Annual Cycle and Depth Penetration of Wind-Generated Near-InertiaI Internal Waves at Ocean Station Papa in the Northeast Pacific

摘要

The downward propagation of near-inertial internal waves following winter storms is examined in the context of a 2-yr record of velocity in the upper 800 m at Ocean Station Papa. The long time series allow accurate estimation of wave frequency, whereas the continuous data in depth allow separation into upward- and downward-propagating components. Near-inertial kinetic energy (KE_(in)) dominates the record. At all measured depths, energy in downgoing motions exceeds that of upward-propagating motions by factors of 3-7, whereas KE_(in) is elevated by a factor of 3-5 in winter relative to summer. The two successive winters are qualitatively similar but show important differences in timing and depth penetration. Energy is seen radiating downward in a finite number of wave groups, which are tagged and catalogued to determine the vertical group velocity c_(gz) which has a mean of about 1.5 × 10 ~4 m s~(-1) (13 m day~(-1)). Case studies of three of these are presented in detail. Downward energy flux is estimated as c_(gz) X KE_(in) (i) by summing over the set of events, (ii) from time series near the bottom of the record, and (ⅲ) from the wavenumber-frequency spectrum and the dispersion relationship. These estimates are compared to the work done on near-inertial motions in the mixed layer by the wind, which is directly estimated from mixed layer near-inertial currents and winds measured from a surface buoy 10 km away. All three methods yield similar values, indicating that 12%-33% of the energy input into the mixed layer transits 800 m toward the deep sea. This simple picture neglects lateral energy flux carried by the first few vertical modes, which was not measured. The substantial deep penetration implies that near-inertial motions may play a role in mixing the deep ocean, but the strong observed variability calls for a need to better understand the role of lateral mesoscale structures in modulating the vertical propagation.