Boundary mixing. Part 1. Transport of buoyancy and passive tracer

Abstract

Diapycnal mixing plays a key role in the thermohaline circulation of the deep ocean. Field observations have suggested that this mixing is intensified over the rough topography along the boundaries of the ocean. In this study, we experimentally explore the transport of salinity and tracer in a horizontally stirred, stratified fluid with a steady vertical buoyancy flux. The mechanical mixing occurs either uniformly across the tank or near a sidewall. To compare uniform and boundary mixing, first we explore the steady state dynamics and find that, in both cases, the surfaces of constant density are horizontal. With uniform mixing, vertical transport of buoyancy occurs uniformly throughout the tank while, with boundary mixing, transport is confined to the turbulent region and the interior space remains quiescent and does not mix or have any net movement. We then explore the transient evolution of the stratification when the source of buoyancy is removed from the base of the system. In the boundary mixing case, the resulting divergence of the turbulent diffusive flux in the boundary region leads to a reduction in the buoyancy of the fluid in the boundary region, and a net upflow develops in the boundary region. In turn, this drives a downwelling in the interior. Vertical gradients in the rate of downwelling lead to stretching of isopycnals and, together, these processes enable the interior stratification to evolve. The experiments highlight that, with boundary mixing, the main transport of buoyancy occurs near the boundaries even though the interior is stratified; independent measurements of tracer mixing in the interior show that this fluid may be quiescent and stratified even though there is a large flux being transported in the boundary region. This has important implications for the interpretation of mixing data in the ocean. In a companion paper, (Li and Woods Part 2), we explore the interaction of a net upwelling with such boundary mixing.