The Coastal Boundary Layer in Lake Ontario: Part II. The Summer-Fall Regime

Abstract

Field observations on coastal currents near Oshawa on the north shore of Lake Ontario during summer and fall are described. The technique of observations (flag-station chain) has been given in Part I, together with a description of the dynamic regime during the spring period. Observations during summer and fall show that the kinetic energy level of water movements increases considerably from spring to summer and again from summer to fall. Much as in the spring, a nearshore band (some 10 km wide) becomes a unique kind of “boundary layer” in which mid-lake motions adjust to the presence of the shares. In this shore zone, currents are shore-parallel and relatively persistent. During summer and fall, mid-lake motions are wave-like, consisting mainly of near-inertial oscillations. Within the shore zone, the current-like motions are associated with thermocline displacements, upward if the current flows to the east, downward if it flows to the west, so that the pressure gradients caused by the non-uniform density distribution are at least partly balanced by the Coriolis force. The lakewide flow pattern is mainly determined by a few periods of relatively strong winds, which can reverse an opposing current and lead to a complete man exchange between the shore zone and mid-lake as a consequence of large thermocline movements. Advection of momentum from the outer zone into the coastal zone also plays a significant role in maintaining coastal currents.