for providing the compilation of the river outflow data set, which is based on daily flow measurements by the US Geological Survey (USGS).
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