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  • 2020-2022  (3)
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  • 1
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    Diurnal Cycling of Submesoscale Dynamics: Lagrangian Implications in Drifter Observations and Model Simulations of the Northern Gulf of Mexico (2020)
    American Meteorological Society
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    Publication Date: 2020-05-18
    Description: The diurnal cycling of submesoscale circulations in vorticity, divergence, and strain is investigated using drifter data collected as part of the Lagrangian Submesoscale Experiment (LASER) experiment, which took place in the northern Gulf of Mexico during winter 2016, and ROMS simulations at different resolutions and degree of realism. The first observational evidence of a submesoscale diurnal cycle is presented. The cycling is detected in the LASER data during periods of weak winds, whereas the signal is obscured during strong wind events. Results from ROMS in the most realistic setup and in sensitivity runs with idealized wind patterns demonstrate that wind bursts disrupt the submesoscale diurnal cycle, independently of the time of day at which they happen. The observed and simulated submesoscale diurnal cycle supports the existence of a shift of approximately 1–3 h between the occurrence of divergence and vorticity maxima, broadly in agreement with theoretical predictions. The amplitude of the modeled signal, on the other hand, always underestimates the observed one, suggesting that even a horizontal resolution of 500 m is insufficient to capture the strength of the observed variability in submesoscale circulations. The paper also presents an evaluation of how well the diurnal cycle can be detected as function of the number of Lagrangian particles. If more than 2000 particle triplets are considered, the diurnal cycle is well captured, but for a number of triplets comparable to that of the LASER analysis, the reconstructed diurnal cycling displays high levels of noise both in the model and in the observations.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 2
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    Effects of Stratification on Shoaling Internal Tidal Bores (2021)
    American Meteorological Society
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    Publication Date: 2021-08-19
    Description: Idealized simulations of a shoaling internal tide on a gently sloping, linear shelf provide a tool to investigate systematically the effects of stratification strength, vertical structure, and internal wave amplitude on internal tidal bores. Simulations that prescribe a range of uniform or variable stratifications and wave amplitudes demonstrate a variety of internal tidal bores characterized by shoreward propagating horizontal density fronts with associated overturning circulations. Qualitatively, we observe three classes of solution: 1) bores, 2) bores with trailing wave trains, and 3) no bores. Very strong stratification (small wave) or very weak stratification (large wave) inhibits bore formation. Bores exist in an intermediate zone of stratification strength and wave amplitude. Within this intermediate zone, wave trains can trail bores if the stratification is relatively weak or wave amplitude large. We observe three types of bore that arise dependent on the vertical structure of stratification and wave amplitude: 1) a ‘backward’ downwelling front (near uniform stratification, small to intermediate waves), 2) a ‘forward’ upwelling front (strong pycnocline, small to large waves), and 3) a ‘double’ bore with leading up and trailing downwelling front (intermediate pycnocline, intermediate to large waves). Visualization of local flow structures explores the evolution of each of these bore-types. A frontogenetic diagnostic framework elucidates the previously undiscussed, yet, universal role of vertical straining of a stratified fluid that initiates formation of bores. Bores with wave trains exhibit strong non-hydrostatic dynamics. The results of this study suggest that mid-to-outer shelf measurements of stratification and cross-shore flow can serve as proxies to indicate the class of bore further inshore.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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  • 3
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    Surface gravity wave effects on submesoscale currents in the open ocean (2021)
    American Meteorological Society
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    Publication Date: 2021-09-09
    Description: A set of realistic coastal simulations in California allows for the exploration of surface gravity wave effects on currents (WEC) in an active submesoscale current regime. We use a new method that takes into account the full surface gravity wave spectrum and produces larger Stokes drift than the monochromatic peak-wave approximation. We investigate two high wave events lasting several days — one from a remotely generated swell and another associated with local wind-generated waves — and perform a systematic comparison between solutions with and without WEC at two submesoscale-resolving horizontal grid resolutions (dx = 270 m and 100 m). WEC results in the enhancement of open-ocean surface density and velocity gradients when the averaged significant wave height HS is relatively large (〉 4.2m). For smaller waves, WEC is a minor effect overall. For the remote swell (strong waves and weak winds), WEC maintains submesoscale structures and accentuates the cyclonic vorticity and horizontal convergence skewness of submesoscale fronts and filaments. The vertical enstrophy ζ2 budget in cyclonic regions (ζ/f 〉 2) reveals enhanced vertical shear and enstrophy production via vortex tilting and stretching. Wind-forced waves also enhance surface gradients, up to the point where they generate a small-submesoscale roll-cell pattern with high vorticity and divergence that extends vertically through the entire mixed layer. The emergence of these roll-cells results in a buoyancy gradient sink near the surface that causes a modest reduction in the typically large submesoscale density gradients.
    Print ISSN: 0022-3670
    Electronic ISSN: 1520-0485
    Topics: Geosciences , Physics
    Published by American Meteorological Society
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