Atlantic Thermohaline Circulation in a Coupled General Circulation Model: Unforced Variations versus Forced Changes

A 1200-yr unforced control run and future climate change simulations using the Parallel Climate Model (PCM), a coupled atmosphere–ocean–land–sea ice global model with no flux adjustments and relatively high resolution (2.8° for the atmosphere and 2/3° for the oceans) are analyzed for changes in Atlantic Ocean circulations. For the forced simulations, historical greenhouse gas and sulfate forcing of the twentieth century and projected forcing for the next two centuries are used. The Atlantic thermohaline circulation (THC) shows large multidecadal (15–40 yr) variations with mean-peak amplitudes of 1.5–3.0 Sv (1 Sv 10 6 m 3 s 1 ) and a sharp peak of power around a 24-yr period in the control run. Associated with the THC oscillations, there are large variations in North Atlantic Ocean heat transport, sea surface temperature (SST) and salinity (SSS), sea ice fraction, and net surface water and energy fluxes, which all lag the variations in THC strength by 2–3 yr. However, the net effect of the SST and SSS variations on upper-ocean density in the midlatitude North Atlantic leads the THC variations by about 6 yr, which results in the 24-yr period. The simulated SST and sea ice spatial patterns associated with the THC oscillations resemble those in observed SST and sea ice concentrations that are associated with the North Atlantic Oscillation (NAO). The results suggest a dominant role of the advective mechanism and strong coupling between the THC and the NAO, whose index also shows a sharp peak around the 24-yr time scale in the control run. In the forced simulations, the THC weakens by 12% in the twenty-first century and continues to weaken by an additional 10% in the twenty-second century if CO2 keeps rising, but the THC stabilizes if CO2 levels off. The THC weakening results from stabilizing temperature increases that are larger in the upper and northern Atlantic Ocean than in the deep and southern parts of the basin. In both the control and forced simulations, as the THC gains (loses) strength and depth, the separated Gulf Stream (GS) moves southward (northward) while the subpolar gyre centered at the Labrador Sea contracts from (expands to) the east with the North Atlantic Current (NAC) being shifted westward (eastward). These horizontal circulation changes, which are dynamically linked to the THC changes, induce large temperature and salinity variations around the GS and NAC paths.

[1]  G. Meehl,et al.  Response of the Atlantic Thermohaline Circulation to Increased Atmospheric CO2 in a Coupled Model , 2004 .

[2]  A. Levermann,et al.  Solution of a model for the oceanic pycnocline depth: Scaling of overturning strength and meridional pressure difference , 2004, physics/0408061.

[3]  S. Häkkinen,et al.  Decline of Subpolar North Atlantic Circulation During the 1990s , 2004, Science.

[4]  Wei Cheng,et al.  Multi-decadal thermohaline variability in an ocean–atmosphere general circulation model , 2004 .

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

[6]  Elizabeth C. Kent,et al.  Global analyses of sea surface temperature, sea ice, and night marine air temperature since the late nineteenth century , 2003 .

[7]  A. Weaver,et al.  North Atlantic response to the above‐normal export of sea ice from the Arctic , 2003 .

[8]  Keith W. Dixon,et al.  A comparison of climate change simulations produced by two GFDL coupled climate models , 2003 .

[9]  Charles S. Zender,et al.  A monthly and latitudinally varying volcanic forcing dataset in simulations of 20th century climate , 2003 .

[10]  Tom M. L. Wigley,et al.  Solar and Greenhouse Gas Forcing and Climate Response in the Twentieth Century , 2003 .

[11]  P. Jones,et al.  Hemispheric and Large-Scale Surface Air Temperature Variations: An Extensive Revision and an Update to 2001. , 2003 .

[12]  Naomi Naik,et al.  Is the Gulf Stream responsible for Europe's mild winters? , 2002 .

[13]  S. Rahmstorf.  Ocean circulation and climate during the past 120,000 years , 2002, Nature.

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

[15]  C. Deser,et al.  Decadal variations in Labrador Sea ice cover and North Atlantic sea surface temperatures , 2002 .

[16]  Stephen Dye,et al.  Rapid freshening of the deep North Atlantic Ocean over the past four decades , 2002, Nature.

[17]  Peter U. Clark,et al.  The role of the thermohaline circulation in abrupt climate change , 2002, Nature.

[18]  T. Wigley,et al.  Effects of stabilizing atmospheric CO2 on global climate in the next two centuries , 2001 .

[19]  R. Bleck,et al.  Atlantic thermohaline circulation and its response to increasing CO2 in a coupled atmosphere‐ocean model , 2001 .

[20]  Tom M. L. Wigley,et al.  Ensemble Simulation of Twenty-First Century Climate Changes: Business-as-Usual versus CO2 Stabilization , 2001 .

[21]  T. Wigley,et al.  Climate Changes in the 21st Century over the Asia-Pacific Region Simulated by the NCAR CSM and PCM , 2001 .

[22]  K. Trenberth,et al.  Estimates of Meridional Atmosphere and Ocean Heat Transports , 2001 .

[23]  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 .

[24]  S. Østerhus,et al.  Decreasing overflow from the Nordic seas into the Atlantic Ocean through the Faroe Bank channel since 1950 , 2001, Nature.

[25]  A. Craig,et al.  Factors that affect the amplitude of El Nino in global coupled climate models , 2001 .

[26]  Peter R. Gent,et al.  Will the North Atlantic Ocean thermohaline circulation weaken during the 21st century? , 2001 .

[27]  M. Holland,et al.  The Role of Ice-Ocean Interactions in the Variability of the North Atlantic Thermohaline Circulation , 2001 .

[28]  T. Wigley,et al.  Climates of the Twentieth and Twenty-First Centuries Simulated by the NCAR Climate System Model , 2001 .

[29]  R. Marsh.  Recent Variability of the North Atlantic Thermohaline Circulation Inferred from Surface Heat and Freshwater Fluxes , 2000 .

[30]  U. Mikolajewicz,et al.  The role of the individual air-sea flux components in CO2-induced changes of the ocean's circulation and climate , 2000 .

[31]  C. Deser,et al.  The Relation between Decadal Variability of Subtropical Mode Water and the North Atlantic Oscillation , 2000 .

[32]  G. Boer,et al.  A transient climate change simulation with greenhouse gas and aerosol forcing: projected climate to the twenty-first century , 2000 .

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

[34]  R. Greatbatch,et al.  Multidecadal Thermohaline Circulation Variability Driven by Atmospheric Surface Flux Forcing , 2000 .

[35]  James C. McWilliams,et al.  Decadal Variability and Predictability in the Midlatitude Ocean–Atmosphere System , 2000 .

[36]  John E. Walsh,et al.  Arctic Sea Ice Variability in the Context of Recent Atmospheric Circulation Trends , 2000 .

[37]  A. Weaver,et al.  On the sensitivity of global warming experiments to the parametrisation of sub-grid scale ocean mixing , 1999 .

[38]  T. Peng,et al.  A possible 20th-century slowdown of southern ocean deep water formation , 1999, Science.

[39]  Stefan Rahmstorf,et al.  Long-Term Global Warming Scenarios Computed with an Efficient Coupled Climate Model , 1999 .

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

[41]  R. Stouffer,et al.  The influence of transient surface fluxes on North Atlantic overturning in a coupled GCM Climate Change Experiment , 1999 .

[42]  S. Rahmstorf.  Shifting seas in the greenhouse? , 1999, Nature.

[43]  Thomas F. Stocker,et al.  The Stability of the Thermohaline Circulation in Global Warming Experiments , 1999 .

[44]  A. Gnanadesikan,et al.  A simple predictive model for the structure of the oceanic pycnocline , 1999, Science.

[45]  G. Russell,et al.  Response to CO2 transient increase in the GISS coupled model : Regional coolings in a warming climate , 1999 .

[46]  J. Toggweiler,et al.  On the Ocean’s Large-Scale Circulation near the Limit of No Vertical Mixing , 1998 .

[47]  P. Gent,et al.  The NCAR Climate System Model, Version One* , 1998 .

[48]  A. Weaver,et al.  On the variability of the thermohaline circulation in the GFDL coupled model , 1998 .

[49]  James C. McWilliams,et al.  Advective Ocean–Atmosphere Interaction: An Analytical Stochastic Model with Implications for Decadal Variability , 1998 .

[50]  W. Broecker,et al.  Thermohaline circulation, the achilles heel of our climate system: will man-made CO2 upset the current balance? , 1997, Science.

[51]  Thomas F. Stocker,et al.  Influence of CO2 emission rates on the stability of the thermohaline circulation , 1997, Nature.

[52]  S. Manabe,et al.  Coupled ocean‐atmosphere model response to freshwater input: Comparison to Younger Dryas Event , 1997 .

[53]  Stephen M. Griffies,et al.  Predictability of North Atlantic Multidecadal Climate Variability , 1997, Science.

[54]  James C. McWilliams,et al.  Approach to Equilibrium in Accelerated Global Oceanic Models. , 1996 .

[55]  A. Weaver,et al.  Sea Surface Temperature-Evaporation Feedback and the Ocean's Thermohaline Circulation , 1996 .

[56]  S. Rahmstorf.  Bifurcations of the Atlantic thermohaline circulation in response to changes in the hydrological cycle , 1995, Nature.

[57]  J. Hurrell.  Decadal Trends in the North Atlantic Oscillation: Regional Temperatures and Precipitation , 1995, Science.

[58]  E. Maier‐Reimer,et al.  Mixed boundary conditions in ocean general circulation models and their influence on the stability of the model's conveyor belt , 1994 .

[59]  Reindert J. Haarsma,et al.  Variability and Multiple Equilibria of the Thermohaline Circulation Associated with Deep-Water Formation , 1994 .

[60]  A. Weaver,et al.  Interdecadal variability in an idealized model of the North Atlantic , 1994 .

[61]  J. Dukowicz,et al.  Implicit free‐surface method for the Bryan‐Cox‐Semtner ocean model , 1994 .

[62]  Syukuro Manabe,et al.  Interdecadal Variations of the Thermohaline Circulation in a Coupled Ocean-Atmosphere Model , 1993 .

[63]  Clara Deser,et al.  Surface Climate Variations over the North Atlantic Ocean during Winter: 1900–1989 , 1993 .

[64]  E. Sarachik,et al.  Thermohaline Oscillations Induced by Strong Steady Salinity Forcing of Ocean General Circulation Models , 1993 .

[65]  R. C. Malone,et al.  Parallel ocean general circulation modeling , 1992 .

[66]  J. Marotzke,et al.  Multiple Equilibria of the Global Thermohaline Circulation , 1991 .

[67]  T. Stocker,et al.  Rapid transitions of the ocean's deep circulation induced by changes in surface water fluxes , 1991, Nature.

[68]  Andrew J. Weaver,et al.  Evidence for decadal variability in an ocean general circulation model: An advective mechanism 1 , 1991 .

[69]  Wallace S. Broecker,et al.  Unpleasant surprises in the greenhouse? , 1987, Nature.

[70]  F. Bryan,et al.  High-latitude salinity effects and interhemispheric thermohaline circulations , 1986, Nature.

[71]  K. Bryan.  Accelerating the Convergence to Equilibrium of Ocean-Climate Models , 1984 .

[72]  H. Stommel,et al.  Thermohaline Convection with Two Stable Regimes of Flow , 1961 .

[73]  W. G. Strand,et al.  The ACPI Climate Change Simulations , 2004 .

[74]  Syukuro Manabe,et al.  Equilibrium response of thermohaline circulation to large changes in atmospheric CO2 concentration , 2004 .

[75]  R. Voss,et al.  Long-term climate changes due to increased CO2 concentration in the coupled atmosphere-ocean general circulation model ECHAM3/LSG , 2001 .

[76]  J. Houghton,et al.  Climate change 2001 : the scientific basis , 2001 .

[77]  T. Stocker.  Past and future reorganizations in the climate system , 2000 .

[78]  W. G. Strand,et al.  Parallel climate model (PCM) control and transient simulations , 2000 .

[79]  S. Levitus,et al.  “Great Salinity Anomalies” in the North Atlantic , 1998 .

[80]  Robert R. Dickson,et al.  Long-term coordinated changes in the convective activity of the North Atlantic , 1996 .

[81]  Stefan Rahmstorf,et al.  Stability of the thermohaline circulation , 1996 .

[82]  Syukuro Manabe,et al.  Multiple-Century Response of a Coupled Ocean-Atmosphere Model to an Increase of Atmospheric Carbon Dioxide , 1994 .

[83]  Y. Kushnir,et al.  Interdecadal Variations in North Atlantic Sea Surface Temperature and Associated Atmospheric Conditions , 1994 .

[84]  Patrick F. Cummins,et al.  Stability and Variability of the Thermohaline Circulation , 1993 .