The role of mean ocean salinity in climate

We describe numerical simulations designed to elucidate the role of mean ocean salinity in climate. Using a coupled atmosphere-ocean general circulation model, we study a 100-year sensitivity experiment in which the global-mean salinity is approximately doubled from its present observed value, by adding 35 psu everywhere in the ocean. The salinity increase produces a rapid global-mean seasurface warming of 0.8 ◦C within a few years, caused by reduced vertical mixing associated with changes in cabbeling. The warming is followed by a gradual global-mean sea-surface cooling of 0.4 ◦ C within a few decades, caused by an increase in the vertical (downward) component of the isopycnal diffusive heat flux. We find no evidence of impacts on the variability of the thermohaline circulation (THC) or El Ni ˜ no/Southern Oscillation (ENSO). The mean strength of the Atlantic meridional overturning is reduced by 20% and the North Atlantic Deep Water penetrates less deeply. Nevertheless, our results dispute claims that higher salinities for the world ocean have profound consequences for the thermohaline circulation. In additional experiments with doubled atmospheric carbon dioxide, we find that the amplitude and spatial pattern of the global warming signal are modified in the hypersaline ocean. In particular, the equilibrated global-mean sea-surface temperature increase caused by doubling carbon dioxide is reduced by 10%. We infer the existence of a non-linear interaction between the climate responses to modified carbon dioxide and modified salinity.

[1]  Ross J. Murray,et al.  Explicit Generation of Orthogonal Grids for Ocean Models , 1996 .

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

[3]  P. Delecluse,et al.  A modeling study of salinity variability and its effects in the tropical Pacific Ocean during the 1993–1999 period , 2002 .

[4]  A. Weaver,et al.  Distinguishing the Influence of Heat, Freshwater, and Momentum Fluxes on Ocean Circulation and Climate , 2002 .

[5]  Cecilie Mauritzen,et al.  Dilution of the Northern North Atlantic Ocean in Recent Decades , 2005, Science.

[6]  Michael E. Schlesinger,et al.  Intensity of hydrological cycles in warmer climates , 2003 .

[7]  B. Dong,et al.  Adjustment of the coupled ocean–atmosphere system to a sudden change in the Thermohaline Circulation , 2002 .

[8]  H. Hasumi,et al.  Effects of Freshwater Forcing on the Atlantic Deep Circulation: A Study with an OGCM Forced by Two Different Surface Freshwater Flux Datasets , 2004 .

[9]  B. Dong,et al.  Mechanism of interdecadal thermohaline circulation variability in a coupled ocean : Atmosphere GCM , 2005 .

[10]  B. Soden,et al.  Robust Responses of the Hydrological Cycle to Global Warming , 2006 .

[11]  E. Guilyardi,et al.  On the climate response of the low-latitude Pacific Ocean to changes in the global freshwater cycle , 2006 .

[12]  Gurvan Madec,et al.  A global ocean mesh to overcome the North Pole singularity , 1996 .

[13]  P. Delecluse,et al.  OPA 8.1 Ocean General Circulation Model reference manual , 1998 .

[14]  M. Maqueda,et al.  Sensitivity of a global sea ice model to the treatment of ice thermodynamics and dynamics , 1997 .

[15]  S. Marsland,et al.  On the sensitivity of Southern Ocean sea ice to the surface freshwater flux: A model study , 2001 .

[16]  Enrico Scoccimarro,et al.  Changes in Tropical Cyclone Activity Due to Global Warming: Results from a High-Resolution Coupled General Circulation Model , 2007 .

[17]  Christophe Maes,et al.  Salinity barrier layer and onset of El Niño in a Pacific coupled model , 2002 .

[18]  M. Latif,et al.  Tropical stabilization of the thermohaline circulation in a greenhouse warming simulation , 2000 .

[19]  E. Guilyardi,et al.  Assessment of the tropical Indo-Pacific climate in the SINTEX CGCM , 2003 .

[20]  S. Manabe,et al.  Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean , 1995, Nature.

[21]  J. Picaut,et al.  Importance of the Salinity Barrier Layer for the Buildup of El Nino , 2005 .

[22]  Jonathan M. Gregory,et al.  Mechanisms Determining the Atlantic Thermohaline Circulation Response to Greenhouse Gas Forcing in a Non-Flux-Adjusted Coupled Climate Model , 2001 .

[23]  S. Josey,et al.  Surface freshwater flux variability and recent freshening of the North Atlantic in the eastern subpolar gyre , 2005 .

[24]  N. Bindoff,et al.  Large-scale freshening of intermediate waters in the Pacific and Indian oceans , 1999, Nature.

[25]  S. Rahmstorf.  On the freshwater forcing and transport of the Atlantic thermohaline circulation , 1996 .

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

[27]  J. Marotzke,et al.  Global Thermohaline Circulation. Part I: Sensitivity to Atmospheric Moisture Transport , 1999 .

[28]  Andrei P. Sokolov,et al.  A model intercomparison of changes in the Atlantic thermohaline circulation in response to increasing atmospheric CO2 concentration , 2005 .

[29]  P. Stott,et al.  Detection and attribution of Atlantic salinity changes , 2008 .

[30]  E. Guilyardi,et al.  Mechanisms for ENSO Phase Change in a Coupled GCM , 2003 .

[31]  M. Collins,et al.  Projections of future climate change , 2002 .

[32]  Jonathan M. Gregory,et al.  Changing spatial structure of the thermohaline circulation in response to atmospheric CO2 forcing in a climate model , 1999, Nature.

[33]  P. Delecluse,et al.  An OGCM Study for the TOGA Decade. Part I: Role of Salinity in the Physics of the Western Pacific Fresh Pool , 1998 .

[34]  E. Guilyardi,et al.  The impact of global freshwater forcing on the thermohaline circulation: adjustment of North Atlantic convection sites in a CGCM , 2006 .

[35]  R. Lukas.  Freshening of the upper thermocline in the North Pacific Subtropical Gyre associated with decadal changes of rainfall , 2001 .

[36]  J. Priscu,et al.  Evolution of temperature and salt structure of Lake Bonney, a chemically stratified Antarctic lake , 1996, Hydrobiologia.

[37]  N. Bindoff,et al.  Decadal Changes along an Indian Ocean Section at 32°S and Their Interpretation , 2000 .

[38]  T. McDougall,et al.  Minimal Adjustment of Hydrographic Profiles to Achieve Static Stability , 1995 .

[39]  Valérie Dulière,et al.  On the representation of high latitude processes in the ORCA-LIM global coupled sea ice–ocean model , 2005 .

[40]  Jérôme Vialard,et al.  An OGCM Study for the TOGA Decade. Part II: Barrier-Layer Formation and Variability , 1998 .

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

[42]  E. Guilyardi,et al.  A new feedback on climate change from the hydrological cycle , 2007 .

[43]  S. Levitus.  Climatological Atlas of the World Ocean , 1982 .

[44]  S. Flögel,et al.  Evaporites and the salinity of the ocean during the Phanerozoic: Implications for climate, ocean circulation and life , 2006 .

[45]  R. Pacanowski,et al.  The Effect of Salinity on the Wind-Driven Circulation and the Thermal Structure of the Upper Ocean , 2004 .

[46]  Jonathan M. Gregory,et al.  Processes governing the recovery of a perturbed thermohaline circulation in HadCM3 , 2002 .

[47]  A. Gordon.  Two Stable Modes of Southern Ocean Winter Stratification , 1991 .

[48]  E. Guilyardi,et al.  The role of lateral ocean physics in the upper ocean thermal balance of a coupled ocean-atmosphere GCM , 2001 .

[49]  B. Huang,et al.  Oceanic Response to Idealized Net Atmospheric Freshwater in the Pacific at the Decadal Time Scale , 2005 .

[50]  Stefan Hagemann,et al.  A parametrization of the lateral waterflow for the global scale , 1997 .

[51]  D. Martinson.  Evolution of the southern ocean winter mixed layer and sea ice: Open ocean deepwater formation and ventilation , 1990 .

[52]  S. Levitus,et al.  Steric sea level variations during 1957–1994: Importance of salinity , 2002 .

[53]  Peter H. Stone,et al.  Global Thermohaline Circulation. Part II: Sensitivity with Interactive Atmospheric Transports , 1999 .

[54]  T. Osborn.  The Vertical Component of Epineutral Diffusion and the Dianeutral Component of Horizontal Diffusion , 1998 .

[55]  R. Schmitt.  The ocean component of the global water cycle (95RG00184) , 1995 .

[56]  M. Claussen,et al.  The atmospheric general circulation model ECHAM-4: Model description and simulation of present-day climate , 1996 .