Annual Cycle of Turbulent Dissipation Estimated from Seagliders

Abstract

The rate of dissipation of turbulent kinetic energy is estimated using Seaglider observations of vertical water velocity in the midlatitude North Atlantic. This estimate is based on the large-eddy method, allowing the use of measurements of turbulent energy at large scales O(1-10 m) to diagnose the rate of energy dissipated through viscous processes at scales O(1 mm). The Seaglider data considered here were obtained in a region of high stratification (1 × 10 −4 < N < 1 × 10 −2 s −1 ), where previous implementations of this method fail. The large-eddy method is generalized to high-stratification by high-pass filtering vertical velocity with a cutoff dependent on the local buoyancy frequency, producing a year-long time series of dissipation rate spanning the uppermost 1,000 m with subdaily resolution. This is compared to the dissipation rate estimated from a moored 600 kHz acoustic Doppler current profiler. The variability of the Seaglider-based dissipation correlates with one-dimensional scalings of wind-and buoyancy-driven mixed-layer turbulence. Plain Language SummaryMeasuring ocean turbulence is crucial for understanding how heat and carbon dioxide are transferred from the atmosphere to the deep ocean. However, measurements of ocean turbulence are sparse. Here autonomous Seagliders are used to estimate turbulence in the surface kilometer of the North Atlantic Ocean. Using an estimate of the vertical water velocity from the flight of the Seaglider through the water, we estimate turbulence by assuming the energy of the largest turbulent fluctuations is representative of the energy dissipated at molecular scales. This approach has been used previously in an ocean region where the vertical gradient of density is small. Our results show that this previous approach fails when the vertical density gradient increases, as it does not account for other processes that are unrelated to turbulence. We introduce a generalized method that isolates only the turbulent processes by accounting for the strength of the vertical density gradient. We show that this new estimate agrees with other turbulence measurements. Our estimate also agrees well with a simple estimates of turbulence from atmospheric processes. This study therefore presents method that can be applied to existing and new Seaglider data to greatly increase our measurements of ocean turbulence.

Wind‐Forced Symmetric Instability at a Transient Mid‐Ocean Front

“…The OSMOSIS site lies at the eastern edge of the North Atlantic subtropical gyre, a region of weak mean flow and moderate eddy kinetic energy that is arguably representative of a substantial fraction of the global ocean. While one-dimensional turbulent processes associated with air-sea buoyancy and wind forcing are found to be the dominant driver of upper-ocean turbulent dissipation over an annual cycle (Evans et al, 2018), our case study demonstrates that SI can play an important role in sustaining dissipation at depths beyond the influence of surface waves. Further, our work suggests that SI may be a common feature throughout the quiescent regions of the ocean, where it can be induced by the occasional alignment of surface winds with transient upper-ocean fronts generated by mesoscale frontogenesis.…”

“…The rate of dissipation of turbulent kinetic energy ϵ obs is estimated from high-frequency fluctuations in the glider vertical velocity using a generalization of the large eddy method of Beaird et al (2012) developed by Evans et al (2018) and used here to explore the key contributions to dissipation at the transient front. A filter with a cutoff dependent on the local buoyancy frequency is applied to glider vertical velocity estimates to remove internal wave variability.…”

Breaking of Internal Waves and Turbulent Dissipation in an Anticyclonic Mode Water Eddy

“…In this paper, we present results from a 4-month glider mission that sampled an anticyclonic mesoscale eddy located at the western boundary of the North Atlantic subtropical gyre, at 268N west of the Great Abaco Island (Bahamas). The observed variability of turbulent kinetic energy (TKE) dissipation rates within the eddy, inferred from glider-derived vertical seawater velocities using a large-eddy approximation (Beaird et al 2012;Evans et al 2018), was found to be consistent with the breaking of internal waves due to eddy-wave interactions. After describing the data collection procedures and methodologies (section 2), we present the general hydrographic conditions and the characteristics of the anticyclonic eddy, as well as the distribution of TKE dissipation, in section 3.…”

“…Additionally, there is the implicit assumption of no energy leakage such that, in a stationary state, the rates of energy transfer and dissipation are equivalent (Gargett 1999). The LEM was first applied to glider data by Beaird et al (2012) to study the variability of turbulent dissipation associated with the Nordic Sea inflows, and later by Evans et al (2018) to investigate the seasonal variability of near surface mixing in the North Atlantic at 488N. In both cases, glider-derived « compared favorably with independent direct estimates from microstructure shear and acoustic Doppler current profiler (ADCP) velocity measurements, and indirectly with boundary layer scalings.…”

Persistent Turbulence in the Samoan Passage

“…The pioneering work of Moum et al (2013), who created a 6-yr record of turbulence estimates from high-frequency temperature measurements in the equatorial Pacific, found a seasonal cycle in turbulence that was responsible for cooling sea surface temperature. Glider based measurements find that upper ocean turbulence is modulated by wind and buoyancy forcing (Evans et al 2018). Moored profiler based estimates of turbulent overturns on the Mid-Atlantic Ridge find a correlation between overturn size and the phase of the tide (Clément et al 2017).…”