Internal waves across the Pacific

Abstract

[1] 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,'' l c = 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 l c 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 Ri 16 À1 = 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 l c . 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 l c . Our observations indicate that PSI occurs in the ocean with sufficient intensity to substantially

Subharmonic Energy Transfer from the Semidiurnal Internal Tide to Near-Diurnal Motions over Kaena Ridge, Hawaii

“…Both upward-and downwardpropagating waves are present in roughly equal parts, and with the straightened wave crests they combine to form a characteristic chevron or 3 shape, which is most prominent below 400-m depth. A similar spatial pattern, associated with the opposite-sign propagation of PSI daughter waves, was previously seen in studies of PSI near the M 2 critical latitude, both in numerical simulations (MacKinnon and Winters 2005) and in observations during IWAP (Alford et al 2007). …”

“…The near-inertial waves were found to be significantly bicoherent with the M 2 tide (MacKinnon et al 2012). However, PSI energy transfers inferred from dissipation estimates and the decay of the M 2 tide were only 10%-20% of the incoming tidal energy flux, rather than the 40% or more predicted by models (Alford et al 2007;Simmons 2008;MacKinnon et al 2012).…”

“…Elevated shear variances were reported equatorward of the critical latitude, and close to 28.88N, near-inertial waves were observed in a characteristic standing pattern associated with PSI (Alford et al 2007). The near-inertial waves were found to be significantly bicoherent with the M 2 tide (MacKinnon et al 2012).…”

“…Motivated by these results, the Internal Waves Across the Pacific (IWAP) experiment (Alford et al 2007) attempted to observe the PSI decay of the M 2 internal tide as it propagates northward from the Hawaiian Ridge across the critical latitude. Elevated shear variances were reported equatorward of the critical latitude, and close to 28.88N, near-inertial waves were observed in a characteristic standing pattern associated with PSI (Alford et al 2007).…”

The Finite Element Sea Ice-Ocean Model (FESOM) v.1.4: formulation of an ocean general circulation model

“…We use a constant background vertical viscosity of 10 −4 m 2 s −1 , and there is no observational justification for this value. Enhanced vertical mixing in the thermocline arising from Parametric Subharmonic Instability (PSI) of the M2 tide at the 28.9 • N/S band (Tian et al, 2006;Alford et al, 2007;Hibiya et al, 2007) is not accounted for in our model formulation. The model sensitivity study by Jochum et al (2008) shows that increasing the off-equator background vertical diffusivity in the thermocline toward the observational estimate (0.17×10 −4 m 2 s −1 instead of 0.1×10 −4 m 2 s −1 ) or accounting for the mixing arising from PSI worsens the North Atlantic results in their particular tests.…”