Mixing and Transformation in a Deep Western Boundary Current: A Case Study

Carl P Spingys,Alberto C Naveira Garabato,Kurt L Polzin,Eleanor E Frajka-Williams,Sonya Legg,Alexander Forryan,E Povl Abrahamsen,Christian E Buckingham

doi:10.1175/jpo-d-20-0132.1

Abstract

AbstractWater-mass transformation by turbulent mixing is a key part of the deep-ocean overturning, as it drives the upwelling of dense waters formed at high latitudes. Here, we quantify this transformation and its underpinning processes in a small Southern Ocean basin: the Orkney Deep. Observations reveal a focusing of the transport in density space as a deep western boundary current (DWBC) flows through the region, associated with lightening and densification of the current’s denser and lighter layers, respectively. These transformations are driven by vigorous turbulent mixing. Comparing this transformation with measurements of the rate of turbulent kinetic energy dissipation indicates that, within the DWBC, turbulence operates with a high mixing efficiency, characterized by a dissipation ratio of 0.6 to 1 that exceeds the common value of 0.2. This result is corroborated by estimates of the dissipation ratio from microstructure observations. The causes of the transformation are unraveled through a decomposition into contributions dependent on the gradients in density space of the: dianeutral mixing rate, isoneutral area, and stratification. The transformation is found to be primarily driven by strong turbulence acting on an abrupt transition from the weakly stratified bottom boundary layer to well-stratified off-boundary waters. The reduced boundary layer stratification is generated by a downslope Ekman flow associated with the DWBC’s flow along sloping topography, and is further regulated by submesoscale instabilities acting to restratify near-boundary waters. Our results provide observational evidence endorsing the importance of near-boundary mixing processes to deep-ocean overturning, and highlight the role of DWBCs as hot spots of dianeutral upwelling.

Highlights

The deep ocean exerts a pivotal control on Earth’s climate by storing large quantities of heat, carbon and other climatically important tracers for centuries to millennia (Watson and Naveira Garabato 2006; Purkey and Johnson 2013; Ferrari et al 2014; Desbruyeres et al 2016), as well as influencing the rate and structure of the circulation in the ocean’s upper layers (Patara and Boning 2014)
The rate, structure, and processes of the water-mass transformation in a small Southern Ocean basin crossed by a deep western boundary current (DWBC) conveying Antarctic Bottom Water (AABW) has been assessed using a combination of observations and a high-resolution numerical model
Such transports entail a dianeutral convergence of the densest and lightest AABW classes at intermediate densities, associated with respective lightening and densification of the densest and lightest waters. These dianeutral transports can be linked to water-mass transformation, which is primarily driven by turbulent mixing and is most vigorous at the base of the DWBC flow over the sloping boundary of the basin

Summary

Introduction

The deep ocean exerts a pivotal control on Earth’s climate by storing large quantities of heat, carbon and other climatically important tracers for centuries to millennia (Watson and Naveira Garabato 2006; Purkey and Johnson 2013; Ferrari et al 2014; Desbruyeres et al 2016), as well as influencing the rate and structure of the circulation in the ocean’s upper layers (Patara and Boning 2014). Since the seminal works of Stommel and Arons (1960) and Munk (1966) (see Nikurashin and Vallis 2011, 2012), it has been recognized that the deep ocean’s climatic role is defined by its stratification and overturning circulation, and that these are established by a balance between (i) the sinking of dense waters formed in areas of the North Atlantic and Southern Oceans, and (ii) the upwelling and lightening of those waters by turbulent diapycnal mixing. A step toward the resolution of this problem is suggested by a range of studies (Thompson and Johnson 1996; Huussen et al 2012; de Lavergne et al 2016b; Ferrari et al 2016; McDougall and Ferrari 2017; Callies 2018; Naveira Garabato et al 2019; Cimoli et al 2019) that highlight turbulent mixing in bottom boundary layers as a key, possibly dominant, mechanism driving the lightening of dense waters sourced at high latitudes. Such a possibility is qualitatively endorsed by direct (Naveira Garabato et al 2019) and indirect

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Conclusion