Coastal Boundary Layer Characteristics during Summer Stratification in Lake Ontario

Abstract

Simultaneous measurements of Eulerian and Lagrangian currents along the north shore of Lake Ontario are analyzed to provide the mean flow properties and horizontal turbulent exchange characteristics in the coastal boundary layer (CBL). The summer coastal boundary layer is characterized by a frictional boundary layer (FBL) of a width of ?3 km, in which shore and bottom friction affects the flow. In this regime the currents are predominantly shore parallel and persistent. The outer boundary layer also called an inertial boundary layer (IBL), typically of the order of 5?6 km wide, is a consequence of the adjustment of inertial oscillations to the lateral boundary. During the summer season within the CBL, the current motions are associated with thermocline displacements. The eastward (westward) wind stress causes thermocline elevation (depression) causing upwelling (downwelling). The mean subsurface westward currents associated with downwelling events are typically stronger in comparison to weak eastward flow during upwelling. Further, upwelling events are characterized by reduced low frequency motion (>1 day) and significant near-inertial (?17 h) currents. The width of the CBL decreases during upwelling and increases during downwelling. Internal waves generated by baroclinic seiches during these events have periods from 11 to 17 hours. The near-surface horizontal exchange coefficients calculated from Lagrangian measurements are higher than those from subsurface Eulerian values. Upwelling events show that the turbulent kinetic energy is higher than mean flow kinetic energy (MKE) in the CBL, and cross-shore turbulent exchange increases in the IBL. During downwelling the alongshore exchange coefficients are higher in the FBL, whereas cross-shore exchanges are higher in the IBL. Downwelling events are also characterized by increased contribution from the MKE rather than the turbulent kinetic energy.