Turbulent Mixing in a Far-Field Plume During the Transition to Upwelling Conditions: Microstructure Observations From an AUV

A REMUS 600 autonomous underwater vehicle was used to measure turbulent mixing within the far-field Chesapeake Bay plume during the transition to upwelling. Prior to the onset of upwelling, the plume was mixed by a combination of energetic downwelling winds and bottom-generated shear resulting in a two-layer plume structure. Estimates of turbulent dissipation and buoyancy flux from a nose-mounted microstructure system indicate that scalar exchange within the plume was patchy and transient, with direct wind mixing constrained to the near surface by stratification within the plume. Changing wind and tide conditions contributed to temporal variability. Following the separation of the upper plume from the coast, alongshore shear became a significant driver of mixing on the shoreward edge of the plume. Plain Language Summary Turbulence measurements made by an autonomous underwater vehicle reveal that the wind-driven offshore movement of a coastal river plume leads to intermittent and patchy mixing of plume and shelf waters. These novel, high-resolution observations highlight the importance of capturing transient features when determining the fate of rivers on continental shelves.