Persistent Turbulence in the Samoan Passage

Jesse M Cusack,Glenn S Carter,James B Girton,Matthew H Alford,Kelly A Pearson-Potts,Gunnar Voet,Shuwen Tan,Larry J Pratt

doi:10.1175/jpo-d-19-0116.1

Abstract

AbstractAbyssal 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 in which the Richardson number was often smaller than ¼, 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 from 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 idealized 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.

Highlights

The Samoan Passage at approximately 1708W, 98S is a topographic constriction through which around 6 Sv (1 Sv 5 106 m3 s21) (Rudnick 1997; Voet et al 2016) of abyssal waters below 4000 m flow from the South Pacific to the North Pacific Ocean basins
The location of the long-term moorings on top of sills is such that they sampled turbulence generated as a result of shear instability at increments
Quantities compared between model and observations: (b) depth-integrated dissipation, (c) time-averaged along-stream velocity, and (d) time-averaged dissipation rate

Summary

Introduction

The Samoan Passage at approximately 1708W, 98S is a topographic constriction through which around 6 Sv (1 Sv 5 106 m3 s21) (Rudnick 1997; Voet et al 2016) of abyssal waters below 4000 m flow from the South Pacific to the North Pacific Ocean basins. It is the largest transport pathway for abyssal waters between these basins, with smaller transports occurring in branches to the east and west (Roemmich et al 1996). The existence of a discernible layer with a sharp density gradient at the interface lends itself to understanding the dynamics using single-layer hydraulic theory

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