Energetic Submesoscale Dynamics in the Ocean Interior

Abstract

Submesoscale ocean processes, characterized by order-1 Rossby and Richardson numbers, are currently thought to be mainly confined to the ocean surface mixed layer, whereas the ocean interior is commonly assumed to be in quasigeostrophic equilibrium. Here, a realistic numerical simulation in the Antarctic Circumpolar Current, with a 1/48° horizontal resolution and tidal forcing, is used to demonstrate that the ocean interior departs from the quasigeostrophic regime down to depths of 900 m, that is, well below the mixed layer. Results highlight that, contrary to the classical paradigm, the ocean interior is strongly ageostrophic, with a pronounced cyclone–anticyclone asymmetry and a dominance of frontogenesis over frontolysis. Numerous vortices and filaments, from the surface down to 900 m, are characterized by large Rossby and low Richardson numbers, strong lateral gradients of buoyancy, and vigorous ageostrophic frontogenesis. These deep submesoscales fronts are only weakly affected by internal gravity waves and drive intense upward vertical heat fluxes, consistent with recent observations in the Antarctic Circumpolar Current and the Gulf Stream. As such, deep submesoscale fronts are an efficient pathway for the transport of heat from the ocean interior to the surface, suggesting the presence of an intensified oceanic restratification at depth.

Altimetry-Based Diagnosis of Deep-Reaching Sub-Mesoscale Ocean Fronts

“…Quasi-geostrophic (QG) scaling predicts a k −3 spectral difference between ∇b-amplitude at the surface and in the interior [24,25]. This is clearly not the case here, highlighting the departure from QG dynamics in the ocean interior, consistent with previous numerical studies [4,26].…”

“…It includes tides, has a horizontal resolution of 1/48 • and 90 vertical levels. We refer the reader to Appendix A of Siegelman [4] and Torres et al [9] for a full description of the characteristics of this simulation and its validation with observations. A sub-domain of the ACC, east of the Kerguelen Islands and spanning 86-90 • E, 48-52 • S is analyzed.…”

“…Recent observational and numerical studies revealed the existence of energetic sub-mesoscale fronts (10-50 km width) in regions of elevated Eddy Kinetic Energy (EKE), not only within the surface mixed layer but also below it down to 900 m [1][2][3][4]. These fronts are characterized by Richardson and Rossby numbers of order one, emphasizing their ageostrophic character [1,3,4]. They result from the horizontal stirring of buoyancy anomalies by mesoscale eddies (50-300 km size) and are associated with intense vertical velocities [5].…”

Interannual to Decadal Variations of Submesoscale Motions around the North Pacific Subtropical Countercurrent

“…The other part should be revealed, which remains in the present study. For example, strain-induced frontogenesis [1,27] and turbulent thermal wind [28] are the candidates, which can induce the submesoscale motions. These drive the secondary circulation across the front and then lead to the APE release [27].…”

“…For example, strain-induced frontogenesis [1,27] and turbulent thermal wind [28] are the candidates, which can induce the submesoscale motions. These drive the secondary circulation across the front and then lead to the APE release [27]. Although the OFES2_NP30 probably does not resolve these mechanisms due to its coarse resolution, further analyses of the existences of these mechanisms in the STCC region and their associated APE release are interesting topics.…”