The total rate of work done on the ocean by the wind is of considerable interest for understanding global energy balances, as the energy from the wind drives ocean currents, grows surface waves, and forces vertical mixing. A large but unknown fraction of this atmospheric energy is dissipated by turbulence in the upper ocean. The focus of this work is twofold. First, we describe a framework for evaluating the vertically integrated turbulent kinetic energy (TKE) equation using measurable quantities from a surface mooring, showing the connection to the atmospheric, mean oceanic, and wave energy. Second, we use this framework to evaluate turbulent energetics in the mixed layer using 10 months of mooring data. This evaluation is made possible by recent advances in estimating TKE dissipation rates from long‐enduring moorings. We find that surface fluxes are balanced by TKE dissipation rates in the mixed layer to within a factor of two.
- NSF-PAR ID:
- 10311749
- Date Published:
- Journal Name:
- Journal of Atmospheric and Oceanic Technology
- ISSN:
- 0739-0572
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract -
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Abstract Estimates of turbulence kinetic energy (TKE) dissipation rate (
ε ) are key in understanding how heat, gas, and other climate‐relevant properties are transferred across the air‐sea interface and mixed within the ocean. A relatively new method involving moored pulse‐coherent acoustic Doppler current profilers (ADCPs) allows for estimates ofε with concurrent surface flux and wave measurements across an extensive length of time and range of conditions. Here, we present 9 months of moored estimates ofε at a fixed depth of 8.4 m at the Stratus mooring site (20°S, 85°W). We find that turbulence regimes are quantified similarly using the Obukhov length scaleand the newer Langmuir stability length scale , suggesting that ocean‐side friction velocity implicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes drift and that is likely facilitated by the steady Southeast trade winds regime. In certain regimes, , where is the von Kármán constant and is instrument depth, and surface buoyancy flux capture our estimates of well, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based on and , scales ε well at times but is overall less consistent than. Monin‐Obukhov similarity theory (MOST) relationships from prior studies in a variety of aquatic and atmospheric settings largely agree with our data in conditions where convection and wind‐driven current shear are both significant sources of TKE, but diverge in other regimes. -
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Abstract Several years of moored turbulence measurements from
χ pods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, the DC layer is only indirectly linked to surface forcing. Yet, it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-min averaged data at fixedχ pod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stressτ . These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 m h−1, independent ofτ . We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 m h−1. This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows.Significance Statement Only recently have long time series measurements of ocean turbulence been available anywhere. Important sites for these measurements are the equatorial cold tongues where the nature of upper-ocean turbulence differs from that in most of the world’s oceans and where heat uptake from the atmosphere is concentrated. Critical to heat transported downward from the mixed layer is the diurnally varying deep cycle of turbulence below the mixed layer and above the core of the Equatorial Undercurrent. Even though this layer does not directly contact the surface, here we show the influence of the surface winds on both the magnitude of the deep cycle turbulence and the timing of its descent into the depths below.
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Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.more » « less
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Abstract Multiyear turbulence measurements from oceanographic moorings in equatorial Atlantic and Pacific cold tongues reveal similarities in deep cycle turbulence (DCT) beneath the mixed layer (ML) and above the Equatorial Undercurrent (EUC) core. Diurnal composites of turbulence kinetic energy dissipation rate,
ϵ , clearly show the diurnal cycles of turbulence beneath the ML in both cold tongues. Despite differences in surface forcing, EUC strength and core depth DCT occurs, and is consistent in amplitude and timing, at all three sites. Time‐mean values ofϵ at 30 m depth are nearly identical at all three sites. Variations of averaged values ofϵ in the deep cycle layer below 30 m range to a factor of 10 between sites. A proposed scaling in depth that isolates the deep cycle layers and ofϵ by the product of wind stress and current shear collapses vertical profiles at all sites to within a factor of 2.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1175/jtech-d-21-0005.1
