On the Relationship between Southern Ocean Overturning and ACC Transport

The eddy field in the Southern Ocean offsets the impact of strengthening winds on the meridional overturning circulation and Antarctic Circumpolar Current (ACC) transport. There is widespread belief that the sensitivities of the overturning and ACC transport are dynamically linked, with limitation of the ACC transport response implying limitation of the overturning response. Here, an idealized numerical model is employed to investigate the response of the large-scale circulation in the Southern Ocean to wind stress perturbations at eddy-permitting to eddy-resolving scales. Significant differences are observed between the sensitivities and the resolution dependence of the overturning and ACC transport, indicating that they are controlled by distinct dynamical mechanisms. The modeled overturning is significantly more sensitive to change than the ACC transport, with the possible implication that the Southern Ocean overturning may increase in response to future wind stress changes without measurable changes in the ACC transport. It is hypothesized that the dynamical distinction between the zonal and meridional transport sensitivities is derived from the depth dependence of the extent of cancellation between the Ekman and eddy-induced transports.

[1]  D. Straub.  On the Transport and Angular Momentum Balance of Channel Models of the Antarctic Circumpolar Current , 1993 .

[2]  P. Gent,et al.  Isopycnal mixing in ocean circulation models , 1990 .

[3]  Michael P. Meredith,et al.  Sensitivity of the Overturning Circulation in the Southern Ocean to Decadal Changes in Wind Forcing , 2011 .

[4]  Peter R. Gent,et al.  Response to Increasing Southern Hemisphere Winds in CCSM4 , 2011 .

[5]  K. Speer,et al.  The Diabatic Deacon Cell , 2000 .

[6]  Michael P. Meredith,et al.  Circumpolar response of Southern Ocean eddy activity to a change in the Southern Annular Mode , 2006 .

[7]  R. Hallberg,et al.  Reconciling estimates of the free surface height in Lagrangian vertical coordinate ocean models with mode-split time stepping , 2009 .

[8]  A. Hogg,et al.  Sensitivity of the Southern Ocean overturning circulation to surface buoyancy forcing , 2011 .

[9]  J. Marshall,et al.  Residual-Mean Solutions for the Antarctic Circumpolar Current and Its Associated Overturning Circulation , 2003 .

[10]  W. Munk,et al.  Note on the Dynamics of the Antarctic Circumpolar Current , 1951 .

[11]  R. Hallberg,et al.  On the Relationship of the Circumpolar Current to Southern Hemisphere Winds in Coarse-Resolution Ocean Models , 2000 .

[12]  Alistair Adcroft,et al.  A finite volume discretization of the pressure gradient force using analytic integration , 2008 .

[13]  E. Guilyardi,et al.  Southern Ocean transformation in a coupled model with and without eddy mass fluxes , 2000 .

[14]  S. Doney,et al.  Toward a mechanistic understanding of the decadal trends in the Southern Ocean carbon sink , 2008 .

[15]  Carsten Eden,et al.  Towards the impact of eddies on the response of the Southern Ocean to climate change , 2010 .

[16]  M. A. Morales Maqueda,et al.  The response of Southern Ocean eddies to increased midlatitude westerlies: A non‐eddy resolving model study , 2011 .

[17]  W. Munk,et al.  Note on the Dynamics of the Antarctic Circumpolar Current1 , 1951 .

[18]  Casper Labuschagne,et al.  Saturation of the Southern Ocean CO2 Sink Due to Recent Climate Change , 2007, Science.

[19]  N. Lovenduski,et al.  The transient response of the Southern Ocean pycnocline to changing atmospheric winds , 2010 .

[20]  Robert Hallberg,et al.  The Role of Eddies in Determining the Structure and Response of the Wind-Driven Southern Hemisphere Overturning: Results from the Modeling Eddies in the Southern Ocean (MESO) Project , 2006 .

[21]  Stephen M. Griffies,et al.  The Role of Mesoscale Eddies in the Rectification of the Southern Ocean Response to Climate Change , 2010 .

[22]  D. Olbers,et al.  A Model of the Zonally Averaged Stratification and Overturning in the Southern Ocean , 2005 .

[23]  Ryan Abernathey,et al.  The Dependence of Southern Ocean Meridional Overturning on Wind Stress , 2011 .

[24]  S. Rintoul,et al.  The response of the Antarctic Circumpolar Current to recent climate change , 2008 .

[25]  R. Farneti,et al.  Impacts of wind stress on the Antarctic Circumpolar Current fronts and associated subduction , 2011 .

[26]  Dirk Olbers,et al.  The dynamical balance, transport and circulation of the Antarctic Circumpolar Current , 2004, Antarctic Science.

[27]  J. Toggweiler,et al.  Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages , 2006 .