Tales of the Venerable Honolulu Tide Gauge*

Abstract

Surface expressions of internal tides constitute a significant component of the total recorded tide. The internal component is strongly modulated by the time-variable density structure, and the resulting perturbation of the recorded tide gives a welcome look at twentieth-century interannual and secular variability. Time series of mean sea level h SL (t) and total recorded M 2 vector a TT (t) are extracted from the Honolulu 1905-2000 and Hilo 1947-2000 tide records. Internal tide parameters are derived from the intertidal continuum surrounding the M 2 frequency line and from a Cartesian display of a TT (t), yielding a ST ϭ 16.6 and 22.1 cm, a IT ϭ 1.8 and 1.0 cm for surface and internal tides at Honolulu and Hilo, respectively. The proposed model a TT (t) ϭ a ST ϩ a IT cos IT (t) is of a phase-modulated internal tide generated by the surface tide at some remote point and traveling to the tide gauge with velocity modulated by the underlying variable density structure. Mean sea level h SL (t) [a surrogate for the density structure and hence for IT (t)] is coherent with a IT (t) within the decadal band 0.2-0.5 cycles per year. For both the decadal band and the century drift the recorded M 2 amplitude is high when sea level is high, according to ␦a TT ϭ O(0.1␦h SL ). The authors attribute the recorded secular increase in the Honolulu M 2 amplitude from a TT ϭ 16.1 to 16.9 cm between 1915 and 2000 to a 28°rotation of the internal tide vector in response to ocean warming.

A survey of recent changes in the main components of the ocean tide

“…Changes in the internal tide with corresponding small changes in its surface expression (e.g. Ray and Mitchum, 1997;Mitchum and Chiswell, 2000;Colosi and Munk, 2006). Such changes can occur even though the barotropic tide is invariant.…”

The Cascade of Tidal Energy from Low to High Modes on a Continental Slope

“…Quantifying and predicting tidal-energy conversion from low to high modes requires (i) quantitative expressions for energy conversion between individual modes of the tide and (ii) a framework that incorporates the time variability of internal tides (Wunsch 1975;van Haren 2004;Colosi and Munk 2006) into the cascade of tidal energy. Previous studies have examined the time variability of internal-tide generation (Kelly and Nash 2010;Zilberman et al 2011), propagation (Rainville and Pinkel 2006) and incidence on continental slopes (Martini et al 2011), but little is understood about the time variability of tidally driven TKE dissipation.…”

Are Any Coastal Internal Tides Predictable?

“…Open-ocean sites that are largely predictable from surface forcing and stratification tend to be located in regions of strong generation, such as the Hawaiian Ridge (Colosi and Munk, 2006;Lee et al, 2006;Carter et al, 2008;Zilberman et al, 2011), which produces internal tides that dwarf the "internal swell. "…”