Submesoscale Mixing Across the Mixed Layer in the Gulf of Mexico

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.

The Role of Freshwater Forcing on Surface Predictability in the Gulf of Mexico

“…Our simulations reasonably represent the spatial structure of the HYCOM analysis. For a validation of the representation of stratification in CROCO, the reader is referred to Liu, Bracco, and Sitar (2021). Warm and salty water enters the basin from the southern boundary with the LC, while relatively cold and fresh water can be found along the coast.…”

“…In our simulations, the riverine forcing is implemented in an active or passive way (Figure 3). In the passive river configuration, the total discharge is applied as a bogus precipitation flux around the river mouth, and the amplitude of the flux decreases following a Gaussian function with an e-folding length of 50 km, as done in recent process studies Liu, Bracco, & Sitar, 2021) and similarly to HYCOM. Barkan, McWilliams, and Liu, Bracco, and Sitar (2021) have shown that this passive implementation ensures that the freshwater reaches realistically far from the coast by comparing seasonal distributions with Levitus data or in-situ measurements over the summer season in limited areas.…”

The role of freshwater forcing on surface predictability in the Gulf of Mexico

“…3). In the passive river configuration, the total discharge is applied as a bogus precipitation flux over several ocean grids near the river mouth location, as done in recent process studies Liu, Bracco, & Sitar, 2021). The passive implementation ensures that the freshwater reaches realistically far from the coast but neglects the momentum flux associated with the river outflow.…”

“…The simulations reasonably represent the spatial structure of the HYCOM analysis. For a brief validation of the representation of stratification in CROCO, the reader is referred to (Liu, Bracco, & Sitar, 2021). Warm and salty water enters the basin from the southern boundary with the LC, while relatively cold and fresh water can be found along the coast.…”

“…Furthermore, the juxtaposition of mesoscale eddies and filaments in waters with very different densities contributes to frontal and baroclinic instabilities at scales of few kilometers and to the formation of fronts and lateral convergence zones (Zhong et al, 2012;Zhong & Bracco, 2013;Luo et al, 2016;D'Asaro et al, 2018;Androulidakis et al, 2018). Submesoscale circulations influence mixing processes within and across the mixed layer (Liu, Bracco, & Sitar, 2021), and their impacts vary first and foremost seasonally, as shown through various modeling efforts (e.g., Zhong et al, 2012;Luo et al, 2016;Bracco et al, 2019;Liu, Bracco, & Sitar, 2021) and drifter deployments (Poje et al, 2014;D'Asaro et al, 2018), but also diurnally and interannually (Sun et al, 2020).…”