National Oceanic and
Atmospheric Administration
United States Department of Commerce


FY 2012

Annual cycle and depth penetration of wind-generated near-inertial internal waves at Ocean Station Papa in the northeast Pacific

Alford, M.H., M.F. Cronin, and J. Klymak

J. Phys. Oceanogr., 42(6), 889–909, doi: 10.1175/JPO-D-11-092.1 (2012)

The downward propagation of near-inertial internal waves following winter storms is examined in the context of a two-year record of velocity in the upper 800 m at Ocean Station Papa. The long time series allow accurate estimation of wave frequency, while the continuous data in depth allow separation into upward- and downward-propagating components. Near-inertial kinetic energy (KEin) dominates the record. At all measured depths, energy in downgoing motions exceeds that of upward-propagating motions by factors of 3–7, while KEin is elevated 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, cgz, which has a mean of 1.03×10−4m s−1 (13 meters per day). Case studies of three of these are presented in detail.

Downward energy flux is estimated as cgz×KEin by a) summing over the set of events, b) time series near the bottom of the record, and c) from the wavenumber-frequency spectrum and the dispersion relationship. These 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.

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