Community Earth System Model Last Millennium Ensemble Research Articles

The relative contributions of internal variability and external forcing to Pacific Walker Circulation over the last millennium

Abstract Pacific Walker Circulation (PWC) is an important component of tropical atmospheric circulation. Current studies have mainly focused on PWC changes over recent decades and the near future, while less effort has been devoted to long-term PWC variations. In this study, we investigate PWC changes over the last millennium (LM) based on the Community Earth System Model Last Millennium Ensemble (CESM-LME). The simulated PWC variations show no significant trend but do reveal decadal fluctuations during the LM, which underestimate the strengthened LIA-MCA PWC differences indicated by proxy-based reconstructions. A quantitative estimation of the contributions made to PWC variability from internal variability and external forcing is conducted by using multiple linear regression (MLR) analysis. The internal variabilities contribute approximately 80% to the changes of PWC during the LM, among which the Interdecadal Pacific Oscillation (IPO) has the largest contribution. In the positive phase of the IPO, the Indo-Pacific sea-level pressure (SLP) gradient decreases, and anomalous westerlies occur in the tropical western Pacific, corresponding to a weakened PWC. The relationships between the IPO and PWC show multidecadal to centennial fluctuations, suggesting that other internal modes or external forcings may have modulated the IPO-PWC relationship. Volcanic forcing is also an important contributor to PWC variability during the LM. The simulated PWC significantly weakens and lasts for 1-2 years after large volcanic eruptions. The El Niño-like SST pattern and the corresponding zonal SLP gradient lead to the PWC weakening following large volcanic eruptions.

Quantifying the internal and external drivers of Southeast Asian rainfall extremes on decadal timescales

Rainfall over mainland Southeast Asia experiences variability on seasonal to decadal timescales in response to a multitude of climate phenomena. Historical records and paleoclimate archives that span the last millennium reveal extreme multi-year rainfall variations that significantly affected the societies of mainland Southeast Asia. Here we utilize the Community Earth System Model Last Millennium Ensemble (CESM-LME) to quantify the contributions of internal and external drivers to decadal-scale rainfall extremes in the Southeast Asia region. We find that internal variability was dominant in driving both Southeast Asian drought and pluvial extremes on decadal timescales although external forcing impacts are also detectable. Specifically, rainfall extremes are more sensitive to Pacific Ocean internal variability than the state of the Indian Ocean. This discrepancy is greater for droughts than pluvials which we suggest is attributable to external forcing impacts that counteract the forced Indian Ocean teleconnections to Southeast Asia. Volcanic aerosols, the most effective radiative forcing during the last millennium, contributed to both the Ming Dynasty Drought (1637–1643) and the Strange Parallels Drought (1756–1768). From the Medieval Climate Anomaly to the Little Ice Age, we observe a shift in Indo-Pacific teleconnection strength to Southeast Asia consistent with enhanced volcanism during the latter interval. This work not only highlights asymmetries in the drivers of rainfall extremes but also presents a framework for quantifying multivariate drivers of decadal-scale variability and hydroclimatic extremes.

The Interdecadal Pacific Oscillation is responsible for the linkage of decadal changes in precipitation and moisture in arid central Asia and the humid Asian monsoon region during the last millennium

Abstract. Reconstruction and observational studies imply a potential linkage of moisture and precipitation change in arid central Asia and monsoonal East Asia, in which the evolution of moisture and precipitation in central Asia is out of phase with that in northern China but in phase with that in southern China. In order to ascertain whether there is a robust linkage between the changes in climate in Asian arid regions and monsoon regions and to elucidate the underlying dynamic mechanisms, we analyzed the Last Millennium Reanalysis dataset and outputs from the Community Earth System Model Last Millennium Ensemble (CESM-LME). The results indicate a significant decadal linkage between precipitation changes in central Asia's arid region and the Asian monsoon region during the last millennium, which is primarily driven by the Interdecadal Pacific Oscillation (IPO). In spring, the positive IPO could enhance westerlies over the Mediterranean Sea and to its east, which could transport more water vapor and cause increased precipitation over central Asia. In summer, the positive IPO is accompanied by a weakened Asian monsoon and southward Asian subtropical westerly jet, which can lead to increased (decreased) summer precipitation over southern China (over northern China and South Asia). The IPO plays a dominant role in connecting the decadal variations in precipitation between arid central Asia and monsoonal Asia by modulating the precipitation of their respective major rainy seasons. Model results suggest that this decadal linkage stems entirely from the internal variability present in the CESM-LME control and all single-forcing simulations. Changes in external forcing factors do not alter this inherent linkage caused by the IPO. Moreover, based on analyses of the aridity index and soil moisture content, this relationship of precipitation variation also causes a similar decadal linkage of moisture changes in central Asia and monsoonal Asia. The differences in the multi-centennial-scale moisture and precipitation variations in the Asian arid region and the monsoon region between the Medieval Climate Anomaly and Little Ice Age are also likely caused by IPO-like sea surface temperature anomalies.

Model and proxy evidence for coordinated changes in the hydroclimate of distant regions over the Last Millennium

Abstract. The Medieval Climate Anomaly (MCA; ca. 950–1250 CE) and the Little Ice Age (LIA; ca. 1450–1850 CE) were periods generally characterized by respectively higher and lower temperatures in many regions. However, they have also been associated with drier and wetter conditions in areas around the Intertropical Convergence Zone (ITCZ) and the Asian Monsoon region and in areas impacted by large-scale climatic modes like the Northern Annular Mode and Southern Annular Mode (NAM and SAM respectively). To analyze coordinated changes in large-scale hydroclimate patterns and whether similar changes also extend to other periods of the Last Millennium (LM) outside the MCA and the LIA, reconstruction-based products have been analyzed. This includes the collection of tree-ring-based drought atlases (DAs), the Paleo Hydrodynamics Data Assimilation product (PHYDA) and the Last Millennium Reanalysis (LMR). These analyses have shown coherent changes in the hydroclimate of tropical and extratropical regions, such as northern and central South America, East Africa, western North America, western Europe, the Middle East, Southeast Asia, and the Indo-Pacific, during the MCA, the LIA and other periods of the LM. Comparisons with model simulations from the Community Earth System Model – Last Millennium Ensemble (CESM-LME) and phases 5 and 6 of the Coupled Model Intercomparison Project (CMIP5 and CMIP6) show that both external forcing and internal variability contributed to these changes, with the contribution of internal variability being particularly important in the Indo-Pacific basin and that of external forcing in the Atlantic basin. These results may help to identify not only those areas showing coordinated changes, but also those regions more impacted by the internal variability, where forced model simulations would not be expected to successfully reproduce the evolution of past actual hydroclimate changes.

Potential causes of megapluvials in the Yangtze–Huaihe River valley over the last millennium

The potential causes of megapluvials over Yangtze–Huaihe River valley (YHRV) during the last millennium were investigated using the Community Earth System Model-Last Millennium Ensemble (CESM-LME) simulation data and multiple proxy data (i.e., reconstruction and paleoclimate assimilation data). Herein, we first depicted the spatial hydroclimatic pattern over the Northern Hemisphere when megapluvials occurred in the YHRV during the last millennium. The proxy and simulation data both showed that megapluvials in YHRV were accompanied by extensive decadal drought in western Asia and northwestern India, also located in mid to low latitudes. Then, by diagnosing the dynamic processes, we found that the baroclinic structure over western Asia and northwestern India caused anomalous geopotential heights, and hence triggered the negative phase of the Circumglobal Teleconnection (CGT). The eastwards wave train at mid-latitudes disturbed by the CGT mode, triggered lower-level northerlies and upper-level southerlies over the northern part of the YHRV and, as a consequence, led to anomalous strong convection near the YHRV. Together with abundant water vapour from the Pacific Northwest, there was a synergistic effect on megapluvial events over the YHRV. Finally, we analysed the linkage of surface temperature in western Asia and northwestern India and megapluvials in the YHRV. Land surface heating over western Asia and northwestern India may largely stimulate the formation of a negative CGT mode through local feedback of air–land interactions and, as a result, affect precipitation anomalies downstream, such as in the YHRV. It is therefore necessary to consider anomalous high surface temperatures in western Asia and northwestern India as important predictors for interdecadal severe floods in the YHRV.

Secular Changes of the Decadal Relationship Between the Northern Hemisphere Land Monsoon Rainfall and Sea Surface Temperature Over the Past Millennium in Climate Model Simulations

AbstractUnderstanding the decadal variability of Northern Hemisphere land monsoon rainfall (NHLMR) is crucial to social‐economic development. However, due to the temporal limitation of instrumental data, the decadal relationship between NHLMR and sea surface temperature (SST) remains uncertain. The extended El Niño–Southern Oscillation (XEN) SST index and the North Atlantic–south Indian Ocean dipole (NAID) SST index can be the predictors to help us understand the decadal relationship between NHLMR and SST. In this study, using the Community Earth System Model‐Last Millennium Ensemble (CESM‐LME) simulations, we find a significant and stable decadal correlation between the NHLMR and XEN in the all‐forcing (AF) (with natural and anthropogenic forcings, AF), control (internal variability only), and other external forcings run over the past millennium, which means that this simulated NHLMR‐XEN relationship is caused by internal variability. The AF experiments show that the decadal relationship between NHLMR and NAID is practically non‐existent until an abrupt and significant increase in volcanic eruptions at about 1700, which is also indicated by the Paleoclimate Modeling Intercomparison Project Phase 3 (Paleoclimate Modeling Intercomparison Project Phase 3) simulations. Using the volcanic forcing sensitivity experiments, we find that the successive strong volcanic eruptions in the Northern Hemisphere (NH) significantly strengthen the synchronous changes of NHLMR and NAID after 1700, with a periodicity of 20–40‐year for NHLMR and NAID. Specifically, successive NH eruptions weaken the north‐south hemispheric thermal contrast, contributing to the weakened NAID index. The cross‐equator flow is thus weakened, decreasing the NHLMR, which cause a resonant behavior of NHLMR and NAID.

Drivers of Last Millennium Antarctic Climate Evolution in an Ensemble of Community Earth System Model Simulations

Improved understanding of the drivers of climate variability, particularly over the last millennium, and its influence on Antarctic ice melt have important implications for projecting ice sheet resilience in a changing climate. Here, we investigated the variability in Antarctic climate and sea ice extent during the last millennium (850–1850 CE) by comparing paleoenvironmental reconstructions with simulations from the Community Earth System Model Last Millennium Ensemble (CESM-LME). Atmospheric and oceanic response to external forcing in CESM-LME simulations typically take the form of an Antarctic dipole: cooling over most of Antarctica and warming east of the Antarctic Peninsula. This configuration is also observed in ice core records. Unforced variability and a dipole response to large volcanic eruptions contribute to weaker cooling in the Antarctic than the Arctic, consistent with the absence of a strong volcanic signal in Antarctic ice core records. The ensemble does not support a clear link between the dipole pattern and baseline shifts in the Southern Annular Mode and El Niño-Southern Oscillation proposed by some paleoclimate reconstructions. Our analysis provides a point of comparison for paleoclimate reconstructions and highlights the role of internal climate variability in driving modeled last millennium climate evolution in the Antarctic.

The Role of Internal Variability in ITCZ Changes Over the Last Millennium

AbstractTropical hydroclimate is modulated by the position and intensity of the Intertropical Convergence Zone (ITCZ). Reconstructions and simulations of the Last Millennium (LM) suggest latitudinal variations of the ITCZ that impact the hydroclimate over large regions of the world. These ITCZ shifts have been generally linked to external radiative forcing, but analyses of the Community Earth System Model – Last Millennium Ensemble (CESM‐LME) demonstrate a significant contribution of internal variability over multidecadal and centennial timescales. In contrast to changes driven by external forcing, which are associated with an asymmetric warming between the Northern and Southern Hemispheres, the contribution of internal variability in the CESM‐LME is associated with cooling and warming of the eastern Pacific. While external forcing remains the main driver of ITCZ changes in the Atlantic basin, the contribution of internal variability in the CESM‐LME exceeds that of the forcing for the Pacific and Indian Ocean basins.

Last millennium climate changes over the Antarctic Peninsula and southern Patagonia in CESM-LME simulations: Differences between Medieval Climate Anomaly and present-day temperatures

This paper describes differences between Medieval Climate Anomaly (MCA) and present-day temperatures in the area composed of the Antarctic Peninsula, southern Patagonia, and the surrounding southern oceans. The investigation is conducted with the simulation results from the Community Earth System Model Last Millennium Ensemble (CESM-LME) considering the impact of each natural (volcanic activity and solar variability) and anthropogenic (greenhouse gas, ozone-aerosols and land use/land cover) individual forcing relative to the full forced simulations. Model results show generalized warming during the MCA in the study area. However, the simulated MCA temperatures are significantly colder than present-day mean values due to the influence of increased atmospheric greenhouse gas concentrations since the beginning of the modern Industrial Era in the ∼1850s. In fact, model runs in which only natural forcings were applied show that, in the absence of greenhouse gas forcing, present-day temperatures in the Antarctic Peninsula and southern Patagonia would be lower than or similar to those during the MCA. The study demonstrates the value of paleoclimate proxy–model comparisons but also highlights the limitations of current available proxy information to perform that integration in the study area.

Comparisons on characteristics and mechanisms of the decadal megadroughts over eastern China between MCA and LIA

AbstractIn this study, simulations of 13 all‐forcing experiments from the Community Earth System Model Last Millennium Ensemble (CESM‐LME) archive were used to investigate the different spatiotemporal characteristics and corresponding mechanisms of the decadal megadroughts over the eastern China during two typical periods in the past millennium, that is, Medieval Climate Anomaly (MCA) and Little Ice Age (LIA). The results show that there is no significant difference in either megadrought intensity or duration between the MCA and LIA, but there are significant differences in the megadrought frequencies. The frequency of megadroughts during the MCA is 1.44 times/hundred years, compared with 1.69 times/hundred years in LIA. Results from solar only sensitivity experiments and volcanic only sensitivity experiments show that this difference is mainly induced by the solar radiation. The mechanism analyses show that a spatial pattern of sea surface temperature anomalies (SSTA) resembling the positive Pacific Decadal Oscillation (PDO)‐like pattern have occurred simultaneously with the megadroughts during both the MCA and LIA. While, the megadroughts in MCA are strongly correlated with the strong PDO‐like SSTA patterns, and the megadroughts in LIA are strongly correlated with the weak PDO‐like SSTA patterns. In addition, the anomalous sea level pressure (SLP) field and wind field associate with the megadroughts during the MCA and LIA are consistent with the strengths of SSTA, with significant larger SLP and wind anomalies during the MCA than those during the LIA. Therefore, it can be concluded that the responses of the large‐scale atmospheric circulation patterns to SSTA are similar during these two typical periods, and the difference in the megadrought frequencies is due to the nonlinear response of the precipitation variability to the large‐scale atmospheric circulation patterns.

Simulated and reconstructed atmospheric variability and their relation with large Pre-industrial summer floods in the Hasli-Aare catchment (Swiss Alps) since 1300 CE

Previous studies have suggested a relation between the variability of the climate and flood events, a conclusion supported by recent findings. Indeed, evidence has emerged that the atmospheric variability modulated by solar activity could play a role in driving the flood dynamic. The aim of this paper is to analyse summer paleoflood variability in the Alpine Hasli-Aare catchment (Switzerland) by simulating the atmospheric variability of flood periods identified from both floodplain sediment and historical sources dating from between 1300 and 2005 CE. We use the Community Earth System Model-Last Millennium Ensemble (CESM-LME) to investigate the extent of this atmospheric variability and address the questions of i) how the simulated atmospheric summer variability changed over the Pre-industrial era (1300–1849 CE) and ii) how this model is related to sedimentary floodplain evidence of flood variability. The results for the observed, reconstructed and simulated SLP grids showed a year-to-year summer atmospheric variability (Summer North Atlantic Oscillation; SNAO) related to a northwest-southeast pressure gradient with positive anomalies extending over the Scandinavian Peninsula and the British Isles, while the Mediterranean area was under slight low-pressure anomalies. The phase changes of the simulated SNAO in the Industrial era (1850–2005 CE) are consistent with changes in the reconstructed and observed SNAO. The comparison of flood reconstruction using geochemical proxies from floodplain sediments and paleoclimate simulations provides evidence that the SNAO in negative/positive phase modulated by solar variability (negative/positive anomalies) can play a substantial role in driving flood frequencies in the Hasli-Aare catchment and even in influencing their spatial distribution. • Flood periods were reconstructed from geochemical floodplain proxies since 1300 CE. • Paleoclimate simulations evidence similar atmospheric variability in flood periods. • Flood pulses correspond to negative anomalies of TSI and negative SNAO phase. • A reverse pattern emerges when flood intensities occurred in positive TSI anomalies. • SNAO modulated by solar variability can play a role in driving flood frequencies.

Methodological and physical biases in global to subcontinental borehole temperature reconstructions: an assessment from a pseudo-proxy perspective

Abstract. Borehole-based reconstruction is a well-established technique to recover information of the past climate variability based on two main hypotheses: (1) past ground surface temperature (GST) histories can be recovered from borehole temperature profiles (BTPs); (2) the past GST evolution is coupled to surface air temperature (SAT) changes, and thus, past SAT changes can be recovered from BTPs. Compared to some of the last millennium (LM) proxy-based reconstructions, previous studies based on the borehole technique indicate a larger temperature increase during the last few centuries. The nature of these differences has fostered the assessment of this reconstruction technique in search of potential causes of bias. Here, we expand previous works to explore potential methodological and physical biases using pseudo-proxy experiments with the Community Earth System Model Last Millennium Ensemble (CESM-LME). A heat-conduction forward model driven by simulated surface temperature is used to generate synthetic BTPs that are then inverted using singular value decomposition. This procedure is applied to the set of simulations that incorporates all of the LM external forcing factors as well as those that consider the concentration of the green house gases (GHGs) and the land use land cover (LULC) changes forcings separately. The results indicate that methodological issues may impact the representation of the simulated GST at different spatial scales, with the temporal logging of the BTPs as the main sampling issue that may lead to an underestimation of the simulated GST 20th-century trends. Our analysis also shows that in the surrogate reality of the CESM-LME the GST does not fully capture the SAT warming during the industrial period, and thus, there may be a further underestimation of the past SAT changes due to physical processes. Globally, this effect is mainly influenced by the GHG forcing, whereas regionally, LULC changes and other forcings factors also contribute. These findings suggest that despite the larger temperature increase suggested by the borehole estimations during the last few centuries of the LM relative to some other proxy reconstructions, both the methodological and physical biases would result in a underestimation of the 20th-century warming.

Decadal variability of northern Asian winter monsoon shaped by the 11-year solar cycle

Climate signals associated with 11-year sunspot cycle have been extensively studied in various regions of the northern hemisphere, but the precise mechanisms remain elusive. Asian winter monsoon (AWM) is the most powerful circulation system on the Earth, yet its relationship with the 11-year solar cycle has not been explored. Here the response of AWM to the 11-year solar forcing is explored by analysis of numerical experiment results obtained from the Community Earth System Model-Last Millennium Ensemble (CESM-LME) modeling project. We show that a strong 11-year solar cycle can excite a resonant response of the intrinsic leading mode of the AWM variability, resulting in a significant signal of decadal variation. The leading mode, characterized by a warm Arctic and cold Siberia, responds to the maximum solar irradiance with a peculiar 3 to 4-year delay. We propose a new mechanism to explain this delayed response, in which the 11-year solar cycle affects the AWM via modulating Arctic sea ice variation during the preceding summer. At the peak of the accumulative solar irradiance (i.e., 4 years after the maximum solar irradiance), the Arctic sea ice concentration reaches a minimum over the Barents–Kara Sea region accompanied by an Arctic sea surface warming, which then persists into the following winter, causing Arctic high-pressure extend to the Ural mountain region, which enhances Siberian High and causes a bitter winter over the northern Asia.

South Atlantic Surface Boundary Current System during the Last Millennium in the CESM-LME: The Medieval Climate Anomaly and Little Ice Age

Interocean waters that are carried northward through South Atlantic surface boundary currents get meridionally split between two large-scale systems when meeting the South American coast at the western subtropical portion of the basin. This distribution of the zonal flow along the coast is investigated during the Last Millennium, when natural forcing was key to establish climate variability. Of particular interest are the changes between the contrasting periods of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). The investigation is conducted with the simulation results from the Community Earth System Model Last Millennium Ensemble (CESM-LME). It is found that the subtropical South Atlantic circulation pattern differs substantially between these natural climatic extremes, especially at the northern boundary of the subtropical gyre, where the westward-flowing southern branch of the South Equatorial Current (sSEC) bifurcates off the South American coast, originating the equatorward-flowing North Brazil Undercurrent (NBUC) and the poleward Brazil Current (BC). It is shown that during the MCA, a weaker anti-cyclonic subtropical gyre circulation took place (inferred from decreased southern sSEC and BC transports), while the equatorward transport of the Meridional Overturning Circulation return flow was increased (intensified northern sSEC and NBUC). The opposite scenario occurs during the LIA: a more vigorous subtropical gyre circulation with decreased northward transport.

Decadal Variations of the East Asian Summer Monsoon Forced by the 11-Year Insolation Cycle

AbstractStatistical evidence suggests that solar activity may affect the atmospheric circulation over East Asia (EA), but the way in which the 11-yr solar radiation cycle affects the East Asian summer monsoon (EASM) remains unexplained. Based on one control experiment and four solar-only forcing experiments performed during the Community Earth System Model–Last Millennium Ensemble (CESM-LME) model project, we explore the potential impacts of the 11-yr solar cycle on EASM variability and the physical processes through which solar forcing influences EASM decadal variability. The model results show that the warm season [May–September (MJJAS)] mean precipitation over EA exhibits significant decadal variation with a “northern wet–southern dry” pattern during peak years in the strong 11-yr solar cycle epoch (AD 900–1285), which is in contrast to the absence of decadal signals during the weak 11-yr solar cycle epoch (AD 1400–1535). For the four-member ensemble averaged solar-only forcing experiment, the summer mean precipitation over northern EA is significantly correlated with the solar forcing (r = 0.414, n = 68, p < 0.05) on a decadal time scale during the strong cycle epoch, whereas there is no statistical link between the EASM and solar activity during the weak cycle epoch (r = 0.002, n = 24). A strong, 11-yr solar cycle is also shown to excite an anomalous sea surface temperature (SST) pattern that resembles a cool Pacific decadal oscillation (PDO) phase, which has a significant 11-yr periodicity. The associated anomalous North Pacific anticyclone dominates the entire extratropical North Pacific and enhances the southerly monsoon over EA, which results in abundant rainfall over northern EA. We argue that the 11-yr solar cycle affects the EASM decadal variation through excitation of a coupled decadal mode in the Asia–North Pacific region.

Influence of radiative forcing factors on ground–air temperature coupling during the last millennium: implications for borehole climatology

Abstract. Past climate variations may be uncovered via reconstruction methods that use proxy data as predictors. Among them, borehole reconstruction is a well-established technique to recover the long-term past surface air temperature (SAT) evolution. It is based on the assumption that SAT changes are strongly coupled to ground surface temperature (GST) changes and transferred to the subsurface by thermal conduction. We evaluate the SAT–GST coupling during the last millennium (LM) using simulations from the Community Earth System Model LM Ensemble (CESM-LME). The validity of such a premise is explored by analyzing the structure of the SAT–GST covariance during the LM and also by investigating the evolution of the long-term SAT–GST relationship. The multiple and single-forcing simulations in the CESM-LME are used to analyze the SAT–GST relationship within different regions and spatial scales and to derive the influence of the different forcing factors on producing feedback mechanisms that alter the energy balance at the surface. The results indicate that SAT–GST coupling is strong at global and above multi-decadal timescales in CESM-LME, although a relatively small variation in the long-term SAT–GST relationship is also represented. However, at a global scale such variation does not significantly impact the SAT–GST coupling, at local to regional scales this relationship experiences considerable long-term changes mostly after the end of the 19th century. Land use land cover changes are the main driver for locally and regionally decoupling SAT and GST, as they modify the land surface properties such as albedo, surface roughness and hydrology, which in turn modifies the energy fluxes at the surface. Snow cover feedbacks due to the influence of other external forcing are also important for corrupting the long-term SAT–GST coupling. Our findings suggest that such local and regional SAT–GST decoupling processes may represent a source of bias for SAT reconstructions from borehole measurement, since the thermal signature imprinted in the subsurface over the affected regions is not fully representative of the long-term SAT variations.

Different Impacts of Northern, Tropical, and Southern Volcanic Eruptions on the Tropical Pacific SST in the Last Millennium

The impact of northern, tropical, and southern volcanic eruptions on the Pacific sea surface temperature (SST) and the different response mechanisms arising due to differences in the volcanic forcing structure are investigated using the Community Earth System Model Last Millennium Ensemble (CESM-LME). Analysis of the simulations indicates that the Pacific features a significant El Niño–like SST anomaly 5–10 months after northern and tropical eruptions, and with a weaker such tendency after southern eruptions, possibly reflective of the weaker magnitude of these eruptions. The Niño-3 index peaks with a lag of one and a half years after northern and tropical eruptions. Two years after all three types of volcanic eruptions, a La Niña–like SST anomaly pattern over the equatorial Pacific is observed, which seems to form an El Niño–Southern Oscillation (ENSO) cycle. The westerly wind anomaly over the western to central Pacific plays an essential role in favoring the development of an El Niño following all three types of eruptions. Thus, the key point of the question is to find the causes of the westerly wind enhancement. The shift of the intertropical convergence zone (ITCZ) can explain the El Niño–like response to northern eruptions, which is not applicable for tropical or southern eruptions. The ocean dynamical thermostat mechanism is the fundamental cause of the anomalous westerly wind for all three types of eruptions.

Changes of climate regimes during the last millennium and the twenty-first century simulated by the Community Earth System Model

This study examines the shifts in terrestrial climate regimes using the Köppen–Trewartha (K–T) climate classification by analyzing the Community Earth System Model Last Millennium Ensemble (CESM-LME) simulations for the period 850–2005 and CESM Medium Ensemble (CESM-ME), CESM Large Ensemble (CESM-LE) and CESM with fixed aerosols Medium Ensemble (CESM-LE_FixA) simulations for the period 1920–2080. We compare K–T climate types from the Medieval Climate Anomaly (MCA) (950–1250) with the Little Ice Age (LIA) (1550–1850), from present day (PD) (1971–2000) with the last millennium (LM) (850–1850), and from the future (2050–2080) with the LM in order to place anthropogenic changes in the context of changes due to natural forcings occurring during the last millennium. For CESM-LME, we focused on the simulations with all forcings, though the impacts of individual forcings (e.g., solar activities, volcanic eruptions, greenhouse gases, aerosols and land use changes) were also analyzed. We found that the climate types changed slightly between the MCA and the LIA due to weak changes in temperature and precipitation. The climate type changes in PD relative to the last millennium have been largely driven by greenhouse gas-induced warming, but anthropogenic aerosols have also played an important role on regional scales. At the end of the twenty-first century, the anthropogenic forcing has a much greater effect on climate types than the PD. Following the reduction of aerosol emissions, the impact of greenhouse gases will further promote global warming in the future. Compared to precipitation, changes in climate types are dominated by greenhouse gas-induced warming. The large shift in climate types by the end of this century suggests possible wide-spread redistribution of surface vegetation and a significant change in species distributions.

“El Niño Like” Hydroclimate Responses to Last Millennium Volcanic Eruptions

Abstract The hydroclimate response to volcanic eruptions depends both on volcanically induced changes to the hydrologic cycle and on teleconnections with the El Niño–Southern Oscillation (ENSO), complicating the interpretation of offsets between proxy reconstructions and model output. Here, these effects are separated, using the Community Earth System Model Last Millennium Ensemble (CESM-LME), by examination of ensemble realizations with distinct posteruption ENSO responses. Hydroclimate anomalies in monsoon Asia and the western United States resemble the El Niño teleconnection pattern after “Tropical” and “Northern” eruptions, even when ENSO-neutral conditions are present. This pattern results from Northern Hemisphere (NH) surface cooling, which shifts the intertropical convergence zone equatorward, intensifies the NH subtropical jet, and suppresses the Southeast Asian monsoon. El Niño events following an eruption can then intensify the ENSO-neutral hydroclimate signature, and El Niño probability is enhanced two boreal winters following all eruption types. Additionally, the eruption-year ENSO response to eruptions is hemispherically dependent: the winter following a Northern eruption tends toward El Niño, while Southern volcanoes enhance the probability of La Niña events and Tropical eruptions have a very slight cooling effect. Overall, eruption-year hydroclimate anomalies in CESM disagree with the proxy record in both Southeast Asia and North America, suggesting that model monsoon representation cannot be solely responsible. Possible explanations include issues with the model ENSO response, the spatial or temporal structure of volcanic aerosol distribution, or data uncertainties.