Turbulence and its Drivers in the Weddell Sea Bottom Water Gravity Current

This study investigates the role of wave-induced turbulence in the dynamics of the Weddell Sea Bottom Water gravity current. The current transports dense water from its formation sites on the shelf to the deep sea and is a crucial component of the Southern Ocean overturning circulation. The analysis is based on data from a mooring array deployed across the continental slope between January 2017 and January 2019, and vertical profiles of temperature and salinity measured on various ship expeditions on a transect along the array. Previous studies suggest that internal waves may play a crucial role in driving turbulence within gravity currents. However, this influence has until now not been quantitatively assessed.

To quantify the contribution of internal waves to turbulence in this particular gravity current along the continental slope, I employ three independent methods for estimating dissipation rates. First, I use a Thorpe scale approach to compute total, process-independent dissipation rates from density inversions in density profiles. Second, I apply the finestructure parameterization to estimate wave-induced dissipation rates from vertical profiles of strain, calculated from temperature and salinity profiles. Third, I develop a new method to estimate wave-induced dissipation rates from moored velocity time series. For that, I estimate wave energy levels from kinetic energy spectra and deduce dissipation rates by applying a formulation that is at the heart of the finestructure parameterization. A direct comparison of the time-averaged results from both the wave energy level method and the established finestructure parameterization differ in most cases by less than a factor of 3, below the associated methodological uncertainty.

Turbulence is highest at the shelf break and decreases towards the deep sea, in line with decreasing strength of wave-induced turbulence. I observe a two-layer structure of the gravity current, a strongly turbulent, about 60-80m thick bottom layer and an upper, more quiescent interfacial layer. In the interfacial layer, internal waves induce an important part of the dissipation rate and therefore drive entrainment of warmer upper water into the gravity current. A precise quantification of the contribution is complicated by large method uncertainties. A comparison with turbulence measurements up- and downstream of our study site indicates that the processes dominating turbulence generation may depend on the location along the Weddell Sea Bottom Water gravity current: on the shelf, trapped waves are most important; on the continental slope, breaking internal waves dominate; and in the basin, symmetric instability is likely the main driver of turbulence.

In addition to the horizontal and vertical spatial variability of turbulence along the continental slope, I investigate its seasonal variability. Variations of the turbulence in the Southern Ocean over time are generally ill-described, as there are almost no publications of dissipation rate time series on scales longer than days. Moored measurements are sliced to yield time series of dissipation rates, utilizing the here newly developed method. The near-bottom turbulence is observed to have considerable temporal variability. The seasonal variability correlates thereby with surface stress, with a time lag in the order of days, even at the abyssal mooring sites. The short time period between a signal on the surface and in the deep makes it highly unlikely that barotropic mechanisms are responsible but rather that the surface and the seafloor are linked by wind-generated near-inertial waves. From this, the drivers of wave-induced turbulence can be distinguished. The spatial variability of wave-induced turbulence is determined by internal tides, while the temporal variability is determined by intermittent near-inertial waves, even at the abyssal base of the continental slope. However, the exact mechanisms are still unknown, especially regarding the deep propagation of near-inertial waves and their dissipation mechanisms above the sea floor. Notwithstanding, I hypothesize that near-inertial waves driving near-bottom turbulence are generally more important than their current representation in literature suggests.