FY 2002

On subinertial oscillations trapped by the Juan de Fuca Ridge, Northeast Pacific

Lavelle, J.W., and G.A. Cannon

J. Geophys. Res., 106(C12), 31,099–31,116, doi: 10.1029/2001JC000865 (2001)

Spectra of currents measured along the 2100 m deep Juan de Fuca Ridge in the northeast Pacific have prominent tidal, inertial, and weather-band (3-7 day period) spectral peaks. The weather-band peak, in particular, has a number of interesting features. That peak is strongest in close lateral proximity to the ridge and strongest near ridge depth; intensification near the ridge is characteristic of trapped motion. Spectral peak intensities vary seasonally with largest amplitudes occurring in autumn and winter; seasonal variation suggests that surface weather is forcing flow at depth. Together trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge. At all frequencies, forcing, topography, and stratification together shape current and hydrographic distributions near-ridge. Effects of those interactions for sub-inertial motion are quantified here using a primitive equation numerical model. Forcing period (1-10 days) and friction are the principal dependencies examined. Results show that flow over the Juan de Fuca Ridge can be amplified by factors of 3 to 4 for diurnal and up to a factor of 7 for weather-band frequencies. Amplification is ridge trapped, with effects extending many hundreds of meters upward and 5-10 km laterally for 5-day period flow; the strength and area of amplification increase with increasing period over the 1- to 10-day-period band. Oscillatory weather-band flow leads to vertical velocities on the order of 0.3 cm s on ridge flanks, which in turn causes periodic temperature (T) and salinity (S) variations with amplitudes of as much as 0.05°C and 0.01 PSU, respectively. The vertical motion and consequent vertical displacement (>100 m) lead to isotherms that plunge below the crest alternately each side of the ridge, a distribution observed in CTD transect data. Near the ridge crest, cross-ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near geostrophic balance with the Coriolis force associated with along-ridge flow. Along-ridge current amplification is, therefore, closely tied to isopycnal movement up and down ridge flanks. Since weather-band oscillations lead to much larger cross-ridge baroclinic pressure gradients than do diurnal motions of the same amplitude, weather-band forcing causes greater along-ridge current amplification. Results vary strongly with forcing period and direction of forcing with respect to the ridge, but depend only moderately on friction and turbulent mixing coefficients.