Loop Current Impact on the Transport of Mississippi River Waters

Schiller, R.V. and Kourafalou, V.H., 2014. Loop current impact on the transport of Mississippi river waters.River plume studies have traditionally focused on coastal and shelf circulation. Here, we examine novel interactions of a large river plume with offshore flows, which introduces additional export pathways for the riverine waters. We show the impact of the Loop Current System (LCS) on the offshore transport and fate of Mississippi River (MR) plume waters. The high-resolution Northern Gulf of Mexico (NGoM) model, nested in a data-assimilative, regional, Gulf of Mexico model, was employed for a 5-year, realistically forced simulation. Model results, in tandem with satellite data, demonstrate that the entrainment of MR waters by the LCS varies interannually and extends pathways of shelf cross-marginal transport to the basin interior and beyond. Plume-to-LCS interactions are determined by episodic northward intrusions of the LCS in the NGoM. If the intrusion is very close to the MR Delta, plume waters can be entrained with little contribution from winds. If the intrusion is farther offshore and lasts for months, LCS entrainment may be modulated by wind forcing because plume transport can change from an along-shelf to an exclusively across-shelf regime. Seasonal changes in river discharge did not have a direct influence on the transport induced by LCS intrusions. However, high-frequency (daily) variability of the river discharge induced by the Mississippi and 16 other major rivers was important and played a role in the overall NGoM freshwater transport. During the 5-year period, plume-to-LCS interactions corresponded on average to ∼11% of the yearly net offshore freshwater transport near the MR Delta. Model results were supported by satellite and in situ observations that demonstrate the far-reaching potential of MR waters as they are traced to the SE Gulf of Mexico and the Florida Keys. This study demonstrates how across-basin, connectivity pathways for riverine waters may be affected by local coastal-to-offshore interactions induced by a large-scale current system.