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Leif Thomas and Raffaele Ferrari

Abstract

The generation and destruction of stratification in the surface mixed layer of the ocean is understood to result from vertical turbulent transport of buoyancy and momentum driven by air–sea fluxes and stresses. In this paper, it is shown that the magnitude and penetration of vertical fluxes are strongly modified by horizontal gradients in buoyancy and momentum. A classic example is the strong restratification resulting from frontogenesis in regions of confluent flow. Frictional forces acting on a baroclinic current either imposed externally by a wind stress or caused by the spindown of the current itself also modify the stratification by driving Ekman flows that differentially advect density. Ekman flow induced during spindown always tends to restratify the fluid, while wind-driven Ekman currents will restratify or destratify the mixed layer if the wind stress has a component up or down front (i.e., directed against or with the geostrophic shear), respectively. Scalings are constructed for the relative importance of friction versus frontogenesis in the restratification of the mixed layer and are tested using numerical experiments of mixed layer fronts forced by both winds and a strain field. The scalings suggest and the numerical experiments confirm that for wind stress magnitudes, mixed layer depths, and cross-front density gradients typical of the ocean, wind-induced friction often dominates frontogenesis in the modification of the stratification of the upper ocean. The experiments reveal that wind-induced destratification is weaker in magnitude than restratification because the stratification generated by up-front winds confines the turbulent stress to a depth shallower than the Ekman layer, which enhances the frictional force, Ekman flow, and differential advection of density. Frictional destratification is further reduced over restratification because the stress associated with the geostrophic shear at the surface tends to compensate a down-front wind stress.

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Carl Wunsch and Raffaele Ferrari

Abstract

The central change in understanding of the ocean circulation during the past 100 years has been its emergence as an intensely time-dependent, effectively turbulent and wave-dominated, flow. Early technologies for making the difficult observations were adequate only to depict large-scale, quasi-steady flows. With the electronic revolution of the past 50+ years, the emergence of geophysical fluid dynamics, the strongly inhomogeneous time-dependent nature of oceanic circulation physics finally emerged. Mesoscale (balanced), submesoscale oceanic eddies at 100-km horizontal scales and shorter, and internal waves are now known to be central to much of the behavior of the system. Ocean circulation is now recognized to involve both eddies and larger-scale flows with dominant elements and their interactions varying among the classical gyres, the boundary current regions, the Southern Ocean, and the tropics.

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Raffaele Ferrari and Maxim Nikurashin

Abstract

Geostrophic eddies control the meridional mixing of heat, carbon, and other climatically important tracers in the Southern Ocean. The rate of eddy mixing is typically quantified through an eddy diffusivity. There is an ongoing debate as to whether eddy mixing in enhanced in the core of the Antarctic Circumpolar Current or on its flanks. A simple expression is derived that predicts the rate of eddy mixing, that is, the eddy diffusivity, as a function of eddy and mean current statistics. This novel expression predicts suppression of the cross-jet eddy diffusivity in the core of the Antarctic Circumpolar Current, despite enhanced values of eddy kinetic energy. The expression is qualitatively and quantitatively validated by independent estimates of eddy mixing from altimetry observations. This work suggests that the meridional eddy diffusivity across the Antarctic Circumpolar Current is weaker than presently assumed because of the suppression of eddy mixing by the strong zonal current.

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Jörn Callies and Raffaele Ferrari

Abstract

Submesoscale (1–200 km) wavenumber spectra of kinetic and potential energy and tracer variance are obtained from in situ observations in the Gulf Stream region and in the eastern subtropical North Pacific. In the Gulf Stream region, steep kinetic energy spectra at scales between 200 and 20 km are consistent with predictions of interior quasigeostrophic–turbulence theory, both in the mixed layer and in the thermocline. At scales below 20 km, the spectra flatten out, consistent with a growing contribution of internal-wave energy at small scales. In the subtropical North Pacific, the energy spectra are flatter and inconsistent with predictions of interior quasigeostrophic–turbulence theory. The observed spectra and their dependence on depth are also inconsistent with predictions of surface quasigeostrophic–turbulence theory for the observed ocean stratification. It appears that unbalanced motions, most likely internal tides at large scales and the internal-wave continuum at small scales, dominate the energy spectrum throughout the submesoscale range. Spectra of temperature variance along density surfaces, which are not affected by internal tides, are also inconsistent with predictions of geostrophic-turbulence theories. Reasons for this inconsistency could be the injection of energy in the submesoscale range by small-scale baroclinic instabilities or modifications of the spectra by coupling between surface and interior dynamics or by ageostrophic frontal effects.

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Jörn Callies and Raffaele Ferrari

Abstract

Baroclinic mixed-layer instabilities have recently been recognized as an important source of submesoscale energy in deep winter mixed layers. While the focus has so far been on the balanced dynamics of these instabilities, they occur in and depend on an environment shaped by atmospherically forced small-scale turbulence. In this study, idealized numerical simulations are presented that allow the development of both baroclinic instability and convective small-scale turbulence, with simple control over the relative strength. If the convection is only weakly forced, baroclinic instability restratifies the layer and shuts off convection, as expected. With increased forcing, however, it is found that baroclinic instabilities are remarkably resilient to the presence of convection. Even if the instability is too weak to restratify the layer and shut off convection, the instability still grows in the convecting environment and generates baroclinic eddies and fronts. This suggests that despite the vigorous atmospherically forced small-scale turbulence in winter mixed layers, baroclinic instabilities can persistently grow, generate balanced submesoscale turbulence, and modify the bulk properties of the upper ocean.

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Jörn Callies and Raffaele Ferrari

Abstract

This paper revisits how the restratifying buoyancy flux generated by baroclinic mixed layer instabilities depends on environmental conditions. The frontal spindown is shown to produce buoyancy fluxes that increase significantly beyond the previously proposed and widely used scaling (f is the Coriolis parameter, Λ is the geostrophic shear, and H is the mixed layer depth), irrespective of whether the initial front is broad or narrow. This increase occurs after the initial phase of the nonlinear evolution, when the baroclinic eddies grow in size and develop velocities significantly in excess of the scaling assumption V ~ ΛH. Implications for parameterizing the restratification caused by baroclinic mixed layer instabilities in coarse-resolution models are discussed.

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K. Shafer Smith and Raffaele Ferrari

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Temperature–salinity profiles from the region studied in the North Atlantic Tracer Release Experiment (NATRE) show large isopycnal excursions at depths just below the thermocline. It is proposed here that these thermohaline filaments result from the mesoscale stirring of large-scale temperature and salinity gradients by geostrophic turbulence, resulting in a direct cascade of thermohaline variance to small scales. This hypothesis is investigated as follows: Measurements from NATRE are used to generate mean temperature, salinity, and shear profiles. The mean stratification and shear are used as the background state in a high-resolution horizontally homogeneous quasigeostrophic model. The mean state is baroclinically unstable, and the model produces a vigorous eddy field. Temperature and salinity are stirred laterally in each density layer by the geostrophic velocity and vertical advection is by the ageostrophic velocity. The simulated temperature–salinity diagram exhibits fluctuations at depths just below the thermocline of similar magnitude to those found in the NATRE data. It is shown that vertical diffusion is sufficient to absorb the laterally driven cascade of tracer variance through an amplification of filamentary slopes by small-scale shear. These results suggest that there is a strong coupling between vertical mixing and horizontal stirring in the ocean at scales below the deformation radius.

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Trevor J. McDougall and Raffaele Ferrari

Abstract

Ledwell, in a comment on McDougall and Ferrari, discusses the dianeutral upwelling and downwelling that occurs near isolated topographic features, by performing a buoyancy budget analysis that integrates the diffusive buoyancy fluxes only out to a set horizontal distance from the topography. The consequence of this choice of control volume is that the magnitude of the area-integrated diffusive buoyancy flux decreases to zero at the base of a topographic feature resulting in a net dianeutral upwelling of water. Based on this result, Ledwell argues that isolated topographic features are preferential locations for the upwelling of waters from the abyss. However the assumptions behind Ledwell’s analysis may or may not be typical of abyssal mixing in the ocean. McDougall and Ferrari developed general expressions for the balance between area-integrated dianeutral advection and diffusion, and then illustrated these general expressions using the very simple assumption that the magnitude of the buoyancy flux per unit area at the top of the turbulent boundary layer was constant. In these pedagogical illustrations, McDougall and Ferrari concentrated on the region near the top (rather than near the base) of isolated topographic features, and they found net sinking of abyssal waters. Here we show that McDougall and Ferrari’s conclusion that isolated topographic features cause dianeutral downwelling is in fact a result that applies for general geometries and for all forms of bottom-intensified mixing profiles at heights above the base of such topographic features.

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Louis-Philippe Nadeau and Raffaele Ferrari

Abstract

Eddy-permitting simulations are used to show that basinlike gyres can be observed in the large-scale barotropic flow of a wind-driven channel with a meridional topographic ridge. This is confirmed using both two-layer quasigeostrophic and 25-level primitive equation models at high horizontal resolution. Comparing results from simulations with and without the topographic ridge, it is shown that the zonal baroclinic transport in the channel increases with increasing wind stress when the bottom topography is flat but not when there is a meridional ridge. The saturation of transport for increasing wind occurs in conjunction with the development of recirculating gyres in the large-scale barotropic streamfunction. This suggests that the total circulation can be thought of as a superposition of a gyre mode (which has zero circumpolar transport) and a free circumpolar mode (which contains all of the transport). Basinlike gyres arise in the channel because the topography steers the barotropic streamlines and supports a frictional boundary layer similar to the more familiar ones observed along western boundaries. The gyre mode is thus closely linked with the bottom form stress exerted by the along-ridge flow and provides the sink for the wind momentum input. In this framework, any increase in wind forcing spins a stronger gyre as opposed to feeding the circumpolar transport. This hypothesis is supported with a suite of experiments where key parameters are carried over a wide range: wind stress, wind stress curl, ridge height, channel length, and bottom friction.

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Baylor Fox-Kemper and Raffaele Ferrari

Abstract

The authors propose a parameterization for restratification by mixed layer eddies that develop from baroclinic instabilities of ocean fronts. The parameterization is cast as an overturning streamfunction that is proportional to the product of horizontal buoyancy gradient, mixed layer depth, and inertial period. The parameterization has remarkable skill for an extremely wide range of mixed layer depths, rotation rates, and vertical and horizontal stratifications. In this paper a coarse resolution prognostic model of the parameterization is compared with submesoscale mixed layer eddy resolving simulations. The parameterization proves accurate in predicting changes to the buoyancy. The climate implications of the proposed parameterization are estimated by applying the restratification scaling to observations: the mixed layer depth is estimated from climatology, and the buoyancy gradients are from satellite altimetry. The vertical fluxes are comparable to monthly mean air–sea fluxes in large areas of the ocean and suggest that restratification by mixed layer eddies is a leading order process in the upper ocean. Critical regions for ocean–atmosphere interaction, such as deep, intermediate, and mode water formation sites, are particularly affected.

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