Direct measurements of diapycnal mixing in a fjord reach—Puget Soundʹs Main Basin

In the spring of 1988, time series of microstructure and ADCP current profiles were collected at four locations in the North Main Basin of Puget Sound, Washington. Depth and time averages of diapycnal diffusivity CKrD at the four stations (1:8 67:0!10 4 m2 s 1) were one to three decades above typical open-ocean thermocline levels. The buoyancy frequency-squared N2 was near open-ocean levels, but unlike the open-ocean where N2TS2, finescale shear-squared S2 was three to six times N2 over significant portions of the water column at two of the stations. The time and space mean of all measurements (CKrD ¼ 3:6!10 3 m2 s 1) is close to inferred vertical eddy diffusivity from a primitive equation model for Puget Sound (Kz ¼ 3!10 3 m2 s 1) (J. Geophys. Res. 96 (1991) 16779). Large time and space variability of Kr was found, with differences of inter-station, depthetime means over one decade. A simple scaling argument using the observed Kr suggests significant exchange of mass between the layers of the subtidal flow over the basin’s residence time. Additionally, measurements show that local mixing may be comparable to volume-weighted sill mixing in modifying the Main Basin’s stratification. Both are contrary to the ‘‘advective reach’’ simplification of fjord dynamics. The mixing levels were dominated by the passage of a mid-depth, southward-flowing density intrusion and what we interpret as a strongly advected, non-linear internal tide. These mechanisms elevated profile-averaged Kr by more than 10 times background levels, with sustained patches of KrR1!10 2 m2 s 1. Critical 8-m gradient Richardson numbers (Ri8!0:25) matching regions of overturns (O20 m) and strong turbulence suggest that shear instabilities dominated the turbulence production, though there was support for double-diffusive convection in the warm core of the density intrusion.