Observations of the semidiurnal internal tide in the western North Atlantic Ocean

Abstract

Extensive moored measurements of temperature and horizontal current fluctuations in a Mid-Ocean Dynamics Experiment in the deep western North Atlantic near 28°N, 70°W are analysed for semidiurnal tides. The M2principal lunar tide is dominant, and about 50% of the main thermocline temperature variance in the M2band is coherent with the astronomical forcing and so of a deterministic rather than random nature. The first baroclinic mode is dominant in the internal tide, and propagates to the southeast through the experimental area. The M2first mode internal tide has an energy density of 0.9 x 102J/m2, about 20% as great as the barotropic tide energy density, and the horizontal currents in the barotropic and baroclinic tides are comparable with a characteristic deep amplitude of 1 cm/s. It is proposed that the Blake Escarpment 700 km to the west is a major generation area for the internal tide, and simple theoretical arguments suggest that the Escarpment is especially effective in generating low mode internal waves. Neighbouring tidal bands are much more dominated by randomly phased internal waves, but the S2principal solar band shows evidence for a weak deterministic internal tide. Mooring motion adds incoherent noise to the tidal period temperature fluctuations. Barotropic currents for the major M2, S2and N2semidiurnal constituents are derived from the current measurements, although internal wave noise levels are high.