A Century of Tidal Variability in the North Pacific Extracted From Hourly Geomagnetic Observatory Measurements at Honolulu

Previous analyses of hourly tide-gauge data taken at Honolulu since 1905 have shown that both sea level and the amplitude of the tides have increased synchronously over time, and a process has been proposed whereby the common cause is an increase in ocean temperature. Validation with independent data of this change in tides, as well as the proposed cause, has been lacking. Ocean tides also generate magnetic fields that reach far outside the ocean, and tidal signals are clearly seen in the hourly data since 1905 from the Honolulu geomagnetic observatory. Here, it is found that the tidal amplitudes have increased synchronously in the tide-gauge and magnetic records, providing independent support for the previous results. Further information about the changing ocean is likely contained in the historical data from geomagnetic observatories as well as the global coverage of modern satellite magnetic surveys. Plain Language Summary In previous work, it has been shown that tidal amplitudes as recorded by tide-gauge data at Honolulu show interannual modulations as well as a century long trend toward larger amplitudes. It was proposed that (a) these changes in the tidal amplitudes are indeed reflective of changes in important ocean parameters (rather than uninteresting local effects) and (b) that the increase is indirectly due to increasing ocean temperature. Ocean tides also generate magnetic fields and the long record from the Honolulu geomagnetic observatory provides an opportunity for validation using an independent data set. Here, results for the modulation and trend are supported by the magnetic data and are also consistent with the explanation regarding ocean warming. Key Points Analyses of geomagnetic data at Honolulu shows modulation in the tides consistent with that seen in tide-gauge data The tidal magnetic modulations appear to be due to ocean and not ionospheric tides If confirmed, this study establishes the opportunity for using historical geomagnetic records in physical oceanography