Effects of Stratification on the Tidal Boundary Layer

Sandra R. Werner and Robert C. Beardsley

Temperature and conductivity data taken on the 76-m isobath of the southern flank of Georges Bank indicate an intrusion of warm and saline shelf/slope front (SSF) water in May 1995. Observations show the SSF intrusion was bottom-trapped at heights < 30 m above bottom from approximately May 6 - 13, and extended to the surface at later times. Tidal analysis of VMCM current measurements indicates stratification causes a current maximum in the lower water column marking the transition from the friction dominated near-bottom waters to the pycnocline where turbulent motion is restricted. The location of the current maximum in the water column depends on the vertical distribution of stratification and is closer to the bottom during the bottom-trapped phase of the SSF intrusion than during the surface intensified phase. Comparison of Richardson number estimates computed from the total velocity shear and the high-passed currents with temporal variations >33 hrs (semidiurnal and diurnal tides) suggests subtidal current shear strongly enhances turbulence production near the bottom, if the semidiurnal tidal flow is in direction of the mean around-bank circulation.