A minimal model for wind- and mixing-driven overturning: threshold behavior for both driving mechanisms

We present a minimal conceptual model for the Atlantic meridional overturning circulation which incorporates the advection of salinity and the basic dynamics of the oceanic pycnocline. Four tracer transport processes following Gnanadesikan in Science 283(5410):2077–2079, (1999) allow for a dynamical adjustment of the oceanic pycnocline which defines the vertical extent of a mid-latitudinal box. At the same time the model captures the salt-advection feedback (Stommel in Tellus 13(2):224–230, (1961)). Due to its simplicity the model can be solved analytically in the purely wind- and purely mixing-driven cases. We find the possibility of abrupt transition in response to surface freshwater forcing in both cases even though the circulations are very different in physics and geometry. This analytical approach also provides expressions for the critical freshwater input marking the change in the dynamics of the system. Our analysis shows that including the pycnocline dynamics in a salt-advection model causes a decrease in the freshwater sensitivity of its northern sinking up to a threshold at which the circulation breaks down. Compared to previous studies the model is restricted to the essential ingredients. Still, it exhibits a rich behavior which reaches beyond the scope of this study and might be used as a paradigm for the qualitative behaviour of the Atlantic overturning in the discussion of driving mechanisms.     

The stability of the MOC as diagnosed from model projections for pre-industrial, present and future climates

The stability of the Atlantic meridional overturning circulation (MOC) is investigated for various climate scenario runs, using data from the CMIP3 archive of coupled atmosphere-ocean models. Apart from atmospheric feedbacks, the sign of the salt flux into the Atlantic basin that is carried by the MOC determines whether the MOC is in the single or multiple equilibria regime. This salt advection feedback is analyzed by diagnosing the freshwater and salt budgets for the combined Atlantic and Arctic basins. Consistent with the finding that almost all coupled climate models recover from hosing experiments, it is found that most models feature a negative salt advection feedback in their pre-industrial climate: freshwater perturbations are damped by this feedback, excluding the existence of a stable off-state for the MOC. All models feature enhanced evaporation over the Atlantic basin in future climates, but for a moderate increase in radiative forcing (B1 and 2 CO₂ scenarios), there is a decrease of the fresh water flux carried by the MOC into the Atlantic (the deficit is made up by increased fresh water transport by the gyre circulation). In this forcing regime the salt advection feedback becomes less negative: for three models from an ensemble of eight it is positive in a 2 CO₂ climate, while two models feature a positive feedback in the pre-industrial climate. For even warmer climates (A1B-equilibrium and 4 CO₂) the salt feedback becomes more negative (damping) again. It is shown that the decrease in northward fresh water transport at 34°S by the MOC (in B1-equilibrium and 2 CO₂) is due to a reduction of the inflow of intermediate waters relative to thermocline waters, associated with a robust shoaling of the MOC in future, warmer climates. In A1B and 4 CO₂ climates northward freshwater transport increases again. The MOC keeps shoaling, but both intermediate and thermocline water masses freshen.     

Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria

It has recently been suggested that the structure and strength of the meridional overturning circulation in the global ocean is governed by the input of mechanical energy to the system by winds and tides. However, it is not clear how this suggestion relates to the existence of multiple equilibria of the meridional overturning circulation, which depends on thermohaline feedbacks and is more consistent with a buoyancy-driven view of the circulation. Both theories have been illustrated by box models in the past (Stommel in Tellus 13:224–230, 1961; Gnanadesikan in Science 283:2077–2079, 1999). Here we incorporate these two theories into a single box model in an attempt to reconcile the roles of mechanical and buoyancy forcing in driving the meridional overturning circulation. The box model has two equilibrium solutions, one with sinking at high northern latitudes as in the present-day Atlantic, and one without. The circulation is mechanically driven, but the northern sinking can be thought of as a release valve which acts as a sink of potential energy when the surface water at high northern latitudes is dense enough to convect. While the source of energy comes from mechanical forcing, the presence or otherwise of multiple equilibria is therefore determined by thermohaline feedbacks. In some areas of parameter space an oscillation between the model’s two circulation regimes occurs, reminiscent of a bipolar seesaw.     

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 (K _h = 15 × 103 m²/s) and small vertical (K _v = 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).     

Two climatic states and feedbacks on thermohaline circulation in an Earth system model of intermediate complexity

The McGill Paleoclimate Model-2 (MPM-2) is employed to study climate–thermohaline circulation (THC) interactions in a pre -industrial climate, with a special focus on the feedbacks on the THC from other climate system components. The MPM-2, a new version of the MPM, has an extended model domain from 90S to 90N, active winds and no oceanic heat and freshwater flux adjustments. In the MPM-2, there are mainly two stable modes for the Atlantic meridional overturning circulation (MOC) under the ‘present-day’ forcing (present-day solar forcing and the pre-industrial atmospheric CO₂ level of 280 ppm). The ‘on’ mode has an active North Atlantic deep water formation, while the ‘off’ mode has no such deep water formation. By comparing the ‘off’ mode climate state with its ‘on’ mode analogue, we find that there exist many large differences between the two climate states, which originate from large changes in the oceanic meridional heat transports. By suppressing or isolating each process associated with a continental ice sheet over North America, sea ice, the atmospheric hydrological cycle and vegetation, feedbacks from these components on the Atlantic MOC are investigated. Sensitivity studies investigating the role of varying continental ice growth and sea ice meridional transport in the resumption of the Atlantic MOC are also carried out. The results show that a fast ice sheet growth and an enhanced southward sea ice transport significantly favor the resumption of the Atlantic MOC in the MPM-2. In contrast to this, the feedback from the atmospheric hydrological cycle is a weak positive one. The vegetation-albedo feedback could enhance continental ice sheet growth and thus could also favor the resumption of the Atlantic MOC. However, before the shut-down of the Atlantic MOC, feedbacks from these components on the Atlantic MOC are very weak.     

A global hybrid coupled model based on atmosphere-SST feedbacks

A global hybrid coupled model is developed, with the aim of studying the effects of ocean-atmosphere feedbacks on the stability of the Atlantic meridional overturning circulation. The model includes a global ocean general circulation model and a statistical atmosphere model. The statistical atmosphere model is based on linear regressions of data from a fully coupled climate model on sea surface temperature both locally and hemispherically averaged, being the footprint of Atlantic meridional overturning variability. It provides dynamic boundary conditions to the ocean model for heat, freshwater and wind-stress. A basic but consistent representation of ocean-atmosphere feedbacks is captured in the hybrid coupled model and it is more than 10 times faster than the fully coupled climate model. The hybrid coupled model reaches a steady state with a climate close to the one of the fully coupled climate model, and the two models also have a similar response (collapse) of the Atlantic meridional overturning circulation to a freshwater hosing applied in the northern North Atlantic.     

The drivers of projected North Atlantic sea level change

Sea level change predicted by the CMIP5 atmosphere–ocean general circulation models (AOGCMs) is not spatially homogeneous. In particular, the sea level change in the North Atlantic is usually characterised by a meridional dipole pattern with higher sea level rise north of 40°N and lower to the south. The spread among models is also high in that region. Here we evaluate the role of surface buoyancy fluxes by carrying out simulations with the FAMOUS low-resolution AOGCM forced by surface freshwater and heat flux changes from CO₂-forced climate change experiments with CMIP5 AOGCMs, and by a standard idealised surface freshwater flux applied in the North Atlantic. Both kinds of buoyancy flux change lead to the formation of the sea level dipole pattern, although the effect of the heat flux has a greater magnitude, and is the main cause of the spread of results among the CMIP5 models. By using passive tracers in FAMOUS to distinguish between additional and redistributed buoyancy, we show that the enhanced sea level rise north of 40°N is mainly due to the direct steric effect (the reduction of sea water density) caused by adding heat or freshwater locally. The surface buoyancy forcing also causes a weakening of the Atlantic meridional overturning circulation, and the consequent reduction of the northward ocean heat transport imposes a negative tendency on sea level rise, producing the reduced rise south of 40°N. However, unlike previous authors, we find that this indirect effect of buoyancy forcing is generally less important than the direct one, except in a narrow band along the east coast of the US, where it plays a major role and leads to sea level rise, as found by previous authors.     

The role of the individual air-sea flux components in CO2-induced changes of the ocean's circulation and climate

 In this study we investigate the role of heat, freshwater and momentum fluxes in changing the oceanic climate and thermohaline circulation as a consequence of increasing atmospheric CO₂ concentration. Two baseline integrations with a fully coupled ocean atmosphere general circulation model with either fixed or increasing atmospheric CO₂ concentrations have been performed. In a set of sensitivity experiments either freshwater (precipitation, evaporation and runoff from the continents) and/or momentum fluxes were no longer simulated, but prescribed according to one of the fully coupled baseline experiments. This approach gives a direct estimate of the contribution from the individual flux components. The direct effect of surface warming and the associated feedbacks in ocean circulation are the dominant processes in weakening the Atlantic thermohaline circulation in our model. The relative contribution of momentum and freshwater fluxes to the total response turned out to be less than 25%, each. Changes in atmospheric water vapour transport lead to enhanced freshwater input into middle and high latitudes, which weakens the overturning. A stronger export of freshwater from the Atlantic drainage basin to the Indian and Pacific ocean, on the other hand, intensifies the Atlantic overturning circulation. In total the modified freshwater fluxes slightly weaken the Atlantic thermohaline circulation. The contribution of the modified momentum fluxes has a similar magnitude, but enhances the formation of North Atlantic deep water. Salinity anomalies in the Atlantic as a consequence of greenhouse warming stem in almost equal parts from changes in net freshwater fluxes and from changes in ocean circulation caused by the surface warming due to atmospheric heat fluxes. Important effects of the momentum fluxes are a poleward shift of the front between Northern Hemisphere subtropical and subpolar gyres and a southward shift in the position of the Antarctic circumpolar current, with a clear signal in sea level. Received: 3 May 1999 / Accepted: 11 December 1999     

The relation of meridional pressure gradients to North Atlantic deep water volume transport in an ocean general circulation model

We use a coarse resolution ocean general circulation model to study the relation between meridional pressure and density gradients in the Southern Ocean and North Atlantic and the Atlantic meridional overturning circulation. In several experiments, we artificially modify the meridional density gradients by applying different magnitudes of the Gent–McWilliams isopycnal eddy diffusion coefficients in the Southern Ocean and in the North Atlantic and investigate the response of the simulated Atlantic meridional overturning to such changes. The simulations are carried out close to the limit of no diapycnal mixing, with a very small explicit vertical diffusivity and a tracer advection scheme with very low implicit diffusivities. Our results reveal that changes in eddy diffusivities in the North Atlantic affect the maximum of the Atlantic meridional overturning, but not the outflow of North Atlantic Deep Water into the Southern Ocean. In contrast, changes in eddy diffusivities in the Southern Ocean affect both the South Atlantic outflow of North Atlantic Deep Water and the maximum of the Atlantic meridional overturning. Results from these experiments are used to investigate the relation between meridional pressure gradients and the components of the Atlantic meridional overturning. Pressure gradients and overturning are found to be linearly related. We show that, in our simulations, zonally averaged deep pressure gradients are very weak between 20°S and about 30°N and that between 30°N and 60°N the zonally averaged pressure grows approximately linearly with latitude. This pressure difference balances a westward geostrophic flow at 30–40°N that feeds the southbound deep Atlantic western boundary current. We extend our analysis to a large variety of experiments in which surface freshwater forcing, vertical mixing and winds are modified. In all experiments, the pycnocline depth, assumed to be the relevant vertical scale for the northward volume transport in the Atlantic, is found to be approximately constant, at least within the coarse vertical resolution of the model. The model behaviour hence cannot directly be related to conceptual models in which changes in the pycnocline depth determine the strength of Atlantic meridional flow, and seems conceptually closer to Stommel’s box model. In all our simulations, the Atlantic overturning seems to be mainly driven by Southern Ocean westerlies. However, the actual strength of the Atlantic meridional overturning is not determined solely by the Southern Ocean wind stress but as well by the density/pressure gradients created between the deep water formation regions in the North Atlantic and the inflow/outflow region in the South Atlantic.     

The stability of the North Atlantic thermohaline circulation in a coupled ocean-atmosphere general circulation model

 The stability of the Atlantic thermohaline circulation against meltwater input is investigated in a coupled ocean-atmosphere general circulation model. The meltwater input to the Labrador Sea is increased linearly for 250 years to a maximum input of 0.625 Sv and then reduced again to 0 (both instantaneously and linearly decreasing over 250 years). The resulting freshening forces a shutdown of the formation of North Atlantic deepwater and a subsequent reversal of the thermohaline circulation of the Atlantic, filling the deep Atlantic with Antarctic bottom water. The change in the overturning pattern causes a drastic reduction of the Atlantic northward heat transport, resulting in a strong cooling with maximum amplitude over the northern North Atlantic and a southward shift of the sea-ice margin in the Atlantic. Due to the increased meridional temperature gradient, the intertropical convergence zone over the Atlantic is displaced southward and the westerlies in the Northern Hemisphere gain strength. We identify four main feedbacks affecting the stability of the thermohaline circulation: the change in the overturning circulation of the Atlantic leads to longer residence times of the surface water in high-northern latitudes, which allows them to accumulate more precipitation and runoff from the continents. As a consequence the stratification in the North Atlantic becomes more stable. This effect is further amplified by an enhanced northward atmospheric water vapour transport, which increases the freshwater input into the North Atlantic. The reduced northward oceanic heat transport leads to colder sea-surface temperatures and an intensification of the atmospheric cyclonic circulation over the Norwegian Sea. The associated Ekman transports cause increased upwelling and increased freshwater export with the East Greenland Current. Both the cooling and the wind-driven circulation changes largely compensate for the effects of the first two feedbacks. The wind-stress feedback destabilizes modes without deep water formation in the North Atlantic, but has been neglected in almost all studies so far. After the meltwater input stops, the North Atlantic deepwater formation resumed in all experiments and the meridional overturning returned within 200 years to a conveyor belt pattern. This happened although the formation of North Atlantic deep water was suppressed in one experiment for more than 300 years and the Atlantic overturning had settled into a circulation pattern with Antarctic bottom water as the only source of deep water. It is a clear indication that cooling and wind-stress feedback are more effective, at least in our model, than advection feedback and increased atmospheric water vapour transport. We conclude that the conveyor belt-type thermohaline circulation seems to be much more stable than hitherto assumed from experiments with simpler models. Received 31 January 1996/Accepted 22 August 1996     

Tropical versus high latitude freshwater influence on the Atlantic circulation

We investigate the model sensitivity of the Atlantic meridional overturning circulation (AMOC) to anomalous freshwater flux in the tropical and northern Atlantic. Forcing in both locations leads to the same qualitative response: a positive freshwater anomaly induces a weakening of the AMOC and a negative freshwater anomaly strengthens the AMOC. Strong differences arise in the temporal characteristics and amplitude of the response. The advection of the tropical anomaly up to the deep water formation area leads to a time delayed response compared to a northern forcing. Thus, in its transient response, the AMOC is less sensitive to a constant anomalous freshwater flux in the tropics than in the north. This difference decreases with time and practically vanishes in equilibrium with constant freshwater forcing. The equilibrium response of the AMOC shows a non-linear dependence on freshwater forcing in both locations, with a stronger sensitivity to positive freshwater forcing. As a consequence, competitive forcing in both regions is balanced when the negative forcing is about 1.5 times larger than the positive forcing. The relaxation time of the AMOC after termination of a freshwater perturbation depends significantly on the AMOC strength itself. A strong overturning exhibits a faster relaxation to its unperturbed state. By means of a set of complementary experiments (pulse-perturbations, constant and stochastic forcing) we quantify these effects and discuss the corresponding time scales and physical processes.     

The role of the Atlantic freshwater balance in the hysteresis of the meridional overturning circulation

We have studied the response of the Atlantic meridional overturning circulation to surface freshwater forcing using an ocean GCM coupled to an energy-moisture-balance atmosphere model. The overturning collapses rapidly when a slowly increasing forcing applied to the North Atlantic passes a positive threshold, and spins up equally quickly when the forcing falls below a negative threshold. This well-known behaviour is referred to as hysteresis because the thresholds in forcing are different for the transitions in opposite directions. However, we argue that the behaviour of the Atlantic salinity is more fundamental than the forcing. Hysteresis as a function of freshwater forcing occurs because the states with North Atlantic overturning on and off each tend to reinforce their associated salinity distributions and inhibit the transition to the other state. During the collapse, the Atlantic becomes less saline because of the import of 80 Sv year of freshwater by ocean transports across 30°S; during the spin-up this freshwater is exported again. We show that qualitatively similar hysteresis behaviour can be produced by perturbing the system without any net freshwater forcing. The salinity flip-flop is associated with the appearance and disappearance of a shallow reverse overturning circulation south of the Equator, which is present while the northern overturning is absent, and may provide the mechanism for the ocean freshwater influx during collapse.     

On the freshwater forcing and transport of the Atlantic thermohaline circulation

The 'conveyor belt' circulation of the Atlantic Ocean transports large amounts of heat northward, acting as a heating system for the northern North Atlantic region. It is widely thought that this circulation is driven by atmospheric freshwater export from the Atlantic catchment region, and that it transports freshwater northward to balance the loss to the atmosphere. Using results from a simple conceptual model and a global circulation model, it is argued here that the freshwater loss to the atmosphere arises mainly in the subtropical South Atlantic and is balanced by northward freshwater transport in the wind-driven subtropical gyre, while the thermohaline circulation transports freshwater southward. It is further argued that the direction of freshwater transport is closely linked to the dynamical regime and stability of the 'conveyor belt': if its freshwater transport is indeed southward, then its flow is purely thermally driven and inhibited by the freshwater forcing. In this case the circulation is not far from Stommel's saddle-node bifurcation, and a circulation state without NADW formation would also be stable. Received: 10 February 1996 / Accepted: 30 May 1996     

Kinetic energy analysis of the response of the Atlantic meridional overturning circulation to CO2-forced climate change

Atmosphere?Cocean general circulation models (AOGCMs) predict a weakening of the Atlantic meridional overturning circulation (AMOC) in response to anthropogenic forcing of climate, but there is a large model uncertainty in the magnitude of the predicted change. The weakening of the AMOC is generally understood to be the result of increased buoyancy input to the north Atlantic in a warmer climate, leading to reduced convection and deep water formation. Consistent with this idea, model analyses have shown empirical relationships between the AMOC and the meridional density gradient, but this link is not direct because the large-scale ocean circulation is essentially geostrophic, making currents and pressure gradients orthogonal. Analysis of the budget of kinetic energy (KE) instead of momentum has the advantage of excluding the dominant geostrophic balance. Diagnosis of the KE balance of the HadCM3 AOGCM and its low-resolution version FAMOUS shows that KE is supplied to the ocean by the wind and dissipated by viscous forces in the global mean of the steady-state control climate, and the circulation does work against the pressure-gradient force, mainly in the Southern Ocean. In the Atlantic Ocean, however, the pressure-gradient force does work on the circulation, especially in the high-latitude regions of deep water formation. During CO₂-forced climate change, we demonstrate a very good temporal correlation between the AMOC strength and the rate of KE generation by the pressure-gradient force in 50?C70°N of the Atlantic Ocean in each of nine contemporary AOGCMs, supporting a buoyancy-driven interpretation of AMOC changes. To account for this, we describe a conceptual model, which offers an explanation of why AOGCMs with stronger overturning in the control climate tend to have a larger weakening under CO₂ increase.     

Assessing the role of North Atlantic freshwater forcing in millennial scale climate variability: a tropical Atlantic perspective

This study analyzes a three-member ensemble of experiments, in which 0.1 Sv of freshwater was applied to the North Atlantic for 100 years in order to address the potential for large freshwater inputs in the North Atlantic to drive abrupt climate change. The model used is the GFDL R30 coupled ocean–atmosphere general circulation model. We focus in particular on the effects of this forcing on the tropical Atlantic region, which has been studied extensively by paleoclimatologists. In response to the freshwater forcing, North Atlantic meridional overturning circulation is reduced to roughly 40% by the end of the 100 year freshwater pulse. Consequently, the North Atlantic region cools by up to 8°C. The extreme cooling of the North Atlantic increases the pole-to-equator temperature gradient and requires more heat be provided to the high latitude Atlantic from the tropical Atlantic. To accommodate the increased heat requirement, the ITCZ shifts southward to allow for greater heat transport across the equator. Accompanying this southward ITCZ shift, the Northeast trade winds strengthen and precipitation patterns throughout the tropical Atlantic are altered. Specifically, precipitation in Northeast Brazil increases, and precipitation in Africa decreases slightly. In addition, we find that surface air temperatures warm over the tropical Atlantic and over Africa, but cool over northern South America. Sea-surface temperatures in the tropical Atlantic warm slightly with larger warm anomalies developing in the thermocline. These responses are robust for each member of the ensemble, and have now been identified by a number of freshwater forcing studies using coupled OAGCMs. The model responses to freshwater forcing are generally smaller in magnitude, but have the same direction, as paleoclimate data from the Younger Dryas suggest. In certain cases, however, the model responses and the paleoclimate data directly contradict one another. Discrepancies between the model simulations and the paleoclimate data could be due to a number of factors, including inaccuracies in the freshwater forcing, inappropriate boundary conditions, and uncertainties in the interpretation of the paleoclimate data. Despite these discrepancies, it is clear from our results that abrupt climate changes in the high latitude North Atlantic have the potential to significantly impact tropical climate. This warrants further model experimentation into the role of freshwater forcing in driving climate change.     

Multimodel analysis on the response of the AMOC under an increase of radiative forcing and its symmetrical reversal

Climate Dynamics - The response of the Atlantic meridional overturning circulation (AMOC) to an increase of radiative forcing (ramp-up) and a subsequent reversal of radiative forcing (ramp-down) is...     

The UVic earth system climate model: Model description,climatology, and applications to past,present and future climates

Abstract

A new earth system climate model of intermediate complexity has been developed and its climatology compared to observations. The UVic Earth System Climate Model consists of a three‐dimensional ocean general circulation model coupled to a thermodynamic/dynamic sea‐ice model, an energy‐moisture balance atmospheric model with dynamical feedbacks, and a thermomechanical land‐ice model. In order to keep the model computationally efficient a reduced complexity atmosphere model is used. Atmospheric heat and freshwater transports are parametrized through Fickian diffusion, and precipitation is assumed to occur when the relative humidity is greater than 85%. Moisture transport can also be accomplished through advection if desired. Precipitation over land is assumed to return instantaneously to the ocean via one of 33 observed river drainage basins. Ice and snow albedo feedbacks are included in the coupled model by locally increasing the prescribed latitudinal profile of the planetary albedo. The atmospheric model includes a parametrization of water vapour/planetary longwave feedbacks, although the radiative forcing associated with changes in atmospheric CO₂ is prescribed as a modification of the planetary longwave radiative flux. A specified lapse rate is used to reduce the surface temperature over land where there is topography. The model uses prescribed present‐day winds in its climatology, although a dynamical wind feedback is included which exploits a latitudinally‐varying empirical relationship between atmospheric surface temperature and density. The ocean component of the coupled model is based on the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model 2.2, with a global resolution of 3.6° (zonal) by 1.8° (meridional) and 19 vertical levels, and includes an option for brine‐rejection parametrization. The sea‐ice component incorporates an elastic‐viscous‐plastic rheology to represent sea‐ice dynamics and various options for the representation of sea‐ice thermodynamics and thickness distribution. The systematic comparison of the coupled model with observations reveals good agreement, especially when moisture transport is accomplished through advection.

Global warming simulations conducted using the model to explore the role of moisture advection reveal a climate sensitivity of 3.0°C for a doubling of CO₂, in line with other more comprehensive coupled models. Moisture advection, together with the wind feedback, leads to a transient simulation in which the meridional overturning in the North Atlantic initially weakens, but is eventually re‐established to its initial strength once the radiative forcing is held fixed, as found in many coupled atmosphere General Circulation Models (GCMs). This is in contrast to experiments in which moisture transport is accomplished through diffusion whereby the overturning is reestablished to a strength that is greater than its initial condition.

When applied to the climate of the Last Glacial Maximum (LGM), the model obtains tropical cooling (30°N‐30°S), relative to the present, of about 2.1°C over the ocean and 3.6°C over the land. These are generally cooler than CLIMAP estimates, but not as cool as some other reconstructions. This moderate cooling is consistent with alkenone reconstructions and a low to medium climate sensitivity to perturbations in radiative forcing. An amplification of the cooling occurs in the North Atlantic due to the weakening of North Atlantic Deep Water formation. Concurrent with this weakening is a shallowing of, and a more northward penetration of, Antarctic Bottom Water.

Climate models are usually evaluated by spinning them up under perpetual present‐day forcing and comparing the model results with present‐day observations. Implicit in this approach is the assumption that the present‐day observations are in equilibrium with the present‐day radiative forcing. The comparison of a long transient integration (starting at 6 KBP), forced by changing radiative forcing (solar, CO₂, orbital), with an equilibrium integration reveals substantial differences. Relative to the climatology from the present‐day equilibrium integration, the global mean surface air and sea surface temperatures (SSTs) are 0.74°C and 0.55°C colder, respectively. Deep ocean temperatures are substantially cooler and southern hemisphere sea‐ice cover is 22% greater, although the North Atlantic conveyor remains remarkably stable in all cases. The differences are due to the long timescale memory of the deep ocean to climatic conditions which prevailed throughout the late Holocene. It is also demonstrated that a global warming simulation that starts from an equilibrium present‐day climate (cold start) underestimates the global temperature increase at 2100 by 13% when compared to a transient simulation, under historical solar, CO₂ and orbital forcing, that is also extended out to 2100. This is larger (13% compared to 9.8%) than the difference from an analogous transient experiment which does not include historical changes in solar forcing. These results suggest that those groups that do not account for solar forcing changes over the twentieth century may slightly underestimate (～3% in our model) the projected warming by the year 2100.     

Revisiting meridional overturing bistability using a minimal set of state variables: stochastic theory

A stochastic analytical model of the Atlantic meridional overturning circulation (AMOC) is presented and tested against climate model data. AMOC stability is characterised by an underlying deterministic differential equation describing the evolution of the central state variable of the system, the average Atlantic salinity. Stability of an equilibrium implies that infinitesimal salinity perturbations are damped, and violation of this requirement yields a range of unoccupied salinity states. The range of states is accurately predicted by the analytical model for a coupled climate model of intermediate complexity. The introduction of climatic noise yields an equation describing the evolution of the probability density function of the state variable, and therefore the AMOC. Given the hysteresis behaviour of the steady AMOC versus surface freshwater forcing, the statistical model is able to describe the variability of the AMOC based on knowledge of the variability in the forcing. The method accurately describes the wandering between AMOC-On and AMOC-Off states in the climate model. The framework presented is a first step in relating the stability of the AMOC to more observable aspects of its behaviour, such as its transient response to variable forcing.     

Response of the South Atlantic circulation to an abrupt collapse of the Atlantic meridional overturning circulation

The South Atlantic response to a collapse of the North Atlantic meridional overturning circulation (AMOC) is investigated in the ECHAM5/MPI-OM climate model. A reduced Agulhas leakage (about 3.1?Sv; 1?Sv?=?10⁶?m³?s^?1) is found to be associated with a weaker Southern Hemisphere (SH) supergyre and Indonesian throughflow. These changes are due to reduced wind stress curl over the SH supergyre, associated with a weaker Hadley circulation and a weaker SH subtropical jet. The northward cross-equatorial transport of thermocline and intermediate waters is much more strongly reduced than Agulhas leakage in relation with an AMOC collapse. A cross-equatorial gyre develops due to an anomalous wind stress curl over the tropics that results from the anomalous sea surface temperature gradient associated with reduced ocean heat transport. This cross-equatorial gyre completely blocks the transport of thermocline waters from the South to the North Atlantic. The waters originating from Agulhas leakage flow somewhat deeper and most of it recirculates in the South Atlantic subtropical gyre, leading to a gyre intensification. This intensification is consistent with the anomalous surface cooling over the South Atlantic. Most changes in South Atlantic circulation due to global warming, featuring a reduced AMOC, are qualitatively similar to the response to an AMOC collapse, but smaller in amplitude. However, the increased northward cross-equatorial transport of intermediate water relative to thermocline water is a strong fingerprint of an AMOC collapse.     

Influence of Low-frequency Solar Forcing on the East Asian Winter Monsoon Based on HadCM3 and Observations

In this study, we investigate the influence of low-frequency solar forcing on the East Asian winter monsoon(EAWM)by analyzing a four-member ensemble of 600-year simulations performed with Had CM3(Hadley Centre Coupled Model,version 3). We find that the EAWM is strengthened when total solar irradiance(TSI) increases on the multidecadal time scale. The model results indicate that positive TSI anomalies can result in the weakening of Atlantic meridional overturning circulation, causing negative sea surface temperature(SST) anomalies in the North Atlantic. Especially for the subtropical North Atlantic, the negative SST anomalies can excite an anomalous Rossby wave train that moves from the subtropical North Atlantic to the Greenland Sea and finally to Siberia. In this process, the positive sea-ice feedback over the Greenland Sea further enhances the Rossby wave. The wave train can reach the Siberian region, and strengthen the Siberian high. As a result, low-level East Asian winter circulation is strengthened and the surface air temperature in East Asia decreases. Overall,when solar forcing is stronger on the multidecadal time scale, the EAWM is typically stronger than normal. Finally, a similar linkage can be observed between the EAWM and solar forcing during the period 1850–1970.