Eddy contributions to the meridional transport of salt in the North Atlantic

[1] The meridional transport of salt in the Atlantic ocean is an important process for climate, controlling the stability of the meridional overturning circulation. The contribution of transient eddies to this transport is quantified in an eddy resolving North Atlantic model at 1/12° resolution (NATL12), and compared with lower resolution North-Atlantic and global 1/4° models. In NATL12 between 10°N and 40°N, there is a volume loss by evaporation of 0.6 Sverdrups (Sv). The divergence of the eddy flux of salt (normalized by a reference salinity of 34.8) is 0.2 Sv over the region, a significant fraction of the total air-seawater exchange, but it is compensated by an opposite convergent transport of salt by the mean flow, so that the total transport of salt is small. The compensation between eddy and mean salt transport is almost complete in a multicentury long global model experiment, but less effective in NATL12 because the short integration time does not allow the salt content to equilibrate and the model drift is large. Eddies arising from baroclinic instability contribute to the meridional salt transports at the northern and southern boundary of the subtropical gyre, where they appear consistent with a lateral diffusion acting on the mean salinity gradient. However, the eddy transport of salt is the sum of two terms: an advective contribution (arising from the correlations of velocity and isopycnal thicknesses) and a diffusion along isopycnals. Both components have the same amplitude at the southern boundary of the subtropical gyre, while diffusion is dominant at the northern boundary.

[1]  William E. Johns,et al.  Continuous, Array-Based Estimates of Atlantic Ocean Heat Transport at 26.5°N , 2011 .

[2]  Kevin E. Trenberth,et al.  Covariability of Components of Poleward Atmospheric Energy Transports on Seasonal and Interannual Timescales , 2003 .

[3]  S. Somot,et al.  21st century Mediterranean sea level rise: steric and atmospheric pressure contributions from a regional model , 2008 .

[4]  Gurvan Madec,et al.  Salt conservation, free surface, and varying levels: A new formulation for ocean general circulation models , 2000 .

[5]  Sanjay S. P. Rattan,et al.  Towards an understanding of Labrador Sea salinity drift in eddy-permitting simulations , 2010 .

[6]  B. Dickson,et al.  A change in the freshwater balance of the Atlantic Ocean over the past four decades , 2003, Nature.

[7]  Richard Smith,et al.  Toward a Physical Understanding of the North Atlantic: A Review of Model Studies in an Eddying Regime , 2013 .

[8]  R. Schmitt,et al.  The Goldsbrough–Stommel Circulation of the World Oceans , 1993 .

[9]  H. Kuo,et al.  FORCED AND FREE MERIDIONAL CIRCULATIONS IN THE ATMOSPHERE , 1956 .

[10]  C. Eden,et al.  A diagnosis of isopycnal mixing by mesoscale eddies , 2009 .

[11]  Thierry Penduff,et al.  Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy-permitting resolution , 2006 .

[12]  C. Wunsch,et al.  Large-Scale Ocean Heat and Freshwater Transports during the World Ocean Circulation Experiment , 2003 .

[13]  M. Cox.  An Eddy Resolving Numerical Model of the Ventilated Thermocline , 1985 .

[14]  Thierry Penduff,et al.  Eddy-permitting ocean circulation hindcasts of past decades , 2007 .

[15]  J. Pedlosky,et al.  On the Indeterminacy of Rotational and Divergent Eddy Fluxes , 2003 .

[16]  Gurvan Madec,et al.  Modifications of gyre circulation by sub-mesoscale physics , 2010 .

[17]  M. Karcher,et al.  Current estimates of freshwater flux through Arctic and subarctic seas , 2007 .

[18]  Carl Wunsch,et al.  Where do ocean eddy heat fluxes matter , 1999 .

[19]  Randy Showstack,et al.  World Ocean Database , 2009 .

[20]  D. Stammer.  On Eddy Characteristics, Eddy Transports, and Mean Flow Properties , 1998 .

[21]  M. England,et al.  Southern Ocean overturning across streamlines in an eddying simulation of the Antarctic Circumpolar Current , 2007 .

[22]  Ed Hawkins,et al.  Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport , 2011 .

[23]  K. Aagaard,et al.  Revising the Bering Strait freshwater flux into the Arctic Ocean , 2005 .

[24]  G. Madec,et al.  An eddy-permitting model of the Atlantic circulation: evaluating open boundary conditions , 2001 .

[25]  Gilles Reverdin,et al.  Global high-resolution mapping of ocean circulation from TOPEX/Poseidon and ERS-1 and -2 , 2000 .

[26]  G. Madec.  NEMO ocean engine , 2008 .

[27]  A. Nurser,et al.  Eddy Advective and Diffusive Transports of Heat and Salt in the Southern Ocean , 2007 .

[28]  P. Gent,et al.  Parameterizing eddy-induced tracer transports in ocean circulation models , 1995 .

[29]  Stephen G. Yeager,et al.  The global climatology of an interannually varying air–sea flux data set , 2009 .

[30]  Richard J. Greatbatch,et al.  A Diagnosis of Thickness Fluxes in an Eddy-Resolving Model , 2007 .

[31]  R. Dussin,et al.  Buoyancy-Driven Flows: Buoyancy-driven currents in eddying ocean models , 2012 .

[32]  C. Eden,et al.  Effects of mesoscale eddy/wind interactions on biological new production and eddy kinetic energy , 2009 .

[33]  Nathaniel L. Bindoff,et al.  On the Total, Mean, and Eddy Heat and Freshwater Transports in the Southern Hemisphere of a ⅛° × ⅛° Global Ocean Model , 2007 .

[34]  M. Maltrud,et al.  Numerical simulation of the North Atlantic Ocean at 1/10 degrees , 2000 .

[35]  Heat Transport by Mesoscale Eddies in an Ocean Circulation Model , 1994 .

[36]  G. Madec,et al.  The roles of surface heat flux and ocean heat transport convergence in determining Atlantic Ocean temperature variability , 2010 .

[37]  R. Chervin,et al.  Transports and budgets of volume, heat, and salt from a global eddy-resolving ocean model , 1994 .

[38]  C. Lique,et al.  A model-based study of ice and freshwater transport variability along both sides of Greenland , 2009 .

[39]  Isaac M. Held,et al.  Parameterization of Quasigeostrophic Eddies in Primitive Equation Ocean Models. , 1997 .

[40]  H. Loon.  The Association between Latitudinal Temperature Gradient and Eddy Transport. Part I: Transport of Sensible Heat in Winter , 1979 .

[41]  J. Molines,et al.  Response of the Southern Ocean to the Southern Annular Mode: Interannual Variability and Multidecadal Trend , 2010 .

[42]  S. Wijffels.  Chapter 6.2 Ocean transport of fresh water , 2001 .

[43]  Thierry Penduff,et al.  An ERA40-based atmospheric forcing for global ocean circulation models , 2010 .

[44]  Thierry Penduff,et al.  Sea Level Expression of Intrinsic and Forced Ocean Variabilities at Interannual Time Scales , 2011 .

[45]  S. Drijfhout,et al.  An Indicator of the Multiple Equilibria Regime of the Atlantic Meridional Overturning Circulation , 2010 .

[46]  Kevin E. Trenberth,et al.  Estimates of Freshwater Discharge from Continents: Latitudinal and Seasonal Variations , 2002 .

[47]  P. L. Traon,et al.  A comparison of surface eddy kinetic energy and Reynolds stresses in the Gulf Stream and the Kuroshio Current systems from merged TOPEX/Poseidon and ERS-1/2 altimetric data , 2001 .

[48]  K. Bryan.  Poleward Buoyancy Transport in the Ocean and Mesoscale Eddies , 1986 .

[49]  L. Talley.  Freshwater transport estimates and the global overturning circulation: Shallow, deep and throughflow components , 2008 .