This study examines the mesoscale and submesoscale circulations along the continental 22 slope in the northern Gulf of Mexico at depths greater than 1000 m. The investigation is 23 performed using a regional model run at two horizontal grid resolutions, 5 km and 1.6 km, 24 over a three year period, from January 2010 to December 2012. 25Ageostrophic submesoscale eddies and vorticity filaments populate the continental slope, 26 and they are stronger and more abundant in the simulation at higher resolution, as to be 27 expected. They are formed from horizontal shear layers at the edges of highly intermittent, 28 bottom-intensified, along-slope boundary currents and in the cores of those currents where 29 they are confined to steep slopes. Two different flow regimes are identified. The first applies 30 to the De Soto Canyon that is characterized by weak mean currents and, in the high-31 resolution run, by intense but few submesoscale eddies that form near preferentially along 32 the Florida continental slope. The second is found in the remainder of the domain, where the 33 mean currents are stronger and the circulation is highly variable in both space and time, and 34 the vorticity field is populated, in the high-resolution case, by numerous vorticity filaments 35 and short-lived eddies. 36Lagrangian tracers are deployed at different times along the continental shelf below 1000 m 37 depth to quantify the impact of the submesoscale currents on transport and mixing. The 38 modeled absolute dispersion is, on average, independent of horizontal resolution, while 39 mixing, quantified by finite-size Lyapunov exponents and vertical relative dispersion, 40 increases when submesoscale processes are present. Dispersion in the De Soto Canyon is 41 smaller than in the rest of the model domain and less affected by resolution. This is further 42 confirmed comparing the evolution of passive dye fields deployed in De Soto Canyon near 43 the Macondo Prospect, where the Deepwater Horizon rig exploded in 2010, and at the 44 largest known natural hydrocarbon seep in the northern Gulf, known as GC600, located a 45 few hundred kilometers to the west of the rig wellhead. 46
Realistic, submesoscale-resolving numerical simulations are used to characterize the flow’s statistics and the geography of surface submesoscale currents in the northern Gulf of Mexico. This study examines the role of the Mississippi–Atchafalaya River system in driving submesoscale currents during winter and summer, on and off the shelf, by investigating two sets of statistically equilibrated solutions, with and without river forcing. In this paper, the first of three, the authors analyze vorticity ζ, horizontal divergence δ, and available potential energy to eddy kinetic energy conversion and show that river forcing has an important effect on the spatial distribution and magnitudes of submesoscale currents in both seasons. During winter, solutions without river forcing display an increase in seasonal-mean values of ζ, δ and compared to solutions with river forcing, particularly east of the Mississippi River delta and offshore. On the contrary, during summer, seasonal-mean values are larger in solutions with river forcing throughout the entire region. The river effects can be rationalized in terms of scaling arguments that relate submesoscale current magnitudes to the surface boundary layer depth and lateral buoyancy gradients. River outflow enhances submesoscale currents by increasing lateral buoyancy gradients but suppresses them by decreasing the boundary layer depth. A discussion of the submesoscale-generating mechanisms that in each season may determine whether the enhancement effect overcomes the suppression effect or vice versa is presented. Regional comparisons of horizontal velocity spectra, root-mean-square ζ, root-mean-square δ, and root-mean-square across different resolutions show no sign of convergence even at 150-m horizontal resolution. This demonstrates the numerical challenge of modeling the full range of submesoscale currents.
Four numerical simulations are used to characterize the impact of submesoscale circulations on surface Lagrangian statistics in the northern Gulf of Mexico over 2 months, February and August, representative of winter and summer. The role of resolution and riverine forcing is explored focusing on surface waters in regions where the water column is deeper than 50 m. Whenever submesoscale circulations are present, the probability density functions (PDFs) of dynamical quantities such as vorticity and horizontal velocity divergence for Eulerian and Lagrangian fields differ, with particles preferentially mapping areas of elevated negative divergence and positive vorticity. The stronger the submesoscale circulations are, the more skewed the Lagrangian distributions become, with greater differences between Eulerian and Lagrangian PDFs. In winter, Lagrangian distributions are modestly impacted by the presence of the riverine outflow, while increasing the model resolution from submesoscale permitting to submesoscale resolving has a more profound impact. In summer, the presence of riverine-induced buoyancy gradients is the key to the development of submesoscale circulations and different Eulerian and Lagrangian PDFs. Finite-size Lyapunov exponents (FSLEs) are used to characterize lateral mixing rates. Whenever submesoscale circulations are resolved and riverine outflow is included, FSLEs slopes are broadly consistent with local stirring. Simulated slopes are close to −0.5 and support a velocity field where the ageostrophic and frontogenetic components contribute stirring at scales between about 5 and 7 times the model resolution and 100 km. The robustness of Lagrangian statistics is further discussed in terms of their spatial and temporal variability and of the number of particles available.
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