Century-scale variability in a randomly forced, two-dimensional thermohaline ocean circulation model

The response of a two-dimensional thermohaline ocean circulation model to a random freshwater flux superimposed on the usual mixed boundary conditions for temperature and salinity is considered. It is shown that for a wide range of vertical and horizontal diffusivities and a box geometry that approximates the Atlantic Ocean, 200–300 yr period oscillations exist in the basic-state, interhemispheric meridional overturning circulation with deep convection in the north. These fluctuations can also be described in terms of propagating salinity anomalies which travel in the direction of the thermohaline flow. For large horizontal (Kh = 15 × 103 m2/s) and small vertical (Kv = 0.5 × 10−4 m2/s) diffusivities, the random forcing also excites deca-millennial oscillations in the basic structure of the thermohaline circulation. In this case, the meridional circulation pattern slowly oscillates between three different stages: a large positive cell, with deep convection in the North Atlantic and upwelling in the south; a symmetric two-cell circulation, with deep convection in both polar regions and upwelling near the equator; and a large negative cell, with deep convection in the South Atlantic and upwelling in the north. Each state can persist for 0 (10 kyr).

[1]  J. Mélice,et al.  A strategy for frequency spectra of quaternary climate records , 1991 .

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

[3]  Uwe Mikolajewicz,et al.  Experiments with an OGCM on the cause of the Younger Dryas , 1989 .

[4]  M. Hollins,et al.  Does the central human retina stretch during accommodation? , 1974, Nature.

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

[6]  Thomas F. Stocker,et al.  A Zonally Averaged, Coupled Ocean-Atmosphere Model for Paleoclimate Studies , 1992 .

[7]  The role of ocean-atmosphere reorganizations in glacial cycles , 1989 .

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

[9]  Arnold L. Gordon,et al.  Interocean Exchange of Thermocline Water , 1986 .

[10]  E. Sarachik,et al.  The Role of Mixed Boundary Conditions in Numerical Models of the Ocean's Climate , 1991 .

[11]  J. Marotzke,et al.  Instability and multiple steady states in a meridional-plane model of the thermohaline circulation , 1988 .

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

[13]  L. Mysak,et al.  Sea-ice anomalies observed in the Greenland and Labrador seas during 1901–1984 and their relation to an interdecadal Arctic climate cycle , 1990 .

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

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

[16]  T. Stocker,et al.  Climatic fluctuations on the century time scale: A review of high-resolution proxy data and possible mechanisms , 1992 .

[17]  Andrew J. Weaver,et al.  Freshwater flux forcing of decadal and interdecadal oceanic variability , 1991, Nature.

[18]  T. Stocker,et al.  A Zonally Averaged Ocean Model for the Thermohaline Circulation. Part II: Interocean Circulation in the Pacific-Atlantic Basin System , 1991 .

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

[20]  D. B. Preston.  Spectral Analysis and Time Series , 1983 .

[21]  W. Broecker,et al.  The chronology of the last deglaciation: implications to the cause of the Younger Dryas event , 1988 .

[22]  Michael Ghil,et al.  High‐frequency paleovariability in climate and CO2 levels from Vostok Ice Core Records , 1991 .

[23]  L. Mysak,et al.  Arctic Sea‐Ice extent and anomalies, 1953–1984 , 1989 .

[24]  Thomas F. Stocker,et al.  Sensitivities of a zonally averaged global ocean circulation model , 1992 .

[25]  R. Dickson,et al.  The great salinity anomaly in the northern North Atlantic 1968-1982 , 1988 .

[26]  Henry Stommel.  Thermohaline Convection with Two Stable Regimes of Flow , 1961 .

[27]  Uwe Mikolajewicz,et al.  Internal secular variability in an ocean general circulation model , 1990 .

[28]  Barry Saltzman,et al.  Three basic problems of paleoclimatic modeling: a personal perspective and review , 1990 .

[29]  Carl Wunsch,et al.  Two transatlantic sections: meridional circulation and heat flux in the subtropical North Atlantic Ocean , 1985 .

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

[31]  J. Duplessy,et al.  Paleoclimatic variability at frequencies ranging from 1 cycle per 10 000 years to 1 cycle per 1000 years: Evidence for nonlinear behaviour of the climate system , 1988 .

[32]  Edward A. Boyle,et al.  North Atlantic thermohaline circulation during the past 20,000 years linked to high-latitude surface temperature , 1987, Nature.