Jonas Nycander

Breaking internal gravity waves cause small-scale turbulent mixing, which changes water mass properties, affects biogeochemical cycles, and contributes to driving the large-scale overturning circulation. Ocean general circulation models do not resolve this process and thus rely on a parameterization. The state-of-the-art IDEMIX (Internal wave Dissipation, Energy and MIXing) model predicts the propagation and dissipation of internal wave energy based on external forcing functions that represent the main generation mechanisms, notably the internal tide generation at the sea floor and the near-inertial wave generation at the sea surface. By linking small-scale mixing to internal wave energetics, IDEMIX allows the consistent parameterization of wave-induced mixing in ocean models. Its basic incarnation treats all internal waves as part of a horizontally homogeneous continuum and was shown to successfully reproduce observed turbulent kinetic energy dissipation rates and internal wave energy levels. In a newer configuration (IDEMIX2), the internal wave field is compartmentalized, distinguishing between a high-mode continuum on the one hand and low-mode near-inertial wave and internal tide compartments, whose horizontal propagation is explicitly resolved in wavenumber angle space, on the other hand. We present the evaluation of the IDEMIX2 model with a particular focus on the impact of applying an anisotropic internal tide forcing. So far, parameterizations of internal tide-driven mixing have not taken the strong anisotropy of the internal tide generation process into account. We demonstrate the need for doing so, showing a notable impact on the modeled internal wave energetics and predicted mixing when changing from the previous isotropic to the new anisotropic tidal forcing in IDEMIX2.

The largest source of internal waves in the ocean is tidal currents flowing across rough bottom topography, resulting in internal tides. When these waves break, they release some of their energy to the background ocean in the form of diapycnal mixing. The distribution of this internal tide-driven mixing has a significant impact on the ocean state. The parameterization of internal tide-driven mixing in climate models is based on estimates of the generation of internal tides, or tidal conversion. Semi-analytical methods based on linear wave theory have been used to compute the tidal conversion from the observed bottom topography, ocean stratification, and tidal currents. However, such linear calculations fail completely at steep continental slopes. Here, we construct a computationally inexpensive method to estimate the tidal conversion into vertical normal modes by continental slopes and shelves, and apply it at the global scale. It uses the usual observational data as inputs but relies on a reduced-physics numerical model rather than on linear theory to compute the tidal conversion. The method also resolves the onshore and offshore energy fluxes. The output is validated with the conversion diagnosed from a realistic simulation. The results are useful for parameterizing the subsequent propagation and breaking of the internal tides and their resulting diapycnal mixing.

The main forcing of the ocean’s internal gravity wave field is the interaction of the barotropic tide with the rough seafloor. This process is inherently anisotropic: the orientation of the topographic obstacles and the direction of the tidal currents determine the amount and direction of the generated internal wave of tidal frequency, the internal tide. Available global estimates of the internal tide generation, however, do not take this directionality into account. We present estimates of the global M2-tide generation into the first 10 vertical normal modes using a new method based on linear theory that resolves both magnitude and direction. Linear theory breaks down once the slope of the topographic obstacle exceeds that of the generated tidal beam. We discuss the role of such supercritical slopes at continental shelves and in the open ocean. Finally, we will use the anisotropic M2-tide generation as forcing of the internal wave model IDEMIX, the backbone of an energetically consistent parameterization of wave-induced turbulent mixing for ocean general circulation models. Both wave energy levels and turbulent kinetic energy dissipation differ substantially compared to the reference scenario with the previously used isotropic tidal forcing. This underlines the importance of resolving the directionality of the internal tide generation in parameterizations of wave-induced turbulent mixing.

We construct a computationally inexpensive semi‐analytical method to compute the tidal conversion into vertical modes by continental slopes and shelves, and apply it at the global scale. It relies on a vertically two‐dimensional reduced‐physics numerical model and uses the observed bottom topography, ocean stratification, and tidal currents as inputs. The method is applicable no matter how steep the slope is and it resolves the onshore and offshore baroclinic tidal energy fluxes. The output is validated with the conversion diagnosed from a global general circulation model simulation.

The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper-ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn–winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas Current and East Australian Current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations in the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn – winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north, the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas current and East Australian current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations of the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn – winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north, the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas current and East Australian current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations of the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

The Southern Ocean hosts a winter deep mixing band (DMB) near the Antarctic Circumpolar Current's (ACC) northern boundary, playing a pivotal role in Subantarctic Mode Water formation. Here, we investigate what controls the presence and geographical extent of the DMB. Using observational data, we construct seasonal climatologies of surface buoyancy fluxes, Ekman buoyancy transport, and upper stratification. The strength of the upper ocean stratification is determined using the columnar buoyancy index, defined as the buoyancy input necessary to produce a 250 m deep mixed layer. It is found that the DMB lies precisely where the autumn – winter buoyancy loss exceeds the columnar buoyancy found in late summer. The buoyancy loss decreases towards the south, while in the north, the stratification is too strong to produce deep mixed layers. Although this threshold is also crossed in the Agulhas current and East Australian current regions, advection of buoyancy is able to stabilise the stratification. The Ekman buoyancy transport has a secondary impact on the DMB extent due to the compensating effects of temperature and salinity transports on buoyancy. Changes in surface temperature drive spatial variations of the thermal expansion coefficient (TEC). These TEC variations are necessary to explain the limited meridional extent of the DMB. We demonstrate this by comparing buoyancy budgets derived using varying TEC values with those derived using a constant TEC value. Reduced TEC in colder waters leads to decreased winter buoyancy loss south of the DMB, yet substantial heat loss persists. Lower TEC values also weaken the effect of temperature stratification, partially compensating for the effect of buoyancy loss damping. TEC modulation impacts both the DMB characteristics and its meridional extent.

In this study, we examine the factors that influence upper ocean pycnocline (UOP) stratification in the Southern Ocean. The UOP is a layer located just below the mixed layer and its stratification controls the rates of exchange of heat, carbon, and nutrients between the ocean interior and the atmosphere. We classify regions of the UOP based on the relative roles of temperature and salinity in stabilizing the layer, resulting in alpha (temperature-stabilized), beta (salinity-stabilized), or transition (temperature and salinity-stabilized) zones. Our analysis uses observation profiles from the EN4.2 database and calculates annual mean buoyancy fluxes by combining existing heat and freshwater flux products and accounting for the effect of Ekman transport. Our results show that the polar transition zone has a complex structure, with interlocking beta pools and local intrusions into alpha zones. Deep mixed layers are found in the southernmost flank of the alpha region, with the exception of the southeast Pacific sector where they are located in the polar transition zone. Regions of negative buoyancy flux show mixed layer deepening along the water path, but deep mixed layers only form when the buoyancy flux is negative throughout the path. Ekman transport contributes also to the formation of deeper mixed layers throughout the Southern Ocean by bringing cold water northward. Overall, our findings reveal that boundaries between alpha, transition and beta regions are generally consistent with more traditional frontal definitions and provide a comprehensive view of upper ocean pycnocline stratification in the Southern Ocean.