Submesoscale horizontal wavenumber spectra from the upper Eastern North Pacific

Anda Vladoiu, Applied Physics Laboratory University of Washington, Ocean Physics Department, Seattle, WA, United States, Ren-Chieh Lien, Applied Physics Lab, Univ of Washington, Seattle, WA, United States and Eric L Kunze, NorthWest Research Associates, Redmond, WA, United States
Abstract:
Isopycnal water-mass variability in the upper 200 m is investigated using towed CTD chain and ADCP measurements collected in the Eastern North Pacific off Baja California in July 2018. The surveys followed the water motion using two drogued drifters. Ship speed varied between about 0.4 and 4 kt, resolving horizontal wavelengths of 10 m to 100 km. Six EM-APEX floats were deployed within the survey box with about 1 km horizontal resolution, profiling the upper 200 m. They moved coherently with the drifters, providing temperature microstructure and additional temperature, salinity and velocity measurements.

Observed isopycnal slope horizontal wavenumber spectra have spectral slopes from 0 to +1/3 at low and intermediate wavenumbers (about 10-2 - 10 cpkm) to +1 at the highest horizontal wavenumbers (greater than about 10 cpkm). Spectral levels are as much as an order of magnitude below the GM model.

Observed normalized isopycnal salinity-gradient spectra have spectral slopes of +1/3 consistent with predictions for horizontal strain spectrum for anisotropic stratified turbulence (Kunze 2019) at horizontal wavelengths greater than O(100 m). Between 100 and 10 m, the spectral slopes range from +1 to -1 and scale with the turbulent kinetic energy dissipation rate inferred from the floats. Horizontal strain spectra inferred from the normalized-salinity gradient spectra have spectral levels up to an order of magnitude above the GM model.

Frequency spectra for horizontal kinetic energy, available potential energy, clockwise- and counterclockwise velocity from the EM-APEX float measurements have a strong semidiurnal peak, weak inertial peak, and spectral shapes consistent with the GM model. Energy ratios are consistent with linear wave theory, suggesting energy dominance of internal waves.

References:

Kunze, E. (2019). A unified model spectrum for anisotropic stratified and isotropic turbulence in the ocean and atmosphere. J. Phys. Oceanogr., 49, 385-407.