Scaling of Moored Surface Ocean Turbulence Measurements in the Southeast Pacific Ocean. Issue 1 (28th December 2022)

• Record Type: Journal Article
• Authors: Miller, Una Kim; Zappa, Christopher J.; Zippel, Seth F.; Farrar, J. Thomas; Weller, Robert A.
• Is Part Of: Journal of geophysical research. Volume 128:Issue 1(2023)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-12-28
• Subjects: turbulence -- dissipation -- Monin‐Obukhov similarity theory -- Stratus region -- upper ocean -- Langmuir; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2022JC018901
• Languages: English
• ISSNs: 2169-9275

Abstract

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 of ε 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 of ε 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 scale ( L M ) $({L}_{M})$ and the newer Langmuir stability length scale ( L L ) $({L}_{L})$, suggesting that ocean‐side friction velocity u ∗ $\left({u}_{\ast }\right)$ implicitly captures the influence of Langmuir turbulence at this site. This is illustrated by a strong correlation between surface Stokes drift u s $\left({u}_{s}\right)$ and u ∗ ${u}_{\ast }$ that is likely facilitated by the steady Southeast trade winds regime. In certain regimes, u ∗ 3 κ z $\frac{{u}_{\ast }^{3}}{\kappa z}$, where κ $\kappa $ is the von Kármán constant and z $z$ is instrument depth, and surface buoyancy flux capture our estimates of ε $\varepsilon $ well, collapsing data points near unity. We find that a newer Langmuir turbulence scaling, based on u s ${u}_{s}$ and u ∗ ${u}_{\ast }$, scales ε well at times but is overall less consistent than u ∗ 3 κ z $\frac{{u}_{\ast }^{3}}{\kappa z}$ . 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. Plain Language Summary: Surface ocean turbulence is key to the transfer of heat, gas, and other climate‐relevant properties between the ocean and atmosphere. Because turbulence is difficult to measure in the field, it is often parameterized using more easily obtained variables such as wind speed, wave measurements, and surface heat flux. Here, we test such parameterizations against an extensive time series of turbulence measurements collected on a mooring line attached to a surface buoy in the Southeast Pacific Ocean. This region is known to support important South American fisheries as well play a significant role in the global radiation budget, yet is poorly represented in climate models. We find the parameterizations to describe our measurements well, and we explore how conditions at the study site influence their performance. Key Points: Moored instrumentation allows for prolonged time series of turbulence estimates with concurrent in‐situ meteorological and wave measurements Law of the Wall and Monin‐Obukhov Similarity theory are able to predict measurements of ε in certain turbulence regimes In the context of turbulence scaling, it may be unnecessary to distinguish between a wind‐driven and Langmuir‐driven turbulence regime