Abstract
Abyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe scale based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the Passage. Overturns were observed in a density band where the Richardson number was often smaller than 1/4, consistent with shear instability occurring at the upper interface of the fast flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales of days to hours. At some mooring locations near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the Passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealised 2D numerical simulation that includes a barotropic tidal forcing. Depth integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.