WHOI PO

Abstract: River discharges represent asignificant source of both buoyancy and momentum to the coastal ocean and areassociated with intense mixing processes as fresh water is diluted tobackground ocean levels in a series of distinct mixing regimes.  The talk focuses on recent data collected inthe Merrimack River plume using a turbulence microstructure equipped UncrewedUnderwater Vehicle (UUV), focusing on dynamics and mixing at the plume front.  The sampling effort yielded 33 passes throughthe frontal region during the first six hours after initiation of discharge onthe ebbing tide. Observed TKE dissipation rates are used to develop a bulkestimate of mixing occurring in the frontal region.  Earlier studies in the Merrimack River plumeare utilized to develop comparable estimates for the near-field and mid-fieldregions of the plume, where strong stratified shear processes actively mix theplume.  During the mid to high discharge,low wind conditions sampled, the front was responsible for approximately 11% ofthe total mixing required to dilute the discharge, compared to 8-24% in thenear-field region and approximately 7% in the mid-field region.  Collectively, these active, momentumdominated regions of the plume account for 40-50% of total mixing, with theremainder occurring in the estuary, and the plume far-field, which is dominatedby buoyancy with mixing driven predominately by wind.