Persistent near-diurnal internal waves observed above a site of M2 barotropic-to-baroclinic conversion

Near-diurnal internal waves were observed in velocity and shear measurements from a shipboard survey along a 35-km section of the Kaena Ridge, northwest of Oahu. Individual waves with upward phase propagation could be traced for almost 4 days even though the ship transited approximately 20 km. Depth-time maps of shear were dominated by near-diurnal waves, despite the fact that Kaena Ridge is a site of considerable M-2 barotropic-to-baroclinic conversion. Guided by recent numerical and observational studies, it was found that a frequency of 1/2M(2) (i.e., 24.84-h period) was consistent with these waves. Nonlinear processes are able to transfer energy within the internal wave spectrum. Bicoherence analysis, which can distinguish between nonlinearly coupled waves and waves that have been independently excited, suggested that the 1/2M(2) waves were nonlinearly coupled with the dominant M-2 internal tide only between 525- and 595-m depth. This narrow depth range corresponded to an observed M-2 characteristic emanating from the northern edge of the ridge. The observations occurred in close proximity to the internal tide generation region, implying a rapid transfer of energy between frequencies. Strong nonlinear interactions seem a likely mechanism. Nonlinear transfers such as these could complicate attempts to close local single-constituent tidal energy budgets.