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Current global warming results in an uptake of heat by the Earth system, which is distributed among the different components of the climate system. However, current‐generation climate models deliver heat inventory and partitioning estimates of Earth system components that differ from recent observations. Here we investigate the global heat distribution under warming by using fully‐coupled CMIP6 Earth system model experiments, including a version of the MPI‐ESM with a deep land model component, accommodating the required space for more realistic terrestrial heat storage. The results show that sufficiently deep land models exert increased subsurface land heat uptake, leading to a heat uptake partitioning among the Earth system components that is closer to observational estimates. The results are relevant for the understanding of Earth's heat partitioning and highlight the importance of the land heat sink in the Earth heat inventory. Plain Language Summary: Global warming is associated with heat accumulation in the Earth system due to the intensification of the greenhouse effect. The available heat is distributed unevenly throughout the climate subsystems: the ocean, land, atmosphere, and cryosphere. Overall, the current generation of climate models captures this partitioning well but, on average, shows an overestimation of the ocean heat uptake and an underestimation of the land heat uptake. Previous studies have shown that the lack of heat input into the land comes from structural limitations in the land model components used. In this study, we account for these shortcomings, which greatly improve the land heat uptake in simulations of future climate scenarios. We find that, as a result, the fraction of simulated heat taken up by the ocean is reduced. This leads to a heat distribution among the climate subsystems that is closer to observational estimates. Our results highlight that land heat uptake is relevant for the Earth system heat distribution and that future research should consider modeling approaches including a more realistic land heat sink. Key Points: Considering a sufficient land depth in current‐generation Earth system models facilitates realistic land heat uptake estimates under warmingIncreased land heat uptake leads to an adjusted global heat distribution and a reduced overestimation of simulated ocean heat uptakeThe land heat sink is relevant for the projection of the global energy inventory and its partitioning under global warming [ABSTRACT FROM AUTHOR]

The anthropogenically-intensified greenhouse effect has caused a radiative imbalance at the top of the atmosphere during the industrial period. This, in turn, has led to an energy surplus in various components of the Earth system, with the ocean storing the largest part. The land contribution ranks second with the latest observational estimates based on borehole temperature profiles, which quantify the terrestrial energy surplus to be 6 % in the last five decades, whereas studies based on state-of-the-art climate models scale it down to 2 %. This underestimation stems from land surface models (LSMs) having a too shallow subsurface, which severely constrains the land heat uptake simulated by Earth System Models (ESMs). A forced simulation of the last 2000 years with the Max Planck Institute ESM (MPI-ESM) using a deep LSM captures 4 times more heat than the standard shallow MPI-ESM simulations in the historical period, well above the estimates provided by other ESMs. However, deepening the LSM does not remarkably affect the simulated surface temperature. It is shown that the heat stored during the historical period by an ESM using a deep LSM component can be accurately estimated by considering the surface temperatures simulated by the ESM using a shallow LSM and propagating them with a standalone forward model. This result is used to derive estimates of land heat uptake using all available observational datasets, reanalysis products, and state-of-the-art ESM experiments. This approach yields values of 10.5-16.0 ZJ for 1971-2018, slightly smaller than the latest borehole-based estimates (18.2 ZJ). [ABSTRACT FROM AUTHOR]

Small‐to‐moderate volcanic eruptions can lead to significant surface cooling when they occur clustered, as observed in recent decades. In this study, based on new high‐resolution ice‐core data from Greenland, we produce a new volcanic forcing data set that includes several small‐to‐moderate eruptions not included in prior reconstructions and investigate their climate impacts of the early 19th century through ensemble simulations with the Max Planck Institute Earth System Model. We find that clustered small‐to‐moderate eruptions produce significant additional global surface cooling (∼0.07 K) during the period 1812–1820, superposing with the cooling by large eruptions in 1809 (unidentified location) and 1815 (Tambora). This additional cooling helps explain the reconstructed long‐lasting cooling after the large eruptions, but simulated regional impacts cannot be confirmed with reconstructions due to a low signal‐to‐noise ratio. This study highlights the importance of small‐to‐moderate eruptions for climate simulations as their impacts can be comparable with that of solar irradiance changes. Plain Language Summary: Volcanic eruptions can influence global climate through the emission of sulfuric acids shielding Earth from incoming solar radiation. Previous volcanic reconstructions based on ice‐cores from the polar regions, however, only considered very strong volcanic eruptions. In this study, based on new ice‐core measurements from Greenland, we reconstruct for the first time volcanic sulfur emissions from small to medium‐sized eruptions and investigate their impact on climate in the early 19th century through experiments with the Max Planck Institute Earth System Model (MPI‐ESM1.2‐LR). We find that clustering of small to medium‐sized eruptions can cause significant global surface cooling (∼0.07 K), which during the 1812–1820 period amplified the cooling caused by the two known large eruptions of the period (1809 unidentified and 1815 Tambora). This additional surface cooling from small eruptions helps explain the long‐lasting cooling after the two strong eruptions generally found in the reconstruction, but the simulated regional impacts cannot be fully confirmed with reconstructions that are too noisy. This study highlights the importance of including small‐to‐moderate eruptions for climate model simulations as their impacts are comparable with that of solar irradiance forcing. Key Points: A new ice‐core based reconstruction of volcanic sulfate in the atmosphere (1733–1895) includes small‐to‐moderate eruptionsSmall‐to‐moderate eruptions can induce significant surface cooling and help explain the long‐lasting cooling in the early 19th centuryRegional cooling from small‐to‐moderate eruptions may be influenced by the circulation changes from the 1815 Tambora for over 10 years [ABSTRACT FROM AUTHOR]

The Atlantic Meridional Overturning Circulation (AMOC) is a key feature of the North Atlantic with global ocean impacts. The AMOC's response to past changes in forcings during the Holocene provides important context for the coming centuries. Here, we investigate AMOC trends using an emerging set of transient simulations using multiple global climate models for the past 6,000 years. Although some models show changes, no consistent trend in overall AMOC strength during the mid‐to‐late Holocene emerges from the ensemble. We interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions. The decadal variability of the AMOC does not change in ensemble during the mid‐ and late‐Holocene. There are interesting AMOC changes seen in the early Holocene, but their nature depends a lot on which inputs are used to drive the experiment. Plain Language Summary: The Atlantic Meridional Overturning Circulation (AMOC) is a deep ocean circulation system that is both important for climate and vulnerable to climate changes. Here we use a set of multiple climate models to look at how the AMOC responded to changes in climate drivers over the past few thousand years. The changes are only small in all of the models, and do not agree in their direction. The AMOC naturally varies on decadal timescales, but we do not see any strong trends in its variability either. We consider these simulations to indicate that the overall AMOC has not changed over the past 6,000 years, which fits with recent data reconstructions. Key Points: A multi‐model ensemble of Holocene transient simulations by general circulation models has been assembledAlthough some models show changes, no consistent trend in overall Atlantic Meridional Overturning Circulation (AMOC) strength during the mid‐to‐late Holocene emerges from the ensembleWe interpret this result to suggest no overall change in AMOC, which fits with our assessment of available proxy reconstructions [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system. The AMOC is predicted to weaken under climate change; however, theories suggest that it may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. Here, we outline a set of experiments designed to explore AMOC hysteresis and sensitivity to additional freshwater input as part of the North Atlantic Hosing Model Intercomparison Project (NAHosMIP). These experiments include adding additional freshwater (hosing) for a fixed length of time to examine the rate and mechanisms of AMOC weakening and whether the AMOC subsequently recovers once hosing stops. Initial results are shown from eight climate models participating in the Sixth Coupled Model Intercomparison Project (CMIP6). The AMOC weakens in all models as a result of the freshening, but once the freshening ceases, the AMOC recovers in half of the models, and in the other half it stays in a weakened state. The difference in model behaviour cannot be explained by the ocean model resolution or type nor by details of subgrid-scale parameterisations. Likewise, it cannot be explained by previously proposed properties of the mean climate state such as the strength of the salinity advection feedback. Instead, the AMOC recovery is determined by the climate state reached when hosing stops, with those experiments where the AMOC is weakest not experiencing a recovery. [ABSTRACT FROM AUTHOR]

The Atlantic meridional overturning circulation (AMOC) is an important part of our climate system. The AMOC is predicted to weaken under climate change, however there are theories that it may have a tipping point beyond which recovery is difficult, hence showing quasi-irreversibility (hysteresis). Although hysteresis has been seen in simple models, it has been difficult to demonstrate in comprehensive global climate models. Here we outline a set of experiments designed to explore AMOC hysteresis and sensitivity to additional freshwater input as part of the North Atlantic hosing model intercomparison project (NAHosMIP). These experiments include adding additional freshwater (hosing) for a fixed length of time to examine the rate and mechanisms or AMOC weakening, and whether the AMOC subsequently recovers once hosing stops. Initial results are shown from eight climate models participating in the Sixth Coupled Model Intercomparison Project CMIP6). The AMOC weakens in all models from the freshening, but once the freshening ceases, the AMOC recovers in half of the models, and in the other half it stays in a weakened state. The difference in model behaviour cannot be explained by the ocean model resolution or type, or by details of subgridscale parameterizations. Nor can it be explained by previously proposed properties of the mean climate state such as the strength of the salinity advection feedback. Instead the AMOC recovery is determined by the climate state reached when hosing stops, with those experiments where the AMOC is weakest not experiencing a recovery. [ABSTRACT FROM AUTHOR]

The literature offers conflicting findings about which regions contribute most to increases in the global radiative feedback after a forcing increase. This paper explains the disagreement by discriminating between two common definitions of the local feedback, which use either local temperature or global temperature as their basis. Although the two definitions of feedback have been previously compared in aquaplanet models with slab oceans, here the definitions are compared for the first time in an atmosphere-ocean general circulation model (MPI-ESM1.2) integrated over four doublings of atmospheric CO2 concentrations. Large differences between the definitions can be seen in all feedbacks, but especially in the temperature and water vapor feedbacks. Differences of up to 10 W m-2 K-1 over the Southern Ocean can be explained by the pattern of surface warming, which weights the local feedbacks and reduces their contribution to the global mean. This finding is, however, dependent on the resolution of analysis, because the local-temperature definition is mathematically inconsistent across spatial scales. Furthermore, attempts to estimate the effect of "pattern weighting" by separating local feedbacks and warming patterns at the gridcell level fail, because the radiative change in key tropical regions is also determined by tropospheric stability via the global circulation. These findings indicate that studies of regional feedback change are more sensitive to methodological choices than previously thought, and that the tropics most likely dominate regional contributions to global radiative feedback change on decadal to centennial time scales. [ABSTRACT FROM AUTHOR]

The question of whether ocean dynamics are relevant for basin-scale North Atlantic decadal temperature variability is the subject of ongoing discussions. Here, we analyze a set of simulations with a single climate model consisting of a 2000-yr preindustrial control experiment, a 100-member historical ensemble, and a 100-member ensemble forced with an incremental CO2 increase by 1% yr−1. Compared to previous approaches, our setup offers the following advantages: First, the large ensemble size allows us to robustly separate internally and externally forced variability and to robustly detect statistical links between different quantities. Second, the availability of different scenarios allows us to investigate the role of the background state for drivers of the variability. We find strong evidence that ocean dynamics, particularly ocean heat transport variations, form an important contribution to generate the Atlantic multidecadal variability (AMV) in the Max Planck Institute Earth System Model (MPI-ESM). Particularly the northwest North Atlantic is substantially affected by ocean circulation for the historical and preindustrial simulations. Anomalies of the Labrador Sea deep ocean density precede a change of the Atlantic meridional overturning circulation (AMOC) and heat advection to the region south of Greenland. Under strong CO2 forcing, the AMV–SST regression pattern shows crucial changes: SST variability in the northwestern part of the North Atlantic is strongly reduced, so that the AMV pattern in this scenario is dominated by the low-latitude branch. We found a connection to changes in the deep-water formation that cause a strong reduction of the mean AMOC and its variability. Consequently, ocean heat transport convergence becomes less important for the SST variability south of Greenland. [ABSTRACT FROM AUTHOR]

Radiative forcing from volcanic aerosol impacts surface temperatures; however, the background climate state also affects the response. A key question thus concerns whether constraining forcing estimates is more important than constraining initial conditions for accurate simulation and attribution of posteruption climate anomalies. Here we test whether different realistic volcanic forcing magnitudes for the 1815 Tambora eruption yield distinguishable ensemble surface temperature responses. We perform a cluster analysis on a superensemble of climate simulations including three 30‐member ensembles using the same set of initial conditions but different volcanic forcings based on uncertainty estimates. Results clarify how forcing uncertainties can overwhelm initial‐condition spread in boreal summer due to strong direct radiative impact, while the effect of initial conditions predominate in winter, when dynamics contribute to large ensemble spread. In our setup, current uncertainties affecting reconstruction‐simulation comparisons prevent conclusions about the magnitude of the Tambora eruption and its relation to the "year without summer." Plain Language Summary: Strong volcanic eruptions are a major natural forcing of climate, and there is increasing awareness of their importance for understanding and prediction of climate. Much of our current understanding is based on past eruptions for which there is no direct observation and that are therefore subject to substantial uncertainties regarding the eruption characteristics, the associated volcanic forcing, and the climatic response. With these premises, our study tackles the question of whether, for a past or analogously future eruption, it is more important to constrain the uncertainty in the volcanic forcing it generates rather than the initial climate conditions upon which it occurs, in order to obtain robust information about the associated climate responses. We use the 1815 eruption of Mount Tambora as a test case. We demonstrate that the climatic effects of initial condition uncertainties can overwhelm those of forcing uncertainties especially in winter and at the regional scale. We show how the European "year without a summer" seen in climate reconstructions is compatible with very different combinations of forcing magnitude and initial conditions. Key Points: The 1815 Tambora eruption is used to demonstrate how initial conditions affect the simulation of posteruption climate anomaliesForcing uncertainties can overwhelm initial‐condition spread in boreal summer, while the effect of initial conditions predominate in winterCurrent initial‐condition uncertainties hamper inferences on magnitude of the Tambora eruption in relation to the "year without summer" [ABSTRACT FROM AUTHOR]

Precipitation and circulation patterns of Northern Hemisphere monsoons are investigated in Coupled Model Intercomparison Project phase 5 simulations for mid‐Holocene and future climate scenario rcp8.5. Although both climates exhibit Northern Hemisphere warming and enhanced interhemispheric thermal contrast in boreal summer, changes in the spatial extent and rainfall intensity in future climate are smaller than in mid‐Holocene for all Northern Hemisphere monsoons except the Indian monsoon. A decomposition of the moisture budget in thermodynamic and dynamic contributions suggests that under future global warming, the weaker response of the African, Indian, and North American monsoons results from a compensation between both components. The dynamic component, primarily constrained by changes in net energy input over land, determines instead most of the mid‐Holocene land monsoonal rainfall response. Plain Language Summary: Mechanisms mediating the response of the Northern Hemisphere monsoons are investigated in two different simulated warm climates: the mid‐Holocene driven by orbital perturbations and a future global warming scenario due to increased greenhouse gas concentration. In both climates, monsoons wetten and expand relative to present day. In general, they do so more in the past than in the future despite a large warming in the latter. To understand these different responses, we explore whether monsoon changes are mostly related to changes in the amount of water vapor held in the atmosphere or to changes in the mean atmospheric circulation. In the past, intensification of monsoons is due to the reinforcing effects of increased water vapor content and strengthening of the mean circulation. In the future, however, the mean circulation weakens, opposing the water vapor content increase and leading to an overall weaker response than in the past. Causes for this difference can be traced back to changes in the net energy input into the atmosphere: While, in the past, this energy input increases, notably over Northern Hemisphere land, it remains largely unchanged in the future climate. Our results highlight that mid‐Holocene is not an analogue for future global warming scenario. Key Points: Different mechanisms mediate the response of Northern Hemisphere monsoons under future global warming and mid‐Holocene forcingNorthern Hemisphere monsoons intensify more strongly in mid‐Holocene than in future climate despite a larger warming in the latterAs an emergent constraint for future projections, tropical circulation weakening limits monsoon rainfall increase with global warming [ABSTRACT FROM AUTHOR]

This paper is the first of a series of four GMD papers on the PMIP4-CMIP6 experiments. Part 2 (Otto-Bliesner et al., 2017) gives details about the two PMIP4-CMIP6 interglacial experiments, Part 3 (Jungclaus et al., 2017) about the last millennium experiment, and Part 4 (Kageyama et al., 2017) about the Last Glacial Maximum experiment. The mid-Pliocene Warm Period experiment is part of the Pliocene Model Intercomparison Project (PlioMIP) - Phase 2, detailed in Haywood et al. (2016). The goal of the Paleoclimate Modelling Intercomparison Project (PMIP) is to understand the response of the climate system to different climate forcings for documented climatic states very different from the present and historical climates. Through comparison with observations of the environmental impact of these climate changes, or with climate reconstructions based on physical, chemical, or biological records, PMIP also addresses the issue of how well state-of-the-art numerical models simulate climate change. Climate models are usually developed using the present and historical climates as references, but climate projections show that future climates will lie well outside these conditions. Palaeoclimates very different from these reference states therefore provide stringent tests for state-of-the-art models and a way to assess whether their sensitivity to forcings is compatible with palaeoclimatic evidence. Simulations of five different periods have been designed to address the objectives of the sixth phase of the Coupled Model Intercomparison Project (CMIP6): the millennium prior to the industrial epoch (CMIP6 name: past1000); the mid-Holocene, 6000 years ago (midHolocene); the Last Glacial Maximum, 21 000 years ago (lgm); the Last Interglacial, 127 000 years ago (lig127k); and the mid-Pliocene Warm Period, 3.2 million years ago (midPliocene-eoi400). These climatic periods are well documented by palaeoclimatic and palaeoenvironmental records, with climate and environmental changes relevant for the study and projection of future climate changes. This paper describes the motivation for the choice of these periods and the design of the numerical experiments and database requests, with a focus on their novel features compared to the experiments performed in previous phases of PMIP and CMIP. It also outlines the analysis plan that takes advantage of the comparisons of the results across periods and across CMIP6 in collaboration with other MIPs. [ABSTRACT FROM AUTHOR]

The Ocean Model Intercomparison Project (OMIP) is an endorsed project in the Coupled Model Intercomparison Project Phase 6 (CMIP6). OMIP addresses CMIP6 science questions, investigating the origins and consequences of systematic model biases. It does so by providing a framework for evaluating (including assessment of systematic biases), understanding, and improving ocean, seaice, tracer, and biogeochemical components of climate and earth system models contributing to CMIP6. Among the WCRP Grand Challenges in climate science (GCs), OMIP primarily contributes to the regional sea level change and near-term (climate/decadal) prediction GCs. OMIP provides (a) an experimental protocol for global ocean/sea-ice models run with a prescribed atmospheric forcing; and (b) a protocol for ocean diagnostics to be saved as part of CMIP6. We focus here on the physical component of OMIP, with a companion paper (Orr et al., 2016) detailing methods for the inert chemistry and interactive biogeochemistry. The physical portion of the OMIP experimental protocol follows the interannual Coordinated Ocean-ice Reference Experiments (CORE-II). Since 2009, CORE-I (Normal Year Forcing) and CORE-II (Interannual Forcing) have become the standard methods to evaluate global ocean/seaice simulations and to examine mechanisms for forced ocean climate variability. The OMIP diagnostic protocol is relevant for any ocean model component of CMIP6, including the DECK (Diagnostic, Evaluation and Characterization of Klima experiments), historical simulations, FAFMIP (Flux Anomaly Forced MIP), C4MIP (Coupled Carbon Cycle Climate MIP), DAMIP (Detection and Attribution MIP), DCPP (Decadal Climate Prediction Project), ScenarioMIP, High- ResMIP (High Resolution MIP), as well as the ocean/sea-ice OMIP simulations. [ABSTRACT FROM AUTHOR]

An on-line, ensemble-based data assimilation (DA) method is performed to reconstruct the climate for 1750-1850 AD, and the performance is evaluated on large and small spatial scales. We use a low-resolution version of the Max Planck Institute for Meteorology MPI-ESM model and assimilate the PAGES 2K continental mean temperature reconstructions for the Northern Hemisphere (NH). The ensembles are generated sequentially for sub-periods based on the analysis of previous sub-periods. The assimilation has good skill for large-scale temperatures, but there is no agreement between the DA analysis and proxy-based reconstructions for small-scale temperature patterns within Europe or with reconstructions for the North Atlantic Oscillation (NAO) index. To explain the lack of added value in small spatial scales, a maximum covariance analysis (MCA) of links between NH temperature and sea level pressure is performed based on a control simulation with MPI-ESM. For annual values, winter and spring the Northern Annular Mode (NAM) is the pattern that is most closely linked to the NH continental temperatures, while for summer and autumn it is a wave-like pattern. This link is reproduced in the DA for winter, spring and annual means, providing potential for constraining the NAM/NAO phase and in turn regional temperature variability. It is shown that the lack of actual small-scale skill is likely due to the fact that the link might be too weak, as the NH continental mean temperatures are not the best predictors for large-scale circulation anomalies, or that the PAGES 2K temperatures include noise. Both factors can lead to circulation anomalies in the DA analysis that are substantially different from reality, leading to unrealistic representation of small-scale temperature variability. Moreover, we show that even if the true amplitudes of the leading MCA circulation patterns were known, there is still a large amount of unexplained local temperature variance. Based on these results, we argue that assimilating temperature reconstructions with a higher spatial resolution might improve the DA performance. [ABSTRACT FROM AUTHOR]

The goal of the Palaeoclimate Modelling Intercomparison Project (PMIP) is to understand the response of the climate system to changes in different climate forcings and to feedbacks. Through comparison with observations of the environmental impacts of these climate changes, or with climate reconstructions based on physical, chemical or biological records, PMIP also addresses the issue of how well state-of-the-art models simulate climate changes. Palaeoclimate states are radically different from those of the recent past documented by the instrumental record and thus provide an out-of-sample test of the models used for future climate projections and a way to assess whether they have the correct sensitivity to forcings and feedbacks. Five distinctly different periods have been selected as focus for the core palaeoclimate experiments that are designed to contribute to the objectives of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This manuscript describes the motivation for the choice of these periods and the design of the numerical experiments, with a focus upon their novel features compared to the experiments performed in previous phases of PMIP and CMIP as well as the benefits of common analyses of the models across multiple climate states. It also describes the information needed to document each experiment and the model outputs required for analysis and benchmarking. [ABSTRACT FROM AUTHOR]

We investigate how differently-constructed indices for North Atlantic sea-surface temperatures (NASSTs) describe the 'Atlantic Multidecadal Variability' (AMV) in a suite of unperturbed as well as externally-forced millennial (pre-industrial period) climate simulations. The simulations stem from an ensemble of Earth system models differing in both resolution and complexity. Different criteria exist to construct AMV indices capturing different aspects of the phenomenon. Although all representations of the AMV maintain strong multidecadal variability, they depict different characteristics of simulated low-frequency NASST variability, evolve differently in time and relate to different hemispheric teleconnections. Due to such multifaceted signatures in the ocean-surface as well as in the atmosphere, reconstructions of past AMV may not univocally reproduce multidecadal NASST variability. AMV features under simulated externally-forced pre-industrial climate conditions are not unambiguously distinguishable, within a linear framework, from AMV features in corresponding unperturbed simulations. This prevents a robust diagnosis of the simulated pre-industrial AMV as a predominantly internal rather than externally-forced phenomenon. We conclude that a multi-perspective assessment of multidecadal NASSTs variability is necessary for understanding the origin of the AMV, its physics and its climatic implications. [ABSTRACT FROM AUTHOR]

The decadal-centennial variations of East Asian summer monsoon (EASM) and the associated rainfall change during the past millennium are simulated using the earth system model developed at the Max Planck Institute for Meteorology. The model was driven by up-to-date reconstructions of external forcing including the recent low-amplitude estimates of solar variations. Analysis of the simulations indicates that the EASM is generally strong during the Medieval Warm Period (MWP; A.D. 1000-1100) and weak during the Little Ice Age (LIA; A.D. 1600-1700). The monsoon rainband exhibits a meridional tripolar pattern during both epochs. Excessive (deficient) precipitation is found over northern China (35°-42°N, 100°-120°E) but deficient (excessive) precipitation is seen along the Yangtze River valley (27°-34°N, 100°-120°E) during the MWP (LIA). Both similarities and disparities of the rainfall pattern between the model results herein and the proxy data have been compared, and reconstructions from Chinese historical documents and some geological evidence support the results. The changes of the EASM circulation including the subtropical westerly jet stream in the upper troposphere and the western Pacific subtropical high (WPSH) in the middle and lower troposphere are consistent with the meridional shift of the monsoon rain belt during both epochs. The meridional monsoon circulation changes are accompanied with anomalous southerly (northerly) winds between 20° and 50°N during the MWP (LIA). The land-sea thermal contrast change caused by the effective radiative forcing leads to the MWP and LIA monsoon changes. The 'warmer land-colder ocean' anomaly pattern during the MWP favors a stronger monsoon, while the 'colder land-warmer ocean' anomaly pattern during the LIA favors a weaker monsoon. [ABSTRACT FROM AUTHOR]

The influence of phytoplankton on the seasonal cycle and the mean global climate is investigated in a fully coupled climate model. The control experiment uses a fixed attenuation depth for shortwave radiation, while the attenuation depth in the experiment with biology is derived from phytoplankton concentrations simulated with a marine biogeochemical model coupled online to the ocean model. Some of the changes in the upper ocean are similar to the results from previous studies that did not use interactive atmospheres, for example, amplification of the seasonal cycle; warming in upwelling regions, such as the equatorial Pacific and the Arabian Sea; and reduction in sea ice cover in the high latitudes. In addition, positive feedbacks within the climate system cause a global shift of the seasonal cycle. The onset of spring is about 2 weeks earlier, which results in a more realistic representation of the seasons. Feedback mechanisms, such as increased wind stress and changes in the shortwave radiation, lead to significant warming in the midlatitudes in summer and to seasonal modifications of the overall warming in the equatorial Pacific. Temperature changes also occur over land where they are sometimes even larger than over the ocean. In the equatorial Pacific, the strength of interannual SST variability is reduced by about 10%–15% and phase locking to the annual cycle is improved. The ENSO spectral peak is broader than in the experiment without biology and the dominant ENSO period is increased to around 5 yr. Also the skewness of ENSO variability is slightly improved. All of these changes lead to the conclusion that the influence of marine biology on the radiative budget of the upper ocean should be considered in detailed simulations of the earth’s climate. [ABSTRACT FROM AUTHOR]

The presence of large internal variability makes detecting a forced response of rainfall to global warming difficult—in particular for regional studies. Therefore, we use a large ensemble of a comprehensive climate model to disentangle the forced response from internal variability in Sahel rainfall. We use both historical simulations to understand the observed record and different future scenarios to investigate the response of Sahel rainfall to future warming in the Max Planck Institute Grand Ensemble (MPI-GE).Observed Sahel rainfall has been decreasing from the 1960s to the 1980s and recovering in the decades thereafter. Moreover, some studies have suggested that the interannual variability in Sahel rainfall has increased in the recent decades. Using the large ensemble, we are able to disentangle internal variability and the forced response and find an externally forced increase in mean Sahel rainfall in the end of the 20th century. In addition we find evidence for a forced increase in interannual rainfall variability. However, the presence of large internal variability makes it impossible to robustly detect and attribute these forced changes in a single realisation. If the simulated magnitude of change is realistic, this implies that even if we can identify these changes in the observed record, they cannot be clearly attributed to a change in the forcing. Only a large ensemble allows a clean separation of forced changes and internal variability.In a strong forcing scenario, we find a forced increase in Sahel rainfall and rainfall variability in the 21st century. For weaker forcing scenarios, we find no change or even a projected decrease in Sahel rainfall with no forced change in the variability. For future scenarios, a large ensemble not only enables us to identify forced changes, but also to estimate how likely it is that we will be able to detect these changes in the single realisation of the real world. [ABSTRACT FROM AUTHOR]

Variability on centennial to multi-centennial timescales is mentioned as a feature in reconstructions of the Holocene climate. As more long transient model simulations with complex climate models become available and efforts have been made to compile large proxy databases, there is now a unique opportunity to study multi-centennial variability with greater detail and a large amount of data than earlier. This paper presents a spectral analysis of transient Holocene simulations from 9 models and 120 proxy records to find the common signals related to oscillation periods and geographic dependencies and discuss the implications for the potential driving mechanisms. Multi-centennial variability is significant in most proxy records, with the dominant oscillation periods around 120–130 years and an average of 240 years. Spectra of model-based global mean temperature (GMT) agree well with proxy evidence with significant multi-centennial variability in all simulations with the dominant oscillation periods around 120–150 years. It indicates a comparatively good agreement between model and proxy data. A lack of latitudinal dependencies in terms of oscillation period is found in both the model and proxy data. However, all model simulations have the highest spectral density distributed over the Northern hemisphere high latitudes, which could indicate a particular variability sensitivity or potential driving mechanisms in this region. Five models also have differentiated forcings simulations with various combinations of forcing agents. Significant multi-centennial variability with oscillation periods between 100 and 200 years is found in all forcing scenarios, including those with only orbital forcing. The different forcings induce some variability in the system. Yet, none appear to be the predominant driver based on the spectral analysis. Solar irradiance has long been hypothesized to be a primary driver of multi-centennial variability. However, all the simulations without this forcing have shown significant multi-centennial variability. The results then indicate that internal mechanisms operate on multi-centennial timescales, and the North Atlantic-Arctic is a region of interest for this aspect. • Multi-centennial oscillation with 100–200 years periods is evident in proxy data and model simulations during the Holocene. • Multi-centennial oscillation is a global signal and is more significant in the Northern Hemisphere high latitudes. • None of the external forcings is found to be the sole driver of the multi-centennial variability. • It indicates the multi-centennial oscillation may be due to potential internal drivers and essential feedbacks. [ABSTRACT FROM AUTHOR]

Lack of constraint on spatial and long-term temporal variability of the El Niño southern Oscillation (ENSO) and its sensitivity to external forcing limit our ability to evaluate climate models and ENSO future projections. Current knowledge of Holocene ENSO variability derived from paleoclimate reconstructions does not separate the role of insolation forcing from internal climate variability. Using an updated synthesis of coral and bivalve monthly resolved records, we build composite records of seasonality and interannual variability in four regions of the tropical Pacific: Eastern Pacific (EP), Central Pacific (CP), Western Pacific (WP) and South West Pacific (SWP). An analysis of the uncertainties due to the sampling of chaotic multidecadal to centennial variability by short records allows for an objective comparison with transient simulations (mid-Holocene to present) performed using four different Earth System models. Sea surface temperature and pseudo-δ18O are used in model-data comparisons to assess the potential influence of hydroclimate change on records. We confirm the significance of the Holocene ENSO minimum (HEM) 3-6ka compared to low frequency unforced modulation of ENSO, with a reduction of ENSO variance of ∼50 % in EP and ∼80 % in CP. The approach suggests that the increasing trend of ENSO since 6ka can be attributed to insolation, while models underestimate ENSO sensitivity to orbital forcing by a factor of 4.7 compared to data, even when accounting for the large multidecadal variability. Precession-induced change in seasonal temperature range is positively linked to ENSO variance in EP and to a lesser extent in other regions, in both models and observations. Our regional approach yields insights into the past spatial expression of ENSO across the tropical Pacific. In the SWP, today under the influence of the South Pacific Convergence Zone (SPCZ), interannual variability was increased by ∼200 % during the HEM, indicating that SPCZ variability is independent from ENSO on millennial time scales. • Composite paleo-ENSO records compared to transient simulations in 4 Pacific regions. • Holocene ENSO minimum (HEM) 3-6ka is significant compared to centennial variability. • Trend in ENSO since 6ka is robustly attributed to insolation forcing. • Seasonal range and ENSO are positively linked in both models and observations. • The interannual variability in the SPCZ was strongly increased during the HEM. [ABSTRACT FROM AUTHOR]