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1. Wind Dependencies of Deep Cycle Turbulence in the Equatorial Cold Tongues

   https://doi.org/10.1175/JPO-D-22-0203.1  

   Moum, James N.; Smyth, William D.; Hughes, Kenneth G.; Cherian, Deepak; Warner, Sally J.; Bourlès, Bernard; Brandt, Peter; Dengler, Marcus (August 2023, Journal of Physical Oceanography)

   Abstract Several years of moored turbulence measurements fromχpods 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 fixedχpod 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 StatementOnly 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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2. Turbulent shear layers in a uniformly stratified background: DNS at high Reynolds number

   https://doi.org/10.1017/jfm.2021.212  

   VanDine, Alexandra; Pham, Hieu T.; Sarkar, Sutanu (June 2021, Journal of Fluid Mechanics)

   Direct numerical simulations are performed to investigate a stratified shear layer at high Reynolds number ( $Re$ ) in a study where the Richardson number ( $Ri$ ) is varied among cases. Unlike previous work on a two-layer configuration in which the shear layer resides between two layers with constant density, an unbounded fluid with uniform stratification is considered here. The evolution of the shear layer includes a primary Kelvin–Helmholtz shear instability followed by a wide range of secondary shear and convective instabilities, similar to the two-layer configuration. During transition to turbulence, the shear layers at low $Ri$ exhibit a period of thickness contraction (not observed at lower $Re$ ) when the momentum and buoyancy fluxes are counter-gradient. The behaviour in the turbulent regime is significantly different from the case with a two-layer density profile. The transition layers, which are zones with elevated shear and stratification that form at the shear-layer edges, are stronger and also able to support a significant internal wave flux. After the shear layer becomes turbulent, mixing in the transition layers is shown to be more efficient than that which develops in the centre of the shear layer. Overall, the cumulative mixing efficiency ( $E^C$ ) is larger than the often assumed value of 1/6. Also, $E^C$ is found to be smaller than that in the two-layer configuration at moderate Ri . It is relatively less sensitive to background stratification, exhibiting little variation for $$0.08 \leqslant Ri \leqslant 0.2$$ . The dependence of mixing efficiency on buoyancy Reynolds number during the turbulence phase is qualitatively similar to homogeneous sheared turbulence. 

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3. Evolution of Dynamic and Thermodynamic Structures before and during Rapid Intensification of Tropical Cyclones: Sensitivity to Vertical Wind Shear

   https://doi.org/10.1175/MWR-D-18-0173.1  

   Tao, Dandan; Zhang, Fuqing (April 2019, Monthly Weather Review)

   This study explores the spatial and temporal changes in tropical cyclone (TC) thermodynamic and dynamic structures before, near, and during rapid intensification (RI) under different vertical wind shear conditions through four sets of convection-permitting ensemble simulations. A composite analysis of TC structural evolution is performed by matching the RI onset time of each member. Without background flow, the axisymmetric TC undergoes a gradual strengthening of the inner-core vorticity and warm core throughout the simulation. In the presence of moderate environmental shear (5–6 m s⁻¹), both the location and magnitude of the asymmetries in boundary layer radial flow, relative humidity, and vertical motion evolve with the tilt vector throughout the simulation. A budget analysis indicates that tilting is crucial to maintaining the midlevel vortex while stretching and vertical advection are responsible for the upper-level vorticity generation before RI when strong asymmetries arise. Two warm anomalies are observed before the RI onset when the vortex column is tilted. When approaching the RI onset, these two warm anomalies gradually merge into one. Overall, the most symmetric vortex structure is found near the RI onset. Moderately sheared TCs experience an adjustment period from a highly asymmetric structure with updrafts concentrated at the down-tilt side before RI to a more axisymmetric structure during RI as the eyewall updrafts develop. This adjustment period near the RI onset, however, is found to be the least active period for deep convection. TC development under a smaller environmental shear (2.5 m s⁻¹) condition displays an intermediate evolution between ensemble experiments with no background flow and with moderate shear (5–6 m s⁻¹). 

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4. PTR-TOF-MS eddy covariance measurements of isoprene and monoterpene fluxes from an eastern Amazonian rainforest

   https://doi.org/10.5194/acp-20-7179-2020  

   Sarkar, Chinmoy; Guenther, Alex B.; Park, Jeong-Hoo; Seco, Roger; Alves, Eliane; Batalha, Sarah; Santana, Raoni; Kim, Saewung; Smith, James; Tóta, Julio; et al (January 2020, Atmospheric Chemistry and Physics)

   null (Ed.)

   Abstract. Biogenic volatile organic compounds (BVOCs) are important components of the atmosphere due to their contribution to atmospheric chemistry and biogeochemical cycles. Tropical forests are the largest source of the dominant BVOC emissions (e.g. isoprene and monoterpenes). In this study, we report isoprene and total monoterpene flux measurements with a proton transfer reaction time-of-flight mass spectrometer (PTR-TOF-MS) using the eddy covariance (EC) method at the Tapajós National Forest (2.857∘ S, 54.959∘ W), a primary rainforest in eastern Amazonia. Measurements were carried out from 1 to 16 June 2014, during the wet-to-dry transition season. During the measurement period, the measured daytime (06:00–18:00 LT) average isoprene mixing ratios and fluxes were 1.15±0.60 ppb and 0.55±0.71 mg C m−2 h−1, respectively, whereas the measured daytime average total monoterpene mixing ratios and fluxes were 0.14±0.10 ppb and 0.20±0.25 mg C m−2 h−1, respectively. Midday (10:00–14:00 LT) average isoprene and total monoterpene mixing ratios were 1.70±0.49 and 0.24±0.05 ppb, respectively, whereas midday average isoprene and monoterpene fluxes were 1.24±0.68 and 0.46±0.22 mg C m−2 h−1, respectively. Isoprene and total monoterpene emissions in Tapajós were correlated with ambient temperature and solar radiation. Significant correlation with sensible heat flux, SHF (r2=0.77), was also observed. Measured isoprene and monoterpene fluxes were strongly correlated with each other (r2=0.93). The MEGAN2.1 (Model of Emissions of Gases and Aerosols from Nature version 2.1) model could simulate most of the observed diurnal variations (r2=0.7 to 0.8) but declined a little later in the evening for both isoprene and total monoterpene fluxes. The results also demonstrate the importance of site-specific vegetation emission factors (EFs) for accurately simulating BVOC fluxes in regional and global BVOC emission models. 

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5. Assessing a Turbulent Mixing Scheme in Diurnal Warm Layers considering Langmuir Turbulence

   https://doi.org/10.1175/JPO-D-24-0164.1  

   Wang, Xingchi; Kukulka, Tobias (July 2025, Journal of Physical Oceanography)

   Abstract This study investigates how Langmuir turbulence (LT) driven by Stokes drift shear affects the heated ocean surface boundary layer (OSBL) based on turbulence-resolving large-eddy simulations (LES) and assesses an analytic vertical mixing parameterization based on a simplified second-moment closure (SMC) approach. Diurnal solar heating forces OSBL shoaling to generate a diurnal warm layer (DWL) in which heat and momentum are trapped. Without LT, relatively weak turbulent mixing results in a near-surface jet that is associated with enhanced turbulent kinetic energy (TKE) production of shear-driven turbulence (ST), which approximately balances TKE dissipation rates. Conversely, LT maintains strong mixing, delaying the DWL formation and preventing the TKE dissipation enhancement by generating a less sheared jet. However, sufficiently strong heating destroys TKE to ultimately reduce mixing and to create more sheared jets, which effectively shifts the LT to an ST-dominated regime. A second-moment turbulence budget analysis suggests that 1) the near-surface OSBL responds rapidly to the surface forcing, 2) Stokes drift impacts heat and momentum budgets in profoundly different ways, and 3) buoyancy terms are to leading order negligible. Building on these findings and introducing a physics-based mixing length, we develop a simplified SMC model that can be solved for near-surface expressions for key turbulent variables and mixing coefficients in terms of known variables. For ST, these expressions are consistent with the Monin–Obukhov similarity theory. For LT, these expressions reveal a fundamental dependence of turbulent variables on Stokes drift shear. 

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