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A vorticity-divergence view of internal wave generation by tropical cyclones: insights from Super Typhoon Mangkhut
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  • Noel G. Brizuela,
  • T. M. Shaun Johnston,
  • Matthew H Alford,
  • Olivier Asselin,
  • Daniel L. Rudnick,
  • Jim Moum,
  • Elizabeth J Thompson,
  • Shuguang Wang,
  • Chia-Ying Lee
Noel G. Brizuela
University of California, San Diego, University of California, San Diego

Corresponding Author:[email protected]

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T. M. Shaun Johnston
Scripps Institution of Oceanography, Scripps Institution of Oceanography
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Matthew H Alford
Scripps Institution of Oceanography, Scripps Institution of Oceanography
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Olivier Asselin
Scripps Institution of Oceanography, UC San Diego, Scripps Institution of Oceanography, UC San Diego
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Daniel L. Rudnick
Scripps Institution of Oceanography, Scripps Institution of Oceanography
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Jim Moum
Oregon State University, Oregon State University
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Elizabeth J Thompson
NOAA Earth System Research Lab, NOAA Earth System Research Lab
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Shuguang Wang
Nanjing University, Nanjing University
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Chia-Ying Lee
Lamont Doherty Earth Observatory, Lamont Doherty Earth Observatory
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Abstract

Tropical cyclones (TCs) are powered by heat fluxes across the air-sea interface, which are in turn influenced by subsurface physical processes that can modulate storm intensity. Here, we use data from 6 profiling floats to recreate 3D fields of temperature (T), salinity (S), and velocity around Super Typhoon Mangkhut (western North Pacific, September 2018). Observational estimates of vorticity (ζ) and divergence (Γ) agree with output from a 3D coupled model, while their relation to vertical velocities is explained by a linear theoretical statement of inertial pumping. Under this framework, inertial pumping is described as a linear coupling between ζ and Γ, whose cycles cause periodic displacements in the ocean thermocline and generate near-inertial waves (NIWs). Vertical profiles of T and S show gradual mixing of the upper ocean with diffusivities as high as κ~10^-1 m^2 s^-1, which caused an asymmetric cold wake of sea surface temperature (SST). We estimate that rain layer destruction used ~ 10% of energy used for mixing near the TC track, therefore inhibiting SST cooling. Lastly, watermass transformation analyses suggest that κ > 3x10^-3 m^2 s^-1 above ~110 m depth and up to 600 km behind the TC. These analyses provide an observational summary of the ocean response to TCs, demonstrate some advantages of ζ and Γ for the study of internal wave fields, and provide conceptual clarity on the mechanisms that lead to NIW generation behind TCs.