Mixing Induced Vertical Heat and Freshwater Fluxes in the Upper Ocean of the Subpolar North Atlantic

Abstract

This study is focused on an investigation of diapycnal diffusivity induced by internal waves breaking in the upper ocean of the subpolar North Atlantic. Turbulent diffusion plays a significant role in transferring heat and freshwater between the oceanic surface and deep ocean. The upper subpolar North Atlantic is defined as a region between 40-70N and 0-65W below the seasonal thermocline. It is the complex region where mixing is induced by many processes such as double diffusion, turbulence, or isopycnal mixing. This study is an attempt to define the contribution of turbulent diapycnal mixing to the vertical transfers of heat and freshwater. To date, measurements of dissipation rate and diapycnal diffusivity are limited in the upper subpolar North Atlantic. Often, these parameters are parametrized with CTD (Conductivity-Temperature-Depth) and Lowered ADCP (Acoustic Doppler Current Profiler) data on CTD stations. Measurements of the microscale shear and temperature gradient are relatively rare in this region. Here, I added the vertical shear from the shipboard ADCP velocity data to the analysis of parametrized turbulent mixing. The main advantage of this kind of the data is that the dataset is continuous in time and space along the ship track. It helps to increase the number of estimations of diapycnal mixing in the region. I started from the description of a method for post-processing of the collected velocity datasets. After, I describe the variability of diapycnal diffusivity in the subpolar North Atlantic. The next chapter is dedicated to understanding the role of wind forcing in energy transfer to the internal waves field and turbulent mixing. The last two chapters contain results from a numerical model and investigate the role of turbulent diffusion in the vertical transfers of heat and freshwater in comparison with vertical advection, and describe the sensitivity of background stratification to changes in external forcings. Diapycnal diffusivity is used as a measure of turbulent diffusion induced by internal waves’ breaking and estimated from the long-term shipboard observations. Vertical shear is calculated from the shipboard ADCP data, and buoyancy is described with the CTD data collected during 13 research cruises in the subpolar North Atlantic in 2003-2018. Dissipation rate is estimated from a finescale shear-based parameterization based on the properties of the internal waves’ field and the assumption of proportionality of dissipation to internal waves’ energy and buoyancy. Diffusive heat and freshwater fluxes are calculated for the CTD stations. Advective fluxes are computed with a high-resolution NEMO (Nucleus for European Modelling of the Ocean) model based on the configuration ANHA12 (Arctic and Northern Hemispheric Atlantic with 1/12 degree). Additionally, a Hybrid Slab Model is used to estimate the energy flux from wind to the internal waves’ field. Diapycnal diffusivity has no significant temporal (seasonal or interannual) variability in the subpolar North Atlantic. Two main regions with different mixing regimes are described. The Labrador Sea and the region south of Greenland are characterized by low turbulent diffusion rates. Enhanced diffusivities are detected in the central parts of the subpolar North Atlantic (the Western North Atlantic along the 47N and the PIES line along the Mid-Atlantic Ridge). The energy flux coming from wind to the internal wave energy shows strong seasonality. But a direct response of internal waves’ energy or dissipation and diffusivity to wind input energy flux is not found. Vertical advection is driven by the vertical velocity field and also contributes to vertical heat and freshwater transfers. It varies in the upper subpolar North Atlantic and follows the divergence and convergence zones of the general circulation. At the same time, the turbulent heat flux has a downward direction, while the turbulent freshwater flux is upward. Dissipation rate is estimated from a finescale parameterization that depends on two parameters, internal wave energy and background stratification. The high-resolution NEMO model output allows us to describe potential changes of the upper ocean buoyancy depending on changes in different atmospheric and hydrological forcings. In a short-term perspective, buoyancy is the most sensitive to wind and air temperature. Wind has a double contribution as it influences the mixed layer depth and contributes to the internal waves’ energy. Wind also modifies sensible and latent heat fluxes and influences the ocean-atmosphere interactions and oceanic heat content. In a long-term perspective, changes in precipitation and runoff forcings will also have a significant contribution. The signal of internal waves in the upper subpolar North Atlantic is found from the shipboard data, but double diffusion, enhanced mesoscale eddy activity and non-linear interactions of internal waves with topography, general currents or eddies should be taken into account for a complete analysis of vertical mixing in further investigations.