Scott Bachman

Isohaline Salinity Budget of the North Atlantic Salinity Maximum

AbstractIn this study, the salinity budget of the North Atlantic subtropical salinity maximum region for control volumes bounded by isohaline surfaces is analyzed. The authors provide closed budgets based on output from a high-resolution numerical simulation and partial budgets based on analyses of observational climatologies of hydrography and surface fluxes. With this choice of control volume, advection is eliminated from the instantaneous volume-integrated salt budget, and time-mean advection is eliminated from the budget evaluated from time-averaged data. In this way, the role of irreversible mixing processes in the maintenance and variability of the salinity maximum are more readily revealed. By carrying out the analysis with both near-instantaneous and time-averaged model output, the role of mesoscale eddies in stirring and mixing for this water mass is determined. This study finds that the small-scale mixing acting on enhanced gradients generated by the mesoscale eddies is approximately equal to th...

A scale‐aware subgrid model for quasi‐geostrophic turbulence

5 This paper introduces two methods for dynamically prescribing eddy-induced diffusivity, advection, and vis6 cosity appropriate for primitive equation models with resolutions permitting the forward potential enstrophy 7 cascade of quasigeostrophic dynamics, such as operational ocean models and high-resolution climate mod8 els with O(25)km horizontal resolution and finer. Where quasigeostrophic dynamics fail (e.g., the equator, 9 boundary layers, deep convection), the method reverts to scalings based on a matched two-dimensional en10 strophy cascade. A principle advantage is that these subgrid models are scale-aware, meaning that the model 11 is suitable over a range of grid resolutions: from mesoscale grids that just permit baroclinic instabilities to 12 grids below the submesoscale where ageostrophic effects dominate. 13 Two approaches are presented here using Large Eddy Simulation (LES) techniques adapted for three14 dimensional rotating, stratified turbulence. The simpler approach has one non-dimensional parameter, Λ, 15 which has an optimal value near 1. The second approach dynamically optimizes Λ during simulation using 16 a test filter. The new methods are tested in an idealized scenario by varying the grid resolution, and their use 17 improves the spectra of potential enstrophy and energy in comparison to extant schemes. The new meth18 ods keep the gridscale Reynolds and Péclet numbers near one throughout the domain, which confers robust 19 numerical stability and minimal spurious diapycnal mixing. Although there are no explicit parameters in 20 the dynamic approach, there is strong sensitivity to the choice of test filter. Designing test filters for hetero21 geneous ocean turbulence adds cost and uncertainty, and we find the dynamic method does not noticeably 22 improve over setting Λ = 1. 23 ∗Corresponding Author address: Scott Bachman, DAMTP, Centre for Mathematical Sciences, Wilberforce Road, Cambridge CB3 Preprint submitted to Elsevier November 4, 2016

Numerical Simulations of the Equilibrium between Eddy-Induced Restratification and Vertical Mixing

AbstractSubmesoscale dynamics are hypothesized to play a leading-order role in setting the stratification of the mixed layer via the interaction of submesoscale eddies and surface forcing. Previous studies of such interactions have generally focused on the time-evolving characteristics of submesoscale turbulence, such as the spindown of a baroclinically unstable front. This paper focuses instead on the equilibrium dynamics of the oceanic mixed layer, where forcing and dissipation are in balance, through a combination of scaling analysis and numerical simulations. The steady dynamics are well described by a turbulent thermal wind balance, with external forcing parameterized by a strong vertical diffusivity κ. Scaling laws are developed for the characteristic vertical length scale Lυ, ageostrophic velocity scales U and V, buoyancy frequency N2, and eddy buoyancy flux , which are appropriate for a turbulent mixed layer whose stratification is equilibrated against strong vertical mixing. A suite of numerical ...

Frontal circulation and submesoscale variability during the formation of a Southern Ocean mesoscale eddy

Observations made in the Scotia Sea during the May 2015 Surface Mixed Layer Evolution at Submesoscales (SMILES) research cruise captured submesoscale, O(1–10) km, variability along the periphery of a mesoscale O(10–100) km meander precisely as it separated from the Antarctic Circumpolar Current (ACC) and formed a cyclonic eddy ~120 km in diameter. The meander developed in the Scotia Sea, an eddy-rich region east of the Drake Passage where the Subantarctic and Polar Fronts converge and modifications of Subantarctic Mode Water (SAMW) occur. In situ measurements reveal a rich submesoscale structure of temperature and salinity and a loss of frontal integrity along the newly formed southern sector of the eddy. A mathematical framework is developed to estimate vertical velocity from collocated drifter and horizontal water velocity time series, under certain simplifying assumptions appropriate for the current dataset. Upwelling (downwelling) rates of O(100) m day−1 are found in the northern (southern) eddy sector. Favorable conditions for submesoscale instabilities are found in the mixed layer, particularly at the beginning of the survey in the vicinity of density fronts. Shallower mixed layer depths and increased stratification are observed later in the survey on the inner edge of the front. Evolution in temperature–salinity (T–S) space indicates modification of water mass properties in the upper 200 m over 2 days. Modifications along σθ = 27–27.2 kg m−3 have climate-related implications for mode and intermediate water transformation in the Scotia Sea on finer spatiotemporal scales than observed previously.

Mesoscale and Submesoscale Effects on Mixed Layer Depth in the Southern Ocean

The authors gratefully acknowledge support from the Natural Environment Research Council Awards NE/J010472/1 and NE/J009857/1.

Submesoscale Rossby waves on the Antarctic circumpolar current

Submesoscale Rossby waves are found along the Antarctic circumpolar current where they drive strong vertical circulations. The eastward-flowing Antarctic circumpolar current (ACC) plays a central role in the global ocean overturning circulation and facilitates the exchange of water between the ocean surface and interior. Submesoscale eddies and fronts with scales between 1 and 10 km are regularly observed in the upper ocean and are associated with strong vertical circulations and enhanced stratification. Despite their importance in other locations, comparatively little is known about submesoscales in the Southern Ocean. We present results from new observations, models, and theories showing that submesoscales are qualitatively changed by the strong jet associated with the ACC in the Scotia Sea, east of Drake Passage. Growing submesoscale disturbances develop along a dense filament and are transformed into submesoscale Rossby waves, which propagate upstream relative to the eastward jet. Unlike their counterparts in slower currents, the submesoscale Rossby waves do not destroy the underlying frontal structure. The development of submesoscale instabilities leads to strong net subduction of water associated with a dense outcropping filament, and later, the submesoscale Rossby waves are associated with intense vertical circulations.