FY 2003

Ocean currents at Axial Volcano, a northeastern Pacific seamount

Lavelle, J.W., E.T. Baker, and G.A. Cannon

J. Geophys. Res., 108(C2), doi: 10.1029/2002JC001305 (2003)

Axial Volcano is a magmatically active seamount that straddles the Juan de Fuca Ridge (JdFR). With a summit depth of ~1400 m, the volcano intercepts deep ocean flows at a height well above that of the linear ridge adjoining it to the north and south. Multiyear, yearlong observations of currents at Axial show that mean flow circles the seamount in an anticyclonic sense, consistent with dynamical constraints, with means as large as 6.6 cm s^-1. Spectra of currents indicate prominent tidal, inertial, and in some cases weather-band period (3- to 7-day) peaks, as has been seen elsewhere along the JdFR ridge. For K₁, O₁, and S₂ tidal currents at the seamount, amplitudes are several times larger than the corresponding background amplitudes and rotation around the tidal current ellipses is anticyclonic. M₂ tidal currents show none of the same properties. Energy at the inertial frequency decreases by more than an order of magnitude from 100 m above to 400 m below the volcano's summit. In contrast, weather-band (WB) period energy increases tenfold over the same depth range, with WB energy all but unobservable at summit depth. The presence of WB oscillations makes Axial unlike other seamounts where long observations have been made. The current meter observations also provide a picture of seasonal trends. As expected, during the period Summer 1996 to Spring 1997, largest amounts of WB energy are found during autumn and winter; in contrast, WB motion at Axial Volcano during Winter 2000 was summer-like in intensity. A linear, baroclinic model of forced damped azimuthal mode-one oscillatory flow around an idealized axisymmetric seamount shows that amplification of diurnal flow should occur principally at the seamount summit while amplification of the WB should occur, as the observations show, on the seamount flanks. Model and measured phase relationships for temperature and current time series pairs circumscribing Axial support the view that azimuthal mode-one oscillations are prominent. Over the 0.8- to 6-day range for forcing period (T_C) the model indicates that the principal topographic resonance, as indicated by kinetic energy (KE) amplification, occurs for a T_C greater than 1 and less than 2 days with only mild dependence in that range on background stratification and seamount shape. Additional but weaker topographic resonances can occur at longer periods in model results, but as T_C becomes slightly larger than the frictional timescale, topographic resonances no longer appear. High dissipation and thus short frictional timescales reported by others in regions of rough, sloping topography make topographic resonances at Axial at periods greater than a few days somewhat unlikely. The relatively broad WB spectral peaks at some Axial sites are consequently more likely the result of spectral peaks in forcing than of topographic resonance.