Observations of near‐surface currents and hydrography in the Connecticut River plume with the surface current and density array

Abstract

It has been well established that the Connecticut River forms a large plume in the vicinity of its mouth during periods of high river discharge. The plume is bounded on the offshore side by a distinct front where the water color and turbidity changes rapidly and foam and detritus accumulate. Though there have been many observations of similar fronts throughout the world, the details of the density and velocity structure in the vicinity of a plume front and the propagation velocity of the feature have not been adequately resolved. To better observe the properties of these phenomena, we have constructed a ship‐mounted array of instruments to measure the near‐surface currents and density (SCUD) distribution in plumes and fronts. SCUD provides current velocity components at four levels and conductivity and temperature at five levels in the upper 3 m of the water column. By using the global positioning system in differential mode to obtain position, the system has a horizontal resolution of 5 m at a ship speed of 1–2 ms−1. Estimates of the ground referenced water velocity components have an uncertainty of approximately 0.1 ms−1. To demonstrate the performance of the SCUD array, we report the preliminary results of an observation program to examine the variability of the front of the Connecticut River plume. In the spring of 1994, several 200 m across‐front SCUD transects separated by 200 m were obtained during the mid ebb in Long Island Sound. During this period the front advanced to the west with a velocity of the order of 0.1 ms−1 in opposition to an eastward tidal current of 1.0 ms−1. A practical salinity increase of 20 in less than 10 m was observed at the front on crossing into the plume. In concert, the front normal velocity component decreased by 0.5 (±0.1) ms−1 over 10 m, and the vertical shear in the front parallel component decreased dramatically. The performance and potential of the instrumentation and the implication of the observations for the development theories of river plume fronts are discussed.