Meridional Ocean Heat Transport Research Articles

Simulated Warming Hole in Paleo-Pacific Oceans

Abstract Both observations and simulations show that under global warming, there exists a warming deficit in the North Atlantic, known as the North Atlantic warming hole (NAWH). Here, we show that a similar warming hole occurs in the subpolar Pacific Ocean of paleoclimate simulations. As the solar constant is increased, the local surface becomes substantially cooler rather than warmer in the subpolar paleo-Pacific Ocean under the land–sea configurations of 70, 90, and 150 million years ago (Ma). The warming hole has a magnitude of ≈3°C and is located in the Northern Hemisphere in 70 and 90 Ma. The warming hole in 150 Ma has a magnitude of ≈1°C and is located in the Southern Hemisphere. Both atmospheric and oceanic processes contribute to trigger the warming hole. For 70- and 90-Ma experiments, atmospheric teleconnection along a great circle from tropics to extratropics intensifies surface winds over subpolar ocean and thereby increases relatively cool seawater transport from high to low latitudes. Meanwhile, global meridional overturning circulation (GMOC) becomes weaker, causing a divergence of the meridional ocean heat transport in the warming hole region. An increasing regional cloud shortwave cooling effect acts to further enhance the warming hole. For 150-Ma experiments, the warming hole is related to the meridional shift of midlatitude jet stream and the weakening of GMOC in the Southern Hemisphere. The strength and phase of the atmospheric teleconnection and the response of GMOC strongly depend on land–sea configuration, resulting in the paleo-Pacific warming hole to occur in special periods only.

Modulation of Western South Atlantic Marine Heatwaves by Meridional Ocean Heat Transport

AbstractMarine heatwaves and cold spells are extreme surface temperature events that have been associated with adverse societal and ecosystem impacts in several regions around the globe. Predicting these events presents a challenge because of their generally short‐lived nature and dependence on air‐sea interactions, both locally and remotely. Here we analyze oceanic propagating features that promote the occurrence of marine heatwaves and cold spells in the western subtropical South Atlantic. The main interannual feature detected from satellite sea level data since 1993 shows a westward propagating zonal pattern with a periodicity of 3–5 years. The pattern has a significant in‐phase correlation with sea surface temperature (SST) anomalies in the western South Atlantic, explaining 82% of the daily extreme warm (90th percentile) and cold (10th percentile) SST anomalies and consequently modulating interannual variations in the intensity and duration of marine heatwave and cold spell events. It is found that meridional oceanic advection plays an important role in the regional heat budget associated with the westward‐propagating mode, modulating the meridional exchange of tropical (warm) and extratropical (cold) waters in the western subtropical South Atlantic region and thereby setting a baseline for temperature extremes on interannual timescales. This propagating mode is well correlated (r > 0.6) with the strength of the meridional overturning circulation at 25°S and 30°S with a lag of approximately 3–9 months. The lagged response provides a potential source of predictability of extreme events in the western South Atlantic.

Simulated dust activity in typical time periods of the past 250 million years

The dust cycle in three typical time periods of the past 250 Myr, namely 240 Ma, 130 Ma and 80 Ma, is studied using the climate model CESM1.2.2 combined with the vegetation model BIOME4 and compared with that of the present day. The atmospheric dust loading obtained for these three periods is 35.9, 9.7 and 3.2 Tg, respectively. In comparison, the present-day dust loading is ∼24 Tg. It is found that the area of subtropical land is the major controlling factor in the variation of dust loading; the climate and the land plant evolution play a relatively minor role in these experiments where continental ice sheets are not considered and the reliability of BIOME4 in simulating vegetation of deep time is still uncertain. Although the simulated atmospheric dust loading in 240 Ma was much higher than that simulated for today, the fraction of dust deposited into the ocean (26%) was much lower due to its supercontinental configuration. Dust was able to cool the local annual mean surface temperature by a maximum of 2 °C in 240 Ma. A warming is obtained in the northern polar region in both 240 Ma and 130 Ma, likely due to strengthened meridional oceanic heat transport rather than due to snow darkening.

Consensus Around a Common Definition of Atlantic Overturning Will Promote Progress

Quantifying the strength of the Atlantic Meridional Overturning Circulation (AMOC) involves separating the northward and southward limbs and calculating their volume transports. The limbs can be distinguished either by depth level or by density class, but recent results have indicated that this choice of coordinate system leads to divergent results, both in terms of the AMOC mean state and its variability. Here, we demonstrate that the AMOC in density coordinates is more informative of the large-scale, three-dimensional AMOC structure, is more closely aligned with the AMOC’s climatic impact via oceanic meridional heat transport, and retains more information about future AMOC pathways than the depth space definition. Adopting a commonly accepted definition of the AMOC in density coordinates will unify a divided literature and promote progress in the field. This commentary thus highlights that the coordinate system used to define the AMOC matters, not only for understanding physical processes and past variations that remain elusive, but also for physically appropriate monitoring of its future evolution.

Evolution of Meridional Heat Transport by Subtropical Western Boundary Currents in a Warming Climate Predicted by High-Resolution Models

Abstract Subtropical western boundary currents (WBCs) are among the most energetic currents in the global circulation system and play an important role in the oceanic meridional heat transport (OHT). Based on nine high-resolution global coupled climate models, this study investigates the change of OHT by subtropical WBCs (WHT) under global warming. We found that WHT in both hemispheres depicts a weakening trend during 1950–2050, primarily caused by the transport change of WBCs. In the Northern Hemisphere, weakening of the Gulf Stream resulting from the slowing AMOC leads to the hemispheric WHT weakening. In the Southern Hemisphere, the WHT decrease is mainly induced by the sharp decline of Agulhas Current transport, associated with the change in wind field in the southern Indian Ocean and Indonesian Throughflow. Compared to the mean flow, the contribution of mesoscale eddies to OHT change is negligible along with WBCs but is important in their extension regions.

Towards two decades of Atlantic Ocean mass and heat transports at 26.5° N.

Continuous measurements of the Atlantic meridional overturning circulation (AMOC) and meridional ocean heat transport at 26.5° N began in April 2004 and are currently available through December 2020. Approximately 90% of the total meridional heat transport (MHT) at 26.5° N is carried by the zonally averaged overturning circulation, and an even larger fraction of the heat transport variability (approx. 95%) is explained by the variability of the zonally averaged overturning. A physically based separation of the heat transport into large-scale AMOC, gyre and shallow wind-driven overturning components remains challenging and requires new investigations and approaches. We review the major interannual changes in the AMOC and MHT that have occurred over the nearly two decades of available observations and their documented impacts on North Atlantic heat content. Changes in the flow-weighted temperature of the Florida Current (Gulf Stream) over the past two decades are now taken into account in the estimates of MHT, and have led to an increased heat transport relative to the AMOC strength in recent years. Estimates of the MHT at 26.5° N from coupled models and various surface flux datasets still tend to show low biases relative to the observations, but indirect estimates based on residual methods (top of atmosphere net radiative flux minus atmospheric energy divergence) have shown recent promise in reproducing the heat transport and its interannual variability. This article is part of a discussion meeting issue 'Atlantic overturning: new observations and challenges'.

Doubling of surface oceanic meridional heat transport by non-symmetry of mesoscale eddies

Oceanic transport of heat by ubiquitous mesoscale eddies plays a critical role in regulating climate variability and redistributing excess heat absorbed by ocean under global warming. Eddies have long been simplified as axisymmetric vortices and their influence on heat transport remains unclear. Here, we combine satellite and drifter data and show that oceanic mesoscale eddies are asymmetric and directionally-dependent, and are controlled by their self-sustaining nature and their dynamical environment. Both the direction and amplitude of eddy-induced he.at fluxes are significantly influenced by eddy’s asymmetry and directional dependence. When the eddy velocity field is decomposed into asymmetric and symmetric components, the eddy kinetic energy exhibits a nearly equal partition between these two components. The total eddy-induced meridional heat flux similarly doubles the heat flux induced by the symmetric components, highlighting the crucial contribution of eddy asymmetry on the magnitude of eddy-induced oceanic heat transport.

The coupled system response to 250 years of freshwater forcing: Last Interglacial CMIP6–PMIP4 HadGEM3 simulations

Abstract. The lig127k-H11 simulation of the Paleoclimate Modelling Intercomparison Project (PMIP4) is run using the HadGEM3-GC3.1 model. We focus on the coupled system response to the applied meltwater forcing. We show here that the coupling between the atmosphere and the ocean is altered in the hosing experiment compared to a Last Interglacial simulation with no meltwater forcing applied. Two aspects in particular of the atmosphere–ocean coupling are found to be affected: Northern Hemisphere (NH) gyre heat transport and Antarctic sea ice area. We apply 0.2 Sv of meltwater forcing across the North Atlantic during a 250-year-long simulation. We find that the strength of the Atlantic Meridional Overturning Circulation (AMOC) is reduced by 60 % after 150 years of meltwater forcing, with an associated decrease of 0.2 to 0.4 PW in meridional ocean heat transport at all latitudes. The changes in ocean heat transport affect surface temperatures. The largest increase in the meridional surface temperature gradient occurs between 40–50∘ N. This increase is associated with a strengthening of 20 % in 850 hPa winds. The jet stream intensification in the Northern Hemisphere in return alters the temperature structure of the ocean by increasing the gyre circulation at the mid-latitudes and the associated heat transport by +0.1–0.2 PW, and it decreases the gyre circulation at high latitudes with a decrease of ocean heat transport of −0.2 PW. The changes in meridional surface temperature and pressure gradients cause the Intertropical Convergence Zone (ITCZ) to move southward, leading to stronger westerlies and a more positive Southern Annual Mode (SAM) in the Southern Hemisphere (SH). The positive SAM influences sea ice formation, leading to an increase in Antarctic sea ice.

Phase-Locked Impact of the 11-Year Solar Cycle on Tropical Pacific Decadal Variability

Abstract As an important external forcing, the effect of the 11-yr solar cycle on the tropical Pacific decadal variability is an interesting question. Here, we systematically investigate the phase-locking of the atmosphere and ocean covariations to the solar cycle in the tropical Pacific and propose a new mechanism to explain these decadal covariations. In both observation/reanalysis datasets and a solar cycle forced sensitivity experiment (named the SOL experiment), the ocean heat content anomalies (OHCa; 300 m) resemble a La Niña–like pattern in the solar cycle ascending phase, and the Walker circulation shifts westward. In the declining phase, the opposite is true. The accumulative solar irradiation directly contributes to this coherent decadal variability via changing the warm water volume and the solar-related heat is redistributed by the ocean dynamic processes. During the 11-yr solar cycle, the Pacific Walker circulation anomalies maintain the OHCa in the western equatorial Pacific and work as negative feedback for the eastern Pacific to help the OHCa phase transition. In addition, oceanic meridional heat transport via the subtropical cells and the propagation of off-equatorial Rossby waves also provide a lagged negative feedback to the OHCa phase transition according to the 11-yr solar cycle. The decadal coupled responses of the tropical Pacific climate system are 2 years more lag in the SOL experiment than in the observation/reanalysis. Significance Statement Here, we propose a new mechanism that the heating effect of the accumulative solar irradiation during the 11-yr solar cycle can be “integrated” into the tropical Pacific OHC and then provide a bottom-up effect on the atmosphere at decadal time scales. The strongly coupled processes in this region amplify the decadal phase-locking of the covariations to the 11-yr solar cycle. Our study demonstrates the role of the 11-yr solar cycle in the tropical Pacific decadal variability and provides a new explanation for the “bottom-up” mechanism of the solar cycle forcing. Our results update the understanding of the tropical Pacific decadal variability and may help to improve climate predictions at decadal time scales.

Механизм компенсации Бьеркнеса как возможный триггер низкочастотной изменчивости Арктического усиления

The causes of Arctic amplification are widely debated, and a cohesive picture has not been obtained yet. This study has investigated the role of the Atlantic meridional oceanic and atmospheric heat transport into the Arctic in the emergence of Arctic amplification. The integral advective fluxes in the layer of Atlantic waters and in the lower troposphere were considered. The results show a strong coupling between the meridional heat fluxes and regional Arctic amplification in the Eurasian Arctic on the decadal time scales (10–15 years). We argue that the low-frequency variability of Arctic amplification is regulated via the chain of oceanic heat transport — atmospheric heat transport — Arctic amplification. The atmospheric response to the ocean influence occurs with a delay of three years and is attributed to the Bjerknes compensation mechanism. In turn, the atmospheric heat and moisture transport directly affects the magnitude of Arctic amplification, with the latter lagging by one year. Thus, the variability of oceanic heat transport at the southern boundary of the Nordic Seas might be a predictor of the Arctic amplification magnitude over the Eurasian Basin of the Arctic Ocean with a lead time of four years. The results are consistent with the concept of the decadal Arctic climate variability expressed via the Arctic Ocean Oscillation index.

Regional earth system model for CORDEX‐South Asia: A comparative assessment of RESM and ESM over the tropical Indian Ocean

AbstractUnderstanding climate variability requires good quality high resolution spatially and temporally varying ocean fields due to its decisive role in regulating the region's climate, including the Indian summer monsoon. In this regard, we employed a new high‐resolution regional earth system model (RESM), namely ROM over CORDEX South Asia. We demonstrated the performance of the RESM and its added value over the global earth system model, namely MPI‐ESM, in simulating the hydrographic characteristics and associated mechanism over the tropical Indian Ocean (TIO). ROM shows better skill than MPI‐ESM in simulating the near‐surface and subsurface characteristics of the ocean. However, larger added values are noticed for subsurface (>350 m) thermal structure. MPI‐ESM's cold sea surface temperature (SST) bias is reduced in ROM which is associated with weaker winds, anomalous cyclonic wind stress curl, enhanced stratifications, and a shallower mixed layer, resulting in greater upper‐ocean heating. The reduced SST bias is also consistent with improved ocean meridional heat transport (OMHT) in ROM. For example, reduced southward export of OMHT over the Arabian Sea increases the surface warming by ~4%, reducing the RMSE by ~0.2–0.6°C and becoming closer to observation. The anomalous cyclonic wind stress curl, in turn, caused mixed layer stratifications in the western Indian Ocean. The advective heat transfer from the south‐eastern Indian Ocean to the western Indian Ocean reduced oceanic cooling by vertical processes, overcame the cooling by the net loss of surface heat fluxes, and favoured the TIO's surface warming.

Impact of Ocean Heat Transport on Arctic Sea Ice Variability in the GFDL CM2‐O Model Suite

AbstractThe impact of horizontal resolution on meridional Ocean Heat Transport (OHT) and sea ice in the Arctic is investigated using the GFDL CM2‐O climate model suite (1°, 1/4°, and 1/10°) in both preindustrial control and climate change simulations. Results show an increase in OHT associated to a decrease in sea ice extent (SIE) in the Arctic on interannual and decadal timescales. This link, however, is not monotonic with spatial resolution. While OHT increases and SIE decreases from the Low to the Medium resolution, the reverse is true from the Medium to the High resolution. Differences in OHT and SIE between the three model configurations mostly arise from the preindustrial state. As the spatial resolution increases, the Irminger Current is favored at the expense of the North Atlantic Drift. This rerouting of water to the Western side of Greenland results in less heat delivered to the Arctic in the High‐resolution configuration than in its Medium counterpart. As a result, the Medium‐resolution configuration is in best agreement with observed SIE and Atlantic OHT. Concurrent with the change in the partitioning in volume is a change in deep convection centers from the Greenland‐Irminger‐Norwegian Seas in the Low resolution to the Labrador Sea in the Medium and High resolutions. Results suggest a coupling between OHT into the Arctic and deep convection in the North Atlantic.

A regional (land–ocean) comparison of the seasonal to decadal variability of the Northern Hemisphere jet stream 1871–2011

Seasonal to decadal variations in Northern Hemisphere jet stream latitude and speed over land (Eurasia, North America) and oceanic (North Atlantic, North Pacific) regions are presented for the period 1871–2011 from the Twentieth Century Reanalysis dataset. Significant regional differences are seen on seasonal to decadal timescales. Seasonally, the jet latitude range is lower over the oceans compared to land, reduced from 20° over Eurasia to 10° over the North Atlantic where the ocean meridional heat transport is greatest. The mean jet latitude range is at a minimum in winter (DJF), particularly along the western boundary of the North Pacific and North Atlantic, where the land-sea contrast and SST gradients are strongest. The 141-year trends in jet latitude and speed show differences on a regional basis. The North Atlantic has significant increasing jet latitude trends in all seasons, up to 3° in winter. Eurasia has significant increasing trends in winter and summer, however, no increase is seen across the North Pacific or North America. Jet speed shows significant increases evident in winter (up to 4.7 ms−1), spring and autumn over the North Atlantic, Eurasia and North America however, over the North Pacific no increase is observed. Long term trends are generally overlaid by multidecadal variability, particularly evident in the North Pacific, where 20-year variability in jet latitude and jet speed are seen, associated with the Pacific Decadal Oscillation which explains 50% of the winter variance in jet latitude since 1940. The results highlight that northern hemisphere jet variability and trends differ on a regional basis (North Atlantic, North Pacific, Eurasia and North America) on seasonal to decadal timescales, suggesting that different mechanisms are influencing the jet latitude and speed. This is important from a climate modelling perspective and for climate predictions in the near and longer term.

The Role of Eddies in the North Atlantic Decadal Variability

The role of eddies in the North Atlantic decadal variability is investigated in this study by using two ocean reanalyses, including an eddy permitting (or eddy poor) reanalysis with horizontal resolution of 0.25 degree and 75 vertical levels and an eddy resolving (or eddy rich) reanalysis with horizontal resolution of 1/12 degree and 50 vertical levels. The prominent mid-1990s warming and post-2005 cooling trend as part of the North Atlantic decadal variability is well displayed in both reanalyses with no significant difference between them. The main driver of the mid-1990s warming and post-2005 cooling trend is the increase and reduction of the meridional ocean heat transport showing similar patterns in both reanalyses. The relative contribution of the heat transport anomalies from eddies to the total heat transport anomalies is slightly larger in eddy resolving than in eddy permitting ocean reanalysis. However, the total mean ocean meridional heat transport increases by 10% in eddy resolving reanalysis with respect to eddy permitting reanalysis and is mainly due to the associated increase of the mean states (temperature and velocity). Therefore, the increase of eddy population due to the increase of horizontal resolution, found by comparing the two datasets, does not affect the MHT anomalies significantly and, consequently, the North Atlantic decadal variability. It is found that the importance of the model horizontal resolution for the North Atlantic decadal variability depends on the interaction between the eddies (small scale) and the mean state (large scale) at decadal time scales. Although the fast increase of computational power will allow soon for eddy-resolving predictions, the need to use high resolution modeling tools for decadal predictions depends on the importance of initialization methods and the interaction between small scale and large-scale variabilities. This study has pivotal implications for the development of North Atlantic decadal prediction systems.

ENSO Explains the Link Between Indian Ocean Dipole and Meridional Ocean Heat Transport

AbstractIndian Ocean meridional heat transport (MHTIO) drives climate and ecosystem impacts, through changes to ocean temperature. Improved understanding of natural variability in tropical and subtropical MHTIO is needed to contextualize observations and future projections. Previous studies suggest that El Niño‐Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) can drive variability in MHTIO. However, it is unclear whether internally generated IOD can drive variability in MHTIO, or if the apparent relationship between IOD and MHTIO arises because both are modulated by ENSO. Here, we use a model experiment which dynamically removes ENSO to determine the role of internally forced IOD on MHTIO. We find that IOD is not linked to anomalies in MHTIO. Nevertheless, internal atmospheric variability drives significant MHTIO variability. There is little evidence for decadal or multidecadal variability in MHTIO, suggesting this may be a region where an anthropogenic trend rises above the level of internal variability sooner.

The thermodynamic and dynamic control of the sensible heat polynya in the western Cosmonaut Sea

A prominent feature in the west Cosmonaut Sea is the reoccurrence of a sensible heat polynya — the western Cosmonaut Sea Polynya (wCSP), which normally exists in late austral autumn and early winter. The Southern Ocean Sate Estimate (SOSE) reanalysis product is employed to investigate the thermodynamic and dynamic processes controlling the formation and evolution of a real wCSP event in 2009, when the polynya properties revealed from satellite observations are reasonably simulated by SOSE. An oceanic heat budget analysis was conducted for the surface layer above the thermocline, and the results reveal that heat advection is the major term contributing to the surface heat content variation in this area. The precursor of wCSP — an embayment — was formed during cyclonic atmospheric circulations. The negative wind stress curl from a cyclone induced upwelling that brought the warm circumpolar deep water to the surface and presumably melted ice. Meanwhile, the location of the cyclone relative to the wCSP created weaker easterlies over the northern boundary of polynya and stronger easterlies over the southern boundary of polynya. This leads to difference in the meridional oceanic heat transports across the northern and southern boundaries, resulting in net positive meridional oceanic heat transport into the polynya. Both the vertical heat advection and the meridional heat advection contribute significantly to increased heat content in the surface layer of wCSP, causing sea-ice melt and formation of the embayment. The cyclonic wind field promotes convergence of sea ice over the northern area of the embayment and results in the polynya. The formation of wCSP is likely related to the strength and position of Southern Annular Mode on interannual scale, and may potentially affect the biological productivity by controlling the availability of light in the mixed layer.

Dynamic Europa ocean shows transient Taylor columns and convection driven by ice melting and salinity

The deep (~100 km) ocean of Europa, Jupiter’s moon, covered by a thick icy shell, is one of the most probable places in the solar system to find extraterrestrial life. Yet, its ocean dynamics and its interaction with the ice cover have received little attention. Previous studies suggested that Europa’s ocean is turbulent using a global model and taking into account non-hydrostatic effects and the full Coriolis force. Here we add critical elements, including consistent top and bottom heating boundary conditions and the effects of icy shell melting and freezing on ocean salinity. We find weak stratification that is dominated by salinity variations. The ocean exhibits strong transient convection, eddies, and zonal jets. Transient motions organize in Taylor columns parallel to Europa’s axis of rotation, are static inside of the tangent cylinder and propagate equatorward outside the cylinder. The meridional oceanic heat transport is intense enough to result in a nearly uniform ice thickness, that is expected to be observable in future missions.

Asymmetric responses of the meridional ocean heat transport to climate warming and cooling in CESM

Asymmetric responses of the meridional ocean heat transport to climate warming and cooling in CESM

Ocean Surface Flux Algorithm Effects on Earth System Model Energy and Water Cycles

Earth system models parameterize ocean surface fluxes of heat, moisture, and momentum with empirical bulk flux algorithms, which introduce biases and uncertainties into simulations. We investigate the atmosphere and ocean model sensitivity to algorithm choice in the Energy Exascale Earth System Model (E3SM). Flux differences between algorithms are larger in atmosphere simulations (where wind speeds can vary) than ocean simulations (where wind speeds are fixed by forcing data). Surface flux changes lead to global scale changes in the energy and water cycles, notably including ocean heat uptake and global mean precipitation rates. Compared to the control algorithm, both COARE and University of Arizona (UA) algorithms reduce global mean precipitation and top of atmosphere radiative biases. Further, UA may slightly reduce biases in ocean meridional heat transport. We speculate that changes seen here, especially in the ocean, could be even larger in coupled simulations.

Early Eocene vigorous ocean overturning and its contribution to a warm Southern Ocean

Abstract. The early Eocene (∼55 Ma) was the warmest period of the Cenozoic and was most likely characterized by extremely high atmospheric CO2 concentrations. Here, we analyze simulations of the early Eocene performed with the IPSL-CM5A2 Earth system model, set up with paleogeographic reconstructions of this period from the DeepMIP project and with different levels of atmospheric CO2. When compared with proxy-based reconstructions, the simulations reasonably capture both the reconstructed amplitude and pattern of early Eocene sea surface temperature. A comparison with simulations of modern conditions allows us to explore the changes in ocean circulation and the resulting ocean meridional heat transport. At a CO2 level of 840 ppm, the early Eocene simulation is characterized by a strong abyssal overturning circulation in the Southern Hemisphere (40 Sv at 60∘ S), fed by deepwater formation in the three sectors of the Southern Ocean. Deep convection in the Southern Ocean is favored by the closed Drake and Tasmanian passages, which provide western boundaries for the buildup of strong subpolar gyres in the Weddell and Ross seas, in the middle of which convection develops. The strong overturning circulation, associated with subpolar gyres, sustains the poleward advection of saline subtropical water to the convective regions in the Southern Ocean, thereby maintaining deepwater formation. This salt–advection feedback mechanism is akin to that responsible for the present-day North Atlantic overturning circulation. The strong abyssal overturning circulation in the 55 Ma simulations primarily results in an enhanced poleward ocean heat transport by 0.3–0.7 PW in the Southern Hemisphere compared to modern conditions, reaching 1.7 PW southward at 20∘ S, and contributes to keeping the Southern Ocean and Antarctica warm in the Eocene. Simulations with different atmospheric CO2 levels show that ocean circulation and heat transport are relatively insensitive to CO2 doubling.