Storm-Driven Near-Inertial Waves and Mixing in the Western North Pacific

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Quantification and understanding of ocean mixing remains one of the most challenging problems in physical oceanography and ocean climate. The accurate representation (parameterization) and distribution of mixing in ocean circulation models is critical to correctly reproduce a wide range of processes from precipitation in the western equatorial Pacific to the strength of the meridional overturning cell and biogeochemical cycles. Near-inertial waves are one of three major sources of deep-ocean mixing. Little is known about their energy pathways beyond their wind generation and that only 15-25% of the wind-forced near-inertial wave energy radiates equatorward as low mode waves. These waves contain half the kinetic energy and most of the vertical shear in the ocean. About a terawatt of inertial wind power is injected by a few dozen mid-latitude fall and winter storms. While numerical and observational evidence points to the bulk of the inertial wind power being lost in the near-field of storm forcing, dissipating and mixing immediately below the surface layer, there has been little observational work to investigate this major piece of the inertial energy budget in detail. As well as determining the fate of wind-forced near-inertial waves, the proposed work will quantify the climatologically-important depth dependence of turbulent mixing in the pycnocline. The project investigators will consult with numerical modelers to how to improve parameterizations for internal-wave-driven turbulent mixing based on their observational findings. A graduate student will be trained in data analysis and interpretation and the project scientists will continue their local public outreach efforts.An innovative observational strategy is proposed to determine the zero-order near-field fate of inertial wind power and the vertical distribution of upper-ocean mixing following storms. An array of twelve profiling floats with electro-magnetic current meters and temperature microstructure sensors will be deployed in the western North Pacific near the inertial wind power gain maximum for a fall-winter season. Using bi-directional Iridium communications, dynamic sampling will focus on storm responses. The instruments will repeatedly profile every three hours between ~ 3 m of the surface and 500-m depth, collecting roughly five thousand profiles of temperature, salinity, horizontal velocity and turbulent temperature-variance dissipation rate. These measurements will provide about fifty independent depth-time series of the surface mixed-layer and upper pycnocline storm responses following five to ten fall and winter storms for a random variety of ocean eddy states. Harmonic fits will separate near-inertial, sub-inertial and higher-frequency signals. The measurements will quantify where and how near-inertial shear contributes to turbulence mixing, as well as estimate radiative losses.