Structure and variability of semidiurnal internal tides in Mamala Bay, Hawaii

Abstract

Moored current meter and temperature observations and results from a three‐dimensional primitive equation model are used to examine the energetic semidiurnal internal tides present in Mamala Bay on the south coast of Oahu, Hawaii. The steady, harmonic component of the internal tide is characterized by large vertical displacements in the central region of the bay (35 m amplitude for the M2 constituent), and enhanced alongshelf baroclinic currents at the headlands on either end of the bay (0.27 m s−1). Seasonal changes in amplitude and phase are observed. The model captures the qualitative spatial structure of the observations. Baroclinic energy flux estimates, from the mooring observations and the numerical simulations, suggest that internal tide energy propagates into the bay and does not originate within the bay. The model indicates that internal wave generation occurs over the flanks (500–1000 m depth) of the ridge, predominantly on the east side, with perhaps some additional contribution on the west from an energetic internal tide generated north of Oahu. Wave superposition is believed to account for the alongshelf spatial structure of currents and displacements. Incoherent modulations of the internal tide occur that are not related to local changes in stratification, at least on superannual timescales. Factors contributing to this signal may include stratification variations at the deep generation sites, mesoscale activity, and/or the shoaling of a random internal wave field into the bay from the open ocean.