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The most prominent feature of the time-mean global meridional overturning circulation (MOC) is the Atlantic MOC (AMOC). However, interannual variability of the global MOC is shown here to be dominated by Pacific MOC (PMOC) variability over the full depth of the ocean at most latitudes. This dominance of interannual PMOC variability is robust across modern climate models and an observational state estimate. PMOC interannual variability has large-scale organization, its most prominent feature being a cross-equatorial cell spanning the tropics. Idealized experiments show that this variability is almost entirely wind driven. Interannual anomalies of zonal mean zonal wind stress produce zonally integrated Ekman transport anomalies that are larger in the Pacific Ocean than in the Atlantic Ocean, simply because the Pacific is wider than the Atlantic at most latitudes. This contrast in Ekman transport variability implies greater variability in the near-surface branch of the PMOC when compared with the near-surface branch of the AMOC. These near-surface variations in turn drive compensating flow anomalies below the Ekman layer. Because the baroclinic adjustment time is longer than a year at most latitudes, these compensating flow anomalies have baroclinic structure spanning the full depth of the ocean. Additional analysis reveals that interannual PMOC variations are the dominant contribution to interannual variations of the global meridional heat transport. There is also evidence of interaction between interannual PMOC variability and El Niño–Southern Oscillation.

Using a set of experiments with an eddy-parameterizing ocean model, it is found that the strength of the Atlantic meridional overturning circulation (AMOC) intensifies with the decrease of the density-dependent mesoscale eddy transfer. However, the intensification is weaker than that suggested by simple scaling relationships previously applied. Perturbing the model control sea surface temperature (SST) to mimic its change in response to doubling of CO2, it is shown that the associated ocean heat uptake (OHU) increases and penetrates deeper with the decrease of the mesoscale eddy transfer. It is shown that the OHU correlates with the AMOC strength, and both these quantities are affected by the mesoscale eddy transfer. Passive tracer experiments in the ocean model provide a possible explanation for the finding in coupled-model climate simulations that the ocean heat uptake efficiency (OHUE) increases with the AMOC strength and decreases with the eddy energy generated from the mean state. It is also found that the OHU in the SST-perturbation experiments scales with the net downward advection of heat. The contribution of the AMOC to the downward heat flux is illustrated using a streamfunction in depth–temperature space.

The ocean’s meridional overturning circulation is closed by the upwelling of dense, carbon-rich waters to the surface of the Southern Ocean. It has been proposed that upwelling in this region is driven by strong westerly winds, implying that the intensification of Southern Ocean winds in recent decades may have enhanced the rate of upwelling, potentially affecting the global overturning circulation. However, there is no consensus on the sensitivity of upwelling to winds or on the nature of the connection between Southern Ocean processes and the global overturning circulation. In this study, the sensitivity of the overturning circulation to changes in Southern Ocean westerly wind stress is investigated using an eddy-permitting ocean–sea ice model. In addition to a suite of standard circulation metrics, an energy analysis is used to aid dynamical interpretation of the model response. Increased Southern Ocean wind stress enhances the upper cell of the overturning circulation through creation of available potential energy in the Southern Hemisphere, associated with stronger upwelling of deep water. Poleward shifts in the Southern Ocean westerlies lead to a complicated transient response, with the formation of bottom water induced by increased polynya activity in the Weddell Sea and a weakening of the upper overturning cell in the Northern Hemisphere. The energetic consequences of the upper overturning cell response indicate an interhemispheric connection to the input of available potential energy in the Northern Hemisphere.

We present ocean model sensitivity experiments aimed at separating the influence of the projected changes in the “thermal” (near-surface air temperature) and “wind” (near-surface winds) forcing on the patterns of sea level and ocean heat content. In the North Atlantic, the distribution of sea level change is more due to the “thermal” forcing, whereas it is more due to the “wind” forcing in the North Pacific; in the Southern Ocean, the “thermal” and “wind” forcing have a comparable influence. In the ocean adjacent to Antarctica the “thermal” forcing leads to an inflow of warmer waters on the continental shelves, which is somewhat attenuated by the “wind” forcing. The structure of the vertically integrated heat uptake is set by different processes at low and high latitudes: at low latitudes it is dominated by the heat transport convergence, whereas at high latitudes it represents a small residual of changes in the surface flux and advection of heat. The structure of the horizontally integrated heat content tendency is set by the increase of downward heat flux by the mean circulation and comparable decrease of upward heat flux by the subgrid-scale processes; the upward eddy heat flux decreases and increases by almost the same magnitude in response to, respectively, the “thermal” and “wind” forcing. Regionally, the surface heat loss and deep convection weaken in the Labrador Sea, but intensify in the Greenland Sea in the region of sea ice retreat. The enhanced heat flux anomaly in the subpolar Atlantic is mainly caused by the “thermal” forcing.

We estimate changes in the historical ocean carbon sink and their uncertainty using an ocean biogeochemical model driven with wind forcing from six different reanalyses and using two different eddy parameterization schemes. First, we quantify wind-induced changes over the extended period from 1871 to 2010 using the 20th Century Reanalysis winds. Consistent with previous shorter-term studies, we find that the wind changes act to reduce the ocean carbon sink, but the wind-induced trends are subject to large uncertainties. One major source of uncertainty is the parameterization of mesoscale eddies in our coarse resolution simulations. Trends in the Southern Ocean residual meridional overturning circulation and the globally integrated surface carbon flux over 1950 to 2010 are about 2.5 times smaller when using a variable eddy transfer coefficient than when using a constant coefficient in this parameterization. A second major source of uncertainty arises from disagreement on historical wind trends. By comparing six reanalyses over 1980 to 2010, we show that there are statistically significant differences in estimated historical wind trends, which vary in both sign and magnitude amongst the products. Through simulations forced with these reanalysis winds, we show that the influence of historical wind changes on ocean carbon uptake is highly uncertain, and the resulting trends depend on the choice of surface wind product.

It has been estimated that much of the wind energy input to the ocean general circulation is removed by mesoscale eddies via baroclinic instability. While the fate of this energy remains a subject of research, arguments have been presented suggesting that a fraction of it may get transferred to lee waves that, upon breaking, result in bottom-enhanced diapycnal mixing. Here the authors propose several parameterizations of this process and explore their impact in a low-resolution ocean–climate model, focusing on their impact on the abyssal meridional overturning circulation (MOC) of Antarctic Bottom Water. This study shows that, when the eddy energy is allowed to maintain diapycnal mixing, the abyssal MOC generally intensifies with increasing wind energy input to the ocean. In such a case, the whole system is driven by the wind: wind steepens isopycnals and generates eddies, and the (parameterized) eddies generate small-scale mixing, driving the MOC. It is also demonstrated that if the model diapycnal diffusivity, eddy transfer coefficient, and surface climate are decoupled from the winds, then stronger wind stress in the Southern Ocean may lead to a weaker MOC in the abyss—in line with previous results. A simple scaling theory, describing the response of the abyssal MOC strength to wind energy input, is developed, providing a better insight on the numerical results.

The North Atlantic climate response to the catastrophic drainage of proglacial Lake Agassiz into the Labrador Sea is analyzed with coarse and ocean eddy-permitting versions of a global coupled climate model. The North Atlantic climate response is qualitatively consistent in that a large-scale cooling is simulated regardless of the model resolution or region of freshwater discharge. However, the magnitude and duration of the North Atlantic climate response is found to be sensitive to model resolution and the location of freshwater forcing. In particular, the long-term entrainment of freshwater along the boundary at higher resolution and its gradual, partially eddy-driven escape into the interior leads to low-salinity anomalies persisting in the subpolar Atlantic for decades longer. As a result, the maximum decline of the Atlantic meridional overturning circulation (AMOC) and the ocean meridional heat transport (MHT) is amplified by about a factor of 2 at ocean eddy-permitting resolution, and the recovery is delayed relative to the coarse grid model. This, in turn, increases the long-term cooling in the high-resolution simulations. A decomposition of the MHT response reveals an increased role for transients and the horizontal mean component of MHT at higher resolution. With fixed wind stress curl, it is a stronger response of bottom pressure torque to the freshwater forcing at higher resolution that leads to a larger anomaly of the depth-integrated circulation.

This study uses a global ocean eddy-permitting climate model to explore the export of abyssal water from the Southern Ocean and its sensitivity to projected twenty-first-century poleward-intensifying Southern Ocean wind stress. The abyssal flow pathways and transport are investigated using a combination of Lagrangian and Eulerian techniques. In an Eulerian format, the equator- and poleward flows within similar abyssal density classes are increased by the wind stress changes, making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework. Lagrangian particle analyses are used to identify the major export pathways of Southern Ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around Antarctica in response to the wind forcing. Both the Lagrangian particle and Eulerian analyses identify transients as playing a key role in the abyssal export of water from the Southern Ocean. Wind-driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the Southern Ocean barotropic circulation.

The meridional ocean heat transport (MOHT), its seasonal variability, and projected changes simulated by the second generation Canadian Earth System Model (CanESM2) are presented. The global mean MOHT is within the uncertainty of the observational estimates. However, a correct simulation of the MOHT for individual ocean basins is more challenging, and the Atlantic MOHT south of 30°N is underestimated. The partitioning of the MOHT into the overturning and gyre components is generally consistent with such partitioning in an observationally optimized ocean model. At low latitudes, the time-mean MOHT is dominated by its overturning component, whereas in the Southern Ocean and, especially, in the subpolar North Atlantic, it is the gyre component that plays a more important role. In the projected warmer climates, CanESM2 simulates a weakening of the poleward MOHT essentially in both hemispheres. The projected MOHT changes are largely determined by the overturning component, except in the subpolar Atlantic where it is dominated by the gyre component. Consistent with (the limited number of) previous studies, the seasonal variability of the MOHT is large and is mostly driven by the seasonal variability of the meridional Ekman transport. In the simulated warmer climates, the seasonal cycle of the MOHT is projected to change, mostly in the tropics and also in the Southern Hemisphere midlatitudes. The eddy contribution to the MOHT is broadly consistent with that in the observationally optimized eddy-permitting model. However, in the tropics a significant fraction of the eddy energy is converted back to the mean circulation, and the heat transports due to the parameterized and permitted eddies differ.

Several recent studies have shown that ocean western boundaries are the primary regions of eddy energy dissipation. Globally, the eddy energy sinks have been estimated to integrate to about 0.2 TW. This is a sizable fraction of the tidal energy dissipation in the deep oceanic interior, estimated at about 1.0 TW and contributing to diapycnal mixing. The authors conduct sensitivity experiments with an ocean general circulation model assuming that the eddy energy is scattered into high-wavenumber vertical modes, resulting in energy dissipation and locally enhanced diapycnal mixing. When only the tidal energy dissipation maintains diapycnal mixing, the overturning circulation, and stratification in the deep ocean are too weak. With the addition of the eddy dissipation, the deep-ocean thermal structure becomes closer to that observed and the overturning circulation and stratification in the abyss become stronger. Furthermore, the mixing associated with the eddy dissipation can, on its own, drive a relatively strong overturning. The stratification and overturning in the deep ocean are sensitive to the vertical structure of diapycnal mixing. When most of this energy dissipates within 300 m above the bottom, the abyssal overturning and stratification are too weak. Allowing for the dissipation to penetrate higher in the water column, such as suggested by recent observations, results in stronger stratification and meridional circulation. Zonal circulation is also affected. In particular, the Drake Passage transport becomes closer to its observational estimates with the increase in the vertical scale for turbulence above topography. Consistent with some theoretical models, the Drake Passage transport increases with the increase in the mean upper-ocean diffusivity.

Changes in the Southern Ocean lower-limb overturning circulation are analyzed using a set of climate models. In agreement with some recently developed theoretical models, it is found that the overturning can be strongly affected by winds. In particular, the simulated strengthening of large-scale southward transport in the abyss is explicitly driven by zonal wind stress. However, there is a considerable range among the climate models in their projected changes of Southern Ocean wind stress. Furthermore, the strengthening of large-scale southward transport tends to be compensated by eddy-induced northward flows in the abyss, particularly at eddy-permitting resolution. As a result, the net Antarctic Bottom Water (AABW) export may only be weakly affected. However, none of the models considered accounts for the possibility that a fraction of the eddy kinetic energy may be converted to diapycnal mixing. If this were the case, the presented energetic arguments suggest that stronger Southern Ocean winds would result in a stronger AABW transport.

Four versions of the same global climate model, one with horizontal resolution of 1.8° × 3.6° and three with 0.2° × 0.4°, are employed to evaluate the role of ocean bottom topography and viscosity on the spatial structure of the deep circulation. This study is motivated by several recent observational studies that find that subsurface floats injected near the western boundary of the Labrador Sea most often do not continuously follow the deep western boundary current (DWBC), in contrast to the traditional view that the deep water formed in the North Atlantic predominantly follows the DWBC. It is found that, with imposed large viscosity values, the model reproduces the traditional view. However, as viscosity is reduced and the model bathymetry resolution increased, much of the North Atlantic Deep Water (NADW) separates from the western boundary and enters the low-latitude Atlantic via interior pathways distinct from the DWBC. It is shown that bottom pressure torques play an important role in maintaining these interior NADW outflows.

Recent results based on models using prescribed surface wind stress forcing have suggested that the net freshwater transport Σ by the Atlantic meridional overturning circulation (MOC) into the Atlantic basin is a good indicator of the multiple-equilibria regime. By means of a coupled climate model of intermediate complexity, this study shows that this scalar Σ cannot capture the connection between the properties of the steady state and the impact of the wind stress feedback on the evolution of perturbations. This implies that, when interpreting the observed value of Σ, the position of the present-day climate is systematically biased toward the multiple-equilibria regime. The results show, however, that the stabilizing influence of the wind stress feedback on the MOC is restricted to a narrow window of freshwater fluxes, located in the vicinity of the state characterized by a zero freshwater flux divergence over the Atlantic basin. If the position of the present-day climate is farther away from this state, then wind stress feedbacks are unable to exert a persistent effect on the modern MOC. This is because the stabilizing influence of the shallow reverse cell situated south of the equator during the off state rapidly dominates over the destabilizing influence of the wind stress feedback when the freshwater forcing gets stronger. Under glacial climate conditions by contrast, a weaker sensitivity with an opposite effect is found. This is ultimately due to the relatively large sea ice extent of the glacial climate, which implies that, during the off state, the horizontal redistribution of fresh waters by the subpolar gyre does not favor the development of a thermally direct MOC as opposed to the modern case.

Four versions of the same global climate model, with horizontal resolution ranging from 1.8° × 3.6° to 0.2° × 0.4°, are employed to evaluate the resolution dependence of the Southern Ocean meridional overturning circulation. At coarse resolutions North Atlantic Deep Water tends to upwell diabatically at low latitudes, so that the Southern Ocean is weakly coupled with the rest of the ocean. As resolution increases and eddy effects become less parameterized the interior circulation becomes more adiabatic and deep water increasingly upwells by flowing along isopycnals in the Southern Ocean, despite each model having the same vertical diffusivity profile. Separating the overturning circulation into mean and eddy-induced components demonstrates that both the permitted and the parameterized eddies induce overturning cells in the Southern Ocean with mass fluxes across mean isopycnals. It is found that for some density classes the transformation rate derived from surface buoyancy fluxes can provide a proxy for the net meridional transport in the upper Southern Ocean. Changes in the Southern Ocean overturning in response to poleward-intensifying Southern Hemisphere winds concomitant with increasing atmospheric CO2 through the twenty-first century are also investigated. Results suggest that the circulation associated with the formation of Antarctic Intermediate Water is likely to strengthen, or stay essentially unchanged, rather than to slow down.

A climate model is used to study the climatic impact of the stress exerted on the ocean by the atmosphere. When this stress is set to zero everywhere, the climate becomes much colder, with global-mean near-surface air temperature dropping from 14.8° to 6.1°C. The largest temperature decrease occurs in high latitudes, where sea ice advances equatorward to 40° of latitude. Many of these changes are induced by the changes in the oceanic circulation. In particular, with momentum flux set to zero, the meridional transport of buoyancy in the ocean, including that fraction often associated with the buoyancy-driven circulation, essentially vanishes and, hence, so does much of the surface heat flux. Vertical transport of buoyancy in the ocean is also strongly affected. In addition, the model suggests that the flux of momentum to the ocean has a profound indirect influence on the transport of latent heat. However, the total radiative flux entering the planet at low and midlatitudes does not change much. Instead, the net energy transport across 40°S increases, whereas that across 40°N decreases. The poleward energy transport in the atmosphere increases at midlatitudes in both hemispheres, whereas the oceanic heat transport decreases most strongly in the Northern Hemisphere. The climate becomes colder in both hemispheres, which is not easy to infer from the meridional transport of energy either by the climate system as a whole or by its individual components. Furthermore, the model suggests that it is the wind stress driving the midlatitude oceans—that is, where the oceanic heat transport accounts for only a very tiny fraction of the total poleward energy transport by the climate system, which is of more importance for maintaining the mean position of sea ice edge and, hence, much of the global climate.

Observations indicate that intense mixing in the ocean is localized above complex topography and near the boundaries. Model experiments presented here illustrate that accounting for this fact can be important. In particular, it is found that in the case of localized mixing, the rate of overturning circulation is proportional to the net rate of generation of potential energy by the vertical mixing, linked to the net downward heat diffusion, rather than to the value of the mean vertical diffusivity coefficient. Furthermore, it is shown that two climate models, having the same vertical profile of diffusivity but differing in their distribution (horizontally uniform versus topography/boundary intensified) can simulate significantly different meridional oceanic circulations, vertical heat transfers, and responses of simulated climate to atmospheric CO2 increase. This is found for relatively large [O(1.0 cm2 s−1)] horizontal-mean values of vertical diffusivity in the pycnocline. However, in cases of relatively small [O(0.1 cm2 s−1)] mean diffusivity in the pycnocline, the simulated integral quantities such as meridional mass and heat transports do not depend much on the details of the mixing distribution. Even so, it is found that the deep western boundary currents are more localized near the boundaries in the case of topography/boundary-intensified mixing; also, the stratification in the deep ocean is set through the localized regions of intense vertical mixing. In addition, it is shown that reconciling the observed basin-mean values of diffusivity in the abyssal ocean of O(10 cm2 s−1) with realistic stratification can be problematic, unless the regions of enhanced vertical mixing are localized.

A global, oceanic teleconnection of salinity, meridional overturning circulation (MOC), and climate of the North Atlantic and North Pacific is proposed. Simulations with a global climate model show that an extraction of freshwater from the Pacific results not only in an increase of salinity there, but also in a decrease of salinity in the Atlantic. As a result, a Pacific MOC develops while the Atlantic MOC collapses without freshwater perturbation in the Atlantic. Similarly, an input of freshwater to the Atlantic leads not only to a decrease of salinity there, but also to an increase of salinity in the Pacific. The Atlantic MOC collapses, whereas the Pacific MOC develops without freshwater perturbation in the Pacific. The mechanism behind this antiphase Atlantic– Pacific relationship is the positive feedback between ocean circulation and salinity contrasts, originally proposed by Stommel to operate between low and high latitudes. Here the authors show that the same mechanism operates on the Atlan...

Different climate models simulate different behavior of the Atlantic meridional overturning circulation (MOC) under the same global warming scenario. We propose a plausible explanation for this and argue that a proper simulation of the present-day climate in the subpolar North Atlantic is important. This is illustrated using results from idealized global warming experiments, in which both the radiative forcing scenario and the model employed are the same, with the only major difference being the initial subpolar North Atlantic climate. The initial conditions are made progressively colder, with more extensive sea-ice cover in the northern North Atlantic.The key result is that starting from conditions which are too cold in the North Atlantic and with sea-ice that is too extensive leads to an MOC that is more stable to the radiative forcing. Furthermore, under considerably underestimated sea surface temperatures in subpolar regions, the MOC can even intensify. A reduction of freshwater flux associated with the reduction of sea-ice melt is shown to be important for such unusual behavior of the MOC. Other mechanisms are also considered, but not deemed as important in explaining published inter-model differences.

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.

A close link between the formation of global-scale water masses, such as North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW), and two stable modes of the thermohaline circulation (THC) is investigated in a coupled model. In the upper 2–3 km of the Atlantic, the THC modes are characterized by meridional overturning circulations of opposite sign, with either a dominance of the AAIW cell over the NADW cell (“off” THC mode) or vice versa (“on” THC mode). A transition between these THC modes is controlled by the relationship between the densities in the source regions of formation of AAIW and NADW water masses. This is shown by applying a freshwater perturbation in the region of enhanced AAIW formation in the Southern Ocean to obtain a hysteresis loop of the NADW circulation. Transitions between the two modes of the THC occur when the densities in the source regions of AAIW and NADW become comparable to each other.

We combine reconstructions, climate model simulations and a conceptual model of glacial climate change on millennial time scales to examine the relation between the high latitudes of both hemispheres. A lead-lag analysis of synchronised proxy records indicates that temperature changes in Greenland preceded changes of the opposite sign in Antarctica by 400– 500 yr . A composite record of the Dansgaard–Oeschger events shows that rapid warming (cooling) in Greenland was followed by a slow cooling (warming) phase in Antarctica. The amplitudes, rates of change and time lag of the interhemispheric temperature changes found in the reconstructions are in excellent agreement with climate model simulations in which the formation of North Atlantic Deep Water is perturbed. The simulated time lag between high northern and southern latitudes is mainly determined by the slow meridional propagation of the signal in the Southern Ocean. Our climate model simulations also show that increased deep water formation in the North Atlantic leads to a reduction of the Antarctic Circumpolar Current through diminishing meridional density gradients in the Southern Ocean. We construct a simple conceptual model of interhemispheric Dansgaard–Oeschger oscillations. This model explains major features of the recorded temperature changes in Antarctica as well as the general shape of the north–south phase relation found in the observations including a broad peak of positive correlations for a lead of Antarctica over Greenland by 1000– 2000 yr . The existence of this peak is due to the regularity of the oscillations and does not imply a southern hemisphere trigger mechanism, contrary to previous suggestions. Our findings thus further emphasise the role of the thermohaline circulation in millennial scale climate variability.

A simple interhemispheric ocean model is used to examine the sensitivity of water sinking in the northern hemisphere to the equator-to-pole density contrast. The model assumes that the sinking is compensated by upwelling in both the low latitude ocean and the Southern Ocean. We compare two vertical mixing schemes: one with a fixed vertical diffusivity and another with fixed mixing energy. The latter case implies that the vertical diffusivity depends on the simulated oceanic circulation. It is shown that when Southern Ocean upwelling is controlled only by northward Ekman transport, the rate of deep water formation has an opposite dependence on the equator-to-pole density contrast between the two vertical mixing schemes. However, when Southern Ocean upwelling is controlled by both Ekman transport and strong enough eddy-induced transport across the Antarctic Circumpolar Current, the two mixing schemes give qualitatively similar dependence: the rate of water sinking increases with the equator-to-pole density contrast, regardless of whether the diffusivity or the mixing energy is held fixed. It is suggested that the ACC eddies and vertical mixing jointly control the response of the overturning circulation to changes in the equator-to-pole density contrast. DOI: 10.1034/j.1600-0870.2003.201518.x

[1] The mild climate of northern Europe is thought to be in part maintained by the deep water formation in the northern North Atlantic and the associated meridional overturning circulation. It has been argued that this circulation is controlled by the wind stress in the Southern Ocean. Using a coupled climate model it is shown that the subpolar Atlantic wind stress also plays an important role. A partial or a complete suppression of this wind stress results in the low salinity Arctic waters filling the subpolar Atlantic interior. As a result, the regions of deep water sinking shift southward and the associated overturning circulation, convective activity and oceanic heat loss in the northern North Atlantic strongly weaken. The sea ice advances southward and the North Atlantic climate, including over much of Europe, becomes colder.

[1] Observations indicate that turbulent mixing is enhanced in the major fronts of the Antarctic Circumpolar Current (ACC), penetrating through much of the water column. Here we employ a simple representation of this process in a global climate model to evaluate its potential impact on the ocean's overturning circulation. Two effects are obtained. First, the frontally-intensified mixing in the Southern Ocean increases the transformation of NADW to UCDW. Second, by intensifying the dense water upwelling in the south, the vertical mixing in the ACC fronts returns a fraction of the northward flowing AABW/LCDW as UCDW, consistent with recent observational results; the corresponding abyssal overturning weakens. The results are interpreted using a single-basin model.

[1] The traditional view is that widespread discharge of freshwater into the North Atlantic leads to a reduction of the Atlantic Meridional Overturning Circulation (AMOC), a concomitant reduction of poleward heat transport and a cooling of the climate of the North Atlantic. Here we report upon coupled atmosphere-ocean general circulation model experiments that suggest a more non-uniform response of the North Atlantic climate to large freshwater perturbations. We show that in a cold climate with extensive sea ice coverage in the Labrador Sea, a massive discharge of freshwater along coastal boundaries leads to an anomalous warming in the western North Atlantic. The warming persist despite a significant weakening of the AMOC and its associated heat transport. It is maintained by major reorganizations of the large-scale wind field, oceanic circulation and convection in the subpolar Atlantic.

[1] The dependence on the mean climate state of the response of the Atlantic meridional overturning circulation (AMOC) is investigated in 17 increasing greenhouse gas experiments with different initial conditions. The AMOC declines in all experiments by 15% to 31%, with typically the largest declines in those experiments with the strongest initial AMOC. In all cases, changes in surface heat fluxes, rather than changes in surface freshwater fluxes, are the dominant cause for the transient AMOC decrease. Surface freshwater fluxes actually switch from reducing the transient AMOC decrease, for low values of atmospheric CO2, to reinforcing the transient AMOC decrease, for higher values of atmospheric CO2. In addition, we find that due to changes in the strengths of feedbacks associated with water vapour and snow/sea ice, the climate sensitivity and transient climate response of the UVic model strongly depends on the mean climate state.

[1] The meridional overturning circulation (MOC), associated with deep water formation in the North Atlantic, is thought to play an important part in the transport of heat by the climate system. According to observational estimates, and simple scaling arguments, the transport of heat by the MOC is ∼1 PW. However, in one recent modeling study (Boccaletti et al., 2005), this estimate is reduced to 0.4 PW. Using a model with higher resolution, the issue of heat transport partitioning between the circulation in the upper ocean and that due to the flows in the deep ocean is revisited here. It is found that, out of about 3 PW of heat entering the low-latitude ocean through the surface, approximately 65% is transported poleward by the directly wind-driven circulations in the upper ocean. However, 0.85 ± 0.35 PW of the peak northward heat transport at 15–20°N, can be associated with the MOC. In the Southern Hemisphere, the net effect of the deep flows on the heat transport is weaker, so that the corresponding maximum at 15–20°S is essentially due to the flows in the upper ocean.

[1] Climate model simulations are used to demonstrate that there is not a simple link between the mechanical energy supply and heat transport in the Atlantic. Setting the flux of momentum into the ocean to zero between 35–60°S reduces the net mechanical energy input by more than a factor of two without much effect on the heat transport in the Atlantic. The strong westerly winds in the Southern Ocean are found to be more important for the circulation of bottom water and for localizing the upwelling of deep water around Antarctica. Furthermore, it is illustrated that the hydrological cycle plays a significant role in maintaining the global ocean circulation and the associated heat transport in the Atlantic, given sufficient mechanical energy to support the diapycnal mixing. A reduction of the meridional moisture transport to the Southern Ocean by a factor of two reduces heat transport in the Atlantic by about the same factor.

Received 5 May 2003; accepted 18 June 2003; published 24 July 2003. [1] Numerical simulations with a climate model of intermediate complexity are used to illustrate the effect of meridional moisture transport in the Southern Hemisphere mid-latitudes on the meridional overturning circulation (MOC) and heat transport in the Atlantic. A novel feature of the model is a diapycnal mixing scheme in the ocean, which ensures low values of diffusivity (about 10 � 5 m 2 s � 1 ) in the pycnocline. It is shown that the Atlantic MOC, northward oceanic heat transport and the associated air-sea heat flux anomalies are all proportional to the southward moisturetransport from subtropical tosubpolarregions inthe Southern Hemisphere. The effect of the intensified ocean circulation on sea surface temperature and salinity is also illustrated. INDEX TERMS: 4532 Oceanography: Physical: General circulation; 4516 Oceanography: Physical: Eastern boundary currents; 4283 Oceanography: General: Water masses. Citation: Saenko, O. A., A. J. Weaver, and A. Schmittner, Atlantic deep circulation controlled by freshening in the Southern Ocean, Geophys. Res. Lett., 30(14), 1754, doi:10.1029/ 2003GL017681, 2003.

[1] We investigate the role of atmospheric moisture advection in determining Southern Ocean water-mass properties in a coupled climate model. Two sensitivity experiments are presented in which the winds advecting moisture are either reduced or enhanced south of 45°S. It is shown that a variation in meridional winds in the Southern Ocean can generate large anomalies of subsurface potential temperature (T) and salinity (S). A reduction (increase) in southward atmospheric moisture advection tends to be balanced by a corresponding reduction (increase) in northward freshwater transport in Antarctic Intermediate Water. The response of both T and S is larger in the case of reduced southward moisture advection, as this corresponds to an overall reduction of Southern Ocean stratification. The interior ocean T–S response is not always density-compensating, particularly in the Indian and Pacific Oceans. This is unlike zonal wind variations which have a tendency to change T–S in a density-conserving manner. Our study suggests that variability in meridional winds over the Southern Ocean can alter local water-mass formation rates significantly.

The response of the North Atlantic dynamic sea surface height (SSH) and ocean circulation to Greenland Ice Sheet (GrIS) meltwater fluxes is investigated using a high-resolution model. The model is forced with either present-day-like or projected warmer climate conditions. In general, the impact of meltwater on the North Atlantic SSH and ocean circulation depends on the surface climate. In the two major regions of deep water formation, the Labrador Sea and the Nordic Seas, the basin-mean SSH increases with the increase of the GrIS meltwater flux. This SSH increase correlates with the decline of the Atlantic meridional overturning circulation (AMOC). However, while in the Labrador Sea the warming forcing and GrIS meltwater input lead to sea level rise, in the Nordic Seas these two forcings have an opposite influence on the convective mixing and basin-mean SSH (relative to the global mean). The warming leads to less sea-ice cover in the Nordic Seas, which favours stronger surface heat loss and deep mixing, lowering the SSH and generally increasing the transport of the East Greenland Current. In the Labrador Sea, the increased SSH and weaker deep convection are reflected in the decreased transport of the Labrador Current (LC), which closes the subpolar gyre in the west. Among the two major components of the LC transport, the thermohaline and bottom transports, the former is less sensitive to the GrIS meltwater fluxes under the warmer climate. The SSH difference across the LC, which is a component of the bottom velocity, correlates with the long-term mean AMOC rate.