Internal waves across the Pacific

The long‐range propagation of the semidiurnal internal tide northward from the Hawaiian ridge and its susceptibility to parametric subharmonic instability (PSI) at the “critical latitude,” λc = 28.8°N, were examined in spring 2006 with intensive shipboard and moored observations spanning 25–37°N along a tidal beam. Velocity and shear at λc were dominated by intense vertically‐standing, inertially‐rotating bands of several hundred meters vertical wavelength. These occurred in bursts following spring tide, contrasting sharply with the downward‐propagating, wind‐generated features seen at other latitudes. These marginally‐stable layers (which have inverse 16‐meter Richardson number Ri16−1 = 0.7) are interpreted as the inertial waves resulting from PSI of the internal tide. Elevated near‐inertial energy and parameterized diapycnal diffusivity, and reduced asymmetry in upgoing/downgoing energy, were also observed at and equatorward of λc. Yet, simultaneous moored measurements of semidiurnal energy flux and 1‐km‐deep velocity sections measured from the ship indicate that the internal tide propagates at least to 37°N, with no detectable energy loss or phase discontinuity at λc. Our observations indicate that PSI occurs in the ocean with sufficient intensity to substantially alter the inertial shear field at and equatorward of λc, but that it does not appreciably disrupt the propagation of the tide at our location.