River plume source-front connectivity

Abstract River plume source-front connectivity, or the relationship between processes at the source, plume body and front, is investigated using numerical simulations of the Merrimack River plume using the Regional Ocean modeling System (ROMS). Source water, i.e. water discharged from the estuary during an ebb pulse, overtakes the front as it propagates offshore resulting in the strong convergence that exists there. Model results show that much of the water released over an ebb pulse does not interact with the propagating tidal plume front, suggesting that the front decouples from the source and sustains itself as a distinct water mass. When a gate is closed at the estuary mouth during early ebb, the overall plume structure changes; gates closed late in ebb do not significantly effect the along- or cross-shore scale of the plume. The change in plume extent is only evident hours after the early-ebb gate closing, when source water is no longer supplied to the front. These experiments suggest that the growing distance between the highly energetic liftoff region and the radial expansion of the plume front results in a flattening of the surface gradient driving flow in the interior, diminishing source-front connectivity.