The Effects of Convection in Geostrophic Circulation

Abstract

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 non-rotating horizontal convection to circulation governed by geostrophic boundary layer flow. The flow is well described by a convective Rossby number, which compares the strength of horizontal convective to Coriolis accelerations in the boundary layer. For more rapid rotation the momentum budget is dominated by fluctuating vertical accelerations in a ‘chimney’ region of vertical plumes. Chimney convection limits the geostrophic inhibition of basin-scale circulation, halting the increase of temperature difference across the basin (or decrease of Nusselt number) with decreasing Rossby number. The North Atlantic Ocean is estimated to lie in the regime controlled by chimney convection, which is evidence for deep convection being an important limit on heat transport.

The experiments and simulations show that buoyancy forcing produces basin-scale gyres and boundary currents, features of mid-latitude ocean circulation often attributed to wind stress. The simulations are used to examine the contribution of boundary currents and various other flow structures to the heat transport. At the Rayleigh numbers achievable, the turbulent viscous dissipation and irreversible mixing are primarily constrained to the thermal boundary layer. These results can be extrapolated to show the distribution of heat transport and energy dissipation in a buoyancy-driven ocean. Show more