Submesoscale Eddy Vertical Covariances and Dynamical Constraints from High-Resolution Numerical Simulations

Abstract

We analyze high-resolution (1 km) simulations of the western Pacific, Gulf of Mexico, and Arabian Sea to understand submesoscale eddy dynamics. A mask based on the Okubo–Weiss parameter isolates small-scale eddies, and we further classify those with |ζ/f| ≥ 1 as being submesoscale eddies. Cyclonic submesoscale eddies exhibit a vertical depth structure in which temperature anomalies from the large-scale background are negative. Peak density anomalies associated with cyclonic submesoscale eddies are found at a depth approximately twice the mixed layer depth (MLD). Within anticyclonic submesoscale eddies, temperature anomalies are positive and have peak density anomalies at the MLD. The depth–depth covariance structure for the cyclonic and anticyclonic submesoscale eddies have maxima over a shallow region near the surface and weak off diagonal elements. The observed vertical structure suggests that submesoscale eddies have a shallower depth profile and smaller vertical correlation scales when compared to the mesoscale phenomenon. We test a two-dimensional submesoscale eddy dynamical balance. Compared to a geostrophic dynamical balance using only pressure gradient and Coriolis force, including velocity tendency and advection produces lower errors by about 20%. In regions with strong tides and associated internal waves (western Pacific and Arabian Sea), using the mixed layer integrated small-scale steric height within the dynamical equations produces the lowest magnitude errors. In areas with weak tides (Gulf of Mexico), using small-scale sea surface height (SSH) produces the lowest magnitude errors. Recovering a submesoscale eddy with the correct magnitude and rotation requires integration of small-scale specific volume anomalies well below the mixed layer.

Assessing Eddying (1/12°) Ocean Reanalysis GLORYS12 Using the 14‐yr Instrumental Record From 59.5°N Section in the Atlantic

“…The Okubo‐Weiss parameter often used to track coherent mesoscale eddies (D'Addezio et al., 2020; Vortmeyer‐Kley et al., 2016) allows separating the vorticity‐dominated and deformation‐dominated flow areas. It is used in eddy activity analysis to distinguish the coherent eddying structures from ones with intense shear.…”

“…Therefore, the eddy field of the reanalysis may be used to understand specific circulation features seen in the observations. The Okubo-Weiss parameter often used to track coherent mesoscale eddies (D'Addezio et al, 2020;Vortmeyer-Kley et al, 2016) It is used in eddy activity analysis to distinguish the coherent eddying structures from ones with intense shear.…”

Progress in Operational Modeling in Support of Oil Spill Response

“…Present regular observing systems do not resolve smaller-scale features, and much of the GoMRI work has shown the importance of the sub-mesoscale effects on transport and dispersion (D'Asaro et al [56]). The sub-mesoscale presents a new challenge in this area, and efforts have begun to represent the vertical structures in sub-mesoscale physics (D'Addezio et al [67]).…”

“…Airborne observations could be used to provide better high-resolution data on currents for model initialization (Rodriguez et al [70]), while satellites can provide high-resolution data with continuous global coverage (Rodriguez et al [71]). Observations available to the data assimilation have limitations that reflect on the predictive skill of non-deterministic mesoscale and sub-mesoscale features, and these issues are discussed in Section 5. mesoscale presents a new challenge in this area, and efforts have begun to represent the vertical structures in sub-mesoscale physics (D'Addezio et al [67]). In the cases of GoMRI studies, targeted high-density observations allowed corrections at smaller scales.…”

Submesoscale Ageostrophic Motions Within and Below the Mixed Layer of the Northwestern Pacific Ocean

“…The 200‐m depth is mostly below the ML throughout the year. Those small eddies certainly have limited vertical scales and cannot extend to depth (D'addezio et al., 2020; Liu et al., 2020)—submesoscale eddies are mostly contained in the ML (Figure 2, right column). This also restrains the development of submesoscale eddies to be local (Boccaletti et al., 2007; Fox‐Kemper et al., 2008).…”