Abstract
The energetic frontal region of the tidally pulsed Connecticut River plume was sampled with the T-REMUS Autonomous Underwater Vehicle (AUV) which navigated ten cross-front transects at various depths. Synchronous and high resolution hydrographic, microstructure, velocity and backscatter observations allowed the creation of detailed views of frontal structure and turbulent mixing in the reference frame of the propagating front. The front was defined by a sharp horizontal density gradient of ~18.5 kg m-4 across only 0.7 m in the horizontal, and strong downwelling velocities of 0.17 m s-1. Three successive zones were consistently identified in the frontal region. The first was a 10 m wide downwelling zone in which buoyant plume water was forced downward, thereby forming a highly energetic frontal head that extended down to 6 m depth and had TKE dissipation rates (ε) of order 10-3 W kg-1. The second, located in the wake of the frontal head, consisted of a 25 m wide mixing zone which was characterized by substantial entrainment of ambient water. This zone had high amplitude and high frequency density anomaly(σθ) variability with ε, ranging between 10-4 and 10-5 W kg-1. Lastly, the third zone, beginning 35m from the front and beyond, was stratified sufficiently to suppress the growth of instabilities, which diminished turbulent mixing. The plume base shoaled to 2 m depth and ε decreased further, from 10-5 to 10-7 W kg-1. In ambient Long Island Sound waters, ε ranged between 10-6and 10-7 W kg-1. The cross-front distributions of σθ and ε did not change substantially as the front transformed from a lateral boundary, where plume flow was primarily in the along-front direction, to one that was at the leading edge of the plume, where flow was mostly in the cross front direction. The observed ε values and those from four previous studies of large and midsized plumes were nondimensionalized with near-frontal region and river mouth parameters. TKE dissipation rates were more effectively normalized with the velocity and length scales associated with the downwelling region at the front than with the flow speed and cross-section area of the mouth, suggesting that local dynamics primarily control ε in the near-frontal region.