Paleoclimate Modelling Intercomparison Project Research Articles

West African Monsoon Dynamics and Its Control on the Stable Oxygen Isotopic Composition of Precipitation in the Late Cenozoic

AbstractThis study presents an overview of the Late Cenozoic evolution of the West African Monsoon (WAM), and the associated changes in atmospheric dynamics and oxygen isotopic composition of precipitation (δ18Op). This evolution is established by using the high‐resolution isotope‐enabled GCM ECHAM5‐wiso to simulate the climatic responses to paleoenvironmental changes during the Mid‐Holocene (MH), Last Glacial Maximum (LGM), and Mid‐Pliocene (MP). The simulated responses are compared to a set of GCM outputs from Paleoclimate Model Intercomparison Project Phase 4 (PMIP4) to assess the added value of a high resolution and model consistency across different time periods. Results show WAM magnitudes and pattern changes that are consistent with PMIP4 models and proxy reconstructions. ECHAM5‐wiso estimates the highest WAM intensification in the MH, with a precipitation increase of up to 150 mm/month reaching 25°N during the monsoon season. The WAM intensification in the MP estimated by ECHAM5‐wiso (up to 80 mm/month) aligns with the mid‐range of the PMIP4 estimates, while the LGM dryness magnitude matches most of the models. Despite an enhanced hydrological cycle in MP, MH simulations indicate a ∼50% precipitation increase and a greater northward extent of WAM than the MP simulations. Strengthened conditions of the WAM in the MH and MP result from a pronounced meridional temperature gradient driving low‐level westerly, Sahel‐Sahara vegetation expansion, and a northward shift of the Africa Easterly Jet. The simulated δ18Op values patterns and their relationship with temperature and precipitation are non‐stationarity over time, emphasizing the implications of assuming stationarity in proxy reconstruction transfer functions.

Causes of Past African Temperature Change in PMIP Simulations of the Mid‐Holocene

AbstractCurrent‐generation climate models project that Africa will warm by up to 5°C in the coming century, severely stressing African populations. Past and ongoing work indicates, however, that the models used to create these projections do not match proxy records of past temperature in Africa during the mid‐Holocene (MH), raising concerns that their future projections may house large uncertainties. Rather than reproducing proxy‐based reconstructions of MH warming relative to the Pre‐Industrial (PI), models instead simulate MH temperatures very similar to or slightly colder than the PI. This data‐model mismatch could be due to a variety of factors, including biases in model surface energy budgets or inaccurate representation of the feedbacks between temperature and hydrologic change during the “Green Sahara.” We focus on the differences among model simulations in the Paleoclimate Modeling Intercomparison Project Phases 3 and 4 (PMIP3 and PMIP4), examining surface temperature and energy budgets to investigate controls on temperature and the potential model sources of this paleoclimate data‐model mismatch. Our results suggest that colder conditions simulated by PMIP3 and PMIP4 models during the MH are in large part due to the joint impacts of feedback uncertainties in response to increased precipitation, a strengthened West African Monsoon (WAM) in the Sahel, and the Green Sahara. We extend these insights into suggestions for model physics and boundary condition changes, and discuss implications for the accuracy of future climate model projections over Africa.

The variations of ENSO teleconnections during the Last Interglacial based on PMIP4 simulations

How the remote influences of El Niño-Southern Oscillation (ENSO) may vary in a warmer future remains a hot topic but still elusive. Based on the outputs from the Paleoclimate Modeling Intercomparison Project Phase 4, here we investigated the variations of ENSO teleconnections in boreal winter during the Last Interglacial (LIG), which shares large similarities with the projected climatic features in the 21st century. Our results suggested that the ENSO-temperature teleconnections on average weakened by ∼13.4% during the LIG relative to the preindustrial, whereas the response of precipitation amplified by ∼5.0%. However, there are large regional discrepancies in the variations of ENSO teleconnections in terms of the sign and amplitude. For example, we observed significant ENSO-temperature relationship changes at the high latitudes during the LIG, with a depressed linkage over northern North America but enhanced linkage over the high latitudes of Eurasia. The most profound change in ENSO-precipitation teleconnection occurred over the tropics during the LIG, characterized by a weaker response over the equatorial Pacific Ocean whereas a stronger response over the Maritime Continent and adjacent oceans. Regarding atmospheric circulations, we found that the ENSO-driven Walker Circulations over the Pacific and the Indian Ocean shifted westward during the LIG, whereas the descending branch of the ENSO-driven Atlantic Walker Circulation shifted eastward. Our results provide a model-based scenario for the variations of ENSO teleconnections during the LIG, but the underlying mechanisms deserve further investigation in the future.

A multi-model assessment of the early last deglaciation (PMIP4 LDv1): a meltwater perspective

Abstract. The last deglaciation (∼20–11 ka BP) is a period of a major, long-term climate transition from a glacial to interglacial state that features multiple centennial- to decadal-scale abrupt climate variations whose root cause is still not fully understood. To better understand this time period, the Paleoclimate Modelling Intercomparison Project (PMIP) has provided a framework for an internationally coordinated endeavour in simulating the last deglaciation whilst encompassing a broad range of models. Here, we present a multi-model intercomparison of 17 transient simulations of the early part of the last deglaciation (∼20–15 ka BP) from nine different climate models spanning a range of model complexities and uncertain boundary conditions and forcings. The numerous simulations available provide the opportunity to better understand the chain of events and mechanisms of climate changes between 20 and 15 ka BP and our collective ability to simulate them. We conclude that the amount of freshwater forcing and whether it follows the ice sheet reconstruction or induces an inferred Atlantic meridional overturning circulation (AMOC) history, heavily impacts the deglacial climate evolution for each simulation rather than differences in the model physics. The course of the deglaciation is consistent between simulations except when the freshwater forcing is above 0.1 Sv – at least 70 % of the simulations agree that there is warming by 15 ka BP in most places excluding the location of meltwater input. For simulations with freshwater forcings that exceed 0.1 Sv from 18 ka BP, warming is delayed in the North Atlantic and surface air temperature correlations with AMOC strength are much higher. However, we find that the state of the AMOC coming out of the Last Glacial Maximum (LGM) also plays a key role in the AMOC sensitivity to model forcings. In addition, we show that the response of each model to the chosen meltwater scenario depends largely on the sensitivity of the model to the freshwater forcing and other aspects of the experimental design (e.g. CO2 forcing or ice sheet reconstruction). The results provide insight into the ability of our models to simulate the first part of the deglaciation and how choices between uncertain boundary conditions and forcings, with a focus on freshwater fluxes, can impact model outputs. We can use these findings as helpful insight in the design of future simulations of this time period.

Dormant dune fields and ameliorative moisture conditions in the West Gobi Desert in East Asia during the Last Interglacial

Arid and semiarid regions are environmentally vulnerable to global warming. Thus, studying their environmental history during warm periods (e.g., the Last Interglacial, LIG) is crucial for understanding their future changes too. The West Gobi Desert is a transitional zone influenced by the westerlies and the East Asian monsoon system. Therefore, the variation in atmospheric circulation profoundly affects its water vapor transport and landscape evolution. Due to the scarcity of continuous records, the moisture transport process remains ambiguous, which limits further research on paleolake evolution and aeolian activity. Here, we combine paleoclimate simulations from the Coupled Model Intercomparison Project 6 (CMIP6)-Paleoclimate Modeling Intercomparison Project 4 (PMIP4) and geological records to clarify the water vapor transport pathways and aeolian activity in the West Gobi Desert during the LIG, compared to the reference period (the piControl). Our results show that increased rainfall and meltwater were possible supplies for surface water during the LIG and were associated with enhanced East Asian summer monsoon and higher surface air temperatures. Concomitantly, weakening aeolian activity and dormant dune fields occurred in the West Gobi Desert, resulting from higher surface moisture content and weaker surface wind regimes associated with the weakening westerlies and East Asian winter monsoon. Notably, the limited weakening of aeolian activity that occurred in the LIG winter and spring was caused by weaker surface wind and deteriorating surface moisture conditions, while the significant dormancy of dune fields in the LIG summer and autumn was initiated by the combined effects of ameliorative surface moisture conditions and lower surface wind speed.

Last Glacial Maximum climate and atmospheric circulation over the Australian region from climate models

Abstract. The Last Glacial Maximum (LGM; ∼21 kyr ago) was the most recent time that the Earth experienced global maximum ice volume and minimum eustatic sea level. The climate changes over the Australian region at the LGM remain uncertain, including the extent of cooling in the arid interior, changes in the regional atmospheric circulations such as the tropical monsoon and mid-latitude westerlies, and changes in the balance between precipitation and evaporation. In this study, 13 climate model simulations that were included in the Paleoclimate Modelling Intercomparison Project (PMIP) Phases 3 and 4 are used to investigate regional climate (temperature, precipitation, and wind) over Australia at the LGM. The model simulations are compared with existing proxy records and other modelling studies. All models simulate consistent annual and seasonal cooling over the Australian region (defined as 0–45° S, 110–160° E) at the LGM compared to pre-industrial, with a multi-model mean 2.9 °C decrease in annual average surface air temperature over land at the LGM compared to pre-industrial. Models simulate a range of LGM precipitation anomalies over the region. Simulated precipitation changes over tropical Australasia appear to be driven by changes in circulation and moisture transport, which vary greatly between models. Surface moisture balance calculated from precipitation minus evaporation shows little change over much of the Australian land area at the LGM. Changes in the strength and position of the mid-latitude westerlies are uncertain, with wide model disagreement. These results indicate that climate model simulations do not show a robust response in either tropical or mid-latitude circulation to LGM boundary conditions, suggesting that caution is required when interpreting model output in this region. Further analysis based on model evaluation and quantitative model–proxy comparison is required to better understand the drivers of LGM climate and atmospheric circulation changes in this region.

Relative importance of the mechanisms triggering the Eurasian ice sheet deglaciation in the GRISLI2.0 ice sheet model

Abstract. The last deglaciation (21 to 8 ka) of the Eurasian ice sheet (EIS) is thought to have been responsible for a sea level rise of about 20 m. While many studies have examined the timing and rate of the EIS retreat during this period, many questions remain about the key processes that triggered the EIS deglaciation 21 kyr ago. Due to its large marine-based parts in the Barents–Kara (BKIS) and British Isles sectors, the BKIS is often considered to be a potential analogue of the current West Antarctic ice sheet (WAIS). Identifying the mechanisms that drove the EIS evolution might provide a better understanding of the processes at play in the West Antarctic destabilization. To investigate the relative impact of key drivers on the EIS destabilization, we used the three-dimensional ice sheet model GRISLI (GRenoble Ice Shelf and Land Ice) (version 2.0) forced by climatic fields from five Paleoclimate Modelling Intercomparison Project phases 3 and 4 (PMIP3, PMIP4) Last Glacial Maximum (LGM) simulations. In this study, we performed sensitivity experiments to test the response of the simulated Eurasian ice sheets to surface climate, oceanic temperatures (and thus basal melting under floating ice tongues), and sea level perturbations. Our results highlight that the EIS retreat simulated with the GRISLI model is primarily triggered by atmospheric warming. Increased atmospheric temperatures further amplify the sensitivity of the ice sheets to sub-shelf melting. These results contradict those of previous modelling studies mentioning the central role of basal melting on the deglaciation of the marine-based Barents–Kara ice sheet. However, we argue that the differences with previous works are mainly related to differences in the methodology followed to generate the initial LGM ice sheet. Due to the strong sensitivity of EIS to the atmospheric forcing highlighted with the GRISLI model and the limited extent of the confined ice shelves during the LGM, we conclude by questioning the analogy between EIS and the current WAIS. However, because of the expected rise in atmospheric temperatures, the risk of hydrofracturing is increasing and could ultimately put the WAIS in a configuration similar to the past Eurasian ice sheet.

Indian Ocean Dipole Changes During the Last Interglacial Modulated by the Mean Oceanic Climatology

AbstractInvestigating the Indian Ocean Dipole (IOD) during the Last Interglacial (LIG) can advance knowledge of IOD behaviors in orbitally‐induced warmer‐than‐present scenarios. Based on multiple model outputs from the Paleoclimate Modeling Intercomparison Project Phase 4, the analysis suggests reduced frequencies of the IOD compared to the preindustrial period. Spatially, the whole growing and mature stages of the IOD feature suppressed variability over the west and westward expansions of eastern anomalies, while the eastern perturbation enhances only at the initial phase. The overall amplitude assessed by the dipole mode index shows only minor reductions. These changes are attributed to orbitally‐induced enhancement in mean‐state westward currents along the equator, which transport more anomalous cold water westward. In addition, the subdued El Niño–Southern Oscillation (ENSO) during the LIG is unlikely to cause a weakening of the IOD, in association with a less important role of ENSO in the formation of the IOD.

Extratropical circulation associated with Mediterranean droughts during the Last Millennium in CMIP5 simulations

Abstract. The Mediterranean region is expected to experience significant changes in hydroclimate, reflected in increases in the duration and severity of soil moisture droughts. While numerous studies have explored Mediterranean droughts in coupled climate models under present and future scenarios, understanding droughts in past-climate simulations remains relatively underexplored. Such simulations can offer insights into long-term drought variability that observational records cannot capture. Therefore, our study investigates circulation patterns in the Euro-Atlantic domain associated with multi-year soil moisture droughts over the Mediterranean region during the last millennium (850–2005 CE) in climate simulations. For this, we use the fifth phase of the Climate Model Intercomparison Project–Paleoclimate Model Intercomparison Project Phase 3 (CMIP5–PMIP3) and the CESM Last Millennium Ensemble Project. Primarily, we examine the differences among the models in representing drought variability and related circulation patterns. For the analysis, we exclude the anthropogenic trends from 1850–2005 CE, and to detect the circulation patterns, we perform k-means clustering combined with linear correlation analyses. The findings confirm that Mediterranean drought occurrence during the last millennium is associated with internal variability in the climate system. Drought variability, the associated circulation patterns, and the frequencies of these patterns vary across the models. Some climate models exhibit a multidecadal anti-phase occurrence of some drought periods between the western and eastern Mediterranean regions, although the exact periods of coherence differ among the models. This anti-phase co-variability, which agrees with some proxy records, can be explained by the dominant circulation patterns in each region detected by the models: western Mediterranean droughts are dominated by a high-pressure system over central Europe and a North Atlantic Oscillation (NAO)-like pattern, while eastern Mediterranean droughts are linked to positive pressure anomalies in the southern and eastern Mediterranean and negative NAO, East Atlantic, and East Atlantic–western Russia-like patterns. The frequencies of these modes of climate variability are strongly model-dependent; i.e., some patterns occur more frequently or only in some models, suggesting that the main drivers of droughts differ among the models. Although it is complicated to evaluate the representation of droughts and associated circulation among the models, in general, the models with lower horizontal and vertical spatial land resolutions exhibit drought variability and patterns that distinctly differ from other models. These model differences and preferences toward some circulation patterns can be a source of uncertainties in the model–proxy comparison of Mediterranean droughts and potentially influence future climate projections.

Spatial contrasts in mid-Holocene dune activities in deserts of northern China: Results from PMIP4 simulations in comparison with geological data

Numerous studies have explored paleoenvironmental conditions in China's dune fields around the mid-Holocene (MH), ∼6000 calendar years ago, often based on individual paleosol-aeolian sand depositional sequences and local-scale landscape dynamics. So far, continent-scale modeling studies have only indirectly assessed dune activation and stabilization processes via changes in simulated precipitation, net precipitation (precipitation minus evaporation), and effective moisture (precipitation divided by evaporation). Here we conducted a comprehensive study using gridded data on potential evapotranspiration, precipitation, and surface wind velocity from 13 climate models participating in the Paleoclimate Modeling Intercomparison Project phases 4 (PMIP4) and compared the model results with geological records. Our evaluation of spatial patterns of dune activity in northern China around the MH reveals that the intensity of annual MH dune activity exhibited significant differences between the eastern and western parts of Chinese deserts compared to those of the preindustrial period (PI). Dune activity was significantly weaker in the eastern region, while in the western part, it was considerably stronger. The key drivers of this disparity were the distinct spatial variations in near-surface wind speed and effective moisture between the two regions. Compared with the optically stimulated luminescence ages of the paleosol-aeolian sand sedimentary sequences derived from a compilation of geological records from 88 sites across northern China, the model results suggest that the weakening of dune activity and the process of dune stabilization in eastern China around the MH were mainly attributed to a significant increase in effective moisture and a concurrent decrease in near-surface wind speed due to the intensified East Asian summer monsoon. Additionally, the weakening of westerlies in western China resulted in reduced effective moisture, contributing to increased sand availability for dune development and subsequent desert expansion.

Global terrestrial monsoon area variations since Last Glacial Maximum based on TraCE21ka and PMIP4-CMIP6 simulations

Global monsoon over land (GM_L) have significant variations in their dominated area (GMA_L). The quantitative assessment of changes in GMA_L is essential for understanding spatial environment impacts and the underlying driving mechanism of GM_L evolution. Simulation results of a suite of transient climate evolution simulation over the last 21,000 years (TraCE21ka) and five-model ensemble outputs of the Paleoclimate Modelling Intercomparison Project in their latest phase (fourth, PMIP4-CMIP6) are analyzed to study the changes in global, hemispheric and six sub-monsoon areas over the past 21 ka over land. Both datasets can reproduce large-scale characteristics of GMA_L variability, although there are differences in the magnitudes of their changes. Compared to 0 ka, TraCE21ka simulations reveal that GMA_L reduced by about 4.89 × 106 km2 (decreased by 10.89%) during the Last Glacial Maximum (LGM, 21–18 ka), and reached a maximum of 46.61 × 106 km2 (increased by 3.83%) during the middle Holocene (MH, 9–6 ka). In comparison, the results of PMIP4-CMIP6 yield a smaller change of GMA_L during the LGM (decreased by 3.72%) and a larger magnitude of GMA_L during the MH (increased by 4.73%). The analysis of GMA_L over the past 21 ka reveals the varying influences of external forcing, with insolation playing a dominant role. Rising atmospheric greenhouse gas concentrations (GHG) and ice sheet melting have induced a stepwise expansion in GMA_L during the last deglaciation, superimposed with several abrupt millennial-scale weakened monsoon events under meltwater fluxes (MWF) forcing. After that, insolation caused a gradually declining trend in GMA_L during the Holocene. The position of the Intertropical Convergence Zone (ITCZ) under orbital forcing, leading to an anti-phase reaction between Northern Hemisphere monsoon areas (NHMA_L) and Southern Hemisphere monsoon areas (SHMA_L), particularly after 9 ka. Among the six sub-monsoons, they show diverse variations and regional characteristics under different forcings. Although regional monsoons like Indonesia-Australia monsoon (IAM) exhibit nonlinear behavior due to internal variabilities, the overall response of global monsoons to external forcing is predominantly linear. In addition, changes in the hydrological processes can indicate the formation and expansion of monsoon domains. Moisture budget shows that thermodynamic effect has always dominated monsoon hydrological cycles over the past 21 ka. The differences in regional monsoon hydrological processes arise from the dynamic effect. Moreover, δ18O records have been used to test these numerical simulation results. Which, together with diverse thermodynamic and dynamic component configurations of terrestrial paleo-monsoon variations, would help to better understand the underlying mechanism of global monsoon and its influences in the future.

The change in convection over the Indo-Pacific warm pool in the mid-Holocene and its influence on South Asian precipitation

This study investigates how convection over the Indo-Pacific warm pool (IPWP) differed during the mid-Holocene compared to the preindustrial period. The research employs modeling data from the Paleoclimate Modelling Intercomparison Project phases 3 and 4 combined with published hydroclimate records from the Maritime Continent and a new primary productivity reconstruction in the western Celebes Sea. While there is large variability amongst models in terms of simulating precipitation changes, the model AWI-ESM-1-1-LR has the highest agreement with proxy data. This model indicates a dipole pattern consisting of strengthened convection over the eastern equatorial Indian Ocean and Sumatra and weakened convection over the western equatorial Pacific and Borneo-Sulawesi-New Guinea. The reduced convection over the latter region is supported by the primary productivity record illustrating lower precipitation over Borneo and a deepened nutricline in the western Celebes Sea driven by anomalous equatorial easterly winds. This dipole pattern demonstrates an anomalous trans-ocean zonal circulation cell that links the western equatorial Pacific and equatorial Indian Oceans. This cell emerging in summer and peaking in autumn, is proposed to arise from a trans-ocean zonal imbalance of equatorial precipitation change induced by altered summer insolation, and higher equatorial insolation in autumn. The increased precipitation in South Asia during the mid-Holocene is partially attributed to moisture transported from the western equatorial Pacific associated with this anomalous zonal cell. Thus, the dipole pattern of altered IPWP convection is tightly linked to dynamic precipitation changes and can influence regions outside the IPWP.

External Forcings Caused the Tripole Trend of Asian Precipitation During the Holocene

AbstractTo investigate the evolution of precipitation over Asian continent in the Holocene and the associated mechanisms, we used a set of simulations of the transient climate evolution over the past 21,000 years (TraCE‐21ka), multimodel results from the Paleoclimate Modeling Intercomparison Project Phase 4 (PMIP4), and proxy records in Asia. The TraCE‐21ka results showed a tripole pattern in suborbital‐scale precipitation trends over the Asian continent during the Holocene, with a trend of increase over southern parts of the monsoon regions and arid Central Asia (ACA), and a trend of decline over northern parts of the monsoon regions and their areas of transition with ACA. This tripole pattern was corroborated by proxy records from multiple regions and multimodel results from the PMIP4. Further analysis based on single‐forcing simulations of TraCE‐21ka indicated that influences from different external forcings were different on the Asian precipitation in the main rainy seasons in the Holocene and that their combined effects shaped the tripole pattern. In summer, orbital forcing, by reducing solar radiation in mid‐to‐high latitudes and weakening the land‐sea thermal contrast, has been the dominant factor in the long‐term evolution of precipitation in the monsoon region and West Asia. In winter and spring, changes in meltwater flux played dominant roles in intensifying local water cycle and horizontal moisture advection, which drove the trend of increase in precipitation in ACA. Additionally, changes in greenhouse gas concentration and continental ice sheet forcings both also contribute to the increase in precipitation in ACA.

Unraveling the complexities of the Last Glacial Maximum climate: the role of individual boundary conditions and forcings

Abstract. In order to quantify the relative importance of individual boundary conditions and forcings, including greenhouse gases, ice sheets, and Earth's orbital parameters, on determining Last Glacial Maximum (LGM) climate, we have performed a series of LGM experiments using a state-of-the-art climate model AWI-ESM, in which different combinations of boundary conditions and forcings have been applied following the protocol of Paleoclimate Modelling Intercomparison Project phase 4 (PMIP4). In good agreement with observational proxy records, a general colder and drier climate is simulated in our full-forced LGM experiment as compared to the present-day simulation. Our simulated results from non-full-forced sensitivity simulations reveal that both the greenhouse gases and ice sheets play a major role in defining the anomalous LGM surface temperature compared to today. Decreased greenhouse gases in LGM as compared to present day leads to a non-uniform global cooling with polar amplification effect. The presence of LGM ice sheets favors a warming over the Arctic and northern Atlantic oceans in boreal winter, as well as a cooling over regions with the presence of ice sheets. The former is induced by a strengthening in the Atlantic meridional overturning circulation (AMOC) transporting more heat to high latitudes, whilst the latter is due to the increased surface albedo and elevation of ice sheets. We find that the Northern Hemisphere monsoon precipitation is influenced by the opposing effects of LGM greenhouse gases and ice sheets. Specifically, the presence of ice sheets leads to significant drying in the Northern Hemisphere monsoon regions, while a reduction in greenhouse gases results in increased monsoon rainfall. Based on our model results, continental ice sheets exert a major control on atmospheric dynamics and the variability of El Niño–Southern Oscillation (ENSO). Moreover, our analysis also implies a nonlinearity in climate response to LGM boundary conditions and forcings.

Quantifying effects of Earth orbital parameters and greenhouse gases on mid-Holocene climate

Abstract. The mid-Holocene (MH) is the most recent typical climate period and a subject of great interest in global paleocultural research. Following the latest Paleoclimate Modeling Intercomparison Project phase 4 (PMIP4) protocol and using a fully coupled climate model, we simulated the climate during both the MH and the preindustrial (PI) periods and quantified the effects of Earth orbital parameters (ORBs) and greenhouse gases (GHGs) on climate differences, focusing on the simulated differences in the Atlantic meridional overturning circulation (AMOC) between these two periods. Compared to the PI simulation, the ORB effect in the MH simulation led to seasonal enhancement of temperature, consistent with previous findings. In the MH simulation, the ORB effect led to a markedly warmer climate in the mid–high latitudes and increased precipitation in the Northern Hemisphere, which were partially offset by the cooling effect of the lower GHGs. The AMOC in the MH simulation was about 4 % stronger than that in the PI simulation. The ORB effect led to 6 % enhancement of the AMOC in the MH simulation, which was, however, partly neutralized by the GHG effect. Transient simulation from the MH to the PI further demonstrated the opposite effects of ORBs and GHGs on the evolution of the AMOC during the past 6000 years. The simulated stronger AMOC in the MH was mainly due to the thinner sea ice in the polar oceans caused by the ORB effect, which reduced the freshwater flux export to the subpolar Atlantic and resulted in a more saline North Atlantic. This study may help us quantitatively understand the roles of different external forcing factors in Earth's climate evolution since the MH.

Divergent response of eastern China precipitation to the Pacific decadal oscillation during the last Interglacial

Understanding the role of the Pacific Decadal Oscillation (PDO) during the Last Interglacial (LIG, ∼127 ka) may advance our knowledge on decadal climate variability in a warmer world. In this study, we investigate the PDO–precipitation relationship over eastern China during the LIG based on the NorESM2-LM, which shows the best performance in capturing the observed PDO pattern and the PDO–precipitation relationship among 12 models from the Paleoclimate Modelling Intercomparison Project Phase 4. Composite analysis shows that summer precipitation during the LIG is generally increased over eastern China in the positive PDO phases relative to the negative phases, which is different from the “drier-wetter-drier” triple pattern in the present day. The discrepant precipitation patterns between the LIG and preindustrial are caused by different responses of composite atmospheric circulations. Moreover, moisture budget analysis indicates that variations of vertical velocity over eastern China largely explain the PDO-linked precipitation patterns during both LIG and preindustrial, although the main causes for vertical motion changes are different across regions and periods based on the quasi-geostrophic ω equation. Additionally, these discrepancies in composite atmospheric circulations are largely linked to differences in PDO patterns and mean climate states between the LIG and preindustrial. Our results suggest a varying relationship between the PDO and precipitation over eastern China in a warmer-than-present world induced by Earth's orbit, which have important implications for PDO reconstructions during the LIG via the PDO-precipitation relationship reported here and shed light on decadal variability of precipitation as well as the associated decadal predictions in a future warmer world.

Mechanisms Associated With Tropospheric Dry Static Stability Change During the Last Glacial Maximum

AbstractThe processes governing tropospheric stratification alterations during the Last Glacial Maximum (LGM) are investigated using the Paleoclimate Modeling Intercomparison Project Phase 3/4 (PMIP3/PMIP4) simulations. The results demonstrate a decrease in static stability in the tropics during both December‐January‐February (DJF) and June‐July‐August (JJA), while an increase is observed in the extratropics during DJF. Further analysis reveals that the rise in static stability over high‐latitude ocean is driven by shifts in surface frozen lines, whereas the increased static stability over ice sheet margins is due to the cooling effect of ice sheet slopes. The study finds that the dry static stability change in ice sheet‐covered areas remains significant and robust in both PMIP3 and PMIP4. However, a weaker stabilization signal is detected in the North Atlantic in PMIP4. These findings provide valuable insights into the processes shaping tropospheric stratification during the LGM and underscore the importance of accounting for ice sheet effects in paleoclimate simulations.

The PMIP4 simulated dryland aridity changes during the Last Interglacial

The dryland is one of the most important land ecosystems on the planet, and its changes are closely bound up with one-third of the world’s population. Investigating dryland climate change during the Last Interglacial (LIG; ∼127 ka ago) can advance our knowledge of dryland climate behaviors in an orbitally-induced warmer-than-present scenario. Based on the multiple model outputs from the Paleoclimate Modelling Intercomparison Project Phase 4, we suggest that the dryland areas during the LIG are 37% wetter than that during the preindustrial period as measured by the aridity index (AI), and 37% of the preindustrial drylands correspondingly convert to wetter subtypes. Spatially, there are hemispheric differences with drylands wetting and contracting in the Northern Hemisphere but desiccating and expanding in the Southern Hemisphere. Further diagnosis indicates that the altered precipitation is the dominant contributor to more than 72% of the AI changes, and the precipitation change is mainly attributed to the orbitally-induced redistribution of incoming insolation and heat. The secondary factor is the relative humidity change, exaggerating the AI changes in the same direction as the precipitation does. The simulation agrees reasonably with reconstructions for most regions, except Australia and southern Africa. The simulated changes in dryland aridity and the mechanism differ from that in future warming scenarios, so we claim that the LIG as a potential analogue for a warmer future does not fully hold for the dryland climate.

Unfavorable environmental conditions for tropical cyclone genesis over the western North Pacific during the Last Interglacial based on PMIP4 simulations

Investigating the variation in tropical cyclone (TC) activity over the western North Pacific (WNP) in past warm periods helps to better understand TC behaviors in a warming future. In this study, the authors analyze the changes in large-scale TC genesis factors and the associated mechanisms over the WNP during the Last Interglacial (LIG), based on multimodel outputs from the Paleoclimate Modelling Intercomparison Project Phase 4. The results show that potential intensity exhibits a general decrease over the WNP during the LIG in the storm season, dominated by a weakened thermodynamic disequilibrium. The moist entropy deficit shows an overall increase over the WNP, arising from the decreased mid-tropospheric moisture and weakened vertical temperature contrast. Vertical wind shear enhances over the central WNP but weakens over the southwestern WNP, which is induced by the changes in the meridional tropospheric temperature gradient and hence high-level zonal winds. The absolute vorticity shows a general decrease over the WNP, partially linked with the decreased SST over the western tropical Pacific. Based on a genesis potential index, the authors suggest a decrease in genesis potential over the WNP during the LIG, indicating unfavorable conditions for TC genesis. The results highlight the important role of Earth's orbit in regulating TC activity, which may shed light on TC behaviors in a warmer climate.

Enhanced seasonality of surface air temperature over China during the mid-Holocene

Using all available simulations performed by climate models participating in PMIP4 (Paleoclimate Modelling Intercomparison Project – Phase 4), the authors quantify the seasonality change of surface air temperature over China during the mid-Holocene (6000 years ago) and the associated physical mechanisms. Relative to the preindustrial period, all 16 models consistently show an enhanced temperature seasonality (i.e., summer minus winter temperature) across China during that interglacial period, with a nationally averaged enhancement of 2.44 °C or 9% for the multimodel mean. The temperature seasonality change is closely related with the seasonal contrast variation of surface energy fluxes mainly due to the mid-Holocene orbital forcing. Specifically, the summer–winter increase in surface net shortwave radiation dominates the intensified temperature seasonality at the large scale of China during the mid-Holocene; the surface net longwave radiation has a minor positive contribution in most of the Tibetan Plateau and eastern China; and both the surface latent and sensible heat fluxes show partial offset effects in most of the country. There are uncertainties in the reconstructed temperature seasonality over China during the mid-Holocene based on the proxy data that can reflect seasonal signals.摘要利用PMIP4多模式试验数据, 作者量化了中全新世 (距今约6000年) 中国温度季节性变化. 结果表明: 相对于工业革命前期, 所有16个模式一致模拟显示中全新世我国温度季节性 (即夏季与冬季温差) 增强, 平均增幅9%; 这与该时期轨道强迫引起的地表能量通量的季节对比变化密切相关, 其中净短波辐射起主导作用, 净长波辐射作用次之, 感热和潜热为负贡献; 与模拟不同, 重建结果存在不确定性.