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Deep and abyssal layer decadal temperature trends from the mid‐1980s to the mid‐2010s are mapped globally using Deep Argo and historical ship‐based Conductivity‐Temperature‐Depth (CTD) instrument data. Abyssal warming trends are widespread, with the strongest warming observed around Antarctic Bottom Water (AABW) formation regions. The warming strength follows deep western boundary currents transporting abyssal waters north and decreases with distance from Antarctica. Abyssal cooling trends are found in the North Atlantic and eastern South Atlantic, regions primarily ventilated by North Atlantic Deep Water (NADW). Deep warming trends are prominent in the Southern Ocean south of about 50°S, the Greenland‐Iceland‐Norwegian (GIN) Seas and the western subpolar North Atlantic, with cooling in the eastern subpolar North Atlantic and the subtropical and tropical western North Atlantic. Globally integrated decadal heat content trends of 21.6 (±6.5) TW in the deep and 12.9 (±1.8) TW in the abyssal layer are more certain than previous estimates. Plain Language Summary: Even the deepest waters in the ocean, which sink to the abyss around Antarctica after being cooled and made saltier by heat exchange with the atmosphere and sea ice formation, have been shown to be warming around much of the globe in recent decades. The net warming rate below 2000‐m depth accounts for about 10% of total ocean heat uptake, but uncertainties in prior estimates have been about half the size of the signal owing to sparse sampling in the deep ocean. However, new observations from Deep Argo floats, capable of profiling to the ocean floor in most locations, have improved that situation in some regions. Here we analyze these new observations together with historical observations collected from ships since the 1970s to map decadal ocean temperature trends around the globe. As a result, we use more historical data than previous estimates. We refine the local patterns of warming and cooling in the waters deeper than 2,000 m. We confirm the amplitude of the net warming below 2,000 m estimated in previous studies, and extend the time covered by those estimates. The increased data coverage substantially reduces the uncertainty of the net warming estimate. Key Points: Analysis of Deep Argo float and historical ship‐based CTD data reveal global patterns in deep and abyssal layer decadal temperature trendsHigh resolution maps reveal spreading of abyssal warming from Antarctica and cooling from the North Atlantic on sub‐basin spatial scalesGlobally integrated decadal heat content trends are 21.6 (±6.5) TW in the deep and 12.9 (±1.8) TW in the abyssal layer [ABSTRACT FROM AUTHOR]

Most of net energy captured by the increase in greenhouse gasses are subsequently absorbed by the oceans, with the majority being taken up by the subpolar North Atlantic and the Southern Ocean, in the form of ocean heat uptake (OHU). In this study, we examine the temporal evolution of OHU from the Coupled Model Intercomparison Project Phase 5&6 (CMIP 5&6) data, with a focus on the time period that would follow reaching CO2 concentration maximum. Our results show that the Southern Ocean and global mean OHU would show a negative trend following abrupt CO2 quadrupling. In contrast, the OHU in the subpolar North Atlantic would still rise and then remain at a high level for a century or longer. The SSP experiments show the same phenomenon when CO2 concentration starts decreasing. This opposite trend of OHU between the Southern Ocean and subpolar North Atlantic is mainly driven by the surface latent heat flux. Further analysis and a freshwater perturbation experiment using a coupled ocean–atmosphere model suggest that the prolonged period of strong OHU over the North Atlantic in the post-CO2 concentration maximum era would be the result of an overshoot of the Atlantic Meridional Overturning Circulation (AMOC) in response to the increase of CO2 concentration. [ABSTRACT FROM AUTHOR]

We formulate a new conceptual model, named "MT2", to describe global ocean heat uptake, as simulated by atmosphere–ocean general circulation models (AOGCMs) forced by increasing atmospheric CO 2 , as a function of global-mean surface temperature change T and the strength of the Atlantic meridional overturning circulation (AMOC, M). MT2 has two routes whereby heat reaches the deep ocean. On the basis of circumstantial evidence, we hypothetically identify these routes as low- and high-latitude. In low latitudes, which dominate the global-mean energy balance, heat uptake is temperature-driven and described by the two-layer model, with global-mean T as the temperature change of the upper layer. In high latitudes, a proportion p (about 14%) of the forcing is taken up along isopycnals, mostly in the Southern Ocean, nearly like a passive tracer, and unrelated to T. Because the proportion p depends linearly on the AMOC strength in the unperturbed climate, we hypothesise that high-latitude heat uptake and the AMOC are both affected by some characteristic of the unperturbed global ocean state, possibly related to stratification. MT2 can explain several relationships among AOGCM projections, some found in this work, others previously reported: ∙ Ocean heat uptake efficiency correlates strongly with the AMOC. ∙ Global ocean heat uptake is not correlated with the AMOC. ∙ Transient climate response (TCR) is anticorrelated with the AMOC. ∙ T projected for the late twenty-first century under high-forcing scenarios correlates more strongly with the effective climate sensitivity than with the TCR. [ABSTRACT FROM AUTHOR]

The diversity of surface-flux perturbations, especially for heat-flux perturbations, notably leads to uncertainties surrounding the responses of ocean climate under global warming scenarios projected by climate/Earth system models. However, when imposing heat-flux perturbations on the models, strong feedback persists between the atmosphere and the ocean, resulting in nearly doubled heat-flux perturbation over the North Atlantic (NA). Herein, quantitative evaluation of the influences of magnitude change of heat-flux perturbations over the NA on the changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat uptake (OHU) and dynamic sea level (DSL) has been conducted by analysis of eight coupled model responses to the heat-flux perturbation experiments in the Flux-Anomaly-Forced Model Inter-comparison Project. It has been demonstrated that the magnitude of the AMOC change is extremely sensitive to the magnitude change of imposed NA heat-flux perturbation, and the weakening amplitude of the AMOC is nearly halved as the imposed heat-flux perturbation F is halved over the NA. The most remarkable responses of both DSL and OHU to the magnitude changes of NA heat-flux perturbation have been primarily found in the Atlantic and Arctic basins, especially for the NA region. Both the added ocean heat uptake (OHUa) and redistributed ocean heat uptake (OHUr) play key roles in OHU changes among the various NA heat-flux perturbation experiments. The magnitude change of NA-mean OHUa is almost linearly related to the imposed NA heat-flux perturbation, while the magnitude change of NA-mean OHUr, which is primarily caused by AMOC change and redistributed heat flux, is not proportional to the imposed NA heat-flux perturbation. There is a nearly linear relationship between the magnitude of AMOC change and the OHUr in tropical regions, including the regions in the low-latitude South Atlantic, the tropical Pacific Ocean and the Indian Ocean.

Ocean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high‐ (HR) and low‐resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation‐based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960–2020 are more realistic in CESM‐HR than ‐LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM‐HR and ‐LR. However, CESM‐HR has a larger increase in OHC in the upper 250 m compared to CESM‐LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy‐induced vertical heat transport between CESM‐HR and ‐LR in the historical period. Moreover, our results suggest that with the same heat input, upper‐ocean warming is likely to be underestimated by most non‐eddy‐resolving climate models. Plain Language Summary: With the rise in anthropogenic emissions, the ocean has absorbed over 90% of greenhouse gas‐related heat, resulting in well‐known ocean warming. This warming is the main cause of severe deoxygenation, coral bleaching, and sea‐level rise, among others. Furthermore, the upper ocean experiences more warming than the deep ocean, which leads to strengthened ocean stratification. On a global scale, the heat into the ocean is vertically redistributed by a downward mean‐flow‐induced heat transport, upward eddy‐induced heat transport, and vertical turbulent mixing. In this study, we compare vertical structures of ocean heat uptake in response to anthropogenic forcing between high (∼10 km) and low horizontal resolution (∼100 km) future climate simulations. We find that, with similar heat input at the ocean surface, the high‐resolution simulations exhibit more warming in the upper 250 m of the ocean than their low‐resolution counterparts. This difference is caused by the different representations of vertical heat transport by mesoscale ocean eddies in high‐ and low‐resolution models. Key Points: Simulated upper‐ocean ocean heat content anomalies are more realistic in high‐resolution simulations than in low‐resolution counterpartsWith similar ocean surface heating, high‐resolution simulations project stronger upper‐ocean warming than low‐resolution simulationsFuture changes in vertical heat transport depend on the representation of the mean ocean states at present day [ABSTRACT FROM AUTHOR]

Abstract Ocean warming is a key factor impacting future changes in climate. Here we investigate vertical structure changes in globally averaged ocean heat content (OHC) in high‐ (HR) and low‐resolution (LR) future climate simulations with the Community Earth System Model (CESM). Compared with observation‐based estimates, the simulated OHC anomalies in the upper 700 and 2,000 m during 1960–2020 are more realistic in CESM‐HR than ‐LR. Under RCP8.5 scenario, the net surface heat into the ocean is very similar in CESM‐HR and ‐LR. However, CESM‐HR has a larger increase in OHC in the upper 250 m compared to CESM‐LR, but a smaller increase below 250 m. This difference can be traced to differences in eddy‐induced vertical heat transport between CESM‐HR and ‐LR in the historical period. Moreover, our results suggest that with the same heat input, upper‐ocean warming is likely to be underestimated by most non‐eddy‐resolving climate models.

The impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport are investigated by contrasting two ensemble chemistry‐climate model simulations in 2005–2099: one with fixed ozone depleting substances (ODSs) and another with decreasing ODSs. In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. Ozone recovery causes a dipole change of the full‐depth ocean heat content (OHC) with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, OHC decreases in response to ozone recovery. This ocean heat loss is shown to be driven by weakened poleward ocean heat transport (OHT) across 40°S, which is partly canceled by enhanced heat uptake. The weakening of poleward OHT into the Southern Ocean is caused by the ozone‐induced equatorward shift of the meridional overturning circulation. Plain Language Summary: With the phaseout of ozone depleting substances as controlled under the Montreal Protocol and its amendments, the stratospheric ozone layer is projected to recover to the pre‐ozone hole levels in this century. Stratospheric ozone recovery can influence temperature and circulation in the atmosphere and ocean. Here we study how stratospheric ozone recovery affects Southern Ocean temperature and heat content using a coupled atmosphere‐ocean‐chemistry model. The strong surface westerlies over the Southern Ocean play a crucial role in ocean circulation and climate. We find that ozone recovery influences Southern Ocean through its impact on the surface westerlies. Our model results show that ozone recovery causes a weakening and equatorward shift of the strong westerlies over the Southern Ocean. As a result, the ocean circulation in the Southern Ocean weakens, leading to a weaker poleward heat transport from the lower latitude into the Southern Ocean. A weaker heat transport results in Southern Ocean cooling and a decrease of the Southern OHC. Key Points: The decrease of ozone depleting substances in the 21st century causes a decrease of the Southern Ocean heatThe ozone‐induced Southern Ocean heat decrease is driven by weakened poleward ocean heat transport (OHT)The weakened poleward OHT is driven by the equatorward shift of the surface winds and meridional overturning circulation [ABSTRACT FROM AUTHOR]

Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies. Plain Language Summary: Taking up almost all of the excess heat trapped on Earth by anthropogenic greenhouse gases, the ocean exerts a key control on our warming climate. Despite progress, tracking that heat remains an observational challenge. This study presents new measurements of ocean warming by making use of sound waves that are generated by earthquakes and propagate long distances through the ocean. These sound waves propagate faster in warmer seawater, so they arrive slightly earlier if warming has occurred. In this study, we measure such changes in arrival time at different frequencies—or pitches—that are sensitive to different parts of the water column, so warming in the upper ocean can be distinguished from warming in the deep ocean. Key Points: Seismic T waves at different frequencies sample different parts of the water columnFrequency‐dependent travel time changes between repeating earthquakes constrain the depth‐dependent temperature change between the eventsThese data reveal the vertical structure of temperature anomalies produces by equatorial waves, mesoscale eddies, and decadal warming [ABSTRACT FROM AUTHOR]

Abstract The impacts of stratospheric ozone recovery on Southern Ocean surface and interior temperature, heat content, heat uptake, and heat transport are investigated by contrasting two ensemble chemistry‐climate model simulations in 2005–2099: one with fixed ozone depleting substances (ODSs) and another with decreasing ODSs. In our simulations ozone recovery significantly affects Southern Ocean temperature, with large latitudinal and vertical variations. Ozone recovery causes a dipole change of the full‐depth ocean heat content (OHC) with an increase south of 60°S and a decrease between 45°S and 60°S. Integrated over latitudes south of 40°S, OHC decreases in response to ozone recovery. This ocean heat loss is shown to be driven by weakened poleward ocean heat transport (OHT) across 40°S, which is partly canceled by enhanced heat uptake. The weakening of poleward OHT into the Southern Ocean is caused by the ozone‐induced equatorward shift of the meridional overturning circulation.

Abstract Seismically generated sound waves that propagate through the ocean are used to infer temperature anomalies and their vertical structure in the deep East Indian Ocean. These T waves are generated by earthquakes off Sumatra and received by hydrophone stations off Diego Garcia and Cape Leeuwin. Between repeating earthquakes, a T wave's travel time changes in response to temperature anomalies along the wave's path. What part of the water column the travel time is sensitive to depends on the frequency of the wave, so measuring travel time changes at a few low frequencies constrains the vertical structure of the inferred temperature anomalies. These measurements reveal anomalies due to equatorial waves, mesoscale eddies, and decadal warming trends. By providing direct constraints on basin‐scale averages with dense sampling in time, these data complement previous point measurements that alias local and transient temperature anomalies.

Climate models do not explicitly account for the smaller scale processes of ocean surface waves. However, many large-scale phenomena are essentially coupled with the waves. In particular, waves enhance mixing in the upper ocean and thereby accelerate the ocean response to atmospheric changes. Here, we introduced a representation of wave-induced turbulent mixing into the one-way coupled ACCESS-OM2-025 ocean model to study its effect on ocean heat content throughout the 21st century under the RCP4.5 scenario. We made two projections on ocean heat uptake for the end of the century: one which accounts for wave-induced mixing (the 'modified' projection) and the other which does not (the 'standard' projection). Both projections showed upper ocean heat content to increase by more than 2.2 × 1022 J. This projected ocean heat uptake was reduced by about 3% in the modified projection. Whilst the inclusion of wave-induced mixing reduces projected ocean heat uptake globally, some areas are expected to warm considerably faster, particularly the North Atlantic sub-tropics, the Tasman Sea, the Sea of Japan, and parts of the South Atlantic. [ABSTRACT FROM AUTHOR]

Ocean heat uptake is caused by "excess heat" being added to the ocean surface by air‐sea fluxes and then carried to depths by ocean transports. One way to estimate excess heat in the ocean is to propagate observed sea surface temperature (SST) anomalies downward using a Green's function (GF) representation of ocean transports. Taking a "perfect‐model" approach, we test this GF method using a historical simulation, in which the true excess heat is diagnosed. We derive GFs from two approaches: (a) simulating GFs using idealized tracers, and (b) inferring GFs from simulated CFCs and climatological tracers. In the model world, we find that combining simulated GFs with SST anomalies reconstructs the Indo‐Pacific excess heat with a root‐mean‐square error of 26% for depth‐integrated changes; the corresponding number is 34% for inferred GFs. Simulated GFs are inaccurate because they are coarse grained in space and time to reduce computational cost. Inferred GFs are inaccurate because observations are insufficient constraints. Both kinds of GFs neglect the slowdown of the North Atlantic heat uptake as the ocean warms up. SST boundary conditions contain redistributive cooling in the Southern Ocean, which causes an underestimate of heat uptake there. All these errors are of comparable magnitude, and tend to compensate each other partially. Inferred excess heat is not sensitive to: (a) small changes in the shape of prior GFs, or (b) additional constraints from SF6 and bomb 14C. Plain Language Summary: Ocean warming is caused by "excess heat" being added to the ocean surface by air‐sea fluxes and then carried to depths by ocean currents. Tracking global ocean warming is important for monitoring climate change. However, a substantial amount of ocean warming occurs at depths, where temperature measurements are scarce. A workaround is to treat well‐observed surface ocean warming as a heat source, and propagate it downward using ocean currents. This method of estimating the interior ocean warming is called the Green's function (GF) method. But how accurate is the GF method? Here, we address this question by treating a computer simulation of the historical ocean as the real world, and comparing simulated ocean warming (as the "truth") with that estimated using the GF method. We find that the GF method broadly reconstructs simulated ocean warming. The results contain some inaccuracies because: (a) neither computer simulations nor observations give an accurate estimate of ocean currents, and (b) part of the surface temperature changes cannot be treated as a source of the interior ocean warming. Key Points: Green's functions (GFs) broadly reconstruct simulated ocean heat uptake when constrained by simulated observations in a historical simulationErrors in the GF method arise from insufficient observational constraints and forced changes in ocean transportsGFs underestimate the Southern Ocean heat uptake when using sea surface temperature anomalies as boundary conditions [ABSTRACT FROM AUTHOR]

The diversity of surface flux perturbations, especially for heat-flux perturbations, notably results in uncertainties surrounding the responses of ocean climate change under the global warming scenarios projected by climate/earth system models. However, when imposing heat-flux perturbations on the models, there are strong feedbacks between atmosphere and ocean, causing nearly doubled heat-flux perturbation over North Atlantic (NA). In this study, we quantitatively evaluated the impacts of magnitude changes of heat-flux perturbations over NA on the changes in the Atlantic Meridional Overturning Circulation (AMOC), ocean heat uptake (OHU) and dynamic sea level (DSL) by analyzing eight model responses to the heat flux perturbations experiments in Flux-Anomaly-Forced Model Inter-comparison Project (FAFMIP). We found that the magnitude of the AMOC change was very sensitive to the magnitude change of imposed NA heat-flux perturbation, and the weakening amplitude of the AMOC was nearly halved as the imposed heat-flux perturbation F halved over the NA. The most significant responses of both DSL and OHU to the magnitude changes of NA heat-flux perturbation were mainly found in the Atlantic and Arctic (AA) basin, especially for the NA region. Both the added ocean heat uptake (OHUa) and redistributed ocean heat uptake (OHUr) play roles in OHU changes among the different NA heat-flux perturbation experiments. The magnitude change of NA-mean OHUa was almost linearly related to the imposed NA heat-flux perturbation, while the magnitude change of NA-mean OHUr, which is mainly caused by AMOC change and redistributed heat flux, was not proportional to the imposed NA heat-flux perturbation. [ABSTRACT FROM AUTHOR]

The Pre‐Industrial climate model simulation is intended as an equilibrium control experiment that uses constant external forcing, yet in the Coupled Model Intercomparison Project Phase 6 (CMIP6) model HadGEM3‐GC31‐LL a distinct change in climate state occurs around year 500 of the 2000 year simulation. The global mean near surface air temperature increases by almost 0.5 K associated with a reduction in southern hemisphere sea ice area of almost 20%. Here we show this step change in the state of the climate to be a consequence of the onset of deep convection in the Weddell and Ross Sea gyres. The delayed onset of convection in the gyres is a consequence of a positive, downward, top‐of‐atmosphere radiative balance, and continual ocean heat up‐take during the model spin‐up and the initial pre‐industrial control simulation. Consequently, model spin‐up strategy should be revised to initialize pre‐industrial simulations that are in energy balance. Plain Language Summary: The Pre‐industrial control simulation of climate models is a reference for climate change simulations. Consequently it should simulate a stable climate. However, if the model is not in radiative equilibrium, the ocean continually accumulates heat, becoming unstable at high latitudes and generating large climate fluctuations. This results in a control simulation that is not a steady state. Consequently the methodologies used to initialize and formulate climate models should be revised to ensure a steady state is achieved. Key Points: The long timescales associated with deep ocean heat uptake suggests simulations may not be equilibriated before forcing is perturbedSouthern Ocean venting of heat reduces sea ice cover and increases global mean temperature, leading to a warmer climate stateNet top‐of‐atmosphere energy imbalance can have implications for the methodologies used for climate model spin‐up [ABSTRACT FROM AUTHOR]

The formation and spreading of dense deepwater in the polar regions play a key role in one of the most important climate systems, namely ocean meridional overturning circulation, and the deepwater formation is projected to decrease under the global warming. However, the impact of the reduced deepwater formation on the climate system has not been explored in detail. Here, we performed a series of numerical experiments with a climate model where the downward water mass transport through the bottom boundary layer is artificially reduced to quantitatively evaluate its impacts on the transient ocean and climate responses. It is demonstrated that changes in deepwater formation have non‐negligible impacts on not only ocean heat content but also the Earth's radiation budget at the top of the atmosphere: reduction in deepwater formation in high‐latitude oceans causes warming of bottom water, cooling of the ocean surface, and a subsequent decrease in outgoing longwave radiation. Plain Language Summary: The sinking and spreading of cold, dense water into the ocean deep layers at high latitudes plays a crucial role in large‐scale ocean circulation, closely linked to the climate system. In this study, we use a climate model to investigate the impact of the reduced dense deepwater formation on the heat budget of the climate system, aiming at a comprehensive understanding of deepwater formation in a warming climate transition. The results show that changes in the dense water formation have a non‐negligible effect on the heat budget of the atmosphere as well as on the heat content of the ocean. A decrease in deepwater formation in the high‐latitude oceans leads to warming of the bottom waters, cooling the ocean surface, and a concomitant decrease in outgoing longwave radiation. Key Points: We evaluate the impacts of reduced deepwater formation in a climate model by adjusting the parameters of the bottom boundary layerWeakening the downward water mass transport through the bottom boundary layer leads to increased net heat transport to the deep oceanChanges in deepwater formation have non‐negligible impacts on ocean heat content and the atmospheric heat balance [ABSTRACT FROM AUTHOR]

The benefits of implementing negative emission technologies for a century (years 2070-2170) on the global warming response to cumulative carbon emissions until year 2420 are assessed with a comprehensive set of intermediate complexity Earth system model integrations. Model integrations include 82 different model realisations covering a wide range of plausible climate states. The global warming response is assessed in terms of two key climate metrics: the effective transient climate response to cumulative CO2 emissions (eTCRE), measuring the surface warming response to cumulative carbon emissions and associated non-CO2 (RCP4.5) forcing, and the zero emissions commitment (ZEC), measuring the extent of any continued warming after net zero is reached. The TCRE is approximated from eTCRE by removing the contributions of non-CO2 forcing as 2.15 °C EgC-1 (with a 10-90 % range of 1.6 to 2.8 °C EgC-1). During the net positive emission phases, the eTCRE decreases from 2.62 (1.90 to 3.65) to 2.30 (1.73 to 3.23) °C EgC-1 due to a weakening in the increase in radiative forcing with an increase in atmospheric carbon, which is partly opposed by an increasing fraction of the radiative forcing warming the surface as the ocean stratifies. During the negative and zero emission phases, a progressive reduction of the eTCRE to 2.0 (1.4 to 2.8) °C EgC-1 is driven by the reducing airborne fraction as CO2 is drawn down by the ocean. The model uncertainty in the slopes of warming versus cumulative CO2 emissions varies from being controlled by the radiative feedback parameter during positive emissions to also being affected by ocean circulation and carbon-cycle parameters during zero or net-negative emissions. There is hysteresis in atmospheric CO[sub 2] and surface warming, where atmospheric CO2 and surface temperature are higher after peak emissions compared with before peak emissions. The continued warming after emissions cease defining the ZEC for the model mean without carbon capture is -0.01 °C at 25 years and decreases in time to -0.15 °C at 90 years after emissions cease. However, there is a spread in the ensemble with a temperature overshoot occurring in 50 % of the ensemble members at year 25. The ZEC only becomes negative in all ensemble members if modest carbon capture is included. Hence, incorporating negative emissions enhances the ability to meet climate targets and avoid risk of continued warming after net zero is reached. [ABSTRACT FROM AUTHOR]

State-of-the-art coupled general circulation models (CGCMs) are used to predict ocean heat uptake (OHU) and sea-level change under global warming. However, the projections of different models vary, resulting in high uncertainty. Much of the inter-model spread is driven by responses to surface heat perturbations. This study mainly focuses on the response of the ocean to a surface heat flux perturbation F, as prescribed by the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP). The results of ocean model were compared with those of a CGCM with the same ocean component. On the global scale, the changes in global mean temperature, ocean heat content (OHC), and steric sea level (SSL) simulated in the OGCM are generally consistent with CGCM simulations. Differences in changes in ocean temperature, OHC, and SSL between the two models primarily occur in the Arctic and Atlantic Oceans (AA) and the Southern Ocean (SO) basins. In addition to the differences in surface heat flux anomalies between the two models, differences in heat exchange between basins also play an important role in the inconsistencies in ocean climate changes in the AA and SO basins. These discrepancies are largely due to both the larger initial value and the greater weakening change of the Atlantic meridional overturning circulation (AMOC) in CGCM. The greater weakening of the AMOC in the CGCM is associated with the atmosphere—ocean feedback and the lack of a restoring salinity boundary condition. Furthermore, differences in surface salinity boundary conditions between the two models contribute to discrepancies in SSL changes. [ABSTRACT FROM AUTHOR]

Climate models do not explicitly account for the smaller scale processes of ocean surface waves. However, many large-scale phenomena are essentially coupled with the waves. In particular, waves enhance mixing in the upper ocean and thereby accelerate the ocean response to atmospheric changes. Here, we introduced a representation of wave-induced turbulent mixing into the one-way coupled ACCESS-OM2-025 ocean model to study its effect on ocean heat content throughout the 21st century under the RCP4.5 scenario. We made two projections on ocean heat uptake for the end of the century: one which accounts for wave-induced mixing (the ‘modified’ projection) and the other which does not (the ‘standard’ projection). Both projections showed upper ocean heat content to increase by more than 2.2 × 1022 J. This projected ocean heat uptake was reduced by about 3% in the modified projection. Whilst the inclusion of wave-induced mixing reduces projected ocean heat uptake globally, some areas are expected to warm considerably faster, particularly the North Atlantic sub-tropics, the Tasman Sea, the Sea of Japan, and parts of the South Atlantic.

Tropical cyclones (TCs) can influence climate by pumping heat into the ocean, yet little is known about how the TC-induced ocean heat uptake (OHU) has changed in recent decades. Based on ocean reanalysis, we calculated OHU and found a significant decline of TC-induced OHU from 1982 to 2018. If all the ocean heat gain is balanced by poleward heat transport, approximately 15% of peak ocean heat transport would have been reduced during the study period. The decreasing trend of OHU is consistent with the enhanced ocean stratification, the shallowed mixed layer depth and the reduced cold wake size. The reduction of OHU primarily occurs in the Northwest Pacific, where the shortened TC lifespan contributes as well. Furthermore, the decline of OHU might offset about 28% of the upper ocean warming in the subtropical Northwest Pacific.

Abstract Earth's energy imbalance (EEI) represents the rate of global energy accumulation in response to radiative forcings and feedbacks. Ocean heat uptake (OHU) poses a vital constraint on EEI and its uncertainty. Considering recent geodetic observations, geophysical corrections, and new estimates of the ocean's expansion efficiency of heat, we translate steric sea‐level change, the difference of total sea‐level and ocean‐mass change, into an OHU of 0.86 [0.62, 1.10, 5%–95%] Wm−2 for the period 2005–2019. Adding components of non‐oceanic heat uptake, we obtain an EEI of 0.94 [0.70, 1.19] Wm−2, which is at the upper end of previous assessments, but agrees within uncertainty. Interannual geodetic OHU variability exhibits a higher correlation with top‐of‐the‐atmosphere net radiative flux than hydrographic‐only data, but has a three times larger standard deviation. The radiation fluxes and the geodetic approach suggest an increase in heat uptake since 2005, most markedly in recent years.

Tropical cyclones (TC), which are among the most destructive natural phenomena on Earth, have significant impacts on the western boundary current (WBC). We quantify the direct impact of TCs on the surface geostrophic velocity of the Kuroshio, the WBC in the western North Pacific, by analyzing satellite‐derived geostrophic current data and using results from theoretical and numerical models. The study reveals that TCs decrease the surface geostrophic velocity by 14 cm s−1 which is 16% of the mean velocity of the Kuroshio (85 cm s−1), with the reduced velocity maintained for a month. The decrease in velocity of the Kuroshio is mainly (87%) due to the effect of TC‐driven upwelling ("Cooling effect"), the effect of ocean heat uptake after the TC ("Warming effect") being minor (13%); these are basically enabled by the strong thermal gradient around the Kuroshio. Plain Language Summary: The tropical cyclone (TC), also known as hurricane or typhoon, has a strong impact on the ocean as well as the land. In this study, we investigate how TCs affect ocean currents, with special focus on the Kuroshio, the western boundary current (WBC) in the North Pacific region. By analyzing satellite‐derived data, and building theoretical and numerical models, we deduce that when a TC hits the Kuroshio, its surface velocity decreases by 16%. This work makes a significant contribution by providing a quantitative understanding of the influence of TCs on the WBC. Further, given that the WBC is the strongest ocean current system on Earth and plays a key role in maintaining global heat balance, this study emphasizes the potential importance of the TC‐induced change on the current for the global distribution of heat energy. Key Points: Tropical cyclones (TC) decrease the surface geostrophic velocity of the Kuroshio by 14 cm s−1 (16% of the mean velocity)"Cooling Effect" of the TC‐driven upwelling explains 87% of the geostrophic velocity decrease immediately after a TC"Warming Effect" of the ocean heat uptake explains 13% of the geostrophic velocity decrease, which is maintained for ∼30 days after a TC [ABSTRACT FROM AUTHOR]

Earth's energy imbalance (EEI) represents the rate of global energy accumulation in response to radiative forcings and feedbacks. Ocean heat uptake (OHU) poses a vital constraint on EEI and its uncertainty. Considering recent geodetic observations, geophysical corrections, and new estimates of the ocean's expansion efficiency of heat, we translate steric sea‐level change, the difference of total sea‐level and ocean‐mass change, into an OHU of 0.86 [0.62, 1.10, 5%–95%] Wm−2 for the period 2005–2019. Adding components of non‐oceanic heat uptake, we obtain an EEI of 0.94 [0.70, 1.19] Wm−2, which is at the upper end of previous assessments, but agrees within uncertainty. Interannual geodetic OHU variability exhibits a higher correlation with top‐of‐the‐atmosphere net radiative flux than hydrographic‐only data, but has a three times larger standard deviation. The radiation fluxes and the geodetic approach suggest an increase in heat uptake since 2005, most markedly in recent years. Plain Language Summary: As the Earth is warming in response to increasing greenhouse gas concentrations, 90% of the extra heat is deposited in the world's oceans. This heat accumulation in the ocean can be used to estimate Earth's energy imbalance (EEI), which quantifies the energy uptake or deficit of our planet. Profiles of ocean temperature measured with profiling floats are often used to calculate the ocean heat uptake, but this approach does not sample the deep ocean (below 2,000 m) and misses some areas near coasts. Alternatively, the ocean's thermal expansion can be related to the heat uptake and is obtained by subtracting satellite‐observed ocean mass changes due to ice‐sheet and glacier melt, from total sea‐level changes. Using the newest data and methods over the recent two decades, we find that EEI (0.94 Wm−2) derived from the satellite data is slightly larger than that from the ocean float measurements (0.76 Wm−2). But despite the higher central value, both are equivalent within uncertainty. Key Points: We assess the sea‐level budget and its implications on Earth's energy imbalance (EEI) over 2005–2019Space geodesy suggests an EEI of 0.94 ± 0.24 Wm−2Multiple approaches suggest that heat uptake is increasing most markedly in recent years [ABSTRACT FROM AUTHOR]

The ocean has absorbed approximately 90% of the accumulated heat in the climate system since 1970. As global warming accelerates, understanding ocean heat content changes and tracing these to surface heat input is increasingly important. We introduce a novel framework by organizing the ocean into temperature‐percentiles from warmest to coldest, allowing us to trace ocean temperature changes to changes in surface fluxes and mixing. Applying this framework to observations and historical CMIP6 simulations, we find that 50 ± 6% of surface heat uptake between 1970 and 2014 is confined to isotherms in the coldest 90% of the ocean volume. These isotherms outcrop over only 23% of the ocean's surface area in the sub‐polar regions, implying a disproportionately large heat input per unit area. Additionally, a cooling bias in the CMIP6 models is traced to inaccurate sea surface temperatures and surface heat fluxes into the warmest 5%–20% of the ocean volume. Plain Language Summary: The ocean has absorbed approximately 90% of the heat that has built up in the climate system since 1970. Understanding the processes which drive this uptake of heat by the ocean is critical to climate projections. Typically, this has required the use of climate and ocean models, which must be validated with ocean observations. To enable this, we introduce a new method, the tracer‐percentile framework, which allows us to directly use observations to understand the processes dictating heat uptake by the ocean. Using climate models (collectively called CMIP6) and ocean observations, we calculate the heat input into layers of water at the ocean surface, the total heat stored in these layers, and the mixing of heat between these layers. We find that the heat input at the surface and heat stored in the ocean has increased between 1970 and 2014, with 50 ± 6% of the increase in heat uptake at the surface happening over about a quarter of the ocean's surface, which connects to 90% of the world's ocean volume. We also identify inaccuracies in the CMIP6 models and trace these problems to the way the surface properties of the ocean (or surface heat input) are represented. Key Points: We introduce a novel tracer‐percentile framework which relates ocean heat content trends to surface flux and turbulent mixing changesHeat content changes in the coldest 90% of the ocean by volume are linked to surface heat fluxes which enter 23% of the ocean's surface areaUsing this framework, we identify a cooling bias in the 5%–20% warmest ocean volume in CMIP6 climate models traced to surface flux biases [ABSTRACT FROM AUTHOR]

The magnitude of global mean surface temperature (GMST) response to increasing atmospheric CO2 concentrations is affected by the efficiency of ocean heat uptake, which in turn can be affected by oceanic mesoscale eddies. Using the Max Planck Institute ‐ Earth System Model (MPI‐ESM1.2), we find that resolving eddies leads to a −0.1°C cooler response of GMST to an abrupt CO2 quadrupling, which is related to a larger rate of heat uptake by an eddying ocean. This is consistent with changes in the energy budget of the whole climate system induced by increasing ocean resolution under the same radiative forcing and climate feedback. As a fast response, heat is taken up by the deep ocean, independent of resolution. The change in deep ocean heat uptake due to resolved eddies is an amplification in the magnitude of the responses of all heat processes, including eddy heat advection, mean heat advection, and diffusive processes. Key Points: Resolving eddies leads to a slightly cooler but energetically consistent response of global mean surface temperature (GMST) to abrupt 4xCO2 forcing−0.1°C cooler GMST response correlates with stronger heat uptake by eddying mid and deep oceanIn the deep ocean, resolving eddies results in stronger heat uptake due to larger response of all relevant heat processes [ABSTRACT FROM AUTHOR]

Monsoon rainfall anomalies are often organized into dynamical modes that are more predictable than rainfall at specific locations. Here, a linear response function (LRF) is constructed to extract the neutral modes (NM) of the dynamical atmospheric system and their optimal forcing that govern precipitation variability, providing a dynamical underpinning for the empirically derived, recurrent rainfall patterns. The leading mode (NM1) features a shift in the Atlantic ITCZ, a drying over the Indian Ocean and Indian subcontinent, and a "South‐Flood‐North Drought (SFND)" pattern over Eastern China, a pattern inferable from changes in the atmospheric energy flow forced by the corresponding optimal forcing and further amplified by the water vapor and cloud feedbacks. Moreover, the slow evolving component of the NM1 in response to CO2 forcing can be skillfully predicted by running the ocean heat uptake through the LRF, implicating a deterministic ocean dynamic source for the NM1‐like trend under climate warming. Plain Language Summary: The Asian monsoon has long been observed to exhibit coherent patterns of variability, but it remains to be seen if they represent the true dynamical modes of the climate system. By probing the ocean heat flux forcing for a climate model from all representative locations of the global ocean, this study constructs the systematic forcing‐response relationship for the boreal summer precipitation, therefrom the leading dynamical modes of variability known as neutral modes can be extracted. The result confirms that the previous empirically derived monsoon rainfall patterns, particularly the one comprising the "South‐Flood‐North Drought (SFND)" dipole over Eastern China, are dynamically based and explainable by the shift and/or pulsing of the global atmospheric energy flow. The dynamical mode with the SFND dipole as its integral part is found to be acutely responsive to an interhemispherically contrasting ocean heat flux, a likely consequence of the slowly evolving ocean heat uptake (OHU) under increased CO2 forcing. Key Points: The recurrent patterns of the Asian summer‐monsoon rainfall are shown to represent the dynamical modes of the climate systemThe patterns of the leading dynamical modes emerge as the result of the variations in the atmospheric energy flowThe projected long‐term trend towards the "South‐Flood‐North Drought" pattern in China may have a deterministic oceanic origin [ABSTRACT FROM AUTHOR]

The second version of the Chinese Academy of Sciences Earth System Model (CAS-ESM2.0) is participating in the Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) experiments in phase 6 of the Coupled Model Intercomparison Project (CMIP6). The purpose of FAFMIP is to understand and reduce the uncertainty of ocean climate changes in response to increased CO2 forcing in atmosphere-ocean general circulation models (AOGCMs), including the simulations of ocean heat content (OHC) change, ocean circulation change, and sea level rise due to thermal expansion. FAFMIP experiments (including faf-heat, faf-stress, faf-water, faf-all, faf-passiveheat, faf-heat-NA50pct and faf-heat-NA0ct) have been conducted. All of the experiments were integrated over a 70-year period and the corresponding data have been uploaded to the Earth System Grid Federation data server for CMIP6 users to download. This paper describes the experimental design and model datasets and evaluates the preliminary results of CAS-ESM2.0 simulations of ocean climate changes in the FAFMIP experiments. The simulations of the changes in global ocean temperature, Atlantic Meridional Overturning Circulation (AMOC), OHC., and dynamic sea level (DSL), are all reasonably reproduced. [ABSTRACT FROM AUTHOR]

Sea levels of different atmosphere–ocean general circulation models (AOGCMs) respond to climate change forcing in different ways, representing a crucial uncertainty in climate change research. We isolate the role of the ocean dynamics in setting the spatial pattern of dynamic sea-level (ζ) change by forcing several AOGCMs with prescribed identical heat, momentum (wind) and freshwater flux perturbations. This method produces a ζ projection spread comparable in magnitude to the spread that results from greenhouse gas forcing, indicating that the differences in ocean model formulation are the cause, rather than diversity in surface flux change. The heat flux change drives most of the global pattern of ζ change, while the momentum and water flux changes cause locally confined features. North Atlantic heat uptake causes large temperature and salinity driven density changes, altering local ocean transport and ζ. The spread between AOGCMs here is caused largely by differences in their regional transport adjustment, which redistributes heat that was already in the ocean prior to perturbation. The geographic details of the ζ change in the North Atlantic are diverse across models, but the underlying dynamic change is similar. In contrast, the heat absorbed by the Southern Ocean does not strongly alter the vertically coherent circulation. The Arctic ζ change is dissimilar across models, owing to differences in passive heat uptake and circulation change. Only the Arctic is strongly affected by nonlinear interactions between the three air-sea flux changes, and these are model specific. [ABSTRACT FROM AUTHOR]

Abstract There is large uncertainty in the future regional sea level change under anthropogenic climate change. Our study presents and uses a novel design of ocean general circulation model (OGCM) experiments to investigate the ocean's response to surface buoyancy and momentum flux perturbations without atmosphere‐ocean feedbacks (e.g., without surface restoring or bulk formulae), as part of the Flux‐Anomaly‐Forced Model Intercomparison Project (FAFMIP). In an ensemble of OGCMs forced with identical surface flux perturbations, simulated dynamic sea level (DSL) and ocean heat content (OHC) change demonstrate considerable disagreement. In the North Atlantic, the disagreement in DSL and OHC change between models is mainly due to differences in the residual (resolved and eddy) circulation change, with a large spread in the Atlantic meridional overturning circulation (AMOC) weakening (20–50%). In the western North Pacific, OHC change is similar among the OGCM ensemble, but the contributing physical processes differ. For the Southern Ocean, isopycnal and diapycnal mixing change dominate the spread in OHC change. In addition, a component of the atmosphere‐ocean feedbacks are quantified by comparing coupled, atmosphere‐ocean GCM (AOGCM) and OGCM FAFMIP experiments with consistent ocean models. We find that there is 10% more AMOC weakening in AOGCMs relative to OGCMs, since the extratropical North Atlantic SST cooling due to heat redistribution amplifies the surface heat flux perturbation. This component of the atmosphere‐ocean feedbacks enhances the pattern of North Atlantic OHC and DSL change, with relatively stronger increases and decreases in the tropics and extratropics, respectively.

Abstract Anthropogenic emissions of greenhouse gases cause the planet to warm, and the ocean uptake of anthropogenic heat slows the warming, preventing the climate system from equilibrating with the increasing radiative forcing. The uneven ocean surface warming affects regional changes in tropical rainfall, El Niño, and the global climate sensitivity. Thus, the study of ocean warming patterns bridges the ocean‐atmospheric dynamics community focusing on spatial patterns on one hand and the climate change community with a traditional emphasis on the planetary energy budget and radiative feedback on the other.

The surface warming response to carbon emissions is diagnosed using a suite of Earth system models, 9 CMIP6 and 7 CMIP5, following an annual 1% rise in atmospheric CO _2 over 140 years. This surface warming response defines a climate metric, the Transient Climate Response to cumulative carbon Emissions (TCRE), which is important in estimating how much carbon may be emitted to avoid dangerous climate. The processes controlling these intermodel differences in the TCRE are revealed by defining the TCRE in terms of a product of three dependences: the surface warming dependence on radiative forcing (including the effects of physical climate feedbacks and planetary heat uptake), the radiative forcing dependence on changes in atmospheric carbon and the airborne fraction. Intermodel differences in the TCRE are mainly controlled by the thermal response involving the surface warming dependence on radiative forcing, which arise through large differences in physical climate feedbacks that are only partly compensated by smaller differences in ocean heat uptake. The other contributions to the TCRE from the radiative forcing and carbon responses are of comparable importance to the contribution from the thermal response on timescales of 50 years and longer for our subset of CMIP5 models and 100 years and longer for our subset of CMIP6 models. Hence, providing tighter constraints on how much carbon may be emitted based on the TCRE requires providing tighter bounds for estimates of the physical climate feedbacks, particularly from clouds, as well as to a lesser extent for the other contributions from the rate of ocean heat uptake, and the terrestrial and ocean cycling of carbon.

There is large uncertainty in the future regional sea level change under anthropogenic climate change. Our study presents and uses a novel design of ocean general circulation model (OGCM) experiments to investigate the ocean's response to surface buoyancy and momentum flux perturbations without atmosphere‐ocean feedbacks (e.g., without surface restoring or bulk formulae), as part of the Flux‐Anomaly‐Forced Model Intercomparison Project (FAFMIP). In an ensemble of OGCMs forced with identical surface flux perturbations, simulated dynamic sea level (DSL) and ocean heat content (OHC) change demonstrate considerable disagreement. In the North Atlantic, the disagreement in DSL and OHC change between models is mainly due to differences in the residual (resolved and eddy) circulation change, with a large spread in the Atlantic meridional overturning circulation (AMOC) weakening (20–50%). In the western North Pacific, OHC change is similar among the OGCM ensemble, but the contributing physical processes differ. For the Southern Ocean, isopycnal and diapycnal mixing change dominate the spread in OHC change. In addition, a component of the atmosphere‐ocean feedbacks are quantified by comparing coupled, atmosphere‐ocean GCM (AOGCM) and OGCM FAFMIP experiments with consistent ocean models. We find that there is 10% more AMOC weakening in AOGCMs relative to OGCMs, since the extratropical North Atlantic SST cooling due to heat redistribution amplifies the surface heat flux perturbation. This component of the atmosphere‐ocean feedbacks enhances the pattern of North Atlantic OHC and DSL change, with relatively stronger increases and decreases in the tropics and extratropics, respectively. Plain Language Summary: A rise in sea level, as a result of climate change due to human activity, is a major threat to coastal communities and environments. Sea level rise is partially caused by a warming and expansion of the world's oceans, due to a net heat input from the atmosphere to the ocean. Changes in rainfall patterns and surface winds also affect the sea level, but net heat input changes are the most important factor. State‐of‐the‐art computer models disagree on future projections of local sea level rise. It has been suggested that this disagreement comes from differences in the amount of net heat input, and also the different assumptions going into the computer models. We find a large local sea level rise disagreement in the North Atlantic from giving several different computer models the same net heat input change. These differences are linked to uncertainty in how much Atlantic currents will slow in response to a given amount of warming. We also find that computer models that include an interactive ocean and atmosphere slow the Atlantic currents by more than computer models with an interactive ocean but fixed atmosphere. This finding builds our knowledge of the processes that determine the ocean's role in climate change. Key Points: Ensemble of ocean simulations without atmospheric feedbacks reveal spread in heat content changeIn the North Atlantic, spread from identical surface flux perturbation is due to circulation changeAtmosphere‐ocean feedbacks amplify local surface heat flux, AMOC and dynamic sea level change [ABSTRACT FROM AUTHOR]

In this study, the impact of oceanic processes on the sensitivity of transient climate change is investigated using two sets of coupled experiments with and without tidal forcing, which are termed Exp_Tide and Exp_Control, respectively. After introducing tidal forcing, the transient climate response (TCR) decreases from 2.32 K to 1.90 K, and the surface air temperature warming at high latitudes decreases by 29%. Large ocean heat uptake efficiency and heat storage can explain the low TCR in Exp_Tide. Approximately 21% more heat is stored in the ocean in Exp_Tide (1.10×1024J) than in Exp_Control (0.91×1024J). Most of the large ocean warming occurs in the upper 1 000 m between 60°S and 60°N, primarily in the Atlantic and Southern Oceans. This ocean warming is closely related to the Atlantic Meridional Overturning Circulation (AMOC). The initial transport at mid- and high latitudes and the decline in the AMOC observed in Exp_Tide are both larger than those observed in Exp_Control. The spatial structures of AMOC are also different with and without tidal forcing in present experiments. The AMOC in Exp_Tide has a large northward extension. We also investigated the relationship between AMOC and TCR suggested by previous studies using the present experiments. [ABSTRACT FROM AUTHOR]

Mixing of the upper ocean by the wind field associated with tropical cyclones (TCs) creates observable cold wakes in sea surface temperature and may potentially influence ocean heat uptake. The relationship between cold wake size and storm size, however, has yet to be explored. Here we apply two objective methods to observed daily sea surface temperature data to quantify the size of TC‐induced cold wakes. The obtained cold wake sizes agree well with the TC sizes estimated from the QuikSCAT‐R wind field database with a correlation coefficient of 0.51 and 0.59, respectively. Furthermore, our new estimate of the total cooling that incorporates the variations in the cold wake size provides improved estimates of TC power dissipation and TC‐induced ocean heat uptake. This study thus highlights the importance of cold wake size in evaluating the climatological effects of TCs. Plain Language Summary: The wind field of a tropical cyclone can mix warm water downward, leaving a cold wake at the surface. This effect may influence ocean heat uptake within the climate system. We derive a novel oceanic metric of tropical cyclone size based on its induced cold wake using sea surface temperature data for a period from 2002 to 2011. The cold wake size agrees well with the wind field size determined from QuikSCAT surface winds. Furthermore, with the cold wake size incorporated, total cooling provides better quantifications of tropical cyclone power dissipation and cyclone‐induced ocean heat uptake. More reliable estimates of cyclone power dissipation and sea surface cooling may be useful for both understanding and forecasting impacts of tropical cyclones on the climate system. Key Points: The size of the tropical cyclone wind field correlates reasonably well with the size of its mixing‐induced cold wakeWith the cold wake size incorporated, TC‐induced sea surface total cooling has a better correspondence with TC power dissipationCompared to sea surface cooling with a fixed cold wake area, the total cooling provides an improved estimate of TC‐induced ocean heat uptake [ABSTRACT FROM AUTHOR]

Anomaly-diffusing energy balance models (AD-EBMs) are routinely employed to analyze and emulate the warming response of both observed and simulated Earth systems. We demonstrate a deficiency in common multi-layer as well as continuous-diffusion AD-EBM variants: They are unable to, simultaneously, properly represent surface warming and the vertical distribution of heat uptake. We show that this inability is due to the diffusion approximation. On the other hand, it is well understood that transport of water from the surface mixed layer into the ocean interior is achieved, in large part, by the process of ventilation—a process associated with outcropping isopycnals. We, therefore, start from a configuration of outcropping isopycnals and demonstrate how an AD-EBM can be modified to include the effect of ventilation on ocean uptake of anomalous radiative forcing. The resulting EBM is able to successfully represent both surface warming and the vertical distribution of heat uptake, and indeed, a simple four-layer model suffices. The simplicity of the models notwithstanding, the analysis presented and the necessity of the modification highlight the role played by processes related to the down-welling branch of global ocean circulation in shaping the vertical distribution of ocean heat uptake. [ABSTRACT FROM AUTHOR]

In their comment on our paper (Caesar et al 2020 Environ. Res. Lett. 15 024003), Chen and Tung (hereafter C&T) argue that our analysis, showing that over the last decades Atlantic meridional overturning circulation (AMOC) strength and global mean surface temperature (GMST) were positively correlated, is incorrect. Their claim is mainly based on two arguments, neither of which is justified: first, C&T claim that our analysis is based on ‘established evidence’ that was only true for preindustrial conditions—this is not the case. Using data from the modern period (1947–2012), we show that the established understanding (i.e. deep-water formation in the North Atlantic cools the deep ocean and warms the surface) is correct, but our analysis is not based on this fact. Secondly, C&T claim that our results are based on a statistical analysis of only one cycle of data which was furthermore incorrectly detrended. This, too, is not true. Our conclusion that a weaker AMOC delays the current surface warming rather than enhances it, is based on several independent lines of evidence. The data we show to support this covers more than one cycle and the detrending (which was performed to avoid spurious correlations due to a common trend) does not affect our conclusion: the correlation between AMOC strength and GMST is positive. We do not claim that this is strong evidence that the two time series are in phase, but rather that this means that the two time series are not anti-correlated.

Most of the excess energy stored in the climate system is taken up by the oceans leading to thermal expansion and sea level rise. Future sea level projections allow decision-makers to assess coastal risk, develop climate resilient communities and plan vital infrastructure in low-elevation coastal zones. Confidence in these projections depends on the ability of climate models to simulate the various components of future sea level rise. In this study we estimate the contribution from thermal expansion to sea level rise using the simulations of global mean thermosteric sea level (GMTSL) from 15 available models in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We calculate a GMTSL rise of 18.8 cm [12.8–23.6 cm, 90% range] and 26.8 cm [18.6–34.6 cm, 90% range] for the period 2081–2100, relative to 1995–2014 for SSP245 and SSP585 scenarios respectively. In a comparison with a 20 model ensemble from Coupled Model Intercomparison Project Phase 5 (CMIP5), the CMIP6 ensemble mean of future GMTSL (2014–2100) is higher for both scenarios and shows a larger variance. By contrast, for the period 1901–1990, GMTSL from CMIP6 has half the variance of that from CMIP5. Over the period 1940–2005, the rate of CMIP6 ensemble mean of GMTSL rise is 0.2 ± 0.1 mm yr ^−1 , which is less than half of the observed rate (0.5 ± 0.02 mm yr ^−1 ). At a multi-decadal timescale, there is an offset of ∼10 cm per century between observed/modelled thermosteric sea level over the historical period and modelled thermosteric sea level over this century for the same rate of change of global temperature. We further discuss the difference in GMTSL sensitivity to the changes in global surface temperature over the historical and future periods.

According to established understanding, deep-water formation in the North Atlantic and Southern Ocean keeps the deep ocean cold, counter-acting the downward mixing of heat from the warmer surface waters in the bulk of the world ocean. Therefore, periods of strong Atlantic meridional overturning circulation (AMOC) are expected to coincide with cooling of the deep ocean and warming of the surface waters. It has recently been proposed that this relation may have reversed due to global warming, and that during the past decades a strong AMOC coincides with warming of the deep ocean and relative cooling of the surface, by transporting increasingly warmer waters downward. Here we present multiple lines of evidence, including a statistical evaluation of the observed global mean temperature, ocean heat content, and different AMOC proxies, that lead to the opposite conclusion: even during the current ongoing global temperature rise a strong AMOC warms the surface. The observed weakening of the AMOC has therefore delayed global surface warming rather than enhancing it. Social Media Abstract : The overturning circulation in the Atlantic Ocean has weakened in response to global warming, as predicted by climate models. Since it plays an important role in transporting heat, nutrients and carbon, a slowdown will affect global climate processes and the global mean temperature. Scientists have questioned whether this slowdown has worked to cool or warm global surface temperatures. This study analyses the overturning strength and global mean temperature evolution of the past decades and shows that a slowdown acts to reduce the global mean temperature. This is because a slower overturning means less water sinks into the deep ocean in the subpolar North Atlantic. As the surface waters are cold there, the sinking normally cools the deep ocean and thereby indirectly warms the surface, thus less sinking implies less surface warming and has a cooling effect. For the foreseeable future, this means that the slowing of the overturning will likely continue to slightly reduce the effect of the general warming due to increasing greenhouse gas concentrations.

To evaluate uncertainty in the transient climate response (TCR) associated with microscale deep-ocean mixing processes induced by internal tidal wave breaking, a set of idealized climate model experiments with two different implementations of deep-ocean mixing is conducted under increasing atmospheric CO _2 concentration 1% per year. The difference in TCR between the two experiments is 0.16 °C, which is about half as large as the multimodel spread of TCR in the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. The TCR difference can be attributed to the difference in the preindustrial climatological state. In the case where deep-ocean mixing works to enhance ocean stratification in the Pacific intermediate-to-deep layers, because the Pacific water mass is transported to the Southern Ocean by the Pacific meridional overturning circulation, the subsurface stratification in the Southern Ocean is also enhanced and deep wintertime convection there is suppressed. Our study shows that in this case during CO _2 increase, ocean heat uptake from the atmosphere to deeper layers is suppressed and TCR is estimated to be higher than the other case. Diminished accumulation of oceanic heat in the deep layer also leads to the sea level depression of ∼0.4 m in the Southern Ocean when atmospheric CO _2 concentration has quadrupled. Together with convective and cloud-radiative processes in the atmosphere and oceanic mesoscale processes, microscale deep-ocean mixing can be one of the major candidates in explaining uncertainty in future climate projections.

In the aftermath of the Paris Agreement, there is a crucial need for scientists in both thermal physiology and climate change research to develop the integrated approaches necessary to evaluate the health, economic, technological, social, and cultural impacts of 1.5°C warming. Our aim was to explore the fidelity of remote temperature measurements for quantitatively identifying the continuous redistribution of heat within both the Earth and the human body. Not accounting for the regional distribution of warming and heat storage patterns can undermine the results of thermal physiology and climate change research. These concepts are discussed herein using two parallel examples: the so-called slowdown of the Earth’s surface temperature warming in the period 1998-2013; and the controversial results in thermal physiology, arising from relying heavily on core temperature measurements. In total, the concept of heat is of major importance for the integrity of systems, such as the Earth and human body. At present, our understanding about the interplay of key factors modulating the heat distribution on the surface of the Earth and in the human body remains incomplete. Identifying and accounting for the interconnections among these factors will be instrumental in improving the accuracy of both climate models and health guidelines.

In the aftermath of the Paris Agreement, there is a crucial need for scientists in both thermal physiology and climate change research to develop the integrated approaches necessary to evaluate the health, economic, technological, social, and cultural impacts of 1.5°C warming. Our aim was to explore the fidelity of remote temperature measurements for quantitatively identifying the continuous redistribution of heat within both the Earth and the human body. Not accounting for the regional distribution of warming and heat storage patterns can undermine the results of thermal physiology and climate change research. These concepts are discussed herein using two parallel examples: the so-called slowdown of the Earth’s surface temperature warming in the period 1998-2013; and the controversial results in thermal physiology, arising from relying heavily on core temperature measurements. In total, the concept of heat is of major importance for the integrity of systems, such as the Earth and human body. At present, our understanding about the interplay of key factors modulating the heat distribution on the surface of the Earth and in the human body remains incomplete. Identifying and accounting for the interconnections among these factors will be instrumental in improving the accuracy of both climate models and health guidelines.

Global coupled climate models have large long-standing cloud and radiation biases, calling into question their ability to simulate climate and climate change. This study assesses the impact of reducing shortwave radiation biases on climate sensitivity within the Community Earth System Model (CESM). The model is modified by increasing supercooled cloud liquid to better match absorbed shortwave radiation observations over the Southern Ocean while tuning to reduce a compensating tropical shortwave bias. With a thermodynamic mixed-layer ocean, equilibrium warming in response to doubled CO2 increases from 4.1 K in the control to 5.6 K in the modified model. This 1.5 K increase in equilibrium climate sensitivity is caused by changes in two extratropical shortwave cloud feedbacks. First, reduced conversion of cloud ice to liquid at high southern latitudes decreases the magnitude of a negative cloud phase feedback. Second, warming is amplified in the mid-latitudes by a larger positive shortwave cloud feedback. The positive cloud feedback, usually associated with the subtropics, arises when sea surface warming increases the moisture gradient between the boundary layer and free troposphere. The increased moisture gradient enhances the effectiveness of mixing to dry the boundary layer, which decreases cloud amount and optical depth. When a full-depth ocean with dynamics and thermodynamics is included, ocean heat uptake preferentially cools the mid-latitude Southern Ocean, partially inhibiting the positive cloud feedback and slowing warming. Overall, the results highlight strong connections between Southern Ocean mixed-phase cloud partitioning, cloud feedbacks, and ocean heat uptake in a climate forced by greenhouse gas changes. [ABSTRACT FROM AUTHOR]

Abstract: The temporal evolution of the effective climate sensitivity is shown to be influenced by the changing pattern of sea surface temperature (SST) and ocean heat uptake (OHU), which in turn have been attributed to ocean circulation changes. A set of novel experiments are performed to isolate the active role of the ocean by comparing a fully coupled CO2 quadrupling community Earth System Model (CESM) simulation against a partially coupled one, where the effect of the ocean circulation change and its impact on surface fluxes are disabled. The active OHU is responsible for the reduced effective climate sensitivity and weaker surface warming response in the fully coupled simulation. The passive OHU excites qualitatively similar feedbacks to CO2 quadrupling in a slab ocean model configuration due to the similar SST spatial pattern response in both experiments. Additionally, the nonunitary forcing efficacy of the active OHU (1.7) explains the very different net feedback parameters in the fully and partially coupled responses. [ABSTRACT FROM AUTHOR]

The paused warming since ca. 2002 (maybe, 1998) is not satisfactorily reflected by the IPCC WGI (2013) Report. The aim of the present study is to collect, present and discuss the key arguments of the issue, selected strictly from this valuable Report. Our study tackles three aspects: (i) Symptoms of pausing, including atmospheric changes, near-surface oceans, cryosphere and geographical differences. (ii) Possible reasons of the paused warming, including external forcing factors, playing rather minor role, and the enhanced ocean heat uptake. Though missing warming is 0.2 K/decade compared to the model expectations, the whole climate system integrates continuously increasing amount of heat, 95 % of which is locked in the oceans. (iii) Consequences of the pausing for the three main branches of the IPCC activity. For climate science, correct simulation of the enhanced heat uptake is a challenge. Since characteristic time scale of most adaptation measures is 1-2 decades, or shorter, near-term projections may not drive adaptation until climate models become able meet this challenge. On the other hand, pausing warming does not question the need for mitigation, since it is physically unlikely, that oceans can uptake endless amount of heat. Vertical temperature gradients of the upper ocean layers already show stagnation.

A two-pattern decomposition of the mean surface air temperature response to an increase of CO2 is assessed. This decomposition is based on a two-layer global energy-balance model and the hypothesis of separability in space and time. It is shown that this decomposition allows the regional transient warming of a given atmosphere–ocean general circulation model to be well represented. The pattern decomposition is applied to 16 CMIP5 climate models and each of the two retrieved patterns – the equilibrium pattern and the pattern associated with the deep-ocean heat uptake – is described and discussed.

Abstract: Cloud phase improvements in a state‐of‐the‐art climate model produce a large 1.5 K increase in equilibrium climate sensitivity (ECS, the surface warming in response to instantaneously doubled CO2) via extratropical shortwave cloud feedbacks. Here we show that the same model improvements produce only a small surface warming increase in a realistic 21st century emissions scenario. The small 21st century warming increase is attributed to extratropical ocean heat uptake. Southern Ocean mean‐state circulation takes up heat while a slowdown in North Atlantic circulation acts as a feedback to slow surface warming. Persistent heat uptake by extratropical oceans implies that extratropical cloud biases may not be as important to 21st century warming as biases in other regions. Observational constraints on cloud phase and shortwave radiation that produce a large ECS increase do not imply large changes in 21st century warming. [ABSTRACT FROM AUTHOR]

The climate response after cessation of carbon emissions is examined here, exploiting a single equation connecting surface warming to cumulative carbon emissions. The multicentennial response to an idealized pulse of carbon is considered by diagnosing a 1,000 year integration of an Earth system model (Geophysical Fluid Dynamics Laboratory ESM2M) and an ensemble of efficient Earth system model simulations. After emissions cease, surface temperature evolves according to (i) how much of the emitted carbon remains in the atmosphere and (ii) how much of the additional radiative forcing warms the surface rather than the ocean interior. The peak in surface temperature is delayed in time after carbon emissions cease through the decline in ocean heat uptake, which in turn increases the proportion of radiative forcing warming the surface. Eventually, after many centuries, surface temperature declines as the radiative forcing decreases through the excess atmospheric CO2 being taken up by the ocean and land. [ABSTRACT FROM AUTHOR]