The Atlantic Meridional Overturning Circulation entails vigorous thermohaline transformations in the subpolar North Atlantic Ocean (SPNA). There, warm and saline waters originating in the (sub)tropics are converted into cooler and fresher waters by a combination of surface fluxes and sub-surface mixing. Using microstructure measurements and a small-scale variance conservation framework, we quantify the diapycnal and isopycnal contributions --by microscale turbulence and mesoscale eddies, respectively-- to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for the majority of thermal (65%) and haline (84%) variance dissipation in the upper 400~m of the eastern SPNA. A simple dimensional analysis suggests that isopycnal stirring could account for O(5-10) Sv of diahaline volume flux, suggesting an important role of such stirring in regional water-mass transformations. Our mixing measurements are thus consistent with recent indirect estimates in highlighting the importance of isopycnal stirring for North Atlantic overturning.Abstract content goes here

A dissection of the physics of seasonal cycle of oceanic upper layer stratification is necessary to improve climate predictions of biogeochemical cycles. We present a time series of vertical profiles of ε, the dissipation rate of turbulent kinetic energy, obtained from a microstructure profiler during the destratification period (summer-to-winter) at a mid-latitude 75m-deep coastal site. Significant correlation is obtained in the mixed layer depth (MLD) with a model combining effects of wind, wave, and buoyancy forcings, estimated from bulk parameters ~10 hours before observations, and used to identify the dominant forcings leading to MLD deepening. Intermittency at surface is correlated with seasonal storminess, and we observe a quadratic relation between kurtosis and skewness for ε statistics. By splitting the time series into layers, we observe the co-location of patches of higher ε with the shear maxima of the two first baroclinic modes, and significant correlations with surface wind stress in the transitional layer the past 24 hours, and at longer scale (4.25 days) in the baroclinic layer, suggesting that internal waves activity influences the setup of mixing intensity despite the lack of tidal forcing. The low-passed microstructure shear distribution seems to support this hypothesis despite possible signal contamination. In the highly stratified layers associated to salt-fingering (MLD’s basis and below), the buoyancy Reynolds number indicates a buoyancy regime control with low mixing value (0.2 x 10e-5 m²/s). More turbulent flows are identified in both surface and bottom layers (0.6 - 0.8 x 10e-5 m²/s), suggesting a seasonal erosion of the stratification by the boundary processes.

Vertical mixing is often regarded as the Achilles' heel of ocean models. In particular, few models include a comprehensive and energy-constrained parameterization of mixing by internal ocean tides. Here, we present an energy-conserving mixing scheme which accounts for the local breaking of high-mode internal tides and the distant dissipation of low-mode internal tides. The scheme relies on four static two-dimensional maps of internal tide dissipation, constructed using mode-by-mode Lagrangian tracking of energy beams from sources to sinks. Each map is associated with a distinct dissipative process and a corresponding vertical structure. Applied to an observational climatology of stratification, the scheme produces a global three-dimensional map of dissipation which compares well with available microstructure observations and with upper-ocean finestructure mixing estimates. This relative agreement, both in magnitude and spatial structure across ocean basins, suggests that internal tides underpin most of observed dissipation in the ocean interior at the global scale. The proposed parameterization is therefore expected to improve understanding, mapping and modelling of ocean mixing.