Loop Current Impact on the Transport of Mississippi River Waters

Abstract

Schiller, R.V. and Kourafalou, V.H., 2014. Loop current impact on the transport of Mississippi river waters. Journal of Coastal Research, 30(6), 1287-1306. Coconut Creek (Florida), ISSN 0749-0208.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 tõ 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.

The ecosystem baseline for particle flux in the Northern Gulf of Mexico

“…Concentrations of fine sediments are relatively constant throughout the year, with median concentrations of 150-250 ppm (Thorne et al, 2008). The transport of Mississippi River water within the Gulf of Mexico is highly variable, spatially and temporally, and depends strongly on winds, mesoscale eddies and the loop current system (Schiller et al, 2011;Schiller and Kourafalou, 2014). Based on satellite images, the influence of Mississippi waters can be observed as far away as the Florida Straight (Hu et al, 2005).…”

“…Based on satellite images, the influence of Mississippi waters can be observed as far away as the Florida Straight (Hu et al, 2005). The freshwater intrusions of the Mississippi can often be detected by an increase in surface chlorophyll concentrations (Hu et al, 2005;Schiller et al, 2011;Schiller and Kourafalou, 2014). Moreover, the influence of Mississippi-derived sediment is clearly visible in the distribution of CaCO 3 in seafloor sediments, where a 'tongue' with low-CaCO 3 sediments (< 10% CaCO 3 of sediment DW) extends from the mouth of the Mississippi River towards the Southeast (Balsam and Beeson, 2003;Ellwood et al, 2006).…”

“…The central Gulf of Mexico is much less productive than the shelf waters, explaining why the fluxes and the contribution of lithogenic matter to total flux at these sites were overall lower and more variable than those observed at the DWH site (Table 2). Nevertheless, riverine inputs are likely still an important source of particles at the Seep and Reference sites , especially during summer months when waters of riverine origin are more commonly found offshore (Schiller and Kourafalou, 2014).…”

“…The Mississippi also carries large amounts of fine, suspended sediment, which settles near the Mississippi Delta and dominates the seafloor sediment there (Balsam and Beeson, 2003;Ellwood et al, 2006). This eutrophic river plume is advected by the strong currents in the Gulf of Mexico depending on the wind conditions and the position of the loop current (Walker, 1996;Walker et al, 2005;Schiller et al, 2011), and can at times be detected as far south as the Florida Straight (Hu et al, 2005;Schiller and Kourafalou, 2014). The complex interaction between different waters (with differing nutrient and trophic states) and highly variable currents leads to a very heterogeneous and dynamic system.…”

Mississippi waters reaching South Florida reefs under no flood conditions: synthesis of observing and modeling system findings

“…The LC is a branch of the North Atlantic western boundary current, which later forms the Gulf Stream. Episodes of such LC entrainment were documented in the past and were associated with the presence of MR waters as far as the Florida Keys area, or even further downstream the Gulf Stream along the southeastern USA (Ortner et al 1995;Gilbert et al 1996;Hu et al 2005;Schiller and Kourafalou 2014). The LC can thus provide nutrients to the Florida Keys, which host the largest coral reefs in the continental USA, and as such, it is a vector of connectivity between the Mississippi Delta area and South Florida ecosystems.…”