Observations of the Niagara River thermal plume (Lake Ontario, North America)

Abstract

Temperature and velocity data are used to describe the Niagara River discharge into Lake Ontario between April and November 1982. The Niagara River discharge (7000 m3 s−1) is usually slightly warmer and therefore less dense than the ambient lake surface water. The density difference varies throughout the year, with the largest density difference being measured in October. Near the river mouth, where the fluid speeds are roughly 1 m s−1, the plume is vertically homogeneous. Here advective forces dominate the momentum balance, even in the presence of strong wind forcing. Approximately 5 km offshore the plume separates from the lake bottom as the water depth increases and buoyancy forces become important. At the offshore boundary of the plume, advection, Coriolis force, buoyancy, and wind stress are all important. The Coriolis effect acts to turn the plume anticyclonically and form a right‐bounded coastal current with speeds of 0.1–0.2 m s−1. Strong upwelling‐favorable winds force the buoyant plume to spread offshore, thinning the plume as it spreads. Eventually, the plume water is mixed with the lake water by the wind. Strong downwelling‐favorable winds concentrate the plume into a narrow and deep coastal current traveling downwind with the coast on its right. Thus exchange between the plume and the lake is enhanced by upwelling‐favorable winds and inhibited by downwelling‐favorable winds.