Observations of tidal variablity on the New England shelf

Copyrighted by American Geophysical Union.Observations from the Coastal Mixing and Optics experiment moored array, deployed\ud from August 1996 through June 1997, are used to describe barotropic and baroclinic\ud tidal variability over the New England shelf. The dominant M₂ tidal elevations decrease\ud toward the northeast to a minimum over the Nantucket shoals (about 34 cm), and\ud barotropic tidal current amplitudes increase strongly toward the northeast to a maximum\ud over the shoals (about 35 cm s⁻¹). Estimates of the depth-averaged M₂ momentum balance\ud indicate that tidal dynamics are linear, and along-shelf pressure gradients are as large\ud as cross-shelf pressure gradients. In addition, tidal current ellipses are weakly polarized,\ud confirming that the dynamics are more complex than simple plane waves. The vertical\ud structure of the M₂ currents decreases in amplitude and phase (phase lead near bottom)\ud over the bottom 20 m. The M₂ momentum deficit near the bottom approximately\ud matches direct covariance estimates of stress, confirming the effects of stress on current\ud structure in the tidally driven bottom boundary layer. Baroclinic current variability at\ud tidal frequencies is small (2 cm s⁻¹ amplitude), with a predominantly mode 1 vertical\ud structure. High-frequency (approaching the buoyancy frequency) internal solitons are\ud observed following the pycnocline. The internal solitons switch from waves of depression\ud to waves of elevation when the depth of maximum stratification is deeper than half the\ud water column depth. Both low-mode baroclinic tidal and high-frequency internal wave\ud energy decrease linearly with bottom depth across the shelf