Tales of the venerable Honolulu tide gauge

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 (Hawaii) 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) = 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) cos theta(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 theta(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 Omega a(TT) = O(0.1 delta h(SL)). The authors attribute the recorded secular increase in the Honolulu M-2 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.