Submesoscale dynamics and transport properties in the Gulf of Mexico
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Submesoscale processes, characterized by O(1km) horizontal scale and O(1) Rossby number, are ubiquitous in the world ocean and play an important role in the vertical flux of mass, buoyancy and tracers in the upper ocean. However, they have not been intensively studied due to the requirement of uniquely high spatial and temporal resolution in the observation and computer modeling. In this thesis, using a suite of high-resolution numerical experiments in the northwestern Gulf of Mexico, where rich submesoscale structures are accompanied by the strong mesoscale Loop Current eddies, the impact of resolving submesoscales on the tracer distribution and 3-D transport was extensively examined. It was concluded that, submesoscale dynamics aggregated the surface tracers and formed characteristic patterns at scales of kilometers near the ocean surface by enhanced convergence/divergence zones associated with strong ageostrophic processes. This distinctive phenomenon was evident in recent ocean color satellite images which showed similar extensive lines and spirals of floating Sargassum in the western Gulf of Mexico. In addition, better-resolved submesoscale activities increased the horizontal resolution dramatically and elevated local vertical velocity both within and below the mixed layer while leaving the horizontal component almost unaltered. The vertical dispersion increased by several fold with the largest difference close to the surface. Considering the pervasive presence of submesoscale structures at the surface ocean, these models predict that submesoscale processes may serve as an important nutrient supply mechanism in the upper ocean and potentially make a significant contribution on balancing the global biogeochemical tracer budget.