skip to main content
US FlagAn official website of the United States government
dot gov icon
Official websites use .gov
A .gov website belongs to an official government organization in the United States.
https lock icon
Secure .gov websites use HTTPS
A lock ( lock ) or https:// means you've safely connected to the .gov website. Share sensitive information only on official, secure websites.


Title: Ocean Surface Boundary Layer Response to Abruptly Turning Winds
Abstract Turbulence driven by wind and waves controls the transport of heat, momentum, and matter in the ocean surface boundary layer (OSBL). For realistic ocean conditions, winds and waves are often neither aligned nor constant, for example, when winds turn rapidly. Based on a Large Eddy Simulation (LES) method, which captures shear-driven turbulence (ST) and Langmuir turbulence (LT) driven by the Craik-Leibovich vortex force, we investigate the OSBL response to abruptly turning winds. We design idealized LES experiments, whose winds are initially constant to equilibrate OSBL turbulence before abruptly turning 90° either cyclonically or anticyclonically. The transient Stokes drift for LT is estimated from a spectral wave model. The OSBL response includes three successive stages that follow the change in direction. During stage 1, turbulent kinetic energy (TKE) decreases due to reduced TKE production. Stage 2 is characterized by TKE increasing with TKE shear production recovering and exceeding TKE dissipation. Transient TKE levels may exceed their stationary values due to inertial resonance and non-equilibrium turbulence. Turbulence relaxes to its equilibrium state at stage 3, but LT still adjusts due to slowly developing waves. During stages 1 and 2, greatly misaligned wind and waves lead to Eulerian TKE production exceeding Stokes TKE production. A Reynolds stress budget analysis and Reynolds-averaged Navier-Stokes equation models indicate that Stokes production furthermore drives the OSBL response. The Coriolis effects result in asymmetrical OSBL responses to wind turning directions. Our results suggest that transient wind conditions play a key role in understanding realistic OSBL dynamics.  more » « less
Award ID(s):
1634578
PAR ID:
10295658
Author(s) / Creator(s):
;
Date Published:
Journal Name:
Journal of Physical Oceanography
ISSN:
0022-3670
Format(s):
Medium: X
Sponsoring Org:
National Science Foundation
More Like this
  1. This study utilizes a large-eddy simulation (LES) approach to systematically assess the directional variability of wave-driven Langmuir turbulence (LT) in the ocean surface boundary layer (OSBL) under tropical cyclones (TCs). The Stokes drift vector, which drives LT through the Craik–Leibovich vortex force, is obtained through spectral wave simulations. LT’s direction is identified by horizontally elongated turbulent structures and objectively determined from horizontal autocorrelations of vertical velocities. In spite of a TC’s complex forcing with great wind and wave misalignments, this study finds that LT is approximately aligned with the wind. This is because the Reynolds stress and the depth-averaged Lagrangian shear (Eulerian plus Stokes drift shear) that are key in determining the LT intensity (determined by normalized depth-averaged vertical velocity variances) and direction are also approximately aligned with the wind relatively close to the surface. A scaling analysis of the momentum budget suggests that the Reynolds stress is approximately constant over a near-surface layer with predominant production of turbulent kinetic energy by Stokes drift shear, which is confirmed from the LES results. In this layer, Stokes drift shear, which dominates the Lagrangian shear, is aligned with the wind because of relatively short, wind-driven waves. On the contrary, Stokes drift exhibits considerable amount of misalignments with the wind. This wind–wave misalignment reduces LT intensity, consistent with a simple turbulent kinetic energy model. Our analysis shows that both the Reynolds stress and LT are aligned with the wind for different reasons: the former is dictated by the momentum budget, while the latter is controlled by wind-forced waves. 
    more » « less
  2. null (Ed.)
    Abstract This study investigates the dynamics of velocity shear and Reynolds stress in the ocean surface boundary layer for idealized misaligned wind and wave fields using a large-eddy simulation (LES) model based on the Craik–Leibovich equations, which captures Langmuir turbulence (LT). To focus on the role of LT, the LES experiments omit the Coriolis force, which obscures a stress–current-relation analysis. Furthermore, a vertically uniform body force is imposed so that the volume-averaged Eulerian flow does not accelerate but is steady. All simulations are first spun-up without wind-wave misalignment to reach a fully developed stationary turbulent state. Then, a crosswind Stokes drift profile is abruptly imposed, which drives crosswind stresses and associated crosswind currents without generating volume-averaged crosswind currents. The flow evolves to a new stationary state, in which the crosswind Reynolds stress vanishes while the crosswind Eulerian shear and Stokes drift shear are still present, yielding a misalignment between Reynolds stress and Lagrangian shear (sum of Eulerian current and Stokes drift). A Reynolds stress budgets analysis reveals a balance between stress production and velocity–pressure gradient terms (VPG) that encloses crosswind Eulerian shear, demonstrating a complex relation between shear and stress. In addition, the misalignment between Reynolds stress and Eulerian shear generates a horizontal turbulent momentum flux (due to correlations of along-wind and crosswind turbulent velocities) that can be important in producing Reynolds stress (due to correlations of horizontal and vertical turbulent velocities). Thus, details of the Reynolds stress production by Eulerian and Stokes drift shear may be critical for driving upper-ocean currents and for accurate turbulence parameterizations in misaligned wind-wave conditions. 
    more » « less
  3. Abstract Mixing processes in the upper ocean play a key role in transferring heat, momentum, and matter in the ocean. These mixing processes are significantly enhanced by wave‐driven Langmuir turbulence (LT). Based on a paired analysis of observations and simulations, this study investigates wind fetch and direction effects on LT at a coastal site south of the island Martha’s Vineyard (MA, USA). Our results demonstrate that LT is strongly influenced by wind fetch and direction in coastal oceans, both of which contribute to controlling turbulent coastal transport processes. For northerly offshore winds, land limits the wind fetch and wave development, whereas southerly winds are associated with practically infinite fetch. Observed and simulated two‐dimensional wave height spectra reveal persistent southerly swell and substantially more developed wind‐driven waves from the south. For oblique offshore winds, waves develop more strongly in the alongshore direction with less limited fetch, resulting in significant wind and wave misalignments. Observations of coherent near‐surface crosswind velocities indicate that LT is only present for sufficiently developed waves. The fetch‐limited northerly winds inhibit wave developments and the formation of LT. In addition to limited fetch, strong wind–wave misalignments prevent LT development. Although energetic and persistent, swell waves do not substantially influence LT activity during the observation period because these relatively long swell waves are associated with small Stokes drift shear. These observational results agree well with turbulence‐resolving large eddy simulations (LESs) based on the wave‐averaged Navier–Stokes equation, validating the LES approach to coastal LT in the complex wind and wave conditions. 
    more » « less
  4. Abstract This work evaluates the fidelity of various upper-ocean turbulence parameterizations subject to realistic monsoon forcing and presents a finite-time ensemble vector (EV) method to better manage the design and numerical principles of these parameterizations. The EV method emphasizes the dynamics of a turbulence closure multimodel ensemble and is applied to evaluate 10 different ocean surface boundary layer (OSBL) parameterizations within a single-column (SC) model against two boundary layer large-eddy simulations (LES). Both LES include realistic surface forcing, but one includes wind-driven shear turbulence only, while the other includes additional Stokes forcing through the wave-average equations that generate Langmuir turbulence. The finite-time EV framework focuses on what constitutes the local behavior of the mixed layer dynamical system and isolates the forcing and ocean state conditions where turbulence parameterizations most disagree. Identifying disagreement provides the potential to evaluate SC models comparatively against the LES. Observations collected during the 2018 monsoon onset in the Bay of Bengal provide a case study to evaluate models under realistic and variable forcing conditions. The case study results highlight two regimes where models disagree 1) during wind-driven deepening of the mixed layer and 2) under strong diurnal forcing. 
    more » « less
  5. Abstract Dispersion processes in the ocean surface boundary layer (OSBL) determine marine material distributions such as those of plankton and pollutants. Sheared velocities drive shear dispersion, which is traditionally assumed to be due to mean horizontal currents that decrease from the surface. However, OSBL turbulence supports along-wind jets; located in near-surface convergence and downwelling regions, such turbulent jets contain strong local shear. Through wind-driven idealized and large-eddy simulation (LES) models of the OSBL, this study examines the role of turbulent along-wind jets in dispersing material. In the idealized model, turbulent jets are generated by prescribed cellular flow with surface convergence and associated downwelling regions. Numeric and analytic model solutions reveal that horizontal jets substantially contribute to along-wind dispersion for sufficiently strong cellular flows and exceed contributions due to vertical mean shear for buoyant surface-trapped material. However, surface convergence regions also accumulate surface-trapped material, reducing shear dispersion by jets. Turbulence resolving LES results of a coastal depth-limited ocean agree qualitatively with the idealized model and reveal long-lived coherent jet structures that are necessary for effective jet dispersion. These coastal results indicate substantial jet contributions to along-wind dispersion. However, jet dispersion is likely less effective in the open ocean because jets are shorter lived, less organized, and distorted due to spiraling Ekman currents. 
    more » « less