Moored Turbulence Measurements using Pulse-Coherent Doppler Sonar

Abstract

Upper-ocean turbulence is central to the exchanges of heat, momentum, and gasses across the air/sea interface, and therefore plays a large role in weather and climate. Current understanding of upper-ocean mixing is lacking, often leading models to misrepresent mixed-layer depths and sea surface temperature. In part, progress has been limited due to the difficulty of measuring turbulence from fixed moorings which can simultaneously measure surface fluxes and upper-ocean stratification over long time periods. Here we introduce a direct wavenumber method for measuring Turbulent Kinetic Energy (TKE) dissipation rates, ϵ, from long-enduring moorings using pulse-coherent ADCPs. We discuss optimal programming of the ADCPs, a robust mechanical design for use on a mooring to maximize data return, and data processing techniques including phase-ambiguity unwrapping, spectral analysis, and a correction for instrument response. The method was used in the Salinity Processes Upper-ocean Regional Study (SPURS) to collect two year-long data sets. We find the mooring-derived TKE dissipation rates compare favorably to estimates made nearby from a microstructure shear probe mounted to a glider during its two separate two-week missions for (10−8) ≤ ϵ ≤ (10−5) m2 s−3. Periods of disagreement between turbulence estimates from the two platforms coincide with differences in vertical temperature profiles, which may indicate that barrier layers can substantially modulate upper-ocean turbulence over horizontal scales of 1-10 km. We also find that dissipation estimates from two different moorings at 12.5 m, and at 7 m are in agreement with the surface buoyancy flux during periods of strong nighttime convection, consistent with classic boundary layer theory.

Parsing the Kinetic Energy Budget of the Ocean Surface Mixed Layer

“…For the majority of the year, the dissipation term is nearly in balance with the surface driven production and buoyancy terms, with values primarily ranging from 10 −4 to 10 −2 W m −2 . The agreement is typically within the factor of 2 accuracy inherent to measurements of TKE dissipation rate (Moum et al, 1995;S. F. Zippel et al, 2021).…”

“…For the majority of the year, the dissipation term is nearly in balance with the surface driven production and buoyancy terms, with values primarily ranging from 10 −4 to 10 −2 W m −2 . The agreement is typically within the factor of 2 accuracy inherent to measurements of TKE dissipation rate (Moum et al., 1995 ; S. F. Zippel et al., 2021 ). The relatively good agreement here suggests terms left to the residual, R in Equation 11 , including mixing and the wave‐affected layer residual, are indeed small compared with the estimated terms and/or compared to the errors associated with the dissipation measurement.…”

“…Good agreement is seen at all forcing levels (10 −4 –10 −2 W m −2 ), with bin averaged TKE dissipation rates within 95% confidence intervals (twice the standard error) of the estimated forcing. Variability of the daily averaged values is high, but somewhat expected due to the large variance inherent to the TKE dissipation rate estimates (S. F. Zippel et al., 2021 ).…”

“…The key element that enables this analysis is the measurements of TKE dissipation rate throughout the mixed layer, made possible by recent methods development with pulse‐coherent acoustic velocimeters (S. F. Zippel et al., 2021 ). For this study, pulse‐coherent ADCPs were used to estimate TKE dissipation rates at depths of 12.5, 21.5, 41.5, 61.7, 82, 101.6, and 121.6 m, with each burst sampled hourly for nearly the full mooring deployment.…”

“…Here, we examine turbulent energy losses in the oceanic mixed layer directly using a 10‐month time series of TKE dissipation rates made from an ocean mooring (S. F. Zippel et al., 2021 ). These estimates from long‐enduring deep‐ocean platforms showcase a powerful new approach toward understanding mixed layer dynamics in the upper 100 m of the ocean.…”