Mid-Ocean Ridge and Storm Enhanced Mixing in the Central South Atlantic Thermocline

Jingxuan Wei,Kathryn L Gunn,Robert Reece

doi:10.3389/fmars.2021.771973

Abstract

We investigate the spatial distribution of diapycnal mixing and its drivers in the central South Atlantic thermocline between the Rio-Grande Rise to the Mid-Atlantic Ridge. Diapycnal mixing in the ocean interior influences the slowly evolving meridional circulation, yet there are few observations of its variability with space and time or its drivers. To overcome this gap, seismic reflection data are spectrally analyzed to produce a 1,600 km long full-thermocline vertical section of diapycnal diffusivity, that has a vertical and horizontal resolution of O(10) m and spans a period of 4 weeks. We compare seismic-derived diffusivities with CTD-derived diffusivities and direct observations from 1996, 2003, and 2011. In the mean and on decadal scales, we find that thermocline diffusivities have changed little in this region, retaining a background value of 1 × 10–5 m2 s–1. Imprinted upon the background rates, mixing is heterogeneous at mesoscales. Enhanced mixing, exceeding 10 × 10–5 m2 s–1 and spreading between 200 and 700 m depth, is found above the Mid-Atlantic Ridge suggesting the ridge enhances diffusivity by at least one order of magnitude across the entire water column. Rapid decay of diffusivities within 30 km of the ridge implies local dissipation of tidal energy. Above smooth topography, patches of enhanced mixing are possibly caused by a recent storm that injects near-inertial energy into the water column and elevates mixing from 3 × 10–5 m2 s–1 to 50 × 10–5 m2 s–1 down to depths of more than 600 m. The propagation speed of near-inertial energy varies substantially from 17 to 27 m/day. Faster speed, and therefore greater penetration depths of 800 m, are probably facilitated by an eddy. Together, these data extend the observational record of central South Atlantic thermocline mixing and provide insights into drivers of mesoscale variability.

Highlights

Turbulent diapycnal mixing maintains global overturning circulation (Munk and Wunsch, 1998)
Our results extend the observational record of diapycnal mixing in the central South Atlantic thermocline by providing diffusivities in 2003, 2011, and 2016, and provide further insights into the drivers of mesoscale mixing variability
Mesoscale patches of high-amplitude and more continuous reflectivity suggest the presence of eddy-scale processes, these patches extend to depths of 900 m and across tens of kilometers zonally (Figure 4, red boxes)

Summary

Introduction

Turbulent diapycnal mixing maintains global overturning circulation (Munk and Wunsch, 1998). Analytical modeling suggests that an average diffusivity of O(10 × 10−5) m2 s−1 is required to maintain abyssal stratification (Munk and Wunsch, 1998), while O(1 × 10−5) m2 s−1. Less is known about mixing in the ocean interior, away from rough topography and strong coastal winds, in particular in the central South Atlantic thermocline due to a historical lack of observations. To clearly recognize the transition from internal wave regimes to turbulent regimes in log-log space, the vertical displacement spectra are multiplied by (2πkx) to produce the slope spectra.

Results

Discussion

Conclusion