scholarly journals Sensitivity of the Southern Ocean overturning circulation to surface buoyancy forcing

2011 ◽  
Vol 38 (14) ◽  
pp. n/a-n/a ◽  
Author(s):  
Adele K. Morrison ◽  
Andrew M. Hogg ◽  
Marshall L. Ward
Keyword(s):  
Southern Ocean ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
Surface Buoyancy

Available Potential Energy and Irreversible Mixing in the Meridional Overturning Circulation

2009 ◽  
Vol 39 (12) ◽  
pp. 3130-3146 ◽  
Author(s):  
Graham O. Hughes ◽  
Andrew Mc C. Hogg ◽  
Ross W. Griffiths
Keyword(s):  
Kinetic Energy ◽  
Potential Energy ◽  
Meridional Overturning Circulation ◽  
Stratified Turbulence ◽  
Overturning Circulation ◽  
Available Potential Energy ◽  
Buoyancy Forcing ◽  
Energy Transfers ◽  
Meridional Overturning ◽  
Surface Buoyancy

Abstract The overturning circulation of the global oceans is examined from an energetics viewpoint. A general framework for stratified turbulence is used for this purpose; first, it highlights the importance of available potential energy in facilitating the transfer of kinetic energy to the background potential energy (defined as the adiabatically rearranged state with no motion). Next, it is shown that it is the rate of transfer between different energy reservoirs that is important for the maintenance of the ocean overturning, rather than the total amount of potential or kinetic energy. A series of numerical experiments is used to assess which energy transfers are significant in the overturning circulation. In the steady state, the rate of irreversible diapycnal mixing is necessarily balanced by the production of available potential energy sourced from surface buoyancy fluxes. Thus, the external inputs of available potential energy from surface buoyancy forcing and of kinetic energy from other sources (such as surface winds and tides, and leading to turbulent mixing) are both necessary to maintain the overturning circulation.

The Injection of Zonal Momentum by Buoyancy Forcing in a Southern Ocean Model

2015 ◽  
Vol 45 (1) ◽  
pp. 259-271 ◽  
Author(s):  
Emma Howard ◽  
Andrew McC. Hogg ◽  
Stephanie Waterman ◽  
David P. Marshall
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
Volume Transport ◽  
Ocean Model ◽  
Bottom Friction ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
Momentum Budget ◽  
Zonal Momentum

AbstractAn overturning circulation, driven by prescribed buoyancy forcing, is used to set a zonal volume transport in a reentrant channel ocean model with three isopycnal layers. The channel is designed to represent the Southern Ocean such that the forced overturning resembles the lower limb of the meridional overturning circulation (MOC). The relative contributions of wind and buoyancy forcing to the zonal circulation are examined. It is found that the zonal volume transport is strongly dependent on the buoyancy forcing and that the eddy kinetic energy is primarily set by wind stress forcing. The zonal momentum budget integrated over each layer is considered in the buoyancy-forced, wind-forced, and combined forcing case. At equilibrium, sources and sinks of momentum are balanced, but the transient spinup reveals the source of momentum for the current. In the buoyancy-forced case, the forcing creates a baroclinic shear with westward flow in the lower layer, allowing topographic form stress and bottom friction to act as the initial sources of eastward momentum, with bottom friction acting over a longer time frame. In the wind-forced and combined forcing cases, the surface wind stress dominates the initial momentum budget, and the time to reach equilibration is shorter in the combined forcing simulation. These results imply that future changes in the rate of formation of Antarctic Bottom Water may alter the volume transport of the Antarctic Circumpolar Current.

scholarly journals Meridional Overturning Circulation in a multi-basin model. Part II: Sensitivity to diffusivity and wind in warm and cool climates

2021 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Wind Stress ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Primary Determinant ◽  
Meridional Overturning

AbstractThe response of the meridional overturning circulation (MOC) to changes in Southern Ocean (SO) zonal wind forcing and Pacific basin vertical diffusivity is investigated under varying buoyancy forcings, corresponding to ‘warm’, ‘present-day’ and ‘cold’ states, in a two-basin general circulation model connected by a southern circumpolar channel. We find that the Atlantic MOC (AMOC) strengthens with increased SO wind stress or diffusivity in the model Pacific, under all buoyancy forcings. The sensitivity of the AMOC to wind stress increases as the buoyancy forcing is varied from a warm to a present-day or cold state, whereas it is most sensitive to the Pacific diffusivity in a present-day or warm state. Similarly, the AMOC is more sensitive to buoyancy forcing over the Southern Ocean under reduced wind stress or enhanced Pacific diffusivity. These results arise because of the increased importance of the Pacific pathway in the warmer climates, giving an increased linkage between the basins and so the opportunity for the diffusivity in the Pacific to affect the overturning in the Atlantic. In cooler states, such as in glacial climates, the two basins are largely decoupled and the wind strength over the SO is the primary determinant of the AMOC strength. Both wind- and diffusively-driven upwelling sustain the AMOC in the warmer (present-day) state. Changes in SO wind stress alone do not shoal the AMOC to resemble that observed at the last glacial maximum; changes in the buoyancy forcing are also needed to decouple the two basins.

scholarly journals Distinct controls on the strength of the abyssal overturning circulation: channel versus basin dynamics

2021 ◽  
Author(s):  
Chiung-Yin Chang ◽  
Malte F. Jansen
Keyword(s):  
Southern Ocean ◽  
General Circulation ◽  
Bottom Topography ◽  
Strong Dependence ◽  
Circulation Model ◽  
Overturning Circulation ◽  
Basin Scale ◽  
Diapycnal Diffusivity ◽  
Surface Buoyancy ◽  
Buoyancy Loss

AbstractAlthough the reconfiguration of the abyssal overturning circulation has been argued to be a salient feature of Earth’s past climate changes, our understanding of the physical mechanisms controlling its strength remains limited. In particular, existing scaling theories disagree on the relative importance of the dynamics in the Southern Ocean versus the dynamics in the basins to the north. In this study, we systematically investigate these theories and compare them with a set of numerical simulations generated from an ocean general circulation model with idealized geometry, designed to capture only the basic ingredients considered by the theories. It is shown that the disagreement between existing theories can be partially explained by the fact that the overturning strengths measured in the channel and in the basin scale distinctly with the external parameters, including surface buoyancy loss, diapycnal diffusivity, wind stress, and eddy diffusivity. The overturning in the re-entrant channel, which represents the Southern Ocean, is found to be sensitive to all these parameters, in addition to a strong dependence on bottom topography. By contrast, the basin overturning varies with the integrated surface buoyancy loss rate and diapycnal diffusivity but is mostly unaffected by winds and channel topography. The simulated parameter dependence of the basin overturning can be described by a scaling theory that is based only on basin dynamics.

scholarly journals Surface Constraints on the Depth of the Atlantic Meridional Overturning Circulation: Southern Ocean versus North Atlantic

2020 ◽  
Vol 33 (8) ◽  
pp. 3125-3149 ◽  
Author(s):  
Shantong Sun ◽  
Ian Eisenman ◽  
Laure Zanna ◽  
Andrew L. Stewart
Keyword(s):  
Southern Ocean ◽  
North Atlantic ◽  
Surface Density ◽  
Climate Models ◽  
Geometric Model ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Surface Buoyancy

AbstractPaleoclimate proxy evidence suggests that the Atlantic meridional overturning circulation (AMOC) was about 1000 m shallower at the Last Glacial Maximum (LGM) compared to the present. Yet it remains unresolved what caused this glacial shoaling of the AMOC, and many climate models instead simulate a deeper AMOC under LGM forcing. While some studies suggest that Southern Ocean surface buoyancy forcing controls the AMOC depth, others have suggested alternatively that North Atlantic surface forcing or interior diabatic mixing plays the dominant role. To investigate the key processes that set the AMOC depth, here we carry out a number of MITgcm ocean-only simulations with surface forcing fields specified from the simulation results of three coupled climate models that span much of the range of glacial AMOC depth changes in phase 3 of the Paleoclimate Model Intercomparison Project (PMIP3). We find that the MITgcm simulations successfully reproduce the changes in AMOC depth between glacial and modern conditions simulated in these three PMIP3 models. By varying the restoring time scale in the surface forcing, we show that the AMOC depth is more strongly constrained by the surface density field than the surface buoyancy flux field. Based on these results, we propose a mechanism by which the surface density fields in the high latitudes of both hemispheres are connected to the AMOC depth. We illustrate the mechanism using MITgcm simulations with idealized surface forcing perturbations as well as an idealized conceptual geometric model. These results suggest that the AMOC depth is largely determined by the surface density fields in both the North Atlantic and the Southern Ocean.

Energetics of a Rotating Wind-forced Horizontal Convection Model of a Reentrant Channel

2020 ◽  
Author(s):  
Varvara E. Zemskova ◽  
Brian L. White ◽  
Alberto Scotti
Keyword(s):  
Potential Energy ◽  
Southern Ocean ◽  
Wind Stress ◽  
Energy Budget ◽  
Surface Wind ◽  
Meridional Overturning Circulation ◽  
Stress Profile ◽  
Overturning Circulation ◽  
Available Potential Energy ◽  
Buoyancy Forcing

AbstractWe present numerical results for an idealized rotating, buoyancy- and windforced channel as a simple model for the Southern Ocean branch of the Meridional Overturning Circulation (MOC). Differential buoyancy forcing is applied along the top horizontal surface, with surface cooling at one end (to represent the pole) and surface warming at the other (to represent the equatorial region) and a zonally re-entrant channel to represent the Antarctic Circumpolar Current (ACC). Zonally-uniform surface wind forcing is applied with a similar pattern to the westerlies and easterlies with varying magnitude relative to the buoyancy forcing. The problem is solved numerically using a 3D DNS model based on a finite-volume solver for the Boussinesq Navier-Stokes equations with rotation. The overall dynamics, including large-scale overturning, baroclinic eddying, turbulent mixing, and resulting energy cascades are studied by calculating terms in the energy budget using the local Available Potential Energy framework. The basic physics of the overturning in the Southern Ocean are investigated at multiple scales and the output from the fully-resolved DNS simulations is compared with the results from previous studies of the global (ECCO2) and Southern Ocean eddy-permitting state estimates. We find that both the magnitude and shape of the zonal wind stress profile are important to the spatial pattern of the overturning circulation. However, the available potential energy budget and the diapycnal mixing are not significantly affected by the surface wind stress and are primarily set by the buoyancy forcing at the surface.

scholarly journals Response of Southern Ocean Convection and Abyssal Overturning to Surface Buoyancy Perturbations

2015 ◽  
Vol 28 (10) ◽  
pp. 4263-4278 ◽  
Author(s):  
Adele K. Morrison ◽  
Matthew H. England ◽  
Andrew McC. Hogg
Keyword(s):  
Southern Ocean ◽  
Deep Ocean ◽  
Ocean Model ◽  
Surface Relaxation ◽  
Ocean Temperature ◽  
Surface Cooling ◽  
Overturning Circulation ◽  
Surface Warming ◽  
Freshwater Fluxes ◽  
Surface Buoyancy

Abstract This study explores how buoyancy-driven modulations in the abyssal overturning circulation affect Southern Ocean temperature and salinity in an eddy-permitting ocean model. Consistent with previous studies, the modeled surface ocean south of 50°S cools and freshens in response to enhanced surface freshwater fluxes. Paradoxically, upper-ocean cooling also occurs for small increases in the surface relaxation temperature. In both cases, the surface cooling and freshening trends are linked to reduced convection and a slowing of the abyssal overturning circulation, with associated changes in oceanic transport of heat and salt. For small perturbations, convective shutdown does not begin immediately, but instead develops via a slow feedback between the weakened overturning circulation and buoyancy anomalies. Two distinct phases of surface cooling are found: an initial smaller trend associated with the advective (overturning) adjustment of up to ~60 yr, followed by more rapid surface cooling during the convective shutdown period. The duration of the first advective phase decreases for larger forcing perturbations. As may be expected during the convective shutdown phase, the deep ocean warms and salinifies for both types of buoyancy perturbation. However, during the advective phase, the deep ocean freshens in response to freshwater perturbations but salinifies in the surface warming perturbations. The magnitudes of the modeled surface and abyssal trends during the advective phase are comparable to the recent observed multidecadal Southern Ocean temperature and salinity changes.

scholarly journals Meridional Overturning Circulation in a Multibasin Model. Part I: Dependence on Southern Ocean Buoyancy Forcing

2020 ◽  
Vol 50 (5) ◽  
pp. 1159-1178 ◽  
Author(s):  
Jonathan A. Baker ◽  
Andrew J. Watson ◽  
Geoffrey K. Vallis
Keyword(s):  
Southern Ocean ◽  
Deep Water ◽  
General Circulation ◽  
Circulation Model ◽  
Meridional Overturning Circulation ◽  
Pacific Basin ◽  
Overturning Circulation ◽  
Buoyancy Forcing ◽  
The Pacific ◽  
Meridional Overturning

AbstractThe variation in the strength and structure of the overturning circulation under varying Southern Ocean buoyancy forcing, corresponding to present day, a cooler (glacial) state, and a possible future warmer state is analyzed in an idealized two-basin general circulation model connected by a southern circumpolar channel. A connection between the North Atlantic Deep Water (NADW) cell in the Atlantic basin and the Pacific Deep Water (PDW) cell in the Pacific basin occurs with a direct flow of NADW into the channel’s lower cell, while PDW upwelled in the Pacific basin can flow directly into the upper wind-driven cell in the channel. The intersection of these cells along with direct zonal flows between the basins completes the interbasin circulation. The present-day Atlantic meridional overturning circulation (AMOC) in the model is upwelled both by wind-driven upwelling in the Southern Ocean and by diffusion in the Pacific and Atlantic. In a cooler climate with enhanced sea ice, the NADW cell shoals, which can then no longer flow directly into the channel’s lower cell, reducing the Pacific pathway of NADW. This leads to a substantial weakening of the AMOC, suggesting buoyancy forcing changes can play a substantial role in the transition of the AMOC to a glacial state. In contrast, in a warmer equilibrium climate state with reduced AABW formation, the NADW cell strengthens and deepens. NADW is increasingly directed along the Pacific pathway, while the direct upwelling in the channel’s wind-driven upper cell plays a smaller role.

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