Winter variability in the western Gulf of Maine: Part 1: Internal tides

Abstract

During the winter 1997–1998, a field program was conducted in Wilkinson Basin–western Gulf of Maine–as part of a study of winter convective mixing. The field program consisted of (1) Wilkinson basin-scale hydrographic surveys, (2) a tight three-mooring array with ∼100 m separations measured temperature and conductivity at rates of 2–15 min and (3) a single pair of upward/downward-looking pair acoustic Doppler current profiling (ADCP) instruments measured currents with 8 m vertical resolution over the 270 m water column in north-central Wilkinson basin at a rate of 10 min. The moored array measurements below the mixed layer (∼100 m depth) between 11 January and 6 February 1998 were dominated by a combination of the relatively strong semidiurnal external (depth-independent or barotropic) tide; upon which were superposed a weaker phase-locked semidiurnal internal tide and a very weak water column mean currents of about 1 cm/s southward or approximately across the local isobaths. The harmonic analysis of a vertical average of the relatively uniform ADCP velocities in the well-mixed upper 123 m of the water column, defined the external tidal currents which were dominated by a nearly rectilinear, across-isobath (326°T) M 2 semidiurnal tidal current of about 15 cm/s. The depth-dependent residual current field, which was created by subtracting the external tidal current, consisted of (1) clockwise-rotating semidiurnal internal tidal currents of about 5 cm/s below the mixed layer; (2) clockwise-rotating inertial currents; and (3) a considerably less energetic subtidal current variability. The results from both frequency-domain empirical orthogonal function and tidal harmonic analyses of the of isotherm displacement series at each of the three moorings in the 100 m array mutually confirm an approximate east-northeastward phase propagation of the dominant M 2 semidiurnal internal tide across Wilkinson Basin. Further investigation supports the idea that this winter internal tide is very likely generated by the interaction of the external tidal currents and the southwestern wall of Wilkinson Basin. The definitions of the local Wilkinson Basin external tide and phase-locked internal tides will enable us to analyze a less “noisy” set of measurements for the subtle atmospherically forced convective and wind-driven motions.