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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]

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]

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]

We investigate hydrology during a past climate slightly warmer than the present: the last interglacial (LIG). With daily output of preindustrial and LIG simulations from eight new climate models we force hydrological model PCR‐GLOBWB and in turn hydrodynamic model CaMa‐Flood. Compared to preindustrial, annual mean LIG runoff, discharge, and 100‐yr flood volume are considerably larger in the Northern Hemisphere, by 14%, 25%, and 82%, respectively. Anomalies are negative in the Southern Hemisphere. In some boreal regions, LIG runoff and discharge are lower despite higher precipitation, due to the higher temperatures and evaporation. LIG discharge is much higher for the Niger, Congo, Nile, Ganges, Irrawaddy, and Pearl and lower for the Mississippi, Saint Lawrence, Amazon, Paraná, Orange, Zambesi, Danube, and Ob. Discharge is seasonally postponed in tropical rivers affected by monsoon changes. Results agree with published proxies on the sign of discharge anomaly in 15 of 23 sites where comparison is possible. Plain Language Summary: It is still uncertain how the water cycle will respond to a warmer climate in the coming decades. To increase our understanding of the relationships between climate and hydrology, we study the past climate of the last interglacial, which was slightly warmer than the present. We present the results of a modeling approach, showing that while Northern Hemisphere precipitation was higher during the last interglacial, discharge of rivers was even higher, and floods were even larger. On the contrary, in the Southern Hemisphere precipitation, discharge and floods were lower. We show that, for some regions, precipitation, discharge, and floods do not have the same direction of change. The seasonal timing of discharge also changes for some large basins of the Northern Hemisphere. Finally, for 23 sites, we compare our results to geological evidence. These results form a useful term of comparison to both projections of the future and geological studies of past hydrology. Key Points: We present the first modeling of hydrology and floods for the last interglacialBoreal precipitation and runoff are higher than preindustrial, and boreal river discharge and flood volume are (in %) even higherMost of the discharge occurs later in the year in large river basins of the Northern Hemisphere [ABSTRACT FROM AUTHOR]

This study presents the global climate model IPSL‐CM6A‐LR developed at Institut Pierre‐Simon Laplace (IPSL) to study natural climate variability and climate response to natural and anthropogenic forcings as part of the sixth phase of the Coupled Model Intercomparison Project (CMIP6). This article describes the different model components, their coupling, and the simulated climate in comparison to previous model versions. We focus here on the representation of the physical climate along with the main characteristics of the global carbon cycle. The model's climatology, as assessed from a range of metrics (related in particular to radiation, temperature, precipitation, and wind), is strongly improved in comparison to previous model versions. Although they are reduced, a number of known biases and shortcomings (e.g., double Intertropical Convergence Zone [ITCZ], frequency of midlatitude wintertime blockings, and El Niño–Southern Oscillation [ENSO] dynamics) persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. A large ensemble of more than 30 members for the historical period (1850–2018) and a smaller ensemble for a range of emissions scenarios (until 2100 and 2300) are also presented and discussed. Plain Language Summary: Climate models are unique tools to investigate the characteristics and behavior of the climate system. While climate models and their components are developed gradually over the years, the sixth phase of the Coupled Model Intercomparison Project (CMIP6) has been the opportunity for the Institut Pierre‐Simon Laplace to develop, test, and evaluate a new configuration of its climate model called IPSL‐CM6A‐LR. The characteristics and emerging properties of this new model are presented in this study. The model climatology, as assessed from a range of metrics, is strongly improved, although a number of biases common to many models do persist. The equilibrium climate sensitivity and transient climate response have both increased from the previous climate model IPSL‐CM5A‐LR used in CMIP5. Key Points: The IPSL‐CM6A‐LR model climatology is much improved over the previous version, although some systematic biases and shortcomings persistA long preindustrial control and a large number of historical and scenario simulations have been performed as part of CMIP6The effective climate sensitivity of the IPSL model increases from 4.1 to 4.8 K between IPSL‐CM5A‐LR and IPSL‐CM6A‐LR [ABSTRACT FROM AUTHOR]

Based on the fifth phase of the Coupled Model Intercomparison Project (CMIP5)-generation previous Institut Pierre Simon Laplace (IPSL) Earth system model, we designed a new version, IPSL-CM5A2, aiming at running multi-millennial simulations typical of deep-time paleoclimate studies. Three priorities were followed during the setup of the model: (1) improving the overall model computing performance, (2) overcoming a persistent cold bias depicted in the previous model generation and (3) making the model able to handle the specific continental configurations of the geological past. These developments include the integration of hybrid parallelization Message Passing Interface – Open Multi-Processing (MPI-OpenMP) in the atmospheric model of the Laboratoire de Météorologie Dynamique (LMDZ), the use of a new library to perform parallel asynchronous input/output by using computing cores as "I/O servers" and the use of a parallel coupling library between the ocean and the atmospheric components. The model, which runs with an atmospheric resolution of 3.75∘×1.875∘ and 2 to 0.5 ∘ in the ocean, can now simulate ∼100 years per day, opening new possibilities towards the production of multi-millennial simulations with a full Earth system model. The tuning strategy employed to overcome a persistent cold bias is detailed. The confrontation of a historical simulation to climatological observations shows overall improved ocean meridional overturning circulation, marine productivity and latitudinal position of zonal wind patterns. We also present the numerous steps required to run IPSL-CM5A2 for deep-time paleoclimates through a preliminary case study for the Cretaceous. Namely, specific work on the ocean model grid was required to run the model for specific continental configurations in which continents are relocated according to past paleogeographic reconstructions. By briefly discussing the spin-up of such a simulation, we elaborate on the requirements and challenges awaiting paleoclimate modeling in the next years, namely finding the best trade-off between the level of description of the processes and the computing cost on supercomputers. [ABSTRACT FROM AUTHOR]

Based on the CMIP5-generation previous IPSL earth system model, we designed a new version, IPSL-CM5A2, aiming at running multi-millennial simulations typical of deep-time paleoclimates studies. Three priorities were followed during the set-up of the model: (1) improving the overall model computing performance, (2) overcoming a persistent cold bias depicted in the previous model generation, and (3) making the model able to handle the specific continental configurations of the geological past. Technical developments have been performed on separate components and on the coupling system to speed up the whole coupled model. These developments include the integration of hybrid parallelization MPI-OpenMP in LMDz atmospheric component, the use of a new library to perform parallel asynchronous input/output by using computing cores as "IO servers", the use of a parallel coupling library between the ocean and the atmospheric components. The model can now simulate ~100 years per day, opening new possibilities towards the production of multi-millennial simulations with a full earth system model. The tuning strategy employed to overcome a persistent cold bias is detailed. The confrontation of an historical simulation to climatological observations shows overall improved ocean meridional overturning circulation, marine productivity and latitudinal position of zonal wind patterns. We also present the numerous steps required to run the IPSL-CM5A2 for deep-time paleoclimates through a preliminary case-study for the Cretaceous. Namely, a specific work on the ocean model grid was required to run the model for specific continental configurations in which continents are relocated according to past paleogeographic reconstructions. By briefly discussing the spin-up of such a simulation, we elaborate on the requirements and challenges awaiting paleoclimate modelling in the next years, namely finding the best trade-off between the level of description of the processes and the computing cost on supercomputers. [ABSTRACT FROM AUTHOR]

Earth’s climate is a complex dynamical system. Its underlying components interact with eachother in linear or nonlinear ways on multiple several spatial and time scales creating possiblepositive and negative feedback loops. Understanding and studying such a system is a difficultchallenge with crucial implications on our society but requires data mining through anextremely large and complex amount of data. In particular, the investigation ofchanges in dynamical and statistical properties in the climate of the past represents aninvaluable opportunity to understand interactions between external forcing (e.g., orbitalparameters) and the internal variability of the climate system. Given a simulatedclimate field embedded on a two dimensional grid, we propose a new methodology to(i) quantify nonlinear changes in variability in simulated climate fields by meansof an information entropy quantifier, (ii) identify regions that undergo changes inentropy in a homogeneous way and (iii) investigate the time dependency of theconnectivity patterns between these regions. To address these points, we analyze twoglobal transient simulations ran with the IPSL Earth system model from mid- to lateHolocene (6000 years BP to 1950). These simulations differ according to theirhorizontal resolution (3.75ox 1.89ovs. 2.5ox 1.27o), their hydrological component(2 vs. 11 layers) and the way vegetation interacts with climate (prescribed versusdynamic). Specifically, we focus on the variability and connectivity of two fields: seasurface temperature (SST) and precipitation. Time resolution is monthly. For agiven field, the input of the algorithm is a two dimensional grid with detrendedanomalies embedded in it. We consider time windows of 100 years of data every20 years and for each time window we compute the entropy of every time seriesembedded in the grid. The entropy serves as a measure of complexity of time series andits computation depends on recurrence plots, an advanced nonlinear data analysistechnique. The product of this first step is then a spatio-temporal entropy field.We then use δ-MAPS, a dimensionality reduction methodology recently proposedby the authors, to reduce the dimensionality of the entropy field by identifyingsemi-autonomous components, referred to as "domains". Domains are defined as spatiallycontiguous, possibly overlapping, structures that undergo changes in entropy in ahomogeneous way. This allows to study both past climate regime shifts and their spatialheterogeneity. Finally, given two sets of domains (in the SST and precipitation fields),for each time window, we compute their signals and infer a functional network ofnetworks (NoN) between them. The NoN is a weighted and potentially directed graphwhich, in a single framework, quantify the interactions between domains in the samefield and the dependency among domains of different fields. A first analysis of thetime dependency of domains’ complexity and of their NoN topology is presented. [ABSTRACT FROM AUTHOR]

Precipitation projections are usually presented as the change in precipitation between a fixed current baseline and a particular time in the future. However, upcoming generations will be affected in a way probably more related to the moving trend in precipitation patterns, i.e. to the rate and the persistence of regional precipitation changes from one generation to the next, than to changes relative to a fixed current baseline. In this perspective, we propose an alternative characterization of the future precipitation changes predicted by general circulation models, focusing on the precipitation difference between two subsequent 20-year periods. We show that in a business-as-usual emission pathway, the moistening and drying rates increase by 30-40 %, both over land and ocean. As we move further over the twenty-first century, more regions exhibit a significant rate of precipitation change, while the patterns become geographically stationary and the trends persistent. The stabilization of the geographical rate patterns that occurs despite the acceleration of global warming can be physically explained: it results from the increasing contribution of thermodynamic processes compared to dynamic processes in the control of precipitation change. We show that such an evolution is already noticeable over the last decades, and that it could be reversed if strong mitigation policies were quickly implemented. The combination of intensification and increasing persistence of precipitation rate patterns may affect the way human societies and natural ecosystems adapt to climate change, especially in the Mediterranean basin, in Central America, in South Asia and in the Arctic. [ABSTRACT FROM AUTHOR]

The IPSL-CM5A climate model was used to perform a large number of control, historical and climate change simulations in the frame of CMIP5. The refined horizontal and vertical grid of the atmospheric component, LMDZ, constitutes a major difference compared to the previous IPSL-CM4 version used for CMIP3. From imposed-SST (Sea Surface Temperature) and coupled numerical experiments, we systematically analyze the impact of the horizontal and vertical grid resolution on the simulated climate. The refinement of the horizontal grid results in a systematic reduction of major biases in the mean tropospheric structures and SST. The mid-latitude jets, located too close to the equator with the coarsest grids, move poleward. This robust feature, is accompanied by a drying at mid-latitudes and a reduction of cold biases in mid-latitudes relative to the equator. The model was also extended to the stratosphere by increasing the number of layers on the vertical from 19 to 39 (15 in the stratosphere) and adding relevant parameterizations. The 39-layer version captures the dominant modes of the stratospheric variability and exhibits stratospheric sudden warmings. Changing either the vertical or horizontal resolution modifies the global energy balance in imposed-SST simulations by typically several W/m which translates in the coupled atmosphere-ocean simulations into a different global-mean SST. The sensitivity is of about 1.2 K per 1 W/m when varying the horizontal grid. A re-tuning of model parameters was thus required to restore this energy balance in the imposed-SST simulations and reduce the biases in the simulated mean surface temperature and, to some extent, latitudinal SST variations in the coupled experiments for the modern climate. The tuning hardly compensates, however, for robust biases of the coupled model. Despite the wide range of grid configurations explored and their significant impact on the present-day climate, the climate sensitivity remains essentially unchanged. [ABSTRACT FROM AUTHOR]

The LMDZ4 general circulation model is the atmospheric component of the IPSL–CM4 coupled model which has been used to perform climate change simulations for the 4th IPCC assessment report. The main aspects of the model climatology (forced by observed sea surface temperature) are documented here, as well as the major improvements with respect to the previous versions, which mainly come form the parametrization of tropical convection. A methodology is proposed to help analyse the sensitivity of the tropical Hadley–Walker circulation to the parametrization of cumulus convection and clouds. The tropical circulation is characterized using scalar potentials associated with the horizontal wind and horizontal transport of geopotential (the Laplacian of which is proportional to the total vertical momentum in the atmospheric column). The effect of parametrized physics is analysed in a regime sorted framework using the vertical velocity at 500 hPa as a proxy for large scale vertical motion. Compared to Tiedtke’s convection scheme, used in previous versions, the Emanuel’s scheme improves the representation of the Hadley–Walker circulation, with a relatively stronger and deeper large scale vertical ascent over tropical continents, and suppresses the marked patterns of concentrated rainfall over oceans. Thanks to the regime sorted analyses, these differences are attributed to intrinsic differences in the vertical distribution of convective heating, and to the lack of self-inhibition by precipitating downdraughts in Tiedtke’s parametrization. Both the convection and cloud schemes are shown to control the relative importance of large scale convection over land and ocean, an important point for the behaviour of the coupled model. [ABSTRACT FROM AUTHOR]

Abstract: The Asian monsoon is comprised of the Indian and East Asian subsystems. These two components are linked to one another in varying degrees by regions of strong sensible heating (Indo-Asian landmass) and strong latent heat export (the Western Pacific Warm Pool and the southern subtropical Indian Ocean). Variability within the Indian and East Asian subsystems, interactions among them, and the extent to which they interact with other climate phenomena (e.g., ENSO) are current topics of modern and paleoclimate research. This work provides an overview of past and current paleomonsoon research on tectonic to interannual time scales with a primary focus on marine sediment records. While past and current work has contributed greatly to our understanding of paleomonsoon variability at all time scales, additional efforts are required to make further progress in two critical areas. (1) Additional efforts are needed in terms of proxy development and evaluation, requiring concerted efforts at long-term sediment trap deployments in key monsoon-influenced regions as well as development of adequate and widely available core-top databases. These are necessary to assess the impact of modern oceanographic and seafloor processes on potential monsoon proxies. (2) Additional effort is also needed in acquiring a sufficient geographic distribution of downcore records to assess linkages among the subsystems and their role in the context of extratropical climate change. These records should have high sedimentation rates (including varved sections). This requires substantial survey support to identify the most appropriate coring and drilling targets. [Copyright &y& Elsevier]

The typical resolution used in CMIP-class Earth system models limits, among others, the ability to assess climate risks that are associated with smaller-scale weather phenomena such as tropical cyclones. We introduce higher-resolution atmosphere-only simulations following the HighResMIP Tier1 protocol. A baseline simulation (LMDZ-OR) is run with LMD-Z, the atmospheric component of the IPSL-CM Earth System model, with CMIP6 physics including the land-surface model ORCHIDEE, at a resolution equivalent to 50km at mid-latitudes. Large-scale parallel I/O with on-the-fly data processing is enabled by the XIOS I/O server. In two additional simulations (ICO-LMDZ-OR), the LMD-Z latitude-longitude dynamical core is replaced by the more scalable DYNAMICO dynamical core, using icosahedral-hexagonal meshes with quasi-uniform resolution of 50km and 25km.We provide a description and preliminary analysis of these simulations. We present the scalability and throughput of LMDZ-OR and ICO-LMDZ-OR. Some differences between LMDZ-OR and ICO-LMDZ-OR with respect to the processing of input datasets, especially orography, are discussed. The climates in LMDZ-OR and ICO-LMD-OR simulations at similar resolutions are compared. Dependence on resolution of key climate features is examined. [ABSTRACT FROM AUTHOR]