Meridional density gradients do not control the Atlantic overturning circulation

A wide body of modeling and theoretical scaling studies support the concept that changes to the Atlantic meridional overturning circulation (AMOC), whether forced by winds or buoyancy fluxes, can be understood in terms of a simple causative relation between the AMOC and an appropriately defined meridional density gradient (MDG). The MDG is supposed to translate directly into a meridional pressure gradient. Here two sets of experiments are performed using a modular ocean model coupled to an energy‐moisture balance model in which the positive AMOC‐MDG relation breaks down. In the first suite of seven model integrations it is found that increasing winds in the Southern Ocean cause an increase in overturning while the surface density difference between the equator and North Atlantic drops. In the second suite of eight model integrations the equation of state is manipulated so that the density is calculated at the model temperature plus an artificial increment DT that ranges from 238 to 98C. (An increase in DT results in increased sensitivity of density to temperature gradients.) The AMOC in these model integrations drops as the MDG increases regardless of whether the density difference is computed at the surface or averaged over the upper ocean. Traditional scaling analysis can only produce this weaker AMOC if the scale depth decreases enough to compensate for the stronger MDG. Five estimates of the depth scale are evaluated and it is found that the changes in the AMOC can be derived from scaling analysis when using the depth of the maximum overturning circulation or estimates thereof but not from the pycnocline depth. These two depth scales are commonly assumed to be the same in theoretical models of the AMOC. It is suggested that the correlation between the MDG and AMOC breaks down in these model integrations because the depth and strength of the AMOC is influenced strongly by remote forcing such as Southern Ocean winds and Antarctic Bottom Water formation.

[1]  R. Huang,et al.  Stommel’s Box Model of Thermohaline Circulation Revisited—The Role of Mechanical Energy Supporting Mixing and the Wind-Driven Gyration , 2008 .

[2]  J. Toggweiler,et al.  Atlantic Dominance of the Meridional Overturning Circulation , 2008 .

[3]  D. Nof,et al.  Does the Atlantic meridional overturning cell really have more than one stable steady state , 2007 .

[4]  D. Marshall,et al.  Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria , 2007 .

[5]  J. Toggweiler,et al.  Effect of global ocean temperature change on deep ocean ventilation , 2007 .

[6]  Inferring the zonal distribution of measured changes in the meridional overturning circulation , 2006 .

[7]  F. Straneo On the Connection between Dense Water Formation, Overturning, and Poleward Heat Transport in a Convective Basin* , 2006 .

[8]  M. Maqueda,et al.  The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model , 2006 .

[9]  A. Watson,et al.  The role of Southern Ocean mixing and upwelling in glacial-interglacial atmospheric CO2 change , 2006 .

[10]  T. Delworth,et al.  Have anthropogenic aerosols delayed a greenhouse gas‐induced weakening of the North Atlantic thermohaline circulation? , 2006 .

[11]  A. Watson,et al.  Can limited ocean mixing buffer rapid climate change? , 2005 .

[12]  C. Wunsch Thermohaline loops, Stommel box models, and the Sandström theorem , 2005 .

[13]  Simon J. Cox,et al.  Bistability of the thermohaline circulation identified through comprehensive 2-parameter sweeps of an efficient climate model , 2004 .

[14]  R. Kahana,et al.  Global Ocean Circulation Modes Derived from a Multiple Box Model , 2004 .

[15]  S. Griffies,et al.  A Technical Guide to MOM4 , 2004 .

[16]  G. Broström,et al.  The Thermohaline Circulation and Vertical Mixing: Does Weaker Density Stratification Give Stronger Overturning? , 2003 .

[17]  Gerrit Lohmann,et al.  Influence of vertical mixing on the thermohaline hysteresis: Analyses of an OGCM , 2003 .

[18]  Rainer Feistel,et al.  Accurate and Computationally Efficient Algorithms for Potential Temperature and Density of Seawater , 2003 .

[19]  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 .

[20]  M. Spall,et al.  Where Does Dense Water Sink? A Subpolar Gyre Example* , 2001 .

[21]  J. Nilsson,et al.  Freshwater forcing as a booster of thermohaline circulation , 2001 .

[22]  M. Winton,et al.  A Reformulated Three-Layer Sea Ice Model , 2000 .

[23]  Young-Gyu Park,et al.  Comparison of Thermally Driven Circulations from a Depth-Coordinate Model and an Isopycnal-Layer Model. Part I: Scaling-Law Sensitivity to Vertical Diffusivity , 2000 .

[24]  J. Marotzke,et al.  Abrupt climate change and thermohaline circulation: mechanisms and predictability. , 2000, Proceedings of the National Academy of Sciences of the United States of America.

[25]  Young‐Gyu Park,et al.  The Stability of Thermohaline Circulation in a Two-Box Model , 1999 .

[26]  Jeffery R. Scott,et al.  Convective Mixing and the Thermohaline Circulation , 1999 .

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

[28]  W. Munk,et al.  Abyssal recipes II: energetics of tidal and wind mixing , 1998 .

[29]  Neil R. Edwards,et al.  On the Role of Topography and Wind Stress on the Stability of the Thermohaline Circulation , 1998 .

[30]  Jochem Marotzke,et al.  Boundary Mixing and the Dynamics of Three-Dimensional Thermohaline Circulations , 1997 .

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

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

[33]  B. Cushman-Roisin Introduction to Geophysical Fluid Dynamics , 1994 .

[34]  A. Weaver,et al.  Multiple Equilibria of an Asymmetric Two-Basin Ocean Model , 1994 .

[35]  G. Shaffer,et al.  Role of the Bering Strait in controlling North Atlantic ocean circulation and climate , 1994, Nature.

[36]  J. Toggweiler,et al.  Is the Magnitude of the Deep Outflow from the Atlantic Ocean Actually Governed by Southern Hemisphere Winds , 1993 .

[37]  Thomas F. Stocker,et al.  A Zonally Averaged Ocean Model for the Thermohaline Circulation. Part I: Model Development and Flow Dynamics , 1991 .

[38]  K. Trenberth,et al.  The mean annual cycle in global ocean wind stress , 1990 .

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

[40]  Syukuro Manabe,et al.  Two Stable Equilibria of a Coupled Ocean-Atmosphere Model , 1988 .

[41]  Frank O. Bryan,et al.  Parameter sensitivity of primitive equation ocean general circulation models , 1987 .

[42]  P. Welander,et al.  THERMOHALINE EFFECTS IN THE OCEAN CIRCULATION AND RELATED SIMPLE MODELS , 1986 .

[43]  M. Redi Oceanic Isopycnal Mixing by Coordinate Rotation , 1982 .

[44]  Claes Rooth,et al.  Hydrology and ocean circulation , 1982 .

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