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Modulation of near-inertial motions on the Mississippi-Alabama Shelf

Abstract

In this study, we diagnose the time variability and vertical structure of the high- and low-frequency motions on the Mississippi-Alabama Shelf as observed with a bottom-mounted ADCP (Acoustic Doppler Current Profiler) and CTD (Conductivity-Temperature-Depth). The mooring was deployed about 20 km offshore of Mobile Bay for a period from May 17 to August 23, 2018. At this latitude, the diurnal land and sea breeze has the same frequency as the local inertial frequency. Similar to the wind, the observed high-frequency baroclinic velocities (> 30 cm/s) have a broadband diurnal peak and more energy in the clockwise motions. About 60% of the variance in these motions is due to mode 1, which resembles a two-layer structure with surface and bottom velocities that are 180 out of phase. These are all characteristics of wind-driven motions that interact with the coastal wall. The month of June features the best conditions for energetic near-inertial motions: upwelling, consistent sea breeze, and a more continuous instead of a two-layer stratification. This causes near-inertial energy to be also projected on a baroclinic mode 2, featuring a subsurface maximum. This maximum may be attributed to the downward propagation of near-inertial internal wave energy. The observed alongshore low-frequency flows and the up- and downwelling are mostly driven by low-frequency winds. About 83% of the variance in the alongshore low-frequency flows is due to mode 1, which eigenfunction resembles a vertically sheared flow. We find that the amplitude of the near-inertial motions is modulated by the up- and downwelling. During downwelling, the near-inertial baroclinic kinetic energy is greatly reduced because of a reduction in stratification and weaker diurnal winds.

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Acknowledgements

We thank Brian Dzwonkowski for sharing the Dauphin Island Sea Lab mooring CTD data and for providing comments and suggestions on an earlier version of this article. We appreciate Captain Daniel Meek of the M/V Sweet Sue and Davin Wallace and his students Clayton Dike, Rob Hollis, and Nina Gal for their help in deploying and retrieving the USM mooring. Davin Wallace also reviewed an earlier version of this manuscript. Finally, we would like to thank the reviewers for their useful comments and suggestions.

Funding

Maarten Buijsman and Jordan Earls are funded by the Office of Naval Research (ONR) grant N00014-15-1-2288 and the National Science Foundation (NSF) grant OCE1537449. Maarten Buijsman and Helen Weierbach are funded by the National Aeronautic Space Administration (NASA) grant 80NSSC18K0771.

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Buijsman, M.C., Earls, J. & Weierbach, H. Modulation of near-inertial motions on the Mississippi-Alabama Shelf. Ocean Dynamics 71, 1087–1103 (2021). https://doi.org/10.1007/s10236-021-01483-8

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Keywords

  • Coastal processes
  • Near-inertial motions
  • Downwelling
  • Observations