The Effects of Convection in Geostrophic Circulation

• Main Author: Vreugdenhil, Catherine
• Format: Doctoral or Postdoctoral Thesis
• Language: English
• Subjects: Ocean circulation; rotating and stratified flows; horizontal convection; North Atlantic
• Online Access: http://hdl.handle.net/1885/135778; https://doi.org/10.25911/5d6cf9512a0d7; https://openresearch-repository.anu.edu.au/bitstream/1885/135778/4/vreugdenhil_thesis_upload.pdf.jpg

Ocean circulation plays an important role in global climate through the transport of heat and CO2. Surface fluxes of buoyancy and momentum act as primary energy inputs to the circulation, however the surface buoyancy contribution and the effects of vertical convection are not well understood. We examine flow driven by a buoyancy difference applied at a horizontal surface in a closed rotating basin: this is rotating horizontal convection. We use laboratory experiments, direct numerical simulations and scaling analyses to examine the effect of buoyancy and rotation on the mechanical energy budget and dynamical regimes. In one of these regimes the large-scale circulation is coupled to deep ‘chimney’ convection. The direct numerical simulations solve for flow in a rectangular box with a higher temperature applied over half of the base and a lower temperature over the other half, and a uniform Coriolis parameter. The emphasis is on circulation with a turbulent thermal boundary layer and small-scale convection while having a fully-resolved energy budget. The buoyancy forcing and Coriolis parameter are varied to examine the two primary sinks of mechanical energy: irreversible mixing (potential energy sink) and viscous dissipation (kinetic energy sink). Turbulent mixing and heat transport are reduced by rotation, while viscous dissipation is independent of rotation rate. The reduction of heat transport is consistent with existing geostrophic boundary layer scaling, and is inherently linked to the total amount of mixing. Even in the presence of strong rotation, energy from surface buoyancy forcing mostly goes to mixing. For a buoyancy-driven circulation in a basin comparable to the North Atlantic we estimate that mixing is a sink for over 95% of the mechanical energy supply, implying that buoyancy is an efficient driver of ocean circulation. The laboratory experiments closely resemble the simulations, but have an imposed heat flux over the heated region. The experiments further demonstrate the transition from ...