Observations of turbulent structures at the base of the ocean mixed layer

Observed rapid adjustment of the atmospheric boundary layer to submesoscale sea surface temperature fronts.

Current understanding of the role of ocean variability in air-sea exchange is constrained to large and mesoscale dynamics. Oceanic fronts and filaments with horizontal spatial scales of order 0.1 to 10 km-denoted submesoscale-are challenging to observe due to their fast-evolving flow and small spatiotemporal scales of variability. Observations investigating the air-sea fluxes at the submesoscale have shown substantial fluxes of heat, moisture, and momentum, affecting the structure of the overlying atmosphere. Here, modulations of the turbulent atmospheric boundary layer driven by ocean temperature anomalies are investigated using submesoscale-resolving ship and airborne measurements, providing in situ evidence of the atmospheric response to ocean submesoscale temperature variability. Observations suggest near-surface turbulent mixing driven by strong air-sea fluxes of heat and momentum, modifying the vertical structure of the planetary boundary layer. Linear regression coefficients between wind speed and sea surface temperature anomalies reveal a response similar in magnitude to that seen at larger scales, with an integrated change of 0.23 m s-1 °C-1, but occurring over smaller length-scales, implying sharper gradients. Lagged correlations and scaling analysis imply a combined influence of horizontal advection and vertical turbulent mixing of momentum in the atmosphere, previously only described by numerical simulations. Observed cross-frontal wind divergences over the lower 200 m suggest coherent circulations with vertical velocities of order 1 cm s-1. These observations confirm the rapid adjustment of the marine boundary layer to submesoscale ocean temperature variability and the importance of submesoscale-driven air-sea fluxes in changing the properties of the lower atmosphere, processes not resolved in most forecasting and prediction models.

Mixing and sharpening at the interface of a two-layer fluid forced by random jets

Understanding mixing at density interfaces is essential for predicting transport in stratified environmental flows. Laboratory studies have mostly relied on steady, spatially uniform forcing, whereas turbulence in nature is intermittent and heterogeneous. Here, we present experiments on a two-layer salt-stratified fluid forced by random turbulent bursts generated with a randomly actuated synthetic jet array (RASJA). Density fields are recorded with the light attenuation technique, allowing us to resolve the interface evolution. We measure that the upward velocity of the interface decreases with the density jump, in agreement with the power-law found in previous oscillating-grid studies. At large density differences, the interface sharpens during mixing, contrary to the smaller density jump case. Background potential energy analysis demonstrates irreversible mixing in both cases, with comparable energy changes. These results extend classical laboratory observations to a more isotropic forcing, offering new insights into the dynamics of mixing in geophysical settings.

Stratification, turbulence organisation and pressure-strain effects on surface-layer turbulence anisotropy

Abstract Content of image described in text. At large scales, the Reynolds stress tensor exhibits notable anisotropy, a key feature of all wall-bounded turbulent flows. Yet, how the drivers of this anisotropy evolve with shearing and thermal stratification in the atmospheric surface layer (ASL) remains a daunting challenge for theory and models alike. Here, the velocity variance budgets are used to explore the evolution of anisotropy in the daytime ASL close to the surface, a region known to be problematic for large-eddy simulations. A special focus is placed on the importance of slow and rapid pressure-strain correlations, and the role of transport on partitioning the turbulent kinetic energy among the velocity components. Results obtained from near-surface observations of four datasets over flat and horizontally homogeneous terrain show persistent anisotropy over a wide range of flux Richardson numbers italic Ri Subscript f Rif ${\textit{Ri}}_{\!f}$ and wall-normal distances, and highlight the importance of different processes in three distinct flow regimes, roughly related to dynamic ( StartAbsoluteValue italic Ri Subscript f Baseline EndAbsoluteValue much less than 1 |Rif|≪1 $|{\textit{Ri}}_{\!f}|\ll 1$ ), dynamic-convective ( StartAbsoluteValue italic Ri Subscript f Baseline EndAbsoluteValue tilde 1 |Rif|∼1 $|{\textit{Ri}}_{\!f}|\sim 1$ ) and convective ( StartAbsoluteValue italic Ri Subscript f Baseline EndAbsoluteValue much greater than 1 |Rif|≫1 $|{\textit{Ri}}_{\!f}|\gg 1$ ) regimes of the ASL. In particular, close to the surface in the dynamic-convective regime, a drop in wall-normal velocity variance and a substantial increase of spanwise velocity variance are shown to result from the increasing role of pressure transport and rapid distortion, related to turbulence organisation. This behaviour is not captured by the classic Rotta closure but requires the inclusion of both rapid pressure-strain and transport terms. In all regimes, wall blocking is found to influence turbulence close to the surface, thus requiring the adoption of an anisotropic Rotta model to accommodate its effects.

Penetration of surface-generated mesoscale eddies into oceanic abyssal (numerical experiments)

Several series of numerical experiments were conducted to generate a mesoscale eddy in a deep rotating basin (4000 m deep) with stratification typical of mid- and low-latitude of the World Ocean, including an upper mixed layer, a density jump layer, and a main pycnocline. A sea level anomaly driven by Ekman transport convergence was created at the basin’s center by specifying a wind stress configuration typical for a stationary atmospheric anticyclone. Due to geostrophic adjustment, the resulting pressure anomaly generated an anticyclonic baroclinic eddy in the basin, the rotation velocity of which decreased with depth. Numerical experiments varying the horizontal scale of the eddy, the Coriolis frequency, and the stratification parameters yielded a universal relationship between the ratio of the rotation velocity in the surface and bottom layers and the ratio of the horizontal scale of the eddy to the first baroclinic Rossby radius of deformation. Comparing this universal relationship with published estimates of the horizontal scale of mesoscale eddies in the World Ocean from altimetry and shipboard measurements revealed that at low- and mid-latitudes, the rotation velocity typically weakens with depth by more than 80 %. In the presence of a bottom topography disturbance in the form of an abyssal channel under a mesoscale baroclinic eddy, a current arises in the channel whose velocity is significantly higher than the bottom velocity in the undisturbed eddy. The velocity disturbance caused by the abyssal channel is not limited to the channel itself, but reaches the surface, provided that the width of the channel exceeds the first baroclinic Rossby radius of deformation.

Uncovering Ocean Mixed Layer Dynamics From the Sea Surface State

Submesoscale dynamics strongly influence the upper ocean, regulating mixing, air–sea exchange, and vertical heat transport. The recent Surface Water and Ocean Topography mission provides unprecedented high‐resolution observations of sea surface height (SSH), yet linking these surface measurements to subsurface ocean dynamics remains challenging. We develop a theoretical framework for diagnosing key mixed layer (ML) properties from surface‐observable states. We show that horizontal density anomalies induced by mixed layer eddies produce surface imprints that can be effectively captured by spatially filtered SSH. The filtered SSH is integrated into the ML Eddy parameterization to infer the effects of submesoscale restratification. A potential energy budget accounting for the mixing–restratification competition in the ML is diagnosed from surface buoyancy flux, wind stress, and the SSH gradient, enabling reconstruction of the mixed‐layer depth. Vertical eddy heat flux can be further reconstructed from the SSH gradient. This framework offers a promising approach for diagnosing interior submesoscale processes using surface observations.

on Stratified Flows