FY 1995

Ocean tides in the continental margin off the Pacific Northwest Shelf

Mofjeld, H.O., F.I. González, M.C. Eble, and J.C. Newman

J. Geophys. Res., 100(C6), 10,789–10,800, doi: 10.1029/95JC00687 (1995)

A comparison of bottom pressure observations and global tide models shows that the regional distributions of the diurnal and semidiurnal tides off the Oregon and Washington shelves are determined mainly by the large-scale amphidromic systems in the North Pacific Ocean. These cause the tidal amplitudes to increase toward the northeast and the phase lags to increase toward the northwest. Within 200-500 km of the shelfbreak, the curving continental boundary causes the cophase lines to be closely packed near Cape Mendocino and more widely spread in the bight to the north. Farther seaward, the M₂ tide is consistent with a Kelvin wave propagating along the British Columbia boundary (315°T), while the K₁ tide decays offshore more slowly than expected from Kelvin wave theory. A long-term (4.2-year) bottom pressure series at Axial Seamount (45°58′N, 130°00′W; depth 1540 m) yields a nongravitational S′₂ tide that is consistent with a large-scale barotropic tide extending seaward from the coast. At lower frequencies, the observed M_f amplitude matches recent global models, while the M_f phase lag is marginally earlier than model phases. Nearby M_f currents are bottom-trapped and consistent in phase with quasi-geostrophic dynamics; their magnitudes near the bottom are much larger than barotropic M_f currents observed farther west in the North Pacific. The models reproduce the distributions of the diurnal and semidiurnal harmonic constants but with broad-scale offsets in the amplitudes and phase lags. These offsets cause errors (standard deviation = 3.0 cm) in predicted time series of the diurnal and semidiurnal tides that fluctuate on fortnightly to interannual timescales. The ≤29% seasonal variations of the errors are largest in winter and are modulated by 4–5-year fluctuations associated with the nodal cycle. In situ measurements can be used to remove these regional offsets. This would improve the temporal and spatial resolution of nontidal sea level estimates that are based on satellite altimeter data.