Coastal‐trapped behavior of the diurnal internal tide at O'ahu, Hawai'i

Abstract

The influence of rotation on the structure and propagation of internal tides around O'ahu, Hawai'i is investigated using in situ observations and a tidally forced, primitive equation model with realistic bathymetry and stratification. Particular attention is given to the diurnal internal tide, which largely has been de‐emphasized in previous studies of the region because of the dominance of the semidiurnal internal tide but has been determined by recent studies to be a significant contributor to baroclinic variability. Though both diurnal and semidiurnal internal tides are generated primarily over Ka'ena Ridge to the northwest of the island, the diurnal internal tide propagates clockwise around the island as an imperfectly trapped wave, while the semidiurnal internal tide propagates away from the ridge, unaffected by rotation. The diurnal and semidiurnal internal tides fall into the superinertial frequency range; however, the diurnal frequency apparently is sufficiently close to inertial (∼ 1.4f) for rotation to affect internal tide propagation. The in situ observations support the model finding that diurnal trapping provides the primary source of baroclinic variability along the eastern coast of the island, a stretch of coastline otherwise sheltered from the internal tide energy generated over the Hawaiian Ridge. The findings in Hawai'i suggest that coastal trapping of superinertial internal tides may be a significant source of variability and mixing in other nearshore systems around the world.

An Alongshore Momentum Budget Over a Fringing Tropical Fore‐Reef

“…Using the Princeton Ocean Model, Carter et al () estimate that 85% of the semidiurnal barotropic tidal energy lost over the Hawaiian Ridge is converted into internal tides. The energy flux of the semidiurnal internal tide is an order of magnitude larger than that of the diurnal internal tide (Smith et al, ). Specifically, given that our study site is near Kaena Ridge, observed to generate intense internal tides (Nash et al, ), it is likely that the measured pressure gradients and the resulting velocities are primarily driven by the evanescent tail of the M 2 internal tide propagating in deeper stratified waters offshore, which influences the nearshore region, despite the lack of local stratification.…”

Resonant Diurnal Internal Tides in the North Atlantic: 2. Modeling

“…Diurnal internal tides north of Hawaii are not energetic, however (Dushaw et al., 2011), but see Smith et al. (2017). Future avenues of research may be to determine the evolution of the diurnal tidal field eastward across the Atlantis, or to examine internal‐wave‐admitting simulations for the wind‐forced equatorial internal waves of Wunsch and Gill (1976).…”

“…Other than perhaps the Hawaiian Ridge, which extends across 24°N, similar configurations, that is, a substantial topographic feature equatorward of internal-tide turning latitudes that might support resonance, do not exist elsewhere; the Sargasso Sea is unique. Diurnal internal tides north of Hawaii are not energetic, however (Dushaw et al, 2011), but see Smith et al (2017). Future avenues of research may be to determine the evolution of the diurnal tidal field eastward across the Atlantis, or to examine internal-wave-admitting simulations for the wind-forced equatorial internal waves of Wunsch and Gill (1976).…”

Modification of Internal Wave Generation and Energy Conversion in the Nearshore Due To Tide‐Tide and Tide‐Wind Interactions

“…Müller (2013) estimated that the global energy conversion by M 2 , S 2 , K 1 and O 1 to be 1.11 (66%), 0.19 (11%), 0.24 (14%) and 0.13 TW (8%) respectively. The contribution of the K 1 diurnal tide alone has been reported from 8% to 50% of M 2 conversion in Luzon Strait and over Hawaiian Ridge (K. Liu et al., 2017; Smith et al., 2017). Moreover, the interaction of M 2 with other tidal constituents (such as K 1 and O 1 ) over ridges and continental slopes can (a) evoke higher frequency elements in the system, (b) affect the energy conversion rate, and (c) increase the energy dissipation locally through nonlinear interactions (Guo et al., 2020; Liu et al., 2015; X. H. Xie et al., 2008; X. Xie et al., 2010).…”