Investigating Mesoscale, Submesoscale, and Mixing Ocean Dynamics in the Mozambique Channel Using Seismic and Simulation Datasets

Mesoscale and submesoscale structures and turbulent mixing play a vital role in transporting heat, carbon, and nutrients throughout the ocean, impacting ocean circulation and climate. As the bridge between geostrophic balanced mesoscale eddies and smaller scale turbulent dissipation, submesoscale processes (10 m–10 km, e.g. fronts, filaments, vortices, and wakes) facilitate the forward energy cascade. A lack of observations with sufficient spatial and temporal resolution however hamper understanding interior ocean dynamics. Better capturing mesoscale–submesoscale coupling at complex topography is particularly important as such regions act as hotspots of ocean mixing. One promising method is the use of active acoustics or Seismic Oceanography. This approach can map whole swaths of ocean in several hours with a resolution of O(10-100) m and penetrate to the seafloor. Here, a marine seismic dataset, collected in January-March 2016 has been to investigate the detail of subsurface flows in the Mozambique Channel. The Mozambique Channel is an ideal laboratory to study mesoscale-submesoscale interactions using seismics: here water masses from the Indian and Atlantic ocean collide and interact, driving sharp thermohaline gradients and intense current flows interact with complex bathymetry to produce an energetic eddy field. First, a standard processing strategy was applied to seismic sections: frequency filtering, seabed muting, direct wave removal, and f-k filtering, followed by velocity analysis, amplitude correction, stacking and deconvolution. Detailed images of phenomena including anticyclonic and cyclonic eddy distortions, submesoscale lenslike structures, and water mass boundary are presented. A combined inversion was developed and applied using integrated root mean square sound velocity analysis (VA) with iterative Markov Chain Monte Carlo (MCMC) techniques. This approach successfully retrieved thermohaline fields with quantify uncertainties, without the requirement for coincident hydrographic data (ΔT ∼ 1.65 − 2.5◦C, ΔS ∼ 0.08 − 0.5PSU). The dominant error source is converting root-mean-square velocity to interval velocity via the Dix equation. The spatial distributions of five seismic reflection sections capturing anticyclonic eddy (AE), cyclonic eddy (CE), and non-eddy conditions were interpreted using inverted temperature-salinity fields. The AE enhanced downwelling, deepened Sub-Tropical Surface Water/South Indian Central Water watermasses to ∼700 m, and trapped Red Sea Water (RSW)/ Antarctic Intermediate Water (AAIW) as lenses at 800–1200 m. The CE drove upwelling, shoaling Sub-Tropical Surface Water (STSW)/ South Indian Central Water (SICW) and disrupted RSW. In non-eddy conditions, stratification remained more stable and North Atlantic Deep Water (NADW) was present. These results highlight the role of eddies in shaping vertical water mass structure, stratification, and mixing. Diapycnal diffusivity and turbulent dissipation rates, deduced from seismically derived horizontal spectra, were quantitatively investigated and verified. The Mozambique Channel was found to be a mixing hotspot with area mean log10(K) ∼ 0.5 − 1 an order of magnitude above open ocean values. Mixing intensity however differed with eddy polarity. Mixing was week in the AE core (K ∼ 10−6.5 m2 s−1) with localized hotspots near boundaries and submesoscale features (∼ 10−4.2 m2 s−1). The CE exhibited stronger mixing (∼10−3.5 m2 s−1), especially near intense surface stirring and near the seafloor, linked to internal wave generation and bottom boundary layer instabilities. Non-eddy conditions also displayed bottom enhanced mixing (K ≈ 10−4 m2 s−1). Finally, simulations from the Coastal and Regional Ocean Community model (CROCO) were combined with seismic data to analyze AE and CE dynamics. CROCO captured key mesoscale properties, water mass structure, and submesoscale features observed in seismic data. For example, a simulated AE showed deepened isopycnals and lens-like submesoscale structures embedded within the eddy peripheries. One such cyclonic submesoscale structure was tracked and found to be generated by barotropic instabilities associated with AE boundary-slope interaction. Another anticyclonic lens-like structure was generated where the AE encountered the Davies Ridge. Energy conversion diagnostics indicate that mean-to-eddy kinetic energy transfer sustain submesoscale coherent vortices in AEs, while weak or negative conversion in CEs suppresses them. Seismic structure observations support this mechanism. In summary, results of this thesis reveal how eddy polarity and topography jointly regulate stratification, mixing, and interior submesoscale structure formation, advancing understanding of water mass transformation in the Mozambique Channel. Beyond the regional study, the approach establishes a framework for exploiting historical seismic data to investigate submesoscale processes more broadly.<p></p>

Author dc:creator

• Yao Meng (21042116)

• Physical Oceanography
• mesoscale
• submesoscale
• Acoustic measurements
• Mozambique Channel
• mixing

Statement dc:rights

• All rights reserved
• Open Access after 2027-09-15

Identifier

10779/exe.31743235.v1

OAI identifier oai:identifier

oai:figshare.com:article/31743235