On the spatial scales of a river plume

Abstract

[1] We report observations of the structure of the front that surrounds the plume of the Connecticut River in Long Island Sound (LIS). Salinity, temperature, and velocity in the near-surface waters were measured by both towed and ship-mounted sensors and an autonomous underwater vehicle. We find that the plume front extends south from the mouth of the river, normal to the direction of the tidal flow in LIS and then curves to the east to parallel the tidal current. The layer depth at the front and the cross-front jumps in salinity and near-surface velocity all tend to decrease as distance from the source increases. This is qualitatively consistent with the prediction of layer models. In the across-front direction, the plume layer depth increases from zero to the asymptotic value within a few times the plume depth ($5 m). Vertical motion is generated in this zone, and there is evidence of overturning. Farther from the front, the high-frequency salinity standard deviation decays exponentially with a length scale of 30 m. Assuming that the salinity fluctuations are a consequence of turbulence, we find that the rate of turbulent kinetic energy dissipation decreases exponentially in the across-front direction with a decay scale L G % 15 m. Estimates based on AUV-mounted shear probes are consistent with this estimate. We present an explanation of the physics that determines L G and provide a simple formula to guide the choice of resolution in models that are designed to resolve the frontal structure.

Hydraulically controlled front trapping on a tidal flat

“…Boat‐mounted instruments deployed on 18 August 2009 show that, after leaving the channel edge, the front propagated across the flats as a very thin (0.2–0.3 m) gravity current (Figure 5 shows the front after it had propagated 130 m away from the channel edge). The structure at the leading edge of the plume was similar to the structure of larger plumes, e.g., the 2–3 m deep Connecticut River plume [ O'Donnell et al , 2008], and the 5 m deep Chesapeake plume [ Marmorino and Trump , 2000]. The front at x = −30 m is marked by high near‐surface backscatter (Figure 5a) and a 0.2 m/s jump in near‐surface front‐normal velocity (Figure 5b).…”

Frontogenesis and Frontal Progression of a Trapping-Generated Estuarine Convergence Front and Its Influence on Mixing and Stratification

“…Fronts can be regions of high turbulence due to increased shear (Farmer et al 1995;Farmer et al 2002;Gargett and Moum 1995;MacDonald and Geyer 2004). Yet, mixing across fronts can be suppressed or highly localized (O'Donnell et al 2008) and they may inhibit estuarine flushing (Brown et al 1991). In fact, the interaction of shoal/channel exchange, transverse circulation, frontogenesis, and mixing are inherently tied together as demonstrated by Lacy et al (2003).…”

Small-Scale Surface Fronts in Estuaries

“…Because of the advances in instrumentation, considerable progress has been made in measuring and understanding the across-front and along-front variations of plumes both at the estuary mouth (MacDonald and Geyer, 2005) and seaward of the mouth (e.g., Marmorino and Trump, 2000;Pritchard and Huntley, 2002;Nash and Moum, 2005;Orton and Jay, 2005;O'Donnell, 2008;Ostrander et al, 2008;MacCready et al, 2009;Jay et al, 2010). Very high freshwater runoff conditions enhance the possibility for buoyant plume formation.…”

“…These fronts separate low-salinity estuarine or freshwaters from higher-salinity estuarine or coastal waters. Shorter length scales have been measured in the Connecticut River Estuary plume (O'Donnell et al, 2008). Frontal locations are greatly affected by tidal currents.…”