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 M₂ vector a_TT(t) are extracted from the Honolulu 1905–2000 and Hilo 1947–2000 (Hawaii) tide records. Internal tide parameters are derived from the intertidal continuum surrounding the M₂ frequency line and from a Cartesian display of a_TT(t), yielding a_ST = 16.6 and 22.1 cm, a_IT = 1.8 and 1.0 cm for surface and internal tides at Honolulu and Hilo, respectively. The proposed model a_TT(t) = a_ST + a_IT cosθ_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 θ_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 M₂ amplitude is high when sea level is high, according to δa_TT = O(0.1δh_SL). The authors attribute the recorded secular increase in the Honolulu M₂ amplitude from a_TT = 16.1 to 16.9 cm between 1915 and 2000 to a 28° rotation of the internal tide vector in response to ocean warming.