Quantifying the Contribution of Fronts to the Mixing Budget of a Tidally Pulsed River Plume Using the T-REMUS Autonomous Underwater Vehicle (AUV)

Frontal mixing is one of many well-described mechanisms responsible for the dilution of river plumes into coastal ocean water, but there is great uncertainty regarding its magnitude relative to the total mixing of the plume. One reason for this uncertainty is that turbulence at river plume fronts has been difficult to quantify; their transient spatial and temporal nature has a relatively narrow and shallow band of active mixing that poses many challenges for direct measurement. In this study we explore the spatial structure of mixing of the tidally pulsed Connecticut River plume front as it propagates westward into Long Island Sound. We also calculate turbulent kinetic energy dissipation rates in the front to better constrain its contribution to the total mixing of the river plume. In contrast to previous field campaigns that have derived frontal mixing rates during only a snapshot in time or from very small-scale plumes, our methodology is capable of near-synoptic and direct dissipation rate measurements using the UMASS Dartmouth T-REMUS AUV. The leading edge of this custom design Hydroid Inc. AUV is equipped with a Rockland Scientific Micro ASTP and a SBE49 CTD. An upward and downward 1.2MHz RID ADCP provides critical near surface velocity data as well as bottom tracking for precise navigation. In conjunction with a Hydroid Inc. Gateway Buoy that can be free to drift, the T-REMUS can navigate exclusively in its reference frame, thus eliminating the need for fixed pre-deployed transponders. Initially, while the Gateway Buoy was moored less than a kilometer south of the river mouth jetties during the time of diminishing ebb flow, the T-REMUS sampled along an east-west axis crossing the stationary west plume front at multiple constant depth runs ranging from 0.5 to 4 meters. As the front started to propagate westward under the influence of the flooding tidal current, the Gateway Buoy was released and allowed to drift while the T-REMUS continued sampling in the same pattern. Sampling continued for several hours until the front was no longer visible and the eastward ebb current began to flow again. These repeated observations will provide insight into the mixing processes during the transition from a stationary to a leading-edge front and give better estimates of the front’s contribution to the mixing budget of the Connecticut River plume