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This dataset gives the raw model outputs from a pre-industrial (PI) and several Last Glacial Maximum (LGM) sensitivity simulations based on AWIESM. A more detailed description of the individual file is presented below: PI.tar.gz: model outputs for PIsimulation. LGM-full-forced.tar.gz: model outputs for full forced LGM simulation. LGM-with-PI-greenhousegas.tar.gz: model outputs for a sensitivity LGM simulation, in which all boundary conditions and forcings are set to the LGM values but the greenhouse gasesare the same as for PI. LGM-with-PI-icesheets.tar.gz: model outputs for a sensitivity LGM simulation, in which all boundary conditions and forcings are set to the LGM values but the ice sheets are the same as for PI. LGM-with-PI-orbital.tar.gz: model outputs for a sensitivity LGM simulation, in which all boundary conditions and forcings are set to the LGM values but the orbital parameters are the same as for PI. LGM-with-PI-greenhousegas-and-icesheets.nc: model outputs for a sensitivity LGM simulation, in which all boundary conditions and forcings are set to the LGM values but the greenhouse gases and ice sheets are the same as for PI. &nbsp

Numerical simulations employing prognostic stable water isotopes can not only facilitate our understanding of hydrological processes and climate change but also allow for a straightforward comparison between isotope signals obtained from models and various archives. In the current work, we describe the performance and explore the potential of a new version of the Earth system model AWI-ESM, labeled AWI-ESM-wiso, in which we incorporated three isotope tracers into all relevant components of the water cycle. We present here the results of pre-industrial (PI) and mid-Holocene (MH) simulations. The model well reproduces the observed PI isotope compositions in both precipitation and sea water, and captures their major differences from the MH condition. The simulated relationship between the isotope composition in precipitation (δ18Op) and surface air temperature is very similar between the PI and MH conditions, and largely consistent with modern observations despite some regional model biases. The ratio of the MH-PI difference in δ18Op to the MH-PI difference in surface air temperature is reasonable over Greenland and Antarctica only when summertime air temperature is considered. An amount effect is evident over the North Africa monsoon domain, where a negative correlation between δ18Op and the amount of precipitation is simulated. As an example of model applications, we studied the onset date of MH West Africa summer monsoon (WASM) using daily variables. We find that defining the WASM onset based on precipitation alone may yield erroneous results due to the substantial daily variations of precipitation, which may obscure the distinction between pre-monsoon and monsoon seasons. Combining precipitation and isotope indicators, we suggest in this work a novel method for identifying the commencement of the WASM. Moreover, we do not find obvious difference between the MH and PI in terms of the mean onset date of WASM.

The Atlantic meridional overturning circulation (AMOC) plays an important role in regulating global and regional climates, especially the European climate, as it affects northward heat transport. Climate model simulations project a decline in the strength of AMOC under future climate change, although high uncertainties exist across models. The potential slowdown of AMOC could cause large and rapid changes in the climate and is therefore regarded as a ‘low probability-high impact’ event. However, how a weakening AMOC affects temperature and precipitation extremes remains poorly understood. Here we use a state-of-the-art global climate model, a new version of the Alfred Wegener Institute Climate Model (AWI-CM3), to quantify these impacts on Europe. We have performed freshwater hosing experiments to weaken the strength of AMOC and to investigate its sensitivity to different freshwater forcing regions. Our results reveal that the largest decline in AMOC generally appears when additional freshwater is put directly at regions of deepwater formation, especially around the Irminger Sea. As atmospheric responses, widespread cooling and less precipitation are found in the Northern Hemisphere mid-latitudes. We further identify the changes in precipitation and temperature extremes on different timescales. Droughts and cold days are very pronounced under AMOC attenuations, and we argue that they may have a stronger impact than the system's mean responses. Our results provide implications for understanding European weather and climate response to a weakening of AMOC in the past, present, and future.

Model codes: Instruction_AWIESM2_1_wiso.pdf:Instruction on how to downloadAWIESM-wiso and perform simulations. codes_runscripts.tar.gz: Source codes, run-scripts and namelists required for AWIESM-wiso. Model outputs: pi_atmosphere_climatology_monthly.nc: Climatological monthly isotope and climate variables for our PI simulation mh_atmosphere_climatology_monthly.nc: Climatological monthly isotope and climate variables for our MHsimulation pi_h2o16_ocean.nc: Climatological monthly h2o16 tracer in the ocean at all depths for PI. pi_h2o18_ocean.nc: Climatological monthly h2o18tracer in the ocean at all depths for PI. mh_h2o16_ocean.nc: Climatological monthly h2o16 tracer in the ocean at all depths for MH. mh_h2o18_ocean.nc: Climatological monthly h2o18tracer in the ocean at all depths for MH.

The potential climate impact of solar geoengineering is examined using climate model simulations by artificially reducing the incoming solar radiation at the top of the atmosphere. Climate scenario simulations indicate that a doubling of atmospheric carbon dioxide (2xCO2) induces a surface temperature rise which is amplified over the poles primarily during the respective winter. The warming also causes intensification and poleward shift of the global precipitation pattern. In our model, a 2.1% globally uniform solar reduction can largely compensate the global mean warming caused by a doubling of CO2. We find that solar shading is efficient to restore the temperature at the region where the background sunshine is strong, regionally at low-latitudes, seasonally during summer. Solar shading would lead to an overall weakening of the global hydrological cycle, resulting in a large-scale drought. A 3.6% solar reduction in the tropics can largely reduce the tropical and global warming as well. However, it reduces the precipitation at the central tropics, while increase the precipitation over the monsoon region. Comparatively, a 14% solar reduction over the poles can effectively prevent the polar summer temperature increase and sea-ice retreat. However, caused by the increased temperature gradient, polar solar shading increases the storm activity at high-latitudes, especially during summer when the solar reduction reaches its maximum amplitude. Our simulations show that solar shading could be an effective way to stabilize the polar cryosphere. Nevertheless, it has a strong impact on the hydrological cycle and provides a heterogenous regional climate signal.

Recent variability in West African monsoon rainfall 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 6,000 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 6,000 years in response to the gradual weakening of the interhemispheric gradient in sea surface temperatures. The only modes that show a long-term trend are those associated with the strengthening of the El Niño Southern Oscillation from the mid-Holocene onwards. There are marked multi-centennial oscillations superimposed on the long-term trend in monsoon rainfall which are strongly associated with multi-centennial oscillations in the Atlantic Multidecadal Oscillation and in tropical Atlantic sea surface temperatures in all models. However, the influence of different modes on interannual to multi-decadal rainfall variability is not consistent across the models. This is driven in one case by the Atlantic Multidecadal Oscillation and changes in Mediterranean sea surface temperatures, by tropical sea surface temperature changes in another, and in the fourth model by a combination of the Atlantic Multidecadal Oscillation and the El Niño Southern Oscillation. Furthermore, multiple inter-basin teleconnections are associated with significant rainfall anomalies in each model.

pi_atmosphere_climatology_monthly.nc: Climatological monthly isotope and climate variables for our PI simulation mh_atmosphere_climatology_monthly.nc: Climatological monthly isotope and climate variables for our MHsimulation pi_h2o16_ocean.nc: Climatological monthly h2o16 tracer in the ocean at all depths for PI. pi_h2o18_ocean.nc: Climatological monthly h2o18tracer in the ocean at all depths for PI. mh_h2o16_ocean.nc: Climatological monthly h2o16 tracer in the ocean at all depths for MH. mh_h2o18_ocean.nc: Climatological monthly h2o18tracer in the ocean at all depths for MH.

Satellite-observed sea surface temperature (SST) provides an unprecedented opportunity to evaluate the ongoing global warming and has recently reached a milestone of 40 years temporal coverage. One of the major warming patterns captured by satellites is relatively strong subtropical ocean warming across all ocean basins. This pattern was widely interpreted as a manifestation of the Pacific Decadal Oscillation, a natural climate variability. However, we find that the observed warming pattern, characterized by strong warming over the subtropical oceans and mild warming or even cooling over the subpolar oceans, is dynamically consistent with where the surface water converges and diverges. By comparing observations with paleo-reconstructions and simulated short-term and long-term SST response to CO2 forcing, we propose that the observed warming pattern is likely in a transient early warming stage constrained by background ocean circulation. In the long term, ocean warming at high latitudes is expected to exceed the temporally dominating subtropical ocean warming. The committed, but lagged, high-latitude ocean warming has potential to reshape the ocean-atmosphere circulation and threaten the stability of marine-terminating ice sheets.

Growing evidence indicates that the atmospheric and oceanic circulation experiences a systematic poleward shift in a warming climate. However, the complexity of the climate system, including the coupling between the ocean and the atmosphere, natural climate variability and land-sea distribution, tends to obfuscate the causal mechanism underlying the circulation shift. Here, using an idealised coupled aqua-planet model, we explore the mechanism of the shifting circulation, by isolating the contributing factors from the direct CO$$_2$$ 2 forcing, the indirect ocean surface warming, and the wind-stress feedback from the ocean dynamics. We find that, in contrast to the direct CO$$_2$$ 2 forcing, ocean surface warming, in particular an enhanced subtropical ocean warming, plays an important role in driving the circulation shift. This enhanced subtropical ocean warming emerges from the background Ekman convergence of surface anomalous heat in the absence of the ocean dynamical change. It expands the tropical warm water zone, causes a poleward shift of the mid-latitude temperature gradient, hence forces a corresponding shift in the atmospheric circulation and the associated wind pattern. The shift in wind, in turn drives a shift in the ocean circulation. Our simulations, despite being idealised, capture the main features of the observed climate changes, for example, the enhanced subtropical ocean warming, poleward shift of the patterns of near-surface wind, sea level pressure, storm tracks, precipitation and large-scale ocean circulation, implying that increase in greenhouse gas concentrations not only raises the temperature, but can also systematically shift the climate zones poleward.

Numerical simulations allow us to gain a comprehensive understanding of the underlying mechanisms of past, present, and future climate changes. The mid-Holocene (MH) and the last interglacial (LIG) were the two most recent warm episodes of Earth’s climate history and are the focus of paleoclimate research. Here, we present results of MH and LIG simulations with two versions of the state-of-the-art Earth system model AWI-ESM. Most of the climate changes in MH and LIG compared to the preindustrial era are agreed upon by the two model versions, including 1) enhanced seasonality in surface temperature that is driven by the redistribution of seasonal insolation; 2) a northward shift of the intertropical convergence zone (ITCZ) and tropical rain belt; 3) a reduction in annual mean Arctic sea ice concentration; 4) weakening and northward displacement of the Northern Hemisphere Hadley circulation, which is related to the decrease and poleward shift of the temperature gradient from the subtropical to the equator in the Northern Hemisphere; 5) a westward shift of the Indo-Pacific Walker circulation due to anomalous warming over the Eurasia and North Africa during boreal summer; and 6) an expansion and intensification of Northern Hemisphere summer monsoon rainfall, with the latter being dominated by the dynamic component of moisture budget (i.e., the strengthening of wind circulation). However, the simulated responses of the Atlantic meridional overturning circulation (AMOC) in the two models yield different results for both the LIG and the MH. AMOC anomalies between the warm interglacial and preindustrial periods are associated with changes in North Atlantic westerly winds and stratification of the water column at the North Atlantic due to changes in ocean temperature, salinity, and density.

Salinity and drought are major abiotic stresses limiting plant growth and crop productivity. To evaluate the comparative effects of salt and drought stress on castor bean (Ricinus communis L.) germination and early seedling growth, a germination experiment and a seedling experiment were conducted to determine the responses of castor bean (cvs Zibi 5 and Zibi 9) to polyethylene glycol (PEG-6000) and sodium chloride (NaCl) with a range of gradient water potential levels, including 0 (control), –0.13 MPa, –0.2 MPa and –0.6 MPa. Results showed that both stresses caused by NaCl and PEG-6000 decreased germination percentage (GP), germination index (GI), fresh weight (FW), leaf gas exchange parameters (Pn, Ci and Tr) and chlorophyll pigments (Chl a and Chl b). The PEG-treated plants suffered higher restrictions in seed germination and seedlings growth than the NaCl-treated plants. Moreover, the PEG treatments caused more significant reductions in GP, Chl a, FW and Tr, as compared with NaCl treatment. On average, the GP under PEG stress was decreased by 22.3% and 22.0% for Zibi 5 and Zibi 9, respectively, in comparison to NaCl stress. Our study showed that both salt and water stress caused similar reductions in germination percentage, biomass accumulation, leaf gas exchange and chlorophyll pigments, while severer detrimental effects were induced by water stress as compared with salt stress.

Targeting some problems of the RRT_Connect path planning algorithm, such as average search and low efficiency, proposes an improved RRT_Connect algorithm that may optimize the searched nodes and parts of planned paths. Firstly, an improved RRT_Connect algorithm based on destination and searched node bias strategy is proposed. Secondly, an improved RRT_Connect algorithm is put forward for the optimization of the searched nodes and some planned paths to deal with the problem of low quality reflected in the improved RRT_Connect path planning algorithm, and the optimization for the cost of path planning by figuring out valid new nodes and parent nodes of adjacent nodes within a certain range. On this basis, the path planning algorithm is verified by simulation and actual experiments. It is shown by the experimental results that the improved RRT_Connect algorithm proposed in this paper can not only shorten the time and length of path planning but also decrease the number of search iterations and nodes.

Numerical modelling enables a comprehensive understanding not only of the Earth's system today, but also of the past. To date, a significant amount of time and effort has been devoted to paleoclimate modeling and analysis, which involves the latest and most advanced Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4). The definition of seasonality, which is influenced by slow variations in the Earth's orbital parameters, plays a key role in determining the calculated seasonal cycle of the climate. In contrast to the classical calendar used today, where the lengths of the months and seasons are fixed, the angular calendar calculates the lengths of the months and seasons according to a fixed number of degrees along the Earth's orbit. When comparing simulation results for different time intervals, it is essential to account for the angular calendar to ensure that the data for comparison is from the same position along the Earth's orbit. Most models use the classical "fixed-length" calendar, which can lead to strong distortions of the monthly and seasonal values, especially for the climate of the past. Here, by analyzing daily outputs from multiple PMIP4 model simulations, we examine calendar effects on surface air temperature and precipitation under mid-Holocene, last interglacial, and pre-industrial climate conditions. We conclude that: (a) The largest cooling bias occurs in autumn when the classical calendar is applied for the mid-Holocene and last interglacial. (b) The sign of the temperature anomalies between the Last Interglacial and pre-industrial in boreal autumn can be reversed after the switch from classical to angular calendar, particularly over the Northern Hemisphere continents. (c) Precipitation over West Africa is overestimated in boreal summer and underestimated in boreal autumn when the "fixed-length" seasonal cycle is applied. (d) Finally, correcting the calendar based on the monthly model results can reduce the biases to a large extent, but not completely eliminate them. In addition, we examine the calendar effects in 3 transient simulations for 6–0 ka by AWI-ESM, MPI-ESM, and IPSL. We find significant discrepancies between adjusted and unadjusted temperature values over ice-free continents for both hemispheres in boreal autumn. While for other seasons the deviations are relatively small. A drying bias can be found in the summer monsoon precipitation in Africa (in the "fixed-length" calendar), whereby the magnitude of bias becomes smaller over time. Overall, our study underlines the importance of the application of calendar transformation in the analysis of climate simulations. Neglecting the calendar effects could lead to a profound artificial distortion of the calculated seasonal cycle of surface air temperature and precipitation. One important fact to be noted here is that the discrepancy in seasonality under different calendars is an analysis bias and is highly depends on the choice of the reference position/date (usually the vernal equinox, which is set to 31th March) on the Earth's ellipse around the sun. Different model groups may apply different reference dates, so ensuring a consistent reference date and seasonal definition is key when we compare results across multiple models.

Southern hemispheric sea-ice impacts ocean circulation and the carbon exchange between the atmosphere and the ocean. Sea-ice is therefore one of the key processes in past and future climate change and variability. As climate models are the only tool available to project future climate change, it is important to assess their performance against observations for a range of different climate states. The Last Glacial Maximum (LGM, ∼21 000 years ago) represents an interesting target as it is a relatively well-documented period with climatic conditions very different from preindustrial conditions. Here, we analyze the LGM seasonal Southern Ocean sea-ice cover as simulated in numerical simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) phases 3 and 4. We compare the model outputs to a recently updated compilation of LGM seasonal Southern Ocean sea-ice cover and summer sea surface temperature (SST) to assess the most likely LGM Southern Ocean state. Simulations and paleo-proxy records suggest a fairly well-constrained glacial winter sea-ice edge between 50.5 and 51∘ S. However, the spread in simulated glacial summer sea-ice is wide, ranging from almost ice-free conditions to a sea-ice edge reaching 53∘ S. Combining model outputs and proxy data, we estimate a likely LGM summer sea-ice edge between 61 and 62∘ S and a mean summer sea-ice extent of 14–15×106 km2, which is ∼20 %–30 % larger than previous estimates. These estimates point to a higher seasonality of southern hemispheric sea-ice during the LGM than today. We also analyze the main processes defining the summer sea-ice edge within each of the models. We find that summer sea-ice cover is mainly defined by thermodynamic effects in some models, while the sea-ice edge is defined by the position of Southern Ocean upwelling in others. For models included in both PMIP3 and PMIP4, this thermodynamic or dynamic control on sea-ice is consistent across both experiments. Finally, we find that the impact of changes in large-scale ocean circulation on summer sea-ice within a single model is smaller than the natural range of summer sea-ice cover across the models considered here. This indicates that care must be taken when using a single model to reconstruct past climate regimes.

The mid-Holocene (6000 years ago) is a standard time period for the evaluation of the simulated response of global climate models using palaeoclimate reconstructions. The latest mid-Holocene simulations are a palaeoclimate entry card for the Palaeoclimate Model Intercomparison Project (PMIP4) component of the current phase of the Coupled Model Intercomparison Project (CMIP6) – hereafter referred to as PMIP4-CMIP6. Here we provide an initial analysis and evaluation of the results of the experiment for the mid-Holocene. We show that state-of-the-art models produce climate changes that are broadly consistent with theory and observations, including increased summer warming of the Northern Hemisphere and associated shifts in tropical rainfall. Many features of the PMIP4-CMIP6 simulations were present in the previous generation (PMIP3-CMIP5) of simulations. The PMIP4-CMIP6 ensemble for the mid-Holocene has a global mean temperature change of −0.3 K, which is −0.2 K cooler than the PMIP3-CMIP5 simulations predominantly as a result of the prescription of realistic greenhouse gas concentrations in PMIP4-CMIP6. Biases in the magnitude and the sign of regional responses identified in PMIP3-CMIP5, such as the amplification of the northern African monsoon, precipitation changes over Europe, and simulated aridity in mid-Eurasia, are still present in the PMIP4-CMIP6 simulations. Despite these issues, PMIP4-CMIP6 and the mid-Holocene provide an opportunity both for quantitative evaluation and derivation of emergent constraints on the hydrological cycle, feedback strength, and potentially climate sensitivity.

During the Last Glacial Maximum (LGM), a very cold and dry period around 26.5–19 kyr BP, permafrost was widespread across Europe. In this work, we explore the possible benefit of using regional climate model data to improve the permafrost representation in France, decipher how the atmospheric circulation affects the permafrost boundaries in the models, and test the role of ground thermal contraction cracking in wedge development during the LGM. With these aims, criteria for possible thermal contraction cracking of the ground are applied to climate model data for the first time. Our results show that the permafrost extent and ground cracking regions deviate from proxy evidence when the simulated large-scale circulation in both global and regional climate models favours prevailing westerly winds. A colder and, with regard to proxy data, more realistic version of the LGM climate is achieved given more frequent easterly winds conditions. Given the appropriate forcing, an added value of the regional climate model simulation can be achieved in representing permafrost and ground thermal contraction cracking. Furthermore, the model data provide evidence that thermal contraction cracking occurred in Europe during the LGM in a wide latitudinal band south of the probable permafrost border, in agreement with field data analysis. This enables the reconsideration of the role of sand-wedge casts to identify past permafrost regions.

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

The earliest part of the Holocene, from 11.5k to 7k (k = 1000 years before present), is a critical transition period between the relatively cold last deglaciation and the warm middle Holocene. It is marked by more pronounced seasonality and reduced greenhouse gases (GHGs) than the present state, as well as by the presence of the Laurentide Ice Sheet (LIS) and glacial meltwater perturbation. This paper performs experiments under pre-industrial and different early-Holocene regimes with AWI-ESM (Alfred Wegener Institute–Earth System Model), a state-of-the-art climate model with unstructured mesh and varying resolutions, to examine the sensitivity of the simulated Atlantic meridional overturning circulation (AMOC) to early-Holocene insolation, GHGs, topography (including properties of the ice sheet), and glacial meltwater perturbation. In the experiments with early-Holocene Earth orbital parameters and GHGs applied, the AWI-ESM simulation shows a JJA (June–July–August) warming and DJF (December–January–February) cooling over the mid and high latitudes compared with pre-industrial conditions, with amplification over the continents. The presence of the LIS leads to an additional regional cooling over the North America. We also simulate the meltwater event around 8.2k. Big discrepancies are found in the oceanic responses to different locations and magnitudes of freshwater discharge. Our experiments, which compare the effects of freshwater release evenly across the Labrador Sea to a more precise injection along the western boundary of the North Atlantic (the coastal region of LIS), show significant differences in the ocean circulation response, as the former produces a major decline of the AMOC and the latter yields no obvious effect on the strength of the thermohaline circulation. Furthermore, proglacial drainage of Lakes Agassiz and Ojibway leads to a fast spin-down of the AMOC, followed, however, by a gradual recovery. Most hosing experiments lead to a warming over the Nordic Sea and Barents Sea of varying magnitudes, because of an enhanced inflow from lower latitudes and a northward displacement of the North Atlantic deep convection. These processes exist in both of our high- and low-resolution experiments, but with some local discrepancies such as (1) the hosing-induced subpolar warming is much less pronounced in the high-resolution simulations; (2) LIS coastal melting in the high-resolution model leads to a slight decrease in the AMOC; and (3) the convection formation site in the low- and high-resolution experiments differs, in the former mainly over northeastern North Atlantic Ocean, but in the latter over a very shallow subpolar region along the northern edge of the North Atlantic Ocean. In conclusion, we find that our simulations capture spatially heterogeneous responses of the early-Holocene climate.

Constrained by the numbers of action space and state space, Q-learning cannot be applied to continuous state space. Targeting this problem, the double deep Q network (DDQN) algorithm and the corresponding improvement methods were explored. First of all, to improve the accuracy of the DDNQ algorithm in estimating the target Q value in the training process, a multi-step guided strategy was introduced into the traditional DDQN algorithm, for which the single-step reward was replaced with the reward obtained in continuous multi-step interactions of mobile robots. Furthermore, an experience classification training method was introduced into the traditional DDQN algorithm, for which the state transition generated by the mobile robot–environment interaction was divided into two different types of experience pools, and experience pools were trained by the Q network, and the sampling proportions of the two experience pools were updated through the training loss. Afterward, the advantages of a multi-step guided DDQN (MS-DDQN) algorithm and experience classification DDQN (EC-DDQN) algorithm were combined to develop a novel experience classification multi-step DDQN (ECMS-DDQN) algorithm. Finally, the path planning of these four algorithms, including DDQN, MS-DDQN, EC-DDQN, and ECMS-DDQN, was simulated on the OpenAI Gym platform. The simulation results revealed that the ECMS-DDQN algorithm outperforms the other three in the total return value and generalization in path planning.

During the Last Glacial Maximum (LGM), a very cold and dry period around 26.5 to 19 thousand years ago, permafrost was widespread across Europe. In this work, we evaluate the potential of regional climate model simulations to reconstruct the permafrost distribution in western Europe during the LGM. With this aim, criteria for possible thermal contraction cracking of the ground are applied to climate model data for the first time. These criteria serve as a precondition for the development of ice and sand wedges, which are a common proxy for past permafrost. Our results show that the permafrost and ground cracking distribution in Europe during the LGM are not consistent with a large-scale circulation with prevailing westerly winds. However, a colder and with regard to proxy data more realistic version of the LGM climate is achieved given more frequent easterly winds conditions. Whereas the permafrost extent and ground cracking regions in the global climate model simulation deviate from proxy evidence, they are in good agreement in the regional counterpart. Given the appropriate forcing, an added value of the regional climate model simulation can thus be achieved. Furthermore, the model data provide evidence that thermal contraction cracking occurred in Europe during the LGM also south of the probable permafrost border. This enables the reconsideration of the significance of ice wedge pseudomorphs and sand wedge casts to understand past climate variations.

The Last Glacial Maximum (LGM, ∼ 21 000 years ago) has been a major focus for evaluating how well state-of-the-art climate models simulate climate changes as large as those expected in the future using paleoclimate reconstructions. A new generation of climate models has been used to generate LGM simulations as part of the Paleoclimate Modelling Intercomparison Project (PMIP) contribution to the Coupled Model Intercomparison Project (CMIP). Here, we provide a preliminary analysis and evaluation of the results of these LGM experiments (PMIP4, most of which are PMIP4-CMIP6) and compare them with the previous generation of simulations (PMIP3, most of which are PMIP3-CMIP5). We show that the global averages of the PMIP4 simulations span a larger range in terms of mean annual surface air temperature and mean annual precipitation compared to the PMIP3-CMIP5 simulations, with some PMIP4 simulations reaching a globally colder and drier state. However, the multi-model global cooling average is similar for the PMIP4 and PMIP3 ensembles, while the multi-model PMIP4 mean annual precipitation average is drier than the PMIP3 one. There are important differences in both atmospheric and oceanic circulations between the two sets of experiments, with the northern and southern jet streams being more poleward and the changes in the Atlantic Meridional Overturning Circulation being less pronounced in the PMIP4-CMIP6 simulations than in the PMIP3-CMIP5 simulations. Changes in simulated precipitation patterns are influenced by both temperature and circulation changes. Differences in simulated climate between individual models remain large. Therefore, although there are differences in the average behaviour across the two ensembles, the new simulation results are not fundamentally different from the PMIP3-CMIP5 results. Evaluation of large-scale climate features, such as land–sea contrast and polar amplification, confirms that the models capture these well and within the uncertainty of the paleoclimate reconstructions. Nevertheless, regional climate changes are less well simulated: the models underestimate extratropical cooling, particularly in winter, and precipitation changes. These results point to the utility of using paleoclimate simulations to understand the mechanisms of climate change and evaluate model performance.

Growing evidence implies that the atmospheric and oceanic circulation experiences a systematic poleward shift in a warming climate. However, the complexity of climate system, including the coupling between the ocean and the atmosphere, natural climate variability and land-sea distribution, tends to obfuscate the causal mechanism underlying the circulation shift. Here, using an idealized coupled aqua-planet model, we explore the mechanism of the shifting circulation, by isolating the contributing factors from the direct CO2 forcing, the indirect ocean surface warming, and the wind-stress feedback from the ocean dynamics. We find that, in contrast to direct CO2 forcing, an enhanced subtropical ocean warming plays a leading role in driving the circulation shift. This enhanced subtropical ocean warming emerges from the background Ekman convergence of surface anomalous heat in the absence of the ocean dynamical change. It expands the tropical warm water zone, causes a poleward shift of the meridional temperature gradients, hence forces a corresponding shift in the atmospheric circulation. The shift in the atmospheric circulation in turn drives a shift in the ocean circulation. Our simulations, despite being idealized, capture the main features of observed climate changes, for example, the enhanced subtropical ocean warming, poleward shift of the patterns of near-surface wind, sea level pressure, storm tracks, precipitation and large-scale ocean circulation, implying that increase in greenhouse gas concentrations not only raises the temperature, but can also systematically shift the climate zones poleward.

Today, precipitation and wind patterns over the equatorial Indian Ocean and surrounding lands are paced by monsoon and Walker circulations that are controlled by the seasonal land-sea temperature contrast and the inter-annual convection over the Indo-Pacific Warm Pool, respectively. The annual mean surface westerly winds are particularly tied to the Walker circulation, showing interannual variability coupled with the gradient of Sea Surface Temperature (SST) anomaly between the tropical western and southeastern Indian Ocean, namely, the Indian Ocean Dipole (IOD). While the Indian monsoon pattern has been widely studied in the past, few works deal with the evolution of Walker circulation despite its crucial impacts on modern and future tropical climate systems. Here, we reconstruct the long-term westerly (summer) and easterly (winter) wind dynamics of the equatorial Indian Ocean (10°S−10°N), since the Last Glacial Maximum (LGM) based on i) primary productivity (PP) records derived from coccolith analyses of sedimentary cores MD77-191 and BAR94-24, retrieved off the southern tip of India and off the northwestern tip of Sumatra, respectively and ii) the calculation of a sea surface temperature (SST) anomaly gradient off (south) western Sumatra based on published SST data. We compare these reconstructions with atmospheric circulation simulations obtained with the general coupled model AWI-ESM-1-1-LR (Alfred Wegener Institute Earth System Model).Our results show that the Indian Ocean Walker circulation was weaker during the LGM and the early/middle Holocene than present. Model simulations suggest that this is due to anomalous easterlies over the eastern Indian Ocean. The LGM mean circulation state may have been comparable to the year 1997 with a positive IOD, when anomalously strong equatorial easterlies prevailed in winter. The early/mid Holocene mean circulation state may have been equivalent to the year 2006 with a positive IOD, when anomalously strong southeasterlies prevailed over Java-Sumatra in summer. The deglaciation can be seen as a transient period between these two positive IOD-like mean states.

Growing evidence suggests that the oceanic and atmospheric circulation experiences a systematic poleward shift under climate change. However, due to the complexity of climate system, such as, the coupling between the ocean and the atmosphere, natural climate variability and land-sea distribution, the dynamical mechanism of such shift is still not fully understood. Here, using an idealized partially coupled ocean and atmosphere aqua-planet model, we explore the mechanism of the shifting oceanic and atmospheric circulation. We find that, in contrast to the rising GHG concentration, the subtropical ocean warming plays a dominant role in driving the shift in the circulation system. More specifically, due to background ocean dynamics, a relatively faster warming over the subtropical ocean drives a poleward shift in the atmospheric circulation. The shift in the atmospheric circulation in turn drives a shift in the oceanic circulation. Our simulations, despite being idealized, capture the main features of observed climate changes, for example, the enhanced subtropical ocean warming, poleward shift of the patterns of near-surface wind, sea level pressure, cloud, precipitation, storm tracks and large-scale ocean circulation, implying that global warming not only raises the temperature, but also systematically shifts the climate zones.​

The importance of Southern Ocean sea ice has come into the focus of polar research in the last couple of decades. Especially in West Antarctica, where sea ice has declined, its distribution and evolution play a critical role for the stability of nearby ice shelves. Organic geochemical analyses of marine surface sediments from the West Antarctic continental shelves permit a biomarker-based reconstruction of sea surface conditions in these vulnerable areas. We analysed highly branched isoprenoids (HBIs), such as the sea-ice proxy IPSO25 and phytoplankton-derived HBI-trienes, but also phytosterols and isoprenoidal glycerol dialkyl glycerol tetraethers (GDGTs), which are established tools for the reconstruction of primary productivity and sea surface temperatures, respectively. The combination of IPSO25 with a phytoplankton marker results in the semi-quantitative sea-ice index PIPSO25, which provides useful reconstructions of sea-ice conditions, avoiding misleading over- or underestimations of sea-ice cover. Comparisons of the biomarker-based sea-ice distribution patterns and GDGT-based temperatures with (1) sea-ice distributions obtained from satellite observations and (2) estimated sea-ice patterns and SSTs deduced from modelled data are in reasonable agreement, but also highlight specific aspects that need to be considered when interpreting biomarker data. We further discuss IPSO25 concentrations in the vicinity of ice shelves, where elevated values could be related to the occurrence of ice shelf basal melt water and platelet ice under landfast sea ice.