Paleoclimate Modelling Intercomparison Project Phase Research Articles

Summer Westerly Jet in Northern Hemisphere during the Mid-Holocene: A Multi-Model Study

The upper-level jet stream, a narrow band of maximum wind speed in the mid-latitude westerlies, exerts a considerable influence on the global climate by modulating the transport and distribution of momentum, heat and moisture. In this study by using four high-resolution models in the Paleoclimate Modelling Intercomparison Project phase 3, the changes of position and intensity of the northern hemisphere westerly jet at 200 hPa in summer during the mid-Holocene (MH), as well as the related mechanisms, are investigated. The four models show similar performance on the westerly jet. At the hemispheric scale, the simulated westerly jet has a poleward shift during the MH compared to the preindustrial period. The warming in arctic and cooling in the tropics during the MH are caused by the orbital changes of the earth and the precipitation changes, and it could lead to the weakened meridional temperature gradient and pressure gradient, which might account for the poleward shift of the westerly jet from the thermodynamic perspective. From the dynamic perspective, two maximum centers of eddy kinetic energy are simulated over the North Pacific and North Atlantic with the north deviation, which could cause the northward movement of the westerly jet. The weakening of the jet stream is associated with the change of the Hadley cell and the meridional temperature gradient. The largest weakening is over the Pacific Ocean where both the dynamic and the thermodynamic processes have weakening effects. The smallest weakening is over the Atlantic Ocean, and it is induced by the offset effects of dynamic processes and thermodynamic processes. The weakening over the Eurasia is mainly caused by the dynamic processes.

A Comparison of the CMIP6midHoloceneandlig127kSimulations in CESM2

AbstractResults are presented and compared for the Community Earth System Model version 2 (CESM2) simulations of the middle Holocene (MH, 6 ka) and Last Interglacial (LIG, 127 ka). These simulations are designated as Tier 1 experiments (midHoloceneandlig127k) for the Coupled Model Intercomparison Project phase 6 (CMIP6) and the Paleoclimate Modeling Intercomparison Project phase 4 (PMIP4). They use the low‐top, standard 1° version of CESM2 contributing to CMIP6 DECK, historical, and future projection simulations, and to other modeling intercomparison projects. ThemidHoloceneandlig127kprovide the opportunity to examine the responses in CESM2 to the orbitally induced changes in the seasonal and latitudinal distribution of insolation. The insolation anomalies result in summer warming over the Northern Hemisphere continents, reduced Arctic summer minimum sea ice, and increased areal extent of the North African monsoon. The Arctic remains warm throughout the year. These changes are greater in thelig127kthanmidHolocenesimulation. Other notable changes are reduction of the Niño3.4 variability and Drake Passage transport and a small increase in the Atlantic Meridional Overturning Circulation from thepiControltomidHolocenetolig127ksimulation. Comparisons to paleo‐data and to simulations from previous model versions are discussed. Possible reasons for mismatches with the paleo‐observations are proposed, including missing processes in CESM2, simplifications in the CMIP6 protocols for these experiments, and dating and calibration uncertainties in the data reconstructions.

Evolution of the meridional shift of the subtropical and subpolar westerly jet over the Southern Hemisphere during the past 21,000 years

Deciphering the variations in the subtropical westerly jet (STJ) and the subpolar westerly jet (SPJ) over the Southern Hemisphere in Earth’s history will help us to better understand how the westerlies may respond to various external forcings in the future. Using Simulation of Transient Climate Evolution over the past 21,000 years simulations, we demonstrate that the STJ migrated equatorward in the Last Glacial Maximum relative to the present, which followed by abrupt poleward, equatorward, and poleward shifts during the Heinrich Stadial I, Antarctic Cold Reversal, and Younger Dryas, respectively. These large-amplitude fluctuations are controlled by changes in meltwater fluxes and polar ice-sheet topography. During the Holocene, the STJ shows no obvious long-term trend, due to the offsetting effect between increased orbital insolation in austral summer and decreased Antarctic ice-sheet topography. The meridional shift of the SPJ shows an in-phase change with the STJ during the past 21 kyr. In terms of the dynamic mechanisms, the meridional shift of the STJ and SPJ is induced by changes in the meridional temperature gradient and the maximum eddy growth rate, respectively; and is also linked to changes in the mean meridional circulations (that is, the Hadley and Ferrel cells). The modeled meridional shift of the southern westerlies is supported by the results from the Paleoclimate Modelling Intercomparison Project Phase Ⅲ and is consistent with several reconstructions, but it is difficult to constrain robustly by geological evidence as the reconstructed westerlies vary across sites and proxies. Nevertheless, our study provides a possible scenario for the evolution of the southern westerlies during the past 21 kyr and identifies the dominant forcing and the associated mechanisms, which may advance our knowledge on the future behavior of the southern westerlies.

Dynamical and hydrological changes in climate simulations of the last millennium

Abstract. Simulations of climate of the last millennium (LM) show that external forcing had a major contribution to the evolution of temperatures; warmer and colder periods like the Medieval Climate Anomaly (MCA; ca. 950–1250 CE) and the Little Ice Age (LIA; ca. 1450–1850 CE) were critically influenced by changes in solar and volcanic activity. Even if this influence is mainly observed in terms of temperatures, evidence from simulations and reconstructions shows that other variables related to atmospheric dynamics and hydroclimate were also influenced by external forcing over some regions. In this work, simulations from the Coupled Model Intercomparison Project Phase 5 and Paleoclimate Modelling Intercomparison Project Phase 3 (CMIP5/PMIP3) are analyzed to explore the influence of external forcings on the dynamical and hydrological changes during the LM at different spatial and temporal scales. Principal component (PC) analysis is used to obtain the modes of variability governing the global evolution of climate and to assess their correlation with the total external forcing at multidecadal to multicentennial timescales. For shorter timescales, a composite analysis is used to address the response to specific events of external forcing like volcanic eruptions. The results show coordinated long-term changes in global circulation patterns, which suggest expansions and contractions of the Hadley cells and latitudinal displacements of westerlies in response to external forcing. For hydroclimate, spatial patterns of drier and wetter conditions in areas influenced by the North Atlantic Oscillation (NAO), Northern Annular Mode (NAM), and Southern Annular Mode (SAM) and alterations in the intensity and distribution of monsoons and convergence zones are consistently found. Similarly, a clear short-term response is found in the years following volcanic eruptions. Although external forcing has a greater influence on temperatures, the results suggest that dynamical and hydrological variations over the LM exhibit a direct response to external forcing at both long and short timescales that is highly dependent on the particular simulation and model.

The PMIP3 Simulated Climate Changes over Arid Central Asia during the Mid‐Holocene and Last Glacial Maximum

AbstractIn this study, the climate changes over Arid Central Asia (ACA) during the mid‐Holocene (approximately 6,000 calendar years ago, MH) and the Last Glacial Maximum (approximately 21,000 calendar years ago, LGM) were investigated using multimodel simulations derived from the Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3). During the MH, the multimodel median (MMM) shows that in the core region of ACA, the regionally averaged annual surface air temperature (SAT) decreases by 0.13°C and annual precipitation decreases by 3.45%, compared with the preindustrial (PI) climate. The MMM of the SAT increases by 1.67/0.13°C in summer/autumn, whereas it decreases by 1.23/1.11°C in spring/winter. The amplitude of the seasonal cycles of the SAT increases over ACA due to different MH orbital parameters. For precipitation, the regionally averaged MMM decreases by 5.77%/5.69%/0.39%/5.24% in spring/summer/autumn/winter, respectively. Based on the analysis of the aridity index (AI), compared with the PI, a drier climate appears in southern Central Asia and western Xinjiang due to decreasing precipitation. During the LGM, the MMM shows that the regionally averaged SAT decreases by 5.04/4.36/4.70/5.12/5.88°C and precipitation decreases by 27.78%/28.16%/31.56%/27.74%/23.29% annually and in the spring, summer, autumn, and winter, respectively. Robust drying occurs throughout almost the whole core area. Decreasing precipitation plays a dominant role in shaping the drier conditions, whereas strong cooling plays a secondary but opposite role. In response to the LGM external forcings, over Central Asia and Xinjiang, the seasonal cycle of precipitation has a smaller amplitude compared with that under the PI climate. In the model‐data comparison, the simulated MH moisture changes over ACA are to some extent consistent with the reconstructions, further confirming that drier conditions occurred during that period than during the PI.

Variability of surface climate in simulations of past and future

Abstract. It is virtually certain that the mean surface temperature of the Earth will continue to increase under realistic emission scenarios, yet comparatively little is known about future changes in climate variability. This study explores changes in climate variability over the large range of climates simulated by the Coupled Model Intercomparison Project Phase 5 and 6 (CMIP5/6) and the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3), including time slices of the Last Glacial Maximum, the mid-Holocene, and idealized experiments (1 % CO2 and abrupt4×CO2). These states encompass climates within a range of 12 ∘C in global mean temperature change. We examine climate variability from the perspectives of local interannual change, coherent climate modes, and through compositing extremes. The change in the interannual variability of precipitation is strongly dependent upon the local change in the total amount of precipitation. At the global scale, temperature variability is inversely related to mean temperature change on intra-seasonal to multidecadal timescales. This decrease is stronger over the oceans, while there is increased temperature variability over subtropical land areas (40∘ S–40∘ N) in warmer simulations. We systematically investigate changes in the standard deviation of modes of climate variability, including the North Atlantic Oscillation, the El Niño–Southern Oscillation, and the Southern Annular Mode, with global mean temperature change. While several climate modes do show consistent relationships (most notably the Atlantic Zonal Mode), no generalizable pattern emerges. By compositing extreme precipitation years across the ensemble, we demonstrate that the same large-scale modes influencing rainfall variability in Mediterranean climates persist throughout paleoclimate and future simulations. The robust nature of the response of climate variability, between cold and warm climates as well as across multiple timescales, suggests that observations and proxy reconstructions could provide a meaningful constraint on climate variability in future projections.

Assessment of PMIP3 model simulations of megadroughts over the eastern China during the last millennium

AbstractThe simulated spatiotemporal characteristics and variability of decadal megadroughts over the eastern China during the last millennium of 7 model simulations from the Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3) archive were assessed, and the corresponding mechanisms of the megadroughts were also investigated. The simulated megadrought features of frequencies, durations, magnitudes and spatial distributions were compared with reconstructed data. Comparison showed that climate models can reasonably reproduce the major features of megadroughts, however, the simulated megadroughts are not temporal synchronous with the reconstructed megadroughts, indicating that the internal variability rather than external forcing initiates the observed megadroughts. The simulated spatial distributions of wind and sea level pressure (SLP) anomalies during megadrought years showed that megadroughts over the eastern China corresponded to weakening of the East Asia Summer Monsoon (EASM) and strengthening of the East Asia Winter Monsoon (EAWM). A pattern of sea surface temperature anomalies (SSTA) with significant negative SSTA over the western central North Pacific and significant positive SSTA over eastern North Pacific and equatorial Pacific, resembling a positive PDO‐like pattern, was found to suppress precipitation in eastern China. Therefore, it could be concluded that that internal climate variability is the main trigger of the megadroughts over the eastern China, while the North Pacific decadal variability is considered the key driving factor influencing the EASM and EAWM.

A multi‐model analysis of the East Asian monsoon changes in the Medieval Climate Anomaly and Little Ice Age

AbstractUsing nine climate model simulations from the Paleoclimate Modelling Intercomparison Project Phase 3, the changes in the East Asian summer and winter monsoon, together with their associated atmospheric circulations in the Medieval Climate Anomaly (MCA) and Little Ice Age (LIA), were investigated in this study. The East Asian summer monsoon strengthened (weakened) as a consequence of the increased (decreased) land–sea thermal contrast between the East Asian continent and the adjacent western North Pacific and South China Sea in the MCA (LIA). The East Asian winter monsoon (EAWM) changes varied across models, and differences were shown between the low latitudes and mid‐to‐high latitudes of the EAWM region. In the northern area, the EAWM intensified slightly due to a reinforced Aleutian Low during the MCA and decreased slightly because of the suppression of the Siberian High and the weakening and northward shift of the Aleutian Low during the LIA. In contrast, there are large inconsistencies among models during both periods in the southern EAWM region. Our results imply that model improvement is needed to reproduce the complex interacting processes over the low latitudes of East Asia.

Variation of the summer Asian westerly jet over the last millennium based on the PMIP3 simulations

Investigation of the evolution of summer Asian westerly jet (AWJ) in Earth history may deepen our understanding of the behavior of AWJ in the present and future. Here, the variation of summer westerlies and the associated mechanisms over West and East Asia are examined using the last millennium simulations from the Paleoclimate Modeling Intercomparison Project Phase III (PMIP3). The multimodel ensemble mean of 11 PMIP3 models reproduces the spatial distribution of modern zonal wind at 200 hPa and captures the location and intensity of the summer AWJ well. The results illustrate that the location of summer westerlies over West and East Asia shows no clear long-term trend over the last millennium, whereas there is an enhancing trend for the intensity, but with significant interannual and multidecadal variability. Summer westerlies over West Asia mainly oscillate within ~25-year band in terms of the location and intensity, while the principal periodicity over East Asia oscillate within ~25- and ~15-year band, respectively. Moreover, summer AWJ broadly moves poleward and weakens during the ‘Medieval Climate Anomaly’ (MCA, AD 1000–1200) relative to the ‘Little Ice Age’ (LIA, AD 1400–1600), which is attributed to the reduced meridional temperature gradient during the MCA. This poleward shift is consistent with the reconstructed moisture conditions over Central Asia, but the lack of direct proxy for westerlies restricts detailed model–data comparisons. Nevertheless, a possible scenario is presented on the spatiotemporal variation of summer AWJ over the last millennium, which may shed light on the response of the AWJ to natural forcings.

Combining a pollen and macrofossil synthesis with climate simulations for spatial reconstructions of European climate using Bayesian filtering

Abstract. Probabilistic spatial reconstructions of past climate states are valuable to quantitatively study the climate system under different forcing conditions because they combine the information contained in a proxy synthesis into a comprehensible product. Unfortunately, they are subject to a complex uncertainty structure due to complicated proxy–climate relations and sparse data, which makes interpolation between samples difficult. Bayesian hierarchical models feature promising properties to handle these issues, like the possibility to include multiple sources of information and to quantify uncertainties in a statistically rigorous way. We present a Bayesian framework that combines a network of pollen and macrofossil samples with a spatial prior distribution estimated from a multi-model ensemble of climate simulations. The use of climate simulation output aims at a physically reasonable spatial interpolation of proxy data on a regional scale. To transfer the pollen data into (local) climate information, we invert a forward version of the probabilistic indicator taxa model. The Bayesian inference is performed using Markov chain Monte Carlo methods following a Metropolis-within-Gibbs strategy. Different ways to incorporate the climate simulations into the Bayesian framework are compared using identical twin and cross-validation experiments. Then, we reconstruct the mean temperature of the warmest and mean temperature of the coldest month during the mid-Holocene in Europe using a published pollen and macrofossil synthesis in combination with the Paleoclimate Modelling Intercomparison Project Phase III mid-Holocene ensemble. The output of our Bayesian model is a spatially distributed probability distribution that facilitates quantitative analyses that account for uncertainties.

The sensitivity of Northern Hemisphere ice sheets to atmospheric forcing during the last glacial cycle using PMIP3 models

ABSTRACTThe evolution of Northern Hemisphere ice sheets through the last glacial cycle is simulated with the glacial index method by using the climate forcing from one General Circulation Model, COSMOS. By comparing the simulated results to geological reconstructions, we first show that the modelled climate is capable of capturing the main features of the ice-sheet evolution. However, large deviations exist, likely due to the absence of nonlinear interactions between ice sheet and other climate components. The model uncertainties of the climate forcing are examined using the output from nine climate models from the Paleoclimate Modelling Intercomparison Project Phase III. The results show a large variability in simulated ice sheets between the different models. We find that the ice-sheet extent pattern resembles summer surface air temperature pattern at the Last Glacial Maximum, confirming the dominant role of surface ablation process for high-latitude Northern Hemisphere ice sheets. This study shows the importance of the upper boundary condition for ice-sheet modelling, and implies that careful constraints on climate output is essential for simulating realistic glacial Northern Hemisphere ice sheets.

Mid-Holocene drylands: A multi-model analysis using Paleoclimate Modelling Intercomparison Project Phase III (PMIP3) simulations

This study examines changes in aridity levels during the mid-Holocene (approximately 6000 cal. yr ago) using multi-model simulations from the Paleoclimate Modelling Intercomparison Project Phase III. Overall, there is little difference in the total area of drylands from the preindustrial period; global drylands are 8% wetter than during the preindustrial period as measured by an aridity index; and 16% of preindustrial drylands convert to a wetter climate subtype, double the sum of zones that are replaced by a drier category. Considerable variations are present among regions with major contractions of each dryland subtype from northern Africa to South Asia and the main expansions of arid, semiarid, and dry subhumid climates in southern hemisphere continents. The difference in precipitation is the leading factor of the aforementioned changes. The second factor is the altered potential evapotranspiration as mainly induced by relative humidity, which contributes to additional aridity changes in a same direction as precipitation does. The collective effects of precipitation and relative humidity account for more than 80% of the dryland variations. In comparison, the simulated aridity change is in reasonable agreement with reconstructions, while there are model–data discrepancies for Australia and uncertainties across proxies for southern Africa.

A multi-model analysis of ‘Little Ice Age’ climate over China

Using the numerical experiments undertaken by nine climate models within the framework of the Paleoclimate Modeling Intercomparison Project Phase 3 (PMIP3), the ensemble simulations with the Community Earth System Model for the last millennium (CESM-LME), and proxy data, we investigate the climate over China during the ‘Little Ice Age’ (LIA; from 1450 to 1850 CE) against the background of the last millennium (from 850 to 1850 CE). The surface air temperature averaged over China generally decreased over time during the last millennium, with several multi-decadal to centennial variations superimposed on the long-term cooling. Relative to the climatology of the last millennium, the annual surface temperature during the LIA decreased over the country, with an average cooling of −0.07°C for the median of the PMIP3 models. Different magnitudes of cooling occurred in all seasons except spring. The cooling over China during the LIA was largely attributed to changes in volcanic eruptions and land use, while the change in orbital parameters played a role on a seasonal scale. The precipitation over China during the LIA decreased for the annual mean and summer and autumn but slightly increased in winter and spring. Model–data comparisons indicate that the models reproduced the colder and drier climate of the LIA reasonably, although there are some differences in certain aspects.

Contribution of diverse monsoon precipitation over Central and Northern India during mid to Late Holocene

Abstracts The Indian summer monsoon (ISM) precipitation pattern over Indian subcontinent is looked through various high resolution speleothem oxygen (δ18O) isotope records from Mid to Late Holocene. The six cave locations distributed from central to northern India describing regional climatic conditions due to stalagmite deposition are considered. The stalagmite deposition reflects precipitation patterns by temporal variations of δ18O values. Oxygen isotope values reflect cumulative precipitation that is basically interpreted by averaged latitudinal site of inter tropical convergence zone (ITCZ). Under the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3) simulation, Commonwealth Scientific and Industrial Research Organisation Mk3L version 1.0 – Part 2 (CSIRO-Mk3L-1-2) model output shows ITCZ movement from Bay of Bengal (BoB) to Arabian Sea (AS) during Mid to Late Holocene which resulted due to solar insolation changes. It reflects precipitation shift from BoB to AS as well as to core monsoon zone (CMZ) to the western Himalayas. The past millennium epoch, which consists little ice age (LIA) and middle warm period (MWP), shows large variations in 850 hPa geopotential height leading to high anomalous precipitation variability all over India during ISM. The high fluctuation in δ18O values during Mid Holocene over northern India led to drought conditions due to which Indus valley civilization vanished. The present study shows that the Himalayas received higher precipitation during late Holocene. That is basically governed by western disturbances (WD) as well as ISM. Present study summarizes that the mean annual precipitation cycle for entire epoch was roughly similar, but spatial distribution of precipitation during various epoch was quite divergent from Mid to Late Holocene. The onset of ISM was also similar but, withdrawal was comparatively unalike. Wherever, peaks of seasonal precipitation were alike over Indian subcontinent region, spatial distribution of precipitation were unlike.

Components and Mechanisms of Hydrologic Cycle Changes over North America at the Last Glacial Maximum

This study examines the differences in the moisture budget over North America between the Last Glacial Maximum (LGM) and modern climate, as simulated by nine models from Paleoclimate Modelling Intercomparison Project phase 3. The results help elucidate the components and mechanisms of the LGM hydrologic cycle. The models predict substantial increases in winter precipitation minus evaporation ( P − E) over the ice-free parts of western North America with respect to the modern climate, primarily because of increases in moisture convergence by the mean flow. In summer they predict P − E increases from the Great Plains to the southeastern margin of the ice sheet—driven by large decreases in E—that are due to a combination of increased convergence by the mean flow and transient eddies. In both seasons, the LGM–modern changes in P − E are dominated by changes in the circulation, rather than by changes in atmospheric water vapor. Compared to a proxy reconstruction of LGM–modern changes in P, the simulated P responses show modest skill. They generally reproduce the reconstruction in the western part of North America but underestimate the indicated drying of the eastern part. The models that score best tend to simulate more drying of the eastern part as a result of increased moisture divergence by the mean flow. In various regions, there are trade-offs between contributions from the mean flow and transient eddies, pointing to changes in variability during the LGM; however, further work to examine such changes requires higher-frequency model output.

A multi-model analysis of moisture changes during the last glacial maximum

This study investigates terrestrial moisture changes and associated mechanisms during the last glacial maximum (LGM; approximately 21,000 calendar years ago) using multi-model simulations from the Paleoclimate Modelling Intercomparison Project phase 3 (PMIP3). Considering that terrestrial moisture is not determined solely by precipitation, an aridity index (AI) is employed for measuring the terrestrial moisture by combining the effects of both precipitation and potential evapotranspiration (PET), where the latter represents atmospheric water demand and is greatly decreased mainly by the intense cooling during the LGM. Compared to the preindustrial period, the magnitude of global mean terrestrial moisture change is small, as the wetness brought by decreased PET counteracts the dryness induced by decreased precipitation. Regionally, the moisture changes depend on the different combinations of changes in precipitation and PET: (i) drying occurs where precipitation deceases and PET hardly changes, such as the northern tropical Americas and Southeast Asia; (ii) wetting is found in regions with precipitation increases and PET decreases (e.g., northwestern Africa and the central Andes), and their contributions are comparable; (iii) in particular, wetting can also occur in regions of decreased precipitation if a sufficient decrease in PET also occurs (i.e., southeastern North America and the northern and southern parts of eastern Asia), with the latter wetting effect reversing the former drying effect. The multi-model median field is consistent with available paleo-records in southern North America, the northern tropical Americas, the Andes, northwestern Africa, the southern Iberian Peninsula, southwestern Africa, the central part of eastern Asia, and Java but disagrees with proxies in Australia, central Brazil, southeastern Africa, the northern Iberian Peninsula, and the southern part of eastern Asia.

Multi-proxy reconstructions of May–September precipitation field in China over the past 500 years

Abstract. The dominant modes of variability of precipitation for the whole of China over the past millennium and the mechanism governing their spatial structure remain unclear. This is mainly due to insufficient high-resolution proxy records of precipitation in western China. Numerous tree-ring chronologies have recently been archived in publicly available databases through PAGES2k activities, and these provide an opportunity to refine precipitation field reconstructions for China. Based on 479 proxy records, including 371 tree-ring width chronologies, a tree-ring isotope chronology, and 107 drought/flood indices, we reconstruct the precipitation field for China for the past half millennium using the optimal information extraction method. A total of 3631 of 4189 grid points in the reconstruction field passed the cross-validation process, accounting for 86.68 % of the total number of grid points. The first leading mode of variability of the reconstruction shows coherent variations over most of China. The second mode is a north–south dipole in eastern China characterized by variations of the same sign in western China and northern China (except for Xinjiang province). It is likely controlled by the El Niño–Southern Oscillation (ENSO) variability. The third mode is a sandwich triple mode in eastern China including variations of the same sign in western China and central China. The last two modes are reproduced by most of the six coupled climate models' last millennium simulations performed in the framework of the Paleoclimate Modelling Intercomparison Project Phase III (PMIP3). In particular, the link of the second mode with ENSO is confirmed by the models. However, there is a mismatch between models and proxy reconstructions in the time development of different modes. This mismatch suggests the important role of internal variability in the reconstructed precipitation mode variations of the past 500 years.

Widening of the Hadley Cell from Last Glacial Maximum to Future Climate

Abstract The Hadley cell (HC) change from paleoclimate to future climate is examined by comparing coupled model simulations archived for the Paleoclimate Modeling Intercomparison Project phase 3 (PMIP3) and phase 5 of the Coupled Model Intercomparison Project (CMIP5). Specifically, HC width and strength are evaluated using 100-yr equilibrium simulations for the Last Glacial Maximum (LGM), preindustrial (PI), and extended concentration pathway 4.5 (ECP4.5) conditions. Where available, ECP8.5 simulations are also examined to increase the sample size. All models show a systematic widening of the HC from the LGM to the PI and to the ECP4.5 and ECP8.5 simulations. Such widening, which is found in both hemispheres with more robust change in the Southern Hemisphere (SH) than in the Northern Hemisphere (NH), is significantly correlated with global-mean surface air temperature change and the associated static stability change in the subtropics. Based on the zero-crossing latitude of 500-hPa mass streamfunction, about 4.5° latitude widening of the HC results from global warming of 10°C. HC strength also exhibits a systematic weakening in the NH. However, in the SH, HC strength shows a rather minor change from LGM to ECP4.5 conditions because of the cancellation between HC weakening during the austral summer–fall and its strengthening during the spring. This result, which suggests no systematic relationship between HC width and strength changes, is discussed in the context of quasigeostrophic zonal-mean dynamics. Overall findings are also compared with recent studies that are based on transient climate model simulations.

On the Reduced North Atlantic Storminess during the Last Glacial Period: The Role of Topography in Shaping Synoptic Eddies

AbstractThe North Atlantic storminess of Last Glacial Maximum (LGM) fully coupled climate simulations is generally less intense than that of their preindustrial (PI) counterparts, despite having stronger baroclinicity. An explanation for this counterintuitive result is presented by comparing two simulations of the IPSL full climate model forced by Paleoclimate Modelling Intercomparison Project Phase 3 (PMIP3) LGM and PI conditions. Two additional numerical experiments using a simplified dry general circulation model forced by idealized topography and a relaxation in temperature provide guidance for the dynamical interpretation. The forced experiment with idealized Rockies and an idealized Laurentide Ice Sheet has a less intense North Atlantic storm-track activity than the forced experiment with idealized Rockies only, despite similar baroclinicity. Both the climate and idealized runs satisfy or support the following statements. The reduced storm-track intensity can be explained by a reduced baroclinic conversion, which itself comes from a loss in eddy efficiency to tap the available potential energy as shown by energetic budgets. The eddy heat fluxes are northeastward oriented in the western Atlantic in LGM and are less well aligned with the mean temperature gradient than in PI. The southern slope of the Laurentide Ice Sheet topography forces the eddy geopotential isolines to be zonally oriented at low levels in its proximity. This distorts the tubes of constant eddy geopotential in such a way that they tilt northwestward with height during baroclinic growth in LGM while they are more optimally westward tilted in PI.

Mid‐Holocene frozen ground in China from PMIP3 simulations

Extensive degradation of frozen ground in the mid‐Holocene is widely assumed on the basis of sparse proxy data. Here, the simulated soil temperature from the Paleoclimate Modelling Intercomparison Project Phase 3 is used to address this issue over China. By comparing with the results of a preindustrial (0 ka, baseline) simulation, we show that frozen ground in the mid‐Holocene (6 ka) simulation is degraded mainly in northeast China and on the northern Tibetan Plateau. The change follows closely orbitally induced variations in insolation. Quantitatively, permafrost area reduces by 0.02×106 km2 in northeast China in response to an orbitally induced increase in boreal summer insolation but increases by 0.08×106 km2 on the southern Tibetan Plateau due to local summer cooling. Changing values of active layer thickness vary greatly amongst different locations. On average, they are 3 and 4 cm thicker than the preindustrial values in northeast China and on the Tibetan Plateau, respectively. No degradation in seasonally frozen ground is detected over China as a whole. Regionally, its coverage increases by 0.21×106 km2 near the middle and lower reaches of the Yangtze River valley. In addition, the maximum depth of seasonal frost penetration is on average 8.5 cm deeper than preindustrial values due to widespread winter cooling. The changes in frozen ground are consistent amongst models. However, the models disagree with proxy data in terms of not only the changes in frozen ground but also climate. Further modelling improvements and adequate proxy data are both needed to fill in the gaps between models and the data in our knowledge of the mid‐Holocene frozen ground.