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El Niño‐Southern Oscillation (ENSO) flavors have been defined to characterize ENSO events and their teleconnections. Studying El Niño flavor evolution during the Holocene period can provide valuable insights into changes over long time scales. We investigated ENSO flavor evolution using simulations spanning the last 6,000 years and present‐day observations. Two approaches to computing ENSO flavors, in agreement in the present, lead to opposite trends in the last 6,000 years. The methods also differ significantly in their representation of ENSO flavor patterns. However, incorporating the sensitivity of the methods to calibration periods and mean state changes yields similar interpretations of ENSO variability changes. Both methods suggest an increase in El Niño variability spreading to the west and east tropical Pacific over the past 6,000 years. Standardizing El Niño flavor definitions is necessary for meaningful comparisons between studies and robust climate variability analysis. Plain Language Summary: El Niño events are the dominant mode of interannual variability. Looking at El Niño events in the past, during the Holocene period, could shed light on the linkages between the characteristics of El Niño patterns and changes in the climate mean state. Different indicators are used to characterize El Niño pattern diversity depending on the available data. While equivalent under present‐day conditions, these indicators lead to opposite conclusions on the evolution of El Niño patterns over the last 6,000 years. An in‐depth analysis indicates that all the methods suggest that El Niño‐related variance has spread to the west and east tropical Pacific over time when accounting for sensitivity to calibration periods and changes in the mean state during the Holocene. Agreeing on a standard definition for El Niño flavors is essential for accurate comparison between studies and for avoiding misleading conclusions. Key Points: Two equivalent indicators of El Niño‐Southern Oscillation (ENSO) flavors in the present give opposite evolution during the HoloceneResults show that the conflict between methods comes from the representation of ENSO flavor patterns on short and long timescalesA proper assessment must account for the indicator's patterns and calibration, as well as the climate mean‐state change over time [ABSTRACT FROM AUTHOR]

The low-frequency evolution of Indian rainfall mean-state and associated interannual-to-decadal variability is discussed for the last 6000 years from a multi-configuration ensemble of fully coupled global transient simulations. This period is marked by a shift of Indian Summer Monsoon Rainfall (ISMR) distribution towards drier conditions, including extremes, and a contraction of the rainy season. The drying is larger in simulations with higher horizontal resolution of the atmosphere and revised land surface hydrology. Vegetation–climate interactions and the way runoff is routed to ocean modulate the timing of the monsoon onset but have negligible effects on the evolution of seasonal rainfall amounts in our modeling framework in which carbon cycling is always active. This drying trend is accompanied by changes in ISMR interannual-to-decadal variability decreasing over north and south India but increasing over central India (20°–25° N). The ISMR interannual-to-decadal variability is decomposed into six physically consistent regimes using a clustering technique to further characterize its changes and associated teleconnections. From 6 to 3.8 kyr bp, the century-to-century modulations in the frequency of occurrence associated to the regimes are asynchronous between the simulations. Orbitally-driven trends can only be detected for two regimes over the whole 6–0 kyr bp period. These two regimes reflect increased influence of ENSO on both ISMR and Indian Ocean Dipole as the inter-hemispheric energy gradient weakens. Severe long-term droughts are also shown to be a combination of long-term drying and internally generated low-frequency modulations of the interannual-to-decadal variability. [ABSTRACT FROM AUTHOR]

Recent variability in West African monsoon rainfall (WAMR) has been shown to be influenced by multiple ocean–atmosphere modes, including the El Niño Southern Oscillation, Atlantic Multidecadal Oscillation and the Interdecadal Pacific Oscillation. How these modes will change in response to long term forcing is less well understood. Here we use four transient simulations driven by changes in orbital forcing and greenhouse gas concentrations over the past 6000 years to examine the relationship between West African monsoon rainfall multiscale variability and changes in the modes associated with this variability. All four models show a near linear decline in monsoon rainfall over the past 6000 years in response to the gradual weakening of the interhemispheric gradient in sea surface temperatures. The only indices that show a long-term trend are those associated with the strengthening of the El Niño Southern Oscillation from the mid-Holocene onwards. At the interannual-to-decadal timescale, WAMR variability is largely influenced by Pacific–Atlantic – Mediterranean Sea teleconnections in all simulations; the exact configurations are model sensitive. The WAMR interannual-to-decadal variability depicts marked multi-centennial oscillations, with La Niña/negative Pacific Decadal Oscillation and a weakening and/or poleward shift of subtropical high-pressure systems over the Atlantic favoring wet WAMR anomalies. The WAMR interannual-to-decadal variability also depicts an overall decreasing trend throughout the Holocene that is consistent among the simulations. This decreasing trend relates to changes in the North Atlantic and Gulf of Guinea Sea Surface Temperature variability. [ABSTRACT FROM AUTHOR]

A single‐column model approach conducted in the context of the Madden–Julian Oscillation through the CINDY2011/Dynamics of the Madden–Julian Oscillation field campaign is used to disentangle the respective role of the parameterizations of surface turbulent fluxes and of model atmospheric physics in controlling the surface latent heat flux. The major differences between the models used in this study occur during the suppressed phases of deep convection. They are attributed to differences in model atmospheric physics which is shown to control the near‐surface relative humidity and thereby the surface latent heat flux. In contrast, during active phases of deep convection, turbulent air‐sea flux parameterizations impact the latent heat flux through the drag coefficient and can represent two thirds of the divergence caused by the different atmospheric physics. The combined effects need to be accounted for to improve both the representation of latent heat flux and the atmospheric variables used to compute it. Key Points: A single‐column model approach is used in the context of the CINDY2011/DYNAMO field campaign to compare models results during a MJO eventNear‐surface relative humidity is a key constraint for the surface latent heat flux and the way it is simulated by different modelsCompared to atmospheric physics, air‐sea flux parameterization has a relatively large impact only during active convective phases [ABSTRACT FROM AUTHOR]

It has been shown that El Niño events contribute to discharge the warm pool excess of energy out of the tropical Pacific. In a different climate, the energetic budget in the tropical Pacific is altered, which might have an effect on the El Niño amplitude and/or occurrence and thereby on the role of El Niño on energy redistribution. The mid-Holocene period (6 ka BP) offers a good example of changes in the distribution of incoming solar energy. In particular, the equator-pole gradient was weaker compared to the modern period. We analyze long stable simulations of the mid-Holocene and the pre-industrial era and discuss the mean- and El Niño-related energy transports in the two climates. We show that the role of global energy pump played by the tropical Pacific is reduced in the mid-Holocene in our simulation, both in long-term mean and during El Niño years. We demonstrate that this is not only a direct response to insolation forcing but this is further amplified by changes in internal processes. We analyze the relative role of El Niño events in the Pacific discharge in the two climates and show that it is reduced in the mid-Holocene, i.e. the fraction of the Pacific discharge that is due to El Niño is reduced. This is mainly due to reduction in the occurrence of El Niño events. This work gives a new approach to address El Niño changes, from the perspective of the role of El Niño in global energy redistribution. [ABSTRACT FROM AUTHOR]

The turbulent fluxes across the ocean/atmosphere interface represent one of the principal driving forces of the global atmospheric and oceanic circulation. Despite decades of effort and improvements, representation of these fluxes still presents a challenge due to the small-scale acting turbulent processes compared to the resolved scales of the models. Beyond this subgrid parameterization issue, a comprehensive understanding of the impact of air-sea interactions on the climate system is still lacking. In this paper we investigates the large-scale impacts of the transfer coefficient used to compute turbulent heat fluxes with the IPSL-CM4 climate model in which the surface bulk formula is modified. Analyzing both atmosphere and coupled ocean-atmosphere general circulation model (AGCM, OAGCM) simulations allows us to study the direct effect and the mechanisms of adjustment to this modification. We focus on the representation of latent heat flux in the tropics. We show that the heat transfer coefficients are highly similar for a given parameterization between AGCM and OAGCM simulations. Although the same areas are impacted in both kind of simulations, the differences in surface heat fluxes are substantial. A regional modification of heat transfer coefficient has more impact than uniform modification in AGCM simulations while in OAGCM simulations, the opposite is observed. By studying the global energetics and the atmospheric circulation response to the modification, we highlight the role of the ocean in dampening a large part of the disturbance. Modification of the heat exchange coefficient modifies the way the coupled system works due to the link between atmospheric circulation and SST, and the different feedbacks between ocean and atmosphere. The adjustment that takes place implies a balance of net incoming solar radiation that is the same in all simulations. As there is no change in model physics other than drag coefficient, we obtain similar latent heat flux between coupled simulations with different atmospheric circulations. Finally, we analyze the impact of model tuning and show that it can offset part of the feedbacks. [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]

Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127 000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern ones. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land–sea contrast and highlatitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional sensitivity experiments, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically. [ABSTRACT FROM AUTHOR]

Two interglacial epochs are included in the suite of Paleoclimate Modeling Intercomparison Project (PMIP4) simulations in the Coupled Model Intercomparison Project (CMIP6). The experimental protocols for Tier 1 simulations of the mid-Holocene (midHolocene, 6000 years before present) and the Last Interglacial (lig127k, 127,000 years before present) are described here. These equilibrium simulations are designed to examine the impact of changes in orbital forcing at times when atmospheric greenhouse gas levels were similar to those of the preindustrial period and the continental configurations were almost identical to modern. These simulations test our understanding of the interplay between radiative forcing and atmospheric circulation, and the connections among large-scale and regional climate changes giving rise to phenomena such as land-sea contrast and high-latitude amplification in temperature changes, and responses of the monsoons, as compared to today. They also provide an opportunity, through carefully designed additional CMIP6 Tier 2 and Tier 3 sensitivity experiments of PMIP4, to quantify the strength of atmosphere, ocean, cryosphere, and land-surface feedbacks. Sensitivity experiments are proposed to investigate the role of freshwater forcing in triggering abrupt climate changes within interglacial epochs. These feedback experiments naturally lead to a focus on climate evolution during interglacial periods, which will be examined through transient experiments. Analyses of the sensitivity simulations will also focus on interactions between extratropical and tropical circulation, and the relationship between changes in mean climate state and climate variability on annual to multi-decadal timescales. The comparative abundance of paleoenvironmental data and of quantitative climate reconstructions for the Holocene and Last Interglacial make these two epochs ideal candidates for systematic evaluation of model performance, and such comparisons will shed new light on the importance of external feedbacks (e.g., vegetation, dust) and the ability of state-of-the-art models to simulate climate changes realistically. [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]

The South Pacific Convergence Zone (SPCZ) is the main climate feature of the Southwest Pacific. It is characterized by a band of intense convective rainfall extending from the western Pacific warm pool to French Polynesia. Strong precipitation gradients within the SPCZ make local hydrologic conditions very sensitive to small displacements of this rainfall band, as those caused by El Niño and La Niña events. The associated rainfall fluctuations strongly impact the vulnerable Southwest Pacific countries. They are recorded in environmental indicators such as corals, used as proxies of past evolution of the El Niño/Southern Oscillation (ENSO). Here we analyze a set of paleoclimate and future climate simulations and present evidence that changes in the background tropical state largely control the mean SPCZ location. In contrast, changes in the background tropical state do not directly control the interannual variability of the SPCZ location. We show that changes in the interannual variability of the SPCZ location cannot be directly imputable to changes in the ENSO amplitude, or rather the relationship between ENSO and the SPCZ location varies from one climate to another. We thus demonstrate that the teleconnection mechanisms inferred from the modern climate cannot be directly extrapolated to other climates. This study therefore calls for a cautious interpretation of climate reconstructions from environmental indicators in the Southwest Pacific with regard to ENSO variations. [ABSTRACT FROM AUTHOR]

The early Holocene, about 9,500 years ago (9.5 ka BP), was still affected by the presence of remnant ice sheets and their melting. Using as a reference a simulation of the early Holocene, the present study explores the relative contribution of these two factors on the climate mean state and ENSO variability in the tropical Pacific with the IPSL coupled model. The melting water flux and remnant ice sheets both induce a North Atlantic cooling and a southward shift of the ITCZ. Atmospheric teleconnections and local coupled ocean-atmosphere feedbacks lead to a remote SST cooling in the eastern equatorial Pacific. Both forcing factors also weaken the SST annual cycle in the eastern equatorial Pacific, closely related to the zonal wind stress anomalies. Compared with the early Holocene reference run, the freshwater flux experiment exhibits enhanced ENSO amplitude. A feedback analysis suggests that it is due to the wind-thermocline feedback. The remnant ice sheet experiment does not show significant change in ENSO. It exhibits a slight SST variability increase at the east coast and a reduction in the middle of the basin driven by the net heat flux feedback. All experiments reproduce the classical Eastern Pacific (EP) El Niño, and the central Pacific El Niño. The freshwater flux forcing strengthens the amplitude of EP El Niño events due to the stronger wind-thermocline feedback in this experiment. These results suggest that ice sheet melting and the remnant ice-sheet have partially counteracted the insolation forcing in the early Holocene. [ABSTRACT FROM AUTHOR]

In order to better understand the evolution of the Afro-Asian monsoon in the early Holocene, we investigate the impact on boreal summer monsoon characteristics of (1) a freshwater flux in the North Atlantic from the surrounding melting ice sheets and (2) a remnant ice sheet over North America and Europe. Sensitivity experiments run with the IPSL_CM4 model show that both the meltwater flux and the remnant ice sheets induce a cooling of similar amplitude of the North Atlantic leading to a southward shift of the Inter-Tropical Convergence Zone over the tropical Atlantic and to a reduction of the African monsoon. The two perturbations have different impacts in the Asian sector. The meltwater flux results in a weakening of the Indian monsoon and no change in the East Asian monsoon, whereas the remnant ice sheets induce a strengthening of the Indian monsoon and a strong weakening of the East Asian monsoon. Despite the similar coolings in the Atlantic Ocean, the ocean heat transport is reduced only in the meltwater flux experiment, which induces slight differences between the two experiments in the role of the surface latent heat flux in the tropical energetics. In the meltwater experiment, the southward shift of the subtropical jet acts to cool the upper atmosphere over the Tibetan Plateau and hence to weaken the Indian monsoon. In the ice sheet experiment this effect is overwhelmed by the changes in extratropical stationary waves induced by the ice sheets, which are associated with a larger cooling over the Eurasian continent than in the meltwater experiment. However these sensitivity experiments suggest that insolation is the dominant factor explaining the relative changes of the African, Indian and East Asian monsoons from the early to the mid-Holocene. [ABSTRACT FROM AUTHOR]

The climates of the mid-Holocene (MH), 6,000 years ago, and of the Last Glacial Maximum (LGM), 21,000 years ago, have extensively been simulated, in particular in the framework of the Palaeoclimate Modelling Intercomparion Project. These periods are well documented by paleo-records, which can be used for evaluating model results for climates different from the present one. Here, we present new simulations of the MH and the LGM climates obtained with the IPSL_CM5A model and compare them to our previous results obtained with the IPSL_CM4 model. Compared to IPSL_CM4, IPSL_CM5A includes two new features: the interactive representation of the plant phenology and marine biogeochemistry. But one of the most important differences between these models is the latitudinal resolution and vertical domain of their atmospheric component, which have been improved in IPSL_CM5A and results in a better representation of the mid-latitude jet-streams. The Asian monsoon's representation is also substantially improved. The global average mean annual temperature simulated for the pre-industrial (PI) period is colder in IPSL_CM5A than in IPSL_CM4 but their climate sensitivity to a CO doubling is similar. Here we show that these differences in the simulated PI climate have an impact on the simulated MH and LGM climatic anomalies. The larger cooling response to LGM boundary conditions in IPSL_CM5A appears to be mainly due to differences between the PMIP3 and PMIP2 boundary conditions, as shown by a short wave radiative forcing/feedback analysis based on a simplified perturbation method. It is found that the sensitivity computed from the LGM climate is lower than that computed from 2 × CO simulations, confirming previous studies based on different models. For the MH, the Asian monsoon, stronger in the IPSL_CM5A PI simulation, is also more sensitive to the insolation changes. The African monsoon is also further amplified in IPSL_CM5A due to the impact of the interactive phenology. Finally the changes in variability for both models and for MH and LGM are presented taking the example of the El-Niño Southern Oscillation (ENSO), which is very different in the PI simulations. ENSO variability is damped in both model versions at the MH, whereas inconsistent responses are found between the two versions for the LGM. Part 2 of this paper examines whether these differences between IPSL_CM4 and IPSL_CM5A can be distinguished when comparing those results to palaeo-climatic reconstructions and investigates new approaches for model-data comparisons made possible by the inclusion of new components in IPSL_CM5A. [ABSTRACT FROM AUTHOR]

The climates of the mid-Holocene (MH, 6,000 years ago) and the Last Glacial Maximum (LGM, 21,000 years ago) have been extensively documented and as such, have become targets for the evaluation of climate models for climate contexts very different from the present. In Part 1 of the present work, we have studied the MH and LGM simulations performed with the last two versions of the IPSL model: IPSL_CM4, run for the PMIP2/CMIP3 (Coupled Model Intercomparion Project) projects and IPSL_CM5A, run for the most recent PMIP3/CMIP5 projets. We have shown that not only are these models different in their simulations of the PI climate, but also in their simulations of the climatic anomalies for the MH and LGM. In the Part 2 of this paper, we first examine whether palaeo-data can help discriminate between the model performances. This is indeed the case for the African monsoon for the MH or for North America south of the Laurentide ice sheet, the South Atlantic or the southern Indian ocean for the LGM. For the LGM, off-line vegetation modelling appears to offer good opportunities to distinguish climate model results because glacial vegetation proves to be very sensitive to even small differences in LGM climate. For other cases such as the LGM North Atlantic or the LGM equatorial Pacific, the large uncertainty on the SST reconstructions, prevents model discrimination. We have examined the use of other proxy-data for model evaluation, which has become possible with the inclusion of the biogeochemistry morel PISCES in the IPSL_CM5A model. We show a broad agreement of the LGM-PI export production changes with reconstructions. These changes are related to the mixed layer depth in most regions and to sea-ice variations in the high latitudes. We have also modelled foraminifer abundances with the FORAMCLIM model and shown that the changes in foraminifer abundance in the equatorial Pacific are mainly forced by changes in SSTs, hence confirming the SST-foraminifer abundance relationship. Yet, this is not the case in all regions in the North Atlantic, where food availability can have a strong impact of foraminifer abundances. Further work will be needed to exhaustively examine the role of factors other than climate in piloting changes in palaeo-indicators. [ABSTRACT FROM AUTHOR]

Simulations of the West African monsoon (WAM) for the present-day climate (0 ka) and the mid-Holocene (6 ka) using the coupled models from the Paleoclimate Modelling Intercomparison Project phase 2 (PMIP2) are assessed in this study. The authors first compare the ensemble simulations with modern observations and proxy estimates of past precipitation, showing that the PMIP2 model median captures the basic features of the WAM for 0 ka and the changes at 6 ka, despite systematic biases in the preindustrial (PI) simulations and underestimates of the northward extent and intensity of precipitation changes. The model spread is then discussed based on a classification of the monsoonal convective regimes for a subset of seven coupled models. Two major categories of model are defined based on their differences in simulating deep and moderate convective regimes in the PI simulations. Changes in precipitation at 6 ka are dominated by changes in the large-scale dynamics for most of the PMIP2 models and are characterized by a shift in the monsoonal circulation toward deeper convective regimes. Consequently, changes in the total precipitation at 6 ka depend on the changes in convective regimes and the characteristics of these regimes in the PI simulations. The results indicate that systematic model biases in simulating the radiation and heat fluxes could explain the damping of the meridional temperature gradient over West Africa and thereby the underestimation of precipitation in the Sahel-Sahara region. [ABSTRACT FROM AUTHOR]

The too diverse representation of ENSO in a coupled GCM limits one’s ability to describe future change of its properties. Several studies pointed to the key role of atmosphere feedbacks in contributing to this diversity. These feedbacks are analyzed here in two simulations of a coupled GCM that differ only by the parameterization of deep atmospheric convection and the associated clouds. Using the Kerry–Emanuel (KE) scheme in the L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL CM4; KE simulation), ENSO has about the right amplitude, whereas it is almost suppressed when using the Tiedke (TI) scheme. Quantifying both the dynamical Bjerknes feedback and the heat flux feedback in KE, TI, and the corresponding Atmospheric Model Intercomparison Project (AMIP) atmosphere-only simulations, it is shown that the suppression of ENSO in TI is due to a doubling of the damping via heat flux feedback. Because the Bjerknes positive feedback is weak in both simulations, the KE simulation exhibits the right ENSO amplitude owing to an error compensation between a too weak heat flux feedback and a too weak Bjerknes feedback. In TI, the heat flux feedback strength is closer to estimates from observations and reanalysis, leading to ENSO suppression. The shortwave heat flux and, to a lesser extent, the latent heat flux feedbacks are the dominant contributors to the change between TI and KE. The shortwave heat flux feedback differences are traced back to a modified distribution of the large-scale regimes of deep convection (negative feedback) and subsidence (positive feedback) in the east Pacific. These are further associated with the model systematic errors. It is argued that a systematic and detailed evaluation of atmosphere feedbacks during ENSO is a necessary step to fully understand its simulation in coupled GCMs. [ABSTRACT FROM AUTHOR]

This study investigates the role of insolation in controlling the Indian and African monsoon evolutions during the Holocene using coupled ocean-atmosphere simulations of 0, 6, 9.5 kyr BP climates, for which only the variations of Earth’s orbital configuration are considered. The two monsoon systems are enhanced at 6 and 9.5 kyr BP, compared to 0 kyr BP, as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. The analysis of daily climatologies indicates that even though the length of the “celestial” summer season is shorter at 9.5 kyr BP, the rainy season is longer than at present. Emphasis is put on the impact of the precession on the seasonality, which partly explains why the relative amplification of the Indian and African monsoon varies between 9.5 and 6 kyr BP. Moreover, the changes in snow cover over the Tibetan Plateau play a critical role in reinforcing the 9.5 kyr BP monsoon in India during spring. The results suggest that the teleconnection between convection over India and subsidence over the Mediterranean regions, through the Rodwell and Hoskins mechanism, has an impact on the development of the African monsoon at 9.5 kyr BP. [ABSTRACT FROM AUTHOR]

Abstract: The outputs of the climate simulated by two General Circulation Models (GCMs), (IPSL and UGAMP) have been used to force a vegetation model (LPJ-GUESS) to analyze the Holocene African humid period (AHP) and related vegetation changes over the 18°W-35°E, 5°S-25°N region. At the continental scale, simulations with the two models confirm the intensified African monsoon during the Holocene as compared to now, and the early but gradual termination of the AHP in eastern regions as compared to western regions. At the regional scale, the two GCMs results present important differences in the timing of the AHP, its spatial extent and the summer rainfall amplitude. Consequently, the vegetation model simulates changes that are globally in agreement with pollen data, but with large differences according to the region and the model considered. During the AHP, the IPSL climate induced proper vegetation changes in the eastern Sahara and in the Sahel, whereas the UGAMP climate induced correct changes in the western Sahara and in the equatorial zone. [Copyright &y& Elsevier]

Abstract: The role of ocean feedback on monsoon variations at 6 and 9.5kyr Before Present (BP) compared to present-day is investigated by using sets of simulations computed with the IPSL–CM4 ocean–atmosphere coupled model and simulations with the atmospheric model only with the SST prescribed to the present-day simulation for the coupled model. This work is complementary to the study by Marzin and Braconnot (2009) who have analyzed in detail the response of Indian and African monsoons to changes in insolation at 6 and 9.5kyr BP using the IPSL–CM4 coupled model. The monsoon rainfall was intensified at 6 and 9.5kyr BP compared to 0kyr BP as a result of the intensified seasonal cycle of insolation in the Northern Hemisphere. In this paper, the impact of the ocean feedback is analysed for the Indian, East-Asian and African monsoons. The response of the ocean to the 6 and 9.5kyr BP insolation forcing shares similarities between the two periods, but we highlight local differences and a delay in the response of the surface ocean between 6 and 9.5kyr BP. The ocean feedback is shown to be positive for the early stage of the African monsoon. A dipole of SST in the tropical Atlantic favouring the earlier build-up of the monsoon in the 6 and 9.5kyr BP coupled simulations. However, it is strongly negative for the Indian and East Asian monsoons, and of stronger amplitude at 9.5 than at 6kyr BP over India. In these Asian regions, the convection is more active over the ocean than over the continent during the late monsoon season due to the ocean feedback. The results are consistent with previous studies about 6kyr BP climate. In addition, it is shown that the ocean feedback is not sufficient to explain the relative amplifications of the different monsoon systems within the three periods of the Holocene, but that the mechanisms such as the effect of the precession on the seasonal cycle of monsoons as discussed in Marzin and Braconnot (2009) are more plausible. [Copyright &y& Elsevier]

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]

This paper briefly surveys areas of paleoclimate modeling notable for recent progress. New ideas, including hypotheses giving a pivotal role to sea ice, have revitalized the low-order models used to simulate the time evolution of glacial cycles through the Pleistocene, a prohibitive length of time for comprehensive general circulation models (GCMs). In a recent breakthrough, however, GCMs have succeeded in simulating the onset of glaciations. This occurs at times (most recently, 115 kyr b.p.) when high northern latitudes are cold enough to maintain a snow cover and tropical latitudes are warm, enhancing the moisture source. More generally, the improvement in models has allowed simulations of key periods such as the Last Glacial Maximum and the mid-Holocene that compare more favorably and in more detail with paleoproxy data. These models now simulate ENSO cycles, and some of them have been shown to reproduce the reduction of ENSO activity observed in the early to middle Holocene. Modeling studies have demonstrated that the reduction is a response to the altered orbital configuration at that time. An urgent challenge for paleoclimate modeling is to explain and to simulate the abrupt changes observed during glacial epochs (i.e., Dansgaard–Oescher cycles, Heinrich events, and the Younger Dryas). Efforts have begun to simulate the last millennium. Over this time the forcing due to orbital variations is less important than the radiance changes due to volcanic eruptions and variations in solar output. Simulations of these natural variations test the models relied on for future climate change projections. They provide better estimates of the internal and naturally forced variability at centennial time scales, elucidating how unusual the recent global temperature trends are. [ABSTRACT FROM AUTHOR]

Past climate conditions represent a many faceted challenge for our understanding of the natural variability of the climate system and of the major interactions between its various components. This paper presents a short review of the evolution of past climate simulations using three-dimensional models for the Last Glacial Maximum and the mid-Holocene climates. Its shows how the increase in model complexity allows to test new ideas and mechanisms, using results from the Paleoclimate Modeling Intercomparison Project (PMIP) and from simulations where the atmospheric model is coupled to an ocean or a vegetation model. The two periods are respectively representative of the response of the climate system to an annual mean forcing and a seasonal forcing. To cite this article: P. Braconnot, C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

Using a fully coupled ocean–atmosphere model, we have investigated the response of the Indian and southeast monsoons to changes in precession, with special emphasis on the Indian Ocean. We designed our experiments in order to explore the differences between periods where the monsoon activity is strong compared to the period of reference (115 ka BP), and the seasonal timing of the insolation forcing is different. The two extreme cases are such that the maximum insolation occurs between the vernal equinox and the summer solstice in one case and between the summer solstice and the autumnal equinox in the other. Our results show that, even though the changes in the land–sea contrast that drive the monsoon flow follow quite well the differences in the insolation forcing, the regional distribution of monsoon characteristics (monsoon flow, precipitation etc.) from Africa to Southeast Asia and over the ocean substantially varies from one simulation to the other. Large differences are found in the simulated surface temperature and salinity in the Indian Ocean. They are mainly due to changes in the hydrological cycle over the basin (precipitation, evaporation and river runoff). Our results strongly emphasise that the timing of the seasonal cycle must be taken into account in analyses of past monsoons, and that it may explain some of the leads and lags reported between insolation and various monsoon proxies reported in the literature [Copyright &y& Elsevier]

This study describes time evolution of the Southern Annular Mode (SAM) in Mid- to Late Holocene simulated with a state-of-the-art transient simulation of the last 6000 years carried out with the IPSL-CM5A2 model. Simulated SAM index exhibits significant long-term linear trends of different sign depending on the season that are closely related to multi-millennial changes in insolation which was the main driver of long-term climate change in the study period. Interactions between changes in insolation and the SAM are linked to temperature and pressure changes developed through the entire Southern Hemisphere. In fact, model results suggest that insolation changes produced significant changes in extratropical temperature gradients that, in turn, induced changes in pressure gradients synthesized by significant long-term linear trends in the SAM index from Mid- to Late-Holocene. Considering that changes in the SAM index synthetize changes in hemispheric patterns of temperature, pressure and winds, results exposed in this study should be considered as reference for reconstructions of SAM evolution in the last 6000 years from climate proxy archives. [ABSTRACT FROM AUTHOR]

State-of-the-art Earth system models, like the ones used in the Coupled Model Intercomparison Project Phase 6 (CMIP6), suffer from temporal inconsistencies at the ocean–atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on the Schwarz iterative method. Despite its large computational cost compared to standard coupling methods, which makes the algorithm implementation impractical for production runs, the Schwarz method is useful to evaluate some of the errors made in state-of-the-art ocean–atmosphere coupled models (e.g., in the representation of the processes related to diurnal cycle), as illustrated by the present study. IPSL-CM6-SW-VLR is a low-resolution version of the IPSL-CM6 coupled model with a simplified land surface model, implementing a Schwarz iterative coupling scheme. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme (referred to as the parallel algorithm) show large differences after sunrise and before sunset, when the external forcing (insolation at the top of the atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100 % of the solution, even with a small coupling period, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method, which leads to a doubling of the computing cost. Besides the parallel algorithm used in IPSL-CM6, we also test a so-called sequential atmosphere-first algorithm, which is used in some coupled ocean–atmosphere models. We show that the sequential algorithm improves the numerical results compared to the parallel one at the expanse of a loss of parallelism. The present study focuses on the ocean–atmosphere interface with no sea ice. The problem with three components (ocean–sea ice–atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

The Indian Summer Monsoon Rainfall (ISMR) varies on a wide range of timescales, from interannual to multi-centennial. Slight variations of seasonal rainfall amounts strongly affect livelihood, which mostly relies on rain-fed agriculture. Improving our understanding of multi-scale variations of ISMR is thus critical to advance its potential predictability and reduce society vulnerability to climate change.Here, we explore such multi-scale variations by analyzing a multi-complexity ensemble of transient simulations ran with the IPSL Earth system model from mid- to late Holocene (6000 years BP to 1950). First, we show that orbital forcing induces low-frequency changes in the ISMR mean state, with near-linear drying from mid- to late Holocene. The relative long term response between land and ocean is compared to available observations from pollen and lakes over land and ocean sediments to assess the realism of the simulation over the broad Indian sector. Second, we show that interannual-to-decadal variability increases along the period despite the drying trend of the ISMR mean state. We isolated a relationship with a concomitant increase in SST variance in the tropical Pacific, especially in its eastern equatorial part. Is the increased variability in precipitation and SST part of the same large-scale response of local variability to long term insolation? Or is the increased variability in precipitation induced by the strengthening of ENSO variability? In addition, we'll use high resolution paleo records from coral and shell over the Indo-Pacific sector and from speleothems and tree rings over land to assess the realism of the simulated changes in long term teleconnections. [ABSTRACT FROM AUTHOR]

The turbulent fluxes across the ocean/atmosphere interface, which represent one of the main drivers of the global atmospheric and oceanic circulations, are parameterized in climate models. To improve the representation of turbulent fluxes, a new parameterization has been developed in the IPSL model. It takes into account the gustiness effects which are calculated thanks to the coupling between the bulk formula (used to compute the surface turbulent fluxes) and the thermal and the convection schemes (that provide the deep and free convection effect on the subgrid variability of surface winds).Here we investigate how this new effect modulates the ocean-atmosphere interactions and feedbacks. We consider both atmosphere only and coupled ocean-atmosphere simulations ran with the IPSL climate model. We focus on the thermodynamical and the dynamical adjustments of the atmospheric and oceanic circulation induced by the coupling between gustiness and turbulent fluxes.In atmospheric stand-alone simulations, the gustiness has a direct effect on turbulent fluxes through the increase of low-level wind used for their calculation. This increases wind stress and leads to indirect effect by reducing the low-level wind. In addition, gustiness significantly impacts latent heat flux in weak wind regions. This modifies the zonal humidity gradient in the Pacific Ocean, which affects the Walker circulation. In coupled ocean-atmosphere simulations, part of the latent heat flux effect is offset by the ocean adjustment through ocean heat storage and sea surface temperature changes. A feedback loop between the wind stress and ocean circulation also affects the overall ocean and atmospheric circulations. The relative amplitude of these feedbacks will be discussed, considering the role of changes in surface turbulent fluxes on ocean and atmospheric circulations, which control heat transports and inter-basin teleconnections. [ABSTRACT FROM AUTHOR]

The Indian and West African monsoons vary on a large variety of timescales, with alternation of extremely wet and dry years having huge impacts on local economy and society. Some of these extreme monsoon events result from the chaotic nature of climate. They are also favored by teleconnection with various modes of climate variability ranging from interannual (e.g., El Nino Southern Oscillation and the Indian Ocean dipole) to decadal (e.g., Atlantic dipole and Atlantic meridional oscillation) timescales. However the relationships between extreme monsoon years and these multi-time scales of varability are not fully understood. The last 6000 years offers a wide range of test cases to analyze these relationships and the diversity of events.The mid-Holocene, 6000 years BP, is characterized by increased monsoon precipitation in West Africa and India compared to modern conditions. This is caused by the Earth's orbital configuration. The seasonal cycle of incoming solar radiation was enhanced in the northern Hemisphere and reduced it in the southern Hemisphere, thereby enhancing the monsoon flow between ocean and land. The retreat of the two monsoon systems back to present day conditions has been punctuated by periods of rapid transitions and periods of mega droughts that had significant imprints on the environment.We consider extreme monsoon events in a multi-complexity ensemble of transient simulations of the last 6000 years ran with the IPSL Earth system model. These simulations allow us to analyze the monsoon response to Earth's orbit and trace gazes, and how multiscale interactions affect extreme African and Indian monsoon events. First results indicate that interannual to decadal variability in Indian precipitation increases with time when its mean amount decreases. This likely relates to increased ENSO variability in the model, the latter being consistent with paleo-reconstructions from coral and bivalves in the Pacific. By sorting the results in different frequency band or type of extreme events, we also explore the relative changes in variability between the Indian and African monsoon, the characteristics of period with high monsoon or prolonged droughts as well as their relationship with modes of climate variability. [ABSTRACT FROM AUTHOR]

State-of-the-art Earth System models, like the ones used in the CMIP6 intercomparison project, suffer from temporal inconsistencies at the ocean-atmosphere interface. Indeed, the coupling algorithms generally implemented in those models do not allow for a correct phasing between the ocean and the atmosphere, and hence between their diurnal cycles. A possibility to remove these temporal inconsistencies is to use an iterative coupling algorithm based on Schwarz methods. Despite the fact that the computational cost is large compared to standard coupling methods, which makes the method impractical as is for production runs, Schwarz algorithms are useful to evaluate some of the errors made in state-of-the-art ocean-atmosphere coupled models (e.g. in the representation of the processes related to diurnal cycle), as illustrated by the present study. A new coupling scheme based on such iterative method has been implemented in the IPSL coupled model. Comparisons between coupled solutions obtained with this new scheme and the standard IPSL coupling scheme show large differences at sunrise and sunset, when the external forcing (insolation at top of atmosphere) has the fastest pace of change. At these times of the day, the difference between the two numerical solutions is often larger than 100% of the solution, even with a small coupling time step, thus suggesting that significant errors are potentially made with current coupling methods. Most of those differences can be strongly reduced by making only two iterations of the Schwarz method which leads to a doubling of the computing cost. A thorough design of the first guess to initialize the iterative process may yield a solution that reduces the error with only one iteration. The present study focuses on the ocean-atmosphere interface, with no sea-ice. The problem with three components (ocean/sea-ice/atmosphere) remains to be investigated. [ABSTRACT FROM AUTHOR]

Northern Hemispheric summer monsoons were more intense during the mid-Holocene (MH ~ 6000 years ago) and coincided with a northward shift of the Intertropical Convergence Zone (ITCZ) compared to the pre-industrial (PI) climate. Ancient civilizations in the Indus valley, Mesopotamia, and Egypt appear to have flourished during this period, thanks to abundant water availability. This study exploits a high-resolution variable grid global atmosphere model to understand the role of orbital forcing and ocean surface conditions in strengthening the monsoons and shifting the ITCZ northward over Africa, India, and East Asia during the MH. The combined impact of orbital forcing and sea surface temperature (SST) boundary conditions led to a change in monsoon rainfall of around 42, 30, 21, and 41% over Africa, East Asia, India, and northwest India (NWI) relative to the PI conditions. Changes in orbital parameters alone account for more than 36 and 26% of total rainfall increases in Africa and East Asia. Over the Indian subcontinent, the strengthening of monsoon was primarily a combined effect of SST and orbital forcing. In contrast, the SST boundary condition alone could explain the 39% of rainfall increase over NWI, where the Indus valley civilization once existed. Through moisture budget analysis, the study further illustrates the role of dynamic and thermodynamic factors responsible for the changes in monsoon precipitation. The enhanced monsoon resulted in a northward shift of ITCZ by around 3°N, 1.9°N, and 2.5°N over Africa, East Asia, and India, respectively, compared to its PI position. Analogous to the precipitation changes, orbital forcing mostly mediated ITCZ changes across Africa and East Asia, but the combined impact of orbital forcing and SST was responsible for the changes over India. [ABSTRACT FROM AUTHOR]

The mechanisms of El Niño–Southern Oscillation (ENSO) variability change during the mid-Holocene (MH) were investigated through analyzing the model simulations from the Paleoclimate Modelling Intercomparison Project Phases (PMIP) phase-2 and phase-3. The majority of PMIP2 and PMIP3 model simulations show a lower level of ENSO activity in the MH simulation compared to the pre-industrial (PI) simulation, which is qualitatively consistent with that inferred from the paleoclimate proxies. Through employing the Bjerknes stability index, we quantified the dynamic and thermodynamic air-sea feedbacks in the PI and MH simulations. The quantitative analyses showed that the reduced ENSO variability in MH arose from the weakening in the thermocline (TH), zonal-advection (ZA) and Ekman (EK) feedback terms. We found that all the weakened TH, ZA, and EK terms are associated with the reduction in the response of anomalous thermocline depth (Dʹ) to the zonal wind stress anomaly ( τ x ′ ) in MH compared to PI. The reduced Dʹ response is attributed to the flattened meridional structure of ENSO-related τ x ′ field in MH, which is linked to the enhanced surface poleward mean meridional current in MH. Among many aspects of the mean state changes under the MH orbital forcing, this study identified that the surface mean meridional current change might be a key factor behind the suppressed ENSO variability in MH. Lastly, through comparing our findings with the ENSO future projection studies, we found that the wind-thermocline feedback is susceptible in a changing climate, which implies that minimizing the uncertainty in the wind-thermocline feedback change may help constrain future ENSO response. [ABSTRACT FROM AUTHOR]

Simulations of the climates of the Last Glacial Maximum (LGM), 21 000 years ago, and of the Mid-Holocene (MH), 6000 years ago, allow an analysis of climate feedbacks in climate states that are radically different from today. The analyses of cloud and surface albedo feedbacks show that the shortwave cloud feedback is a major driver of differences between model results. Similar behaviours appear when comparing the LGM and MH simulated changes, highlighting the fingerprint of model physics. Even though the different feedbacks show similarities between the different climate periods, the fact that their relative strength differs from one climate to the other prevents a direct comparison of past and future climate sensitivity. The land-surface feedback also shows large disparities among models even though they all produce positive sea-ice and snow feedbacks. Models have very different sensitivities when considering the vegetation feedback. This feedback has a regional pattern that differs significantly between models and depends on their level of complexity and model biases. Analyses of the MH climate in two versions of the IPSL model provide further indication on the possibilities to assess the role of model biases and model physics on simulated climate changes using past climates for which observations can be used to assess the model results. [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 Last Glacial Maximum (LGM, 21 000 years ago) is one of the suite of paleoclimate simulations included in the current phase of the Coupled Model Intercomparison Project (CMIP6). It is an interval when insolation was similar to the present, but global ice volume was at a maximum, eustatic sea level was at or close to a minimum, greenhouse gas concentrations were lower, atmospheric aerosol loadings were higher than today, and vegetation and landsurface characteristics were different from today. The LGM has been a focus for the Paleoclimate Modelling Intercomparison Project (PMIP) since its inception, and thus many of the problems that might be associated with simulating such a radically different climate are well documented. The LGM state provides an ideal case study for evaluating climate model performance because the changes in forcing and temperature between the LGM and pre-industrial are of the same order of magnitude as those projected for the end of the 21st century. Thus, the CMIP6 LGM experiment could provide additional information that can be used to constrain estimates of climate sensitivity. The design of the Tier 1 LGM experiment (lgm) includes an assessment of uncertainties in boundary conditions, in particular through the use of different reconstructions of the ice sheets and of the change in dust forcing. Additional (Tier 2) sensitivity experiments have been designed to quantify feedbacks associated with landsurface changes and aerosol loadings, and to isolate the role of individual forcings. Model analysis and evaluation will capitalize on the relative abundance of paleoenvironmental observations and quantitative climate reconstructions already available for the LGM. [ABSTRACT FROM AUTHOR]

Turbulent momentum and heat (sensible heat and latent heat) fluxes at the air-sea interface are key components of the whole energetic of the Earth's climate. The evaluation of these fluxes in the climate models is still difficult because of the large uncertainties associated with the reference products. In this paper we present an objective metric accounting for reference uncertainties to evaluate the annual cycle of the low latitude turbulent fluxes of a suite of IPSL climate models. This metric consists in a Hotelling T test between the simulated and observed field in a reduce space characterized by the dominant modes of variability that are common to both the model and the reference, taking into account the observational uncertainty. The test is thus more severe when uncertainties are small as it is the case for sea surface temperature (SST). The results of the test show that for almost all variables and all model versions the model-reference differences are not zero. It is not possible to distinguish between model versions for sensible heat and meridional wind stress, certainly due to the large observational uncertainties. All model versions share similar biases for the different variables. There is no improvement between the reference versions of the IPSL model used for CMIP3 and CMIP5. The test also reveals that the higher horizontal resolution fails to improve the representation of the turbulent surface fluxes compared to the other versions. The representation of the fluxes is further degraded in a version with improved atmospheric physics with an amplification of some of the biases in the Indian Ocean and in the intertropical convergence zone. The ranking of the model versions for the turbulent fluxes is not correlated with the ranking found for SST. This highlights that despite the fact that SST gradients are important for the large-scale atmospheric circulation patterns, other factors such as wind speed, and air-sea temperature contrast play an important role in the representation of turbulent fluxes. [ABSTRACT FROM AUTHOR]

New pollen data from Omani mangroves (Kwar-al-Jaramah [22.49° N - 59.76° E] and Filim [20.61° N - 58.17° E]), in addition to previously published paleohydrological records from Southern Arabia improve our understanding of the timing and amplitude of the southward retreat of the Indian monsoon influence in Southern Arabia along a north-south transect. Comparison with simulations performed with the IPSL climate model, considering both snapshot experiments and transient simulations from 6000 cal yr BP to the present, confirm the latitudinally time-transgressive nature of the humid-arid transition at the end of the Holocene Humid Period. This occurred in two steps, respectively dated at around 5000 and 2700 cal yr BP. At around 5000 cal yr BP, the southward ITCZ shift was orbitally-driven and led to the abrupt aridification at Kwar-al-Jaramah and the progressive increase of dryness at Filim as the mean position of the ITCZ was centered ca 22.30°N. At 2700 cal yr BP, aridity was fully in place over all of southern Arabia due to increased climate variability. More intense rainy events during the last millennium, however, may have contributed to the discrete hydrological improvement without any impact on the regional vegetation which has remained desert to the present day. [ABSTRACT FROM AUTHOR]

1a we showed the mid- to late Holocene evolution in annual mean insolation at the top of the atmosphere in the Northern and Southern Hemisphere (hereafter NH and SH). Panel a: annual mean incoming solar radiation averaged in the Northern Hemisphere (NH) [0-90 N] and Southern Hemisphere (SH) [0-90 S] in blue and orange (dashed) lines. Panel a: annual mean incoming solar radiation averaged in the Northern Hemisphere (NH) [0-90 N] and Southern Hemisphere (SH) [0-90 S]. [Extracted from the article]

Driven by enhanced summer insolation in the Northern Hemisphere and land-atmosphere feedbacks during the mid-Holocene (MH), most regions of China were characterized by warmer and wetter summer than present. The MH has been recognized as a benchmark period for climate simulations, but proxy-based reconstructions and model results show some key discrepancies in climate reconstructions. A lack of accurate gridded climate data is a major obstacle to land surface modelling, which is contributing to uncertainties in climate simulations. Here we developed a monthly gridded dataset of temperature and precipitation for MH China through applying eight interpolation methods to 159 pollen-based climate reconstructions for January and July from previous studies. The gridded dataset is consistent with these input records and other independent climate reconstructions at 62 sites. Then, the temperature differences between the MH and pre-industrial period are correlated with elevations in China. Results show a significant elevation-dependent temperature amplification of 0.59 °C/km for July during the MH relative to pre-industrial period. In addition, our gridded reconstructions of the MH climate were compared with climate simulations for the MH and for the future medium-emission scenario. Most simulations significantly underestimated the magnitude of climate anomalies during the MH in China. Similar to the MH, a pronounced warming in winter and a strong wetting in summer are projected in China at the end of this century (i.e., 2091–2100), suggesting analogous seasonal changes during the MH and future. This first high-resolution proxy-based gridded climate dataset for MH China can be used in land surface modelling to improve our understanding of terrestrial changes during the MH, which in turn is expected to facilitate the climate simulations. • Gridded temperature and precipitation data are developed for China in the MH. • The dataset is consistent with input records and independent reconstructions. • A significant elevation-dependent MH warming of 0.59 °C/km is revealed for July. • MH seasonal changes of winter warming and summer wetting are similar to the future. [ABSTRACT FROM AUTHOR]

Changes in climate mean state profoundly impact climate variability. Here, we quantify slow changes in the mean climate induced by the variations in the Earth's orbit from mid- to late Holocene, and their feedback on the main modes of climate variability. We focus on the Indo-Pacific system and show that mid-Holocene conditions favored the dominance of an equatorial dipole mode in the Indian Ocean (IO), independent of the El Niño Southern Oscillation (ENSO) and different from the IO Dipole (IOD) observed today. Mean state changes induced a gradual shift to an IO basin mode that along with the IOD modulates most of the IO variance at present. The climate modes evolution and their connectivity changes are investigated over 6000 years using a complex network methodology and principal component analysis. To uncover the causal link between a constantly evolving mean state and such changes in variability we consider two independent analyses. First, we explore changes in the mean state by quantifying the departure of 500-year annual mean snapshots from the annual mean conditions averaged over the mid-Holocene. Second, we leverage ideas from dynamical system theory and characterize the nature of the Indo-Pacifc transition, in a state space representation, by accounting for its spatiotemporal and multivariable dependency. The analysis reveals that a strengthening of the Walker circulation, driven by enhanced convection over the central Pacific and by the weakening of the Asian monsoon, set the stage for a shift in modes in both basins. [ABSTRACT FROM AUTHOR]

Future climate evolution is of primary importance for the societal, economical, political orientations and decision-making. It explains the increasing use of climate projections as input for quantitative impact studies, assessing vulnerability and defining adaptation strategies in different sectors. Here we analyse 17 national and representative use cases so as to identify the diversity of the demand for climate information depending on user profiles as well as the best practices, methods and tools that are needed to answer the different requests. A particular emphasis is put on the workflow that allows to translate climate data into suitable impact data, the way to deal with the different sources of uncertainty and to provide a suited product to users. We identified three complementary tools to close the gap between climate scientists and user needs: an efficient interface between users and providers; an optimized methodology to handle user requests and a portal to facilitate access to data and elaborated products. We detail in the paper how these three tools can limit the intervention of experts, educate users, and lead to the production of useful information. This work provides the basis on which the ENES (European Network for Earth System Modelling) Portal Interface for the Climate Impact Communities is built. [ABSTRACT FROM AUTHOR]

We argue that Arctic sea ice played an important role during early stages of the last glacial inception. Two simulations of the Institut Pierre Simon Laplace coupled model 4 are analyzed, one for the time of maximum high latitude summer insolation during the last interglacial, the Eemian, and a second one for the subsequent summer insolation minimum, at the last glacial inception. During the inception, increased Arctic freshwater export by sea ice shuts down Labrador Sea convection and weakens overturning circulation and oceanic heat transport by 27 and 15%, respectively. A positive feedback of the Atlantic subpolar gyre enhances the initial freshening by sea ice. The reorganization of the subpolar surface circulation, however, makes the Atlantic inflow more saline and thereby maintains deep convection in the Nordic Seas. These results highlight the importance of an accurate representation of dynamic sea ice for the study of past and future climate changes. [ABSTRACT FROM AUTHOR]

The intraseasonal variability (ISV) is an intermittent phenomenon with variable perturbation patterns. To assess the robustness of the simulated ISV in climate models, it is thus interesting to consider the distribution of perturbation patterns rather than only one average pattern. To inspect this distribution, the authors first introduce a distance that measures the similarity between two patterns. The reproducibility (realism) of the simulated intraseasonal patterns is then defined as the distribution of distances between each pattern and the average simulated (observed) pattern. A good reproducibility is required to analyze the physical source of the simulated disturbances. The realism distribution is required to estimate the proportion of simulated events that have a perturbation pattern similar to observed patterns. The median value of this realism distribution is introduced as an ISV metric. The reproducibility and realism distributions are used to evaluate boreal summer ISV of precipitations over the Indian Ocean for 19 phase 3 of the Coupled Model Intercomparison Project (CMIP3) models. The 19 models are classified in increasing ISV metric order. In agreement with previous studies, the four best ISV metrics are obtained for models having a convective closure totally or partly based on the moisture convergence. Models with high metric values (poorly realistic) tend to give (i) poorly reproducible intraseasonal patterns, (ii) rainfall perturbations poorly organized at large scales, (iii) small day-to-day variability with overly red temporal spectra, and (iv) less accurate summer monsoon rainfall distribution. This confirms that the ISV is an important link in the seamless system that connects weather and climate. [ABSTRACT FROM AUTHOR]

The response of climate to freshwater input in the North Atlantic (NA) has raised a lot of concern about the issue of climate stability since the discovery of abrupt coolings during the last glacial period. Such coolings have usually been related to a weakening of the Atlantic meridional overturning circulation (AMOC), probably associated with massive iceberg surges or meltwater pulses. Additionally, the recent increase in greenhouse gases in the atmosphere has also raised the possibility of a melting of the Greenland ice sheet, which may impact the future AMOC, and thereby the climate. In this study, the extent to which the mean climate influences the freshwater release linked to ice sheet melting in the NA and the associated climatic response is explored. For this purpose the simulations of several climatic states [last interglacial, Last Glacial Maximum, mid-Holocene, preindustrial, and future (2 × CO2)] are considered, and the climatic response to a freshwater input computed interactively according to a surface heat flux budget over the ice sheets is analyzed. It is shown that the AMOC response is not linear with the freshwater input and depends on the mean climate state. The climatic responses to these different AMOC changes share qualitative similarities for the general picture, notably a cooling in the Northern Hemisphere and a southward shift of the intertropical convergence zone (ITCZ) in the Atlantic and across the Panama Isthmus. The cooling in the Northern Hemisphere is related to the sea ice cover response, which strongly depends on the responses of the atmospheric circulation, the local oceanic heat transport, and the density threshold of the oceanic convection sites. These feedbacks and the magnitude of temperature and precipitation changes outside the North Atlantic depend on the mean climate. [ABSTRACT FROM AUTHOR]

The temperature response to a greenhouse gas (GHG) concentration change is studied in an ocean–atmosphere coupled model—L’Institut Pierre-Simon Laplace Coupled Model, version 4 (IPSL-CM4)—for both a glacial and an interglacial context. The response to a GHG concentration changing from Last Glacial Maximum (LGM) to preindustrial values is similar for both climatic contexts in terms of temperature pattern, but the magnitude is greater under modern ones. The model simulates the classical amplification of the temperature response in the northern high latitudes compared to lower latitudes and over the land surfaces compared to the ocean. The physical reasons for the differential warming according to the latitude and to the surface type are studied through an analysis of the energy flux changes, which are decomposed to consider and quantify many different physical processes. The results highlight the role of many different factors in the thermal response to a GHG forcing for different regions, and stress, for instance, the large effect of increased water vapor concentration in the atmosphere. Concerning the land–sea warming ratio, several fluxes contribute to the final value of the ratio, with latent flux having the greatest influence. The different contributions are quantified. The comparison of the flux changes between the interglacial and glacial contexts shows that the differences are more than a simple effect of different surface emissions of the base state. It suggests that the climatic context is particularly important for the cloud and oceanic advection responses to the forcing, along with albedo effects. [ABSTRACT FROM AUTHOR]

The Sahara was significantly wetter and greener than today during the mid‐Holocene (∼6,000 years before present), and those conditions were likely maintained by feedbacks from evaporating wetlands and riparian zones. A lack of spatially continuous wetland reconstruction is the major obstacle to investigating their impacts on climate and vegetation during that epoch. Here, we estimate high‐resolution gridded wetland distribution up to 15″ in the mid‐Holocene North Africa obtained with three statistical and hydrological modeling approaches forced by enhanced and calibrated precipitation from climate models. These wetland models have good performance for present‐day conditions and reproduce mid‐Holocene hydrological elements evaluated by 297 paleo‐records. Simulation results show that 18.9 ± 4.0% of land surface in North Africa was covered by wetlands during the mid‐Holocene. Our results highlight the impact of natural climate change on wetland areas and provide a data set for modeling studies to include wetland feedbacks. Plain Language Summary: Several lines of evidence show that northern Africa was considerably wetter and greener than today at ∼6,000 years ago, which is known as the mid‐Holocene Green Sahara (GS). However, most current models could not reproduce climate in the GS. The importance of wetland feedbacks on sustaining a wetter climate has partially been recognized while large uncertainties in wetland coverage make it difficult to examine wetland feedbacks in climate models. We trained several wetland models under present climate and applied them to wetland reconstructions of GS. The produced wetland maps could capture dense wetlands indicated by 297 paleo‐records. The total wetland fraction in the mid‐Holocene North Africa is 18.9 ± 4.0%, which is more than five times the area today (∼2.8%). The relationship between wetland fraction and precipitation is examined in our models. This work improves our understanding of the GS enigma, and has implications for potential greening and wetting of Sahel and Sahara in the future. Key Points: We present high‐resolution reconstructions of wetland distribution up to 15″ for the mid‐Holocene North AfricaDuring the mid‐Holocene, 18.9 ± 4.0% of land surface in North Africa was covered by wetlandsThe relationship between precipitation and wetland fraction in the mid‐Holocene Sahara is explored [ABSTRACT FROM AUTHOR]

Many models in the framework of the Paleoclimate Modelling Intercomparison Project have undertaken simulations of the mid-Holocene (6 kyr ago) climate change. Analysis of the results have mainly focused on the North African summer monsoon that was enhanced 6 kyr ago, in all models, in response to the prescribed enhanced summer insolation. The magnitude of the simulated increase in total rainfall is very different, however, among the models, and so is the prescribed mean hot desert albedo, which varies from 19% to 38%. The appropriate prescription of hot desert's brightness, in the simulation of present-day climate, is known to be a key parameter since the work of Charney, which has been confirmed by many subsequent studies. There is yet no consensus, however, on the albedo climatological values to be used by climate modelers. Here, it is questioned whether changes in the prescription of hot desert albedo may also affect the simulated intensity of climate change. Using the Laboratoire de Météorologie Dynamique atmospheric general circulation model, two sets of simulations, with a mean hot desert albedo of respectively 35% and 28%, have been carried out. The simulated mid-Holocene summer monsoon change in northern Africa is significantly larger when the background hot desert albedo is the lowest (i.e., 28%). The associated increased northward penetration of monsoon rains allows a greater reduction of hot desert area that is in better agreement with paleodata. At least three good reasons have been found to explain these changes, one of them being that when hot desert albedo is relatively low, the atmosphere above is more unstable and the same increase in solar forcing leads to larger changes in precipitable water. The implication of such a study is that differences in models' responses to any external forcing (insolation, increased atmospheric CO[sub 2] , etc.) may be partly explained by differences in the prescription of land surface properties. Th... [ABSTRACT FROM AUTHOR]