Validation of Surface Wave Spectral Measurements from Velocity Profiling Floats

Abstract Electromagnetic autonomous profiling explorer (EM-APEX) floats have primarily been used to measure subsurface ocean velocities for the purpose of studying ocean dynamics and the vertical structure and shear of currents. However, the motionally induced voltage sensed by the EM-APEX also contains signals from surface wave orbital velocities, and the time taken to pass through the top ∼100 m of the water column is sufficient to estimate a spectrum of surface wave amplitudes with periods from 5 to 20 s (i.e., including both wind seas and long swell) with each profile. Following procedures developed by Hsu and D’Asaro, we analyzed the performance of EM-APEX float measurements of surface wave spectra, as well as bulk wave parameters such as significant wave height and energy period, against other in situ wave–measuring platforms during a 2017 field study off the coast of California [ONR’s Langmuir Circulation Department Research Initiative (LC-DRI)]. We discuss the limitations and uncertainties inherent in the EM-APEX wave measurements and determine the uncertainty of each individual 1-Hz velocity measurement to be on the order of 1.6 cm s −1 , resulting in a minimal detectable significant wave height of H s = 0.6 m. Results indicate that the EM-APEX surface wave spectral measurements are in good agreement with the other in situ wave measurements, with correlation coefficients of R 2 = 0.76 for significant wave height and R 2 = 0.65 for the energy period. This work demonstrates the potential for these floats to be used as a useful tool in future direct measurements of surface-wave-driven mixing processes, Langmuir turbulence, and the interactions between surface waves and ocean currents.