Abstract
Submesoscale circulations influence momentum, buoyancy and transport of biological tracers and pollutants within the upper turbulent layer. How much and how far into the water column this influence extends remain open questions in most of the global ocean. This work evaluates the behavior of neutrally buoyant particles advected in simulations of the northern Gulf of Mexico by analyzing the trajectories of Lagrangian particles released multiple times at the ocean surface and below the mixed layer. The relative role of meso- and submesoscale dynamics is quantified by comparing results in submesoscale permitting and mesoscale resolving simulations. Submesoscale circulations are responsible for greater vertical transport across fixed depth ranges and also across the mixed layer, both into it and away from it, in all seasons. The significance of the submesoscale-induced transport, however, is far greater in winter. In this season, a kernel density estimation and a detailed vertical mixing analysis are performed. It is found that in the large mesoscale Loop Current eddy, upwelling into the mixed layer is the major contributor to the vertical fluxes, despite its clockwise circulation. This is opposite to the behavior simulated in the mesoscale resolving case. In the “submesoscale soup,” away from the large mesoscale structures such as the Loop Current and its detached eddies, upwelling into the mixed layer is distributed more uniformly than downwelling motions from the surface across the base of the mixed layer. Maps of vertical diffusivity indicate that there is an order of magnitude difference among simulations. In the submesoscale permitting case values are distributed around 10–3 m2 s–1 in the upper water column in winter, in agreement with recent indirect estimates off the Chilean coast. Diffusivities are greater in the eastern portion of the Gulf, where the submesoscale circulations are more intense due to sustained density gradients supplied by the warmer and saltier Loop Current.
Highlights
In the ocean, mesoscale eddies with horizontal scales of ∼20–300 km and life spans of weeks to months are major contributors to horizontal and vertical transport in regions that are not subjected to large scale upwelling
Couvelard et al (2015) used idealized simulations of a mesoscale front to show that submesoscale eddy fluxes associated with mixed layer instabilities (MLI, Boccaletti et al, 2007) can intensify the upper ocean stratification in winter, which in turn limits tracer mixing in the upper ocean
The submesoscale circulations in the GoM vary seasonally and are modulated by the depth of the mixed layer, by the freshwater discharge, and by the density gradients associated with the Loop Current (LC) and the detached LC Eddies
Summary
Mesoscale eddies with horizontal scales of ∼20–300 km and life spans of weeks to months are major contributors to horizontal and vertical transport in regions that are not subjected to large scale upwelling. Submesoscale circulations are commonly found in the form of small (smaller than 10 km in diameter) eddies and rapidly changing vorticity filaments and fronts (Capet et al, 2008; Thomas and Ferrari, 2008; McWilliams, 2016; Sun et al, 2020). They form preferentially in the upper and bottom turbulent boundary layers (Lévy et al, 2012) and are characterized by O(1) Richardson and Rossby numbers, with horizontal scales between 100 m and a few kilometers, and characteristic time scales from hours to days (Thomas et al, 2008). Capet et al (2008) and Luo et al (2016) did not find any submesoscaleinduced restratification in the Argentinian shelf and in the Gulf of Mexico, respectively
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