Internal-wave-induced dissipation rates in the Weddell Sea Bottom Water gravity current

Abstract. 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 velocity records from a mooring array deployed across the continental slope between January 2017 and January 2019 and salinity and temperature (CTD) profiles measured by various ship expeditions. To quantify the importance of internal waves for entrainment into the gravity current along the continental slope, we employ three independent methods for estimating turbulence. First, we use a Thorpe scale approach to compute turbulence from density inversions in density profiles in order to calculate total, process-independent dissipation rate. Second, we apply the finestructure parameterization to estimate wave-induced mixing from vertical profiles. Third, we estimate wave energy levels from moored velocity time series and deduce turbulent kinetic energy dissipation rates by applying a formulation that is at the heart of the finestructure parameterization. On this transect, turbulence is highest on the shelf break and decreases towards the deep sea, in line with a decreasing strength of wave-induced turbulence. We observe a 2-layer structure of the gravity current, a strongly turbulent about 60–80 m 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 to entrainment of warmer upper water into the gravity current. A literature comparison with turbulence measurements up- and downstream of our study site suggests that the question of which turbulent process is dominant may be dependent on the location along the Weddell Sea Bottom Water gravity current. On the shelf, trapped waves are most important, on the slope, we see the effect of breaking internal waves and in the basin, symmetric instabilities are identified as the main driver of turbulence.