Community Atmosphere Model Version Research Articles

Influence of ENSO on Tropospheric Ozone Variability in East Asia

AbstractAbundance of tropospheric ozone is determined by the interplay among emissions, chemistry, and climate dependent transport. It is important to identify the essential factors that modulate ozone in China and Korea because of increasing ozone trends in this area. In this study, we investigated the correlations of tropospheric ozone in East Asia with El Niño Southern Oscillations (ENSO). The model simulated ozone from 2000 to 2020 with Community Atmosphere Model version 6 with chemistry (CAM6‐chem), ECMWF Atmospheric Composition Reanalysis data, and ozone profiles measured in South Korea, Japan, and Hong Kong. These data were utilized to understand the ozone–ENSO relationship in this region. We found that tropospheric ozone in East Asia is significantly correlated with the wintertime Niño 3.4 index in El Niño/La Niña developing summer and decaying spring, with correlation coefficient up to 0.9. Particularly tropospheric ozone originated from stratosphere is strongly correlated with the Niño 3.4 index. Ozone concentrations in China and the Korean Peninsula are ∼5 ppb higher during El Niño developing summer compared to La Niña in a broad area, because of anomalous cyclonic/anticyclonic flows in the extratropics. In El Niño/La Niña decaying spring, the opposite occurs in the same region. In the decaying spring, stronger variability was observed in the subtropical regions of the lower to mid troposphere, indicating higher ozone concentrations during El Niño compared to La Niña. This variability is associated with the fluctuations in the Walker circulation, large‐scale sinking/rising motions, and the accompanying biomass burning events. The model reproduced the ozone variability associated with ENSO in the ozonesonde observations.

Insights of warm-cloud biases in Community Atmospheric Model 5 and 6 from the single-column modeling framework and Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) observations

Abstract. There has been a growing concern that most climate models predict precipitation that is too frequent, likely due to lack of reliable subgrid variability and vertical variations in microphysical processes in low-level warm clouds. In this study, the warm-cloud physics parameterizations in the singe-column configurations of NCAR Community Atmospheric Model version 6 and 5 (SCAM6 and SCAM5, respectively) are evaluated using ground-based and airborne observations from the Department of Energy (DOE) Atmospheric Radiation Measurement (ARM) Aerosol and Cloud Experiments in the Eastern North Atlantic (ACE-ENA) field campaign near the Azores islands during 2017–2018. The 8-month single-column model (SCM) simulations show that both SCAM6 and SCAM5 can generally reproduce marine boundary layer cloud structure, major macrophysical properties, and their transition. The improvement in warm-cloud properties from the Community Atmospheric Model 5 and 6 (CAM5 to CAM6) physics can be found through comparison with the observations. Meanwhile, both physical schemes underestimate cloud liquid water content, cloud droplet size, and rain liquid water content but overestimate surface rainfall. Modeled cloud condensation nuclei (CCN) concentrations are comparable with aircraft-observed ones in the summer but are overestimated by a factor of 2 in winter, largely due to the biases in the long-range transport of anthropogenic aerosols like sulfate. We also test the newly recalibrated autoconversion and accretion parameterizations that account for vertical variations in droplet size. Compared to the observations, more significant improvement is found in SCAM5 than in SCAM6. This result is likely explained by the introduction of subgrid variations in cloud properties in CAM6 cloud microphysics, which further suppresses the scheme's sensitivity to individual warm-rain microphysical parameters. The predicted cloud susceptibilities to CCN perturbations in CAM6 are within a reasonable range, indicating significant progress since CAM5 which produces an aerosol indirect effect that is too strong. The present study emphasizes the importance of understanding biases in cloud physics parameterizations by combining SCM with in situ observations.

Spatial distribution of aerosols burden and evaluation of changes in aerosol optical depth using multi-approach observations in tropical region

Understanding of Aerosol optical depth (AOD) parameter is important for air quality assessment. This study aims to evaluate and validate AOD measurements from combine datasets to improve air quality for a period 2005–2020 using Aerosol Robotic Network (AERONET) at Ilorin site (8.320° N, 4.340° E) in Nigeria. AOD outputs from Community Atmosphere Model Version 6 with chemistry (CAM6-chem) at 1° horizontal resolution and Modern-Era Retrospective analysis for Research and Applications (MERRA-2) are investigated in addition to validation of two satellites AOD retrievals: Moderate Resolution Imaging Spectroradiometer (MODIS) and Multi-angle Imaging Spectroradiometer (MISR). Result of spatial distribution of AOD shows high values > 1 in the North and Western Sahara compared to Central Africa. Desert dust shows largest contribution in the North and Western Africa that is up to 2 magnitude larger than other aerosol types. Primary organic matter (POM) and secondary organic aerosols (SOAs) both presents high burdens with later been dominant at around 10° band, and black carbon (BC) largest burden (2.6 × 10 − 5 kgm − 2) is seen in the model from oil and gas exploration site in Nigeria. Inter-comparison of MERRA/MISR/MODIS and AERONET AOD using linear correlation of the seasonal dependence demonstrated high correlation (r = 0.864 − 0.973) subjected to Root Mean Square Error (RMSE = 0.069 − 0.211), suggesting good agreement between the datasets. When compared to seasonal mean maximum AERONET AOD value of 0.978 MERRA is ∼5%, MISR ∼28% and MODIS ∼29% lower with stronger correlations observed in the wet and pre-harmattan seasons. Similarly, MODEL AOD at 550 nm and dust burden were found to be ∼34% and ∼67% lower in context to AERONET AOD annual mean value of 0.627. Positive relationships that indicate an upward slope exist between all the computed datasets with moderate value of AERONET/CAM-chem spearman partial correlation, and MERRA/MODIS and MODIS/MISR showing strong and significant relationship with p-value less than 0.05. Low variance is observed with all measurements except in MERRA.

A new simplified parameterization of secondary organic aerosol in the Community Earth System Model Version 2 (CESM2; CAM6.3)

Abstract. The Community Earth System Model (CESM) community has been providing versatile modeling options, with simple to complex chemistry and aerosol schemes in a single model, in order to support the broad scientific community with various research interests. While different model configurations are available in CESM and these can be used for different fields of Earth system science, simulation results that are consistent across configurations are still desirable. Here we develop a new simple secondary organic aerosol (SOA) scheme in the Community Atmosphere Model (CAM) version 6.3, the atmospheric component of the CESM. The main purpose of this simplified SOA scheme is to reduce the differences in aerosol concentrations and radiative fluxes between CAM and CAM with detailed chemistry (CAM-chem) while maintaining the computational efficiency of CAM. CAM simulation results using the default CAM6 and the new SOA schemes are compared to CAM-chem results as a reference. More consistent SOA concentrations are obtained globally when using the new SOA scheme for both temporal and spatial variabilities. The new SOA scheme shows that 62 % of grid cells globally are within a factor of 2 compared to the CAM-chem SOA concentrations, which is improved from 24 % when using the default CAM6 SOA scheme. Furthermore, other carbonaceous aerosols (black carbon and primary organic aerosol) in CAM6 become closer to CAM-chem results due to more similar microphysical aging timescales influenced by SOA coating, which in turn leads to comparable wet deposition fluxes. This results in an improved global atmospheric burden and concentrations at the high latitudes of the Northern Hemisphere compared to the full chemistry version (CAM-chem). As a consequence, the radiative flux differences between CAM-chem and CAM in the Arctic region (up to 6 W m−2) are significantly reduced for both nudged and free-running simulations. We find that the CAM6 SOA scheme can still be used for radiative forcing calculation as the high biases exist both in pre-industrial and present conditions, but studies focusing on the instantaneous radiative effects would benefit from using the SOA scheme developed in this study. The new SOA scheme also has technical advantages including the use of identical SOA precursor emissions as CAM-chem from the online biogenic emissions instead of pre-calculated emissions that may introduce differences. Future parameter updates to the CAM-chem SOA scheme can be easily translated to the new CAM SOA scheme as it is derived from the CAM-chem SOA scheme.

The Most Effective Remote Forcing in Causing U.S.‐Wide Heat Extremes as Revealed by CESM Green's Function Experiments

AbstractWe make use of the Community Atmosphere Model version 5 Green's function q‐flux perturbation experiments to explore the most effective remote forcing in driving U.S.‐wide summer heat extremes. We find that positive q‐flux forcing over the western North Pacific Ocean is the most effective in causing an increased heat extreme frequency. This works by driving increased sea surface temperature and precipitation over western North Pacific and an eastward propagating Rossby wave train with an anomalous ridge over the contiguous U.S. In comparison, negative q‐flux forcing over the eastern tropical Pacific and its resulting surface cooling also leads to an increased heat extreme frequency but is less effective. Furthermore, guided by the Green's function results, we separate the role of western North Pacific warming and eastern tropical Pacific cooling in U.S. heat extremes in prescribed sea surface temperature experiments and ERA5 reanalysis data and find overall consistent conclusions.

Why a large‐scale monsoon does not exist in North America: Orographic effects

AbstractTo understand why a large‐scale monsoon does not exist in North America, we performed a series of sensitivity simulations to investigate orographic effects of the Rocky Mountains (RMs) using the Community Atmosphere Model version 5.1. Results show that the height of the RMs plays a fundamental role in shaping the monsoon over North America that is confined to a small area from northwestern Mexico to the southwestern United States. When the RMs' height is increased by five times their actual height, a larger part of the non‐monsoon region over North America becomes a monsoon region. The mechanical effects of the RMs uplift dominate in winter, while thermal effects dominate in summer. During winter the mechanical effects induce an equivalent barotropic atmospheric response in the troposphere. With the uplift of the RMs, the ridge and trough located on the western and eastern sides of Canada are strengthened. Most areas of North America are influenced by the northerlies during winter such that enhanced descending motion over the eastern RMs favours a dry winter climate. However, thermal effects dominate during summer through enhanced baroclinic atmospheric responses. The Mexico high and lower‐level cyclonic circulation are strengthened with the RMs uplift, triggering large‐scale ascending motion. Eastern North America is mainly controlled by the enhanced southerly wind along the western flank of the North Atlantic subtropical high. Thus, the enhanced water‐vapour transport and upward motion on the eastern side of the RMs increase summer precipitation. As a result, an obvious seasonal variation with the feature of ‘dry winter and wet summer’ finally develops, indicating that the height of the RMs plays a crucial role in shaping the monsoon over the central United States. Additional experiments show that the base area of the RMs has little effect on the large‐scale monsoon formation over North America.

Connection between the Tropical Pacific and Indian Ocean and Temperature Anomaly across West Antarctic

West Antarctic and the Antarctic Peninsula have experienced dramatic warming in austral spring since the 1970s. Using observations and the Community Atmosphere Model version 4 (CAM4), this study explores the physical mechanism by which the tropical Pacific and Indian Ocean temperature anomaly mode (PIM) affects the dipolar surface air temperature (SAT) anomalies across the West Antarctic in austral spring. The positive phase of the PIM, characterized by positive sea surface temperature anomalies (SSTAs) in the tropical central-eastern Pacific and western Indian Ocean and negative SSTAs in the Maritime Continent, can generate two branches of stationary Rossby wave trains propagating from the tropical central Pacific and southeastern Indian Ocean to the West Antarctic, with an anticyclonic anomaly appearing over the Amundsen Sea. The northerlies advect warmer air to the Ross–Amundsen Seas, but southerlies advect colder air to the Antarctic Peninsula–Weddell Sea, resulting in the dipole of SAT anomalies over the West Antarctic. In this process, the role of tropical central-eastern Pacific SSTAs dominate, and it is amplified by the SSTAs around the Maritime Continent. The SSTAs in the western Indian Ocean combined with the SSTAs over the Maritime Continent further contribute to the western pole of the SAT. Only simulation that includes a prescribed PIM forcing can exactly reproduce the observations of the dipolar SAT response across the West Antarctic, indicating the need to treat the tropical Pacific and Indian Oceans as a unified whole.

Decadal variability of extreme high temperature in mid- and high-latitude Asia and its associated North Atlantic air–sea interaction

The decadal variability of extreme high temperature (EHT) in the mid and highlatitudes of continental Asia (MHLCA) and associated mechanisms were investigated. Observational analysis indicated that, after removing global warming, the first leading mode of the EHT events showed a meridional dipole pattern and had significant decadal variability. During the periods 1980–1994 and 2012–2019 (1995–2011), EHT events were more frequent and stronger (less frequent and weaker) in western-central Siberia than normal, whereas they were less frequent and weaker (more frequent and stronger) in the wide area to the south of Lake Baikal. Further Observational and CESM (Community Earth System Model) results based analysis suggested that decadal change in air–sea interaction in the North Atlantic play an important role in shaping the decadal variability of EHT events in MHLCA. On decadal timescales, meridional negative–positive–negative sea surface temperature (SST) anomalies in the North Atlantic and their coupled positive North Atlantic Oscillation can trigger stronger wave activity flux and cause a significant anticyclonic–cyclonic teleconnection wave train in the troposphere over the mid and high latitudes of the Eurasian continent. As a result, an anomalous high-pressure center is evident in western-central Siberia, and thereby reducing total cloud cover and causing stronger solar heating. Thus, EHT events become more frequent and stronger. On the contrary, for the areas south of Lake Baikal, EHT events are weakened by corresponding cyclonic circulation anomalies. Additionally, the CAM5 (Community Atmosphere Model version 5) experiments suggested more important driving role of the decadal North Atlantic tripole SST anomalies in the abovementioned processes.

Climate Impacts of Convective Cloud Microphysics in NCAR CAM5

Abstract We improved the treatments of convective cloud microphysics in the NCAR Community Atmosphere Model version 5.3 (CAM5.3) by 1) implementing new terminal velocity parameterizations for convective ice and snow particles, 2) adding graupel microphysics, 3) considering convective snow detrainment, and 4) enhancing rain initiation and generation rate in warm clouds. We evaluated the impacts of improved microphysics on simulated global climate, focusing on simulated cloud radiative forcing, graupel microphysics, convective cloud ice amount, and tropical precipitation. Compared to CAM5.3 with the default convective microphysics, the too-strong cloud shortwave radiative forcing due primarily to excessive convective cloud liquid is largely alleviated over the tropics and midlatitudes after rain initiation and generation rate is enhanced, in better agreement with the CERES-EBAF estimates. Geographic distributions of graupel occurrence are reasonably simulated over continents; whereas the graupel occurrence remains highly uncertain over the oceanic storm-track regions. When evaluated against the CloudSat–CALIPSO estimates, the overestimation of convective ice mass is alleviated with the improved convective ice microphysics, among which adding graupel microphysics and the accompanying increase in hydrometeor fall speed play the most important role. The probability distribution function (PDF) of rainfall intensity is sensitive to warm rain processes in convective clouds, and enhancement in warm rain production shifts the PDF toward heavier precipitation, which agrees better with the TRMM observations. Common biases of overestimating the light rain frequency and underestimating the heavy rain frequency in GCMs are mitigated.

Orographic Cirrus and Its Radiative Forcing in NCAR CAM6

AbstractCirrus clouds play an important role in the Earth's radiative energy budget, thereby affecting the climate state and climate change. Orographic gravity wave (OGW)‐induced sub‐grid scale vertical velocity (i.e., cooling rate) is not resolved by large‐scale models and its impact on ice formation in cirrus clouds is not well quantified. In this study, one sub‐grid scale OGW scheme (e.g., McFarlane) is used in the Community Atmosphere Model version 6 (CAM6) to generate vertical velocity variance (σw) for cirrus formation. Results from the default model and simulations with the OGW‐induced σw are evaluated against the DOE ARM Small Particles in Cirrus (SPARTICUS) campaign observations. The OGW based on the McFarlane scheme increases the sub‐grid scale σw over mountains compared to the default model and improves the model agreement with the SPARTICUS observations. Larger σw due to OGWs can trigger more frequent homogeneous nucleation in orographic cirrus and generates a higher number concentration of ice crystals observed during the SPARTICUS campaign. Moreover, our evaluation of the model simulations against satellite observations indicates that the McFarlane scheme generates high in‐cloud ice number concentrations (>200 L−1) in the upper troposphere over mountains and high plateaus at mid‐ and high‐latitudes of the winter hemisphere as shown in the observations. More ice crystals with smaller sizes absorb more infrared radiation (+0.523 ± 0.125 W m−2). The net radiative cloud forcing change at the top of the atmosphere is +0.330 W m−2 due to the orographic cirrus.

Extent of the Impact of Arctic Atmospheric Uncertainty on Extended‐Range Forecasting of Cold Events in East Asia

AbstractThe extended‐range predictability of three simulated extreme cold events in East Asia in the Community Atmosphere Model (version 4) control experiment, whose atmospheric circulation backgrounds are similar to two recent observational cases, is investigated. The results show that they have a predictability of four pentads. Then, we evaluate the extent of the forecast uncertainty in the 4th pentad caused by the initial atmospheric uncertainties in the Arctic, which are large due to sparse instrumental observations. The initial uncertainty leading to the largest forecast uncertainty is obtained by the conditional nonlinear optimal perturbation method, and referred to as the CNOP‐type initial uncertainty. The forecast of the cold surge in the 4th pentad fails after adding the CNOP‐type initial uncertainty at day 0. In comparison, the forecast uncertainties are much weaker when the initial conditions are perturbed by noises, and the weaker influence may be due to the noises' lack of spatial structure. In terms of how the CNOP‐type initial uncertainty develops, a baroclinic structure is seen in the uncertainties on the first 2 days, followed by a propagating Rossby wave feature from the 2nd pentad to the 4th pentad. Meanwhile, synoptic transient eddy feedback also plays an essential role. The results suggest that the CNOP‐type initial uncertainty has potential to identify sensitive areas for targeted observations; plus, it could serve as a member of ensemble initial perturbations, since it indicates the largest uncertainty growth.

Exploring the Factors Controlling the Long‐Term Trend (1988–2019) of Surface Organic Aerosols in the Continental United States by Simulations

AbstractObserved surface organic aerosols (OA) concentrations slightly increased in the western US (WUS) but significantly decreased in the eastern US (EUS) in summer, and continuously decreased in winter over the US region. To understand the driving factors for the long‐term surface OA trend, we apply a revised version of the Community Atmosphere Model version 6 with comprehensive tropospheric and stratospheric chemistry representation, considering the heterogeneous formation of isoprene‐epoxydiol‐derived secondary organic aerosols (SOAIE) and fast photolysis rate of monoterpene‐derived secondary organic aerosols (MTSOA) to diagnose the OA evolution in 1988–2019. Compared to older versions, the revised model better reproduces the climatology, seasonal cycle, and long‐term trend of surface OA as evaluated against the Interagency Monitoring of Protected Visual Environments measurements. We find the decrease in EUS summertime OA is likely attributed to the interplay between SOAIE and MTSOA. With anthropogenic emissions reduction, primary organic aerosols (POA) declined, SOAIE decreased along with sulfate, while MTSOA increased along with biogenic emissions driven by a warming climate. POA from wildfires with a significant trend of 2.9% yr−1 and considerable interannual variation of 62.8% drive the statistically insignificant but increasing WUS summertime OA, while anthropogenic POA dominates the decreasing wintertime OA in the US. Through sensitivity experiments, we find MTSOA show linear responses to the increasing monoterpenes emissions and negligible responses to NOx emissions reduction due to the mutual offsets between MTSOA components from different oxidation pathways. This study reveals the increasingly important role of MTSOA in summertime OA under a warming climate.

Optimal Arctic sea ice concentration perturbation in triggering Ural blocking formation

Utilizing the Community Atmospheric Model version 4, the role of Arctic sea ice concentration (SIC) in triggering the Ural Blocking (UB) event formation on subseasonal timescale is investigated. Taking eight zonal flows as the reference states, the optimal Arctic SIC perturbations that trigger zonal flows into UB events on subseasonal timescale are obtained with the conditional nonlinear optimal perturbation approach. The numerical results show that the Arctic SIC decline in the Greenland, Barents and Okhotsk Seas can trigger zonal flows into UB events on a timescale of four pentads (20 days). Further diagnosis shows that the SIC decline in these regions locally warms the low troposphere via diabatic processes in the first pentad. Then, dynamic processes, such as temperature advection, modulate the temperature in the middle troposphere and weaken the meridional temperature gradient between the Arctic and mid-latitudes upstream of the Ural sector. The weakened meridional temperature gradient further decelerates the background zonal flow near the Ural sector and triggers UB formation in four pentads. This investigation highlights the role of Arctic SIC in the Greenland, Barents and Okhotsk Seas in triggering the formation of UB events on subseasonal timescale. The spatial characteristics of the optimal SIC perturbations may provide scientific support for SIC target observations in capturing the precursor of UB formation and improving UB prediction skills on subseasonal timescale.

Use of lidar aerosol extinction and backscatter coefficients to estimate cloud condensation nuclei (CCN) concentrations in the southeast Atlantic

Abstract. Accurately capturing cloud condensation nuclei (CCN) concentrations is key to understanding the aerosol–cloud interactions that continue to feature the highest uncertainty amongst numerous climate forcings. In situ CCN observations are sparse, and most non-polarimetric passive remote sensing techniques are limited to providing column-effective CCN proxies such as total aerosol optical depth (AOD). Lidar measurements, on the other hand, resolve profiles of aerosol extinction and/or backscatter coefficients that are better suited for constraining vertically resolved aerosol optical and microphysical properties. Here we present relationships between aerosol backscatter and extinction coefficients measured by the airborne High Spectral Resolution Lidar 2 (HSRL-2) and in situ measurements of CCN concentrations. The data were obtained during three deployments in the NASA ObseRvations of Aerosols above CLouds and their intEractionS (ORACLES) project, which took place over the southeast Atlantic (SEA) during September 2016, August 2017, and September–October 2018. Our analysis of spatiotemporally collocated in situ CCN concentrations and HSRL-2 measurements indicates strong linear relationships between both data sets. The correlation is strongest for supersaturations (S) greater than 0.25 % and dry ambient conditions above the stratocumulus deck, where relative humidity (RH) is less than 50 %. We find CCN–HSRL-2 Pearson correlation coefficients between 0.95–0.97 for different parts of the seasonal burning cycle that suggest fundamental similarities in biomass burning aerosol (BBA) microphysical properties. We find that ORACLES campaign-average values of in situ CCN and in situ extinction coefficients are qualitatively similar to those from other regions and aerosol types, demonstrating overall representativeness of our data set. We compute CCN–backscatter and CCN–extinction regressions that can be used to resolve vertical CCN concentrations across entire above-cloud lidar curtains. These lidar-derived CCN concentrations can be used to evaluate model performance, which we illustrate using an example CCN concentration curtain from the Weather Research and Forecasting Model coupled with physics packages from the Community Atmosphere Model version 5 (WRF-CAM5). These results demonstrate the utility of deriving vertically resolved CCN concentrations from lidar observations to expand the spatiotemporal coverage of limited or unavailable in situ observations.

Sensitivity of the Southern Hemisphere Wintertime Teleconnection to the Location of ENSO Heating

Abstract The Southern Hemisphere extratropical atmospheric circulation response to anomalous convection in the tropical western and eastern Pacific Ocean is distinctly different. The response to westward-located heating has a meridional dipole in the South Pacific with large zonal scale and appears unable to be interpreted simply as a stationary Rossby wave train that disperses poleward and eastward from a tropical source like the meridionally arched response to eastward-located heating. This study investigates the cause of this asymmetry by examining the daily evolution of the response to suddenly switching on steady diabatic heating over a western and central/eastern equatorial Pacific location using large-ensemble integrations from the Community Atmosphere Model version 5. We focus on the austral winter months when the subtropical jet supports the development of an effective Rossby wave source in the subtropical westerlies and acts as a waveguide. We show that the subtropical jet strongly influences the height response to the western tropical Pacific heating, promoting prominent zonal circumglobal propagation. Development of a transient eddy feedback in the extratropical storm track after approximately 10 days appears to play a primary role in establishing the time-mean response, which we test through comparison with similar experiments conducted using a simplified linear model. Conversely, the height anomalies for eastern tropical Pacific heating, farther away from the subtropical jet core, have larger meridional propagation, dispersing in a typical Hoskins–Karoly manner into the Southern Hemisphere extratropics, while the transient eddy feedback plays a secondary role for the establishment of the steady response.

Current AMO mitigating extreme high temperatures in Central Asia under global warming

AbstractUnder global warming, extreme high temperatures (EHTs) have become frequent and enhanced around the world. However, we found that the frequency and intensity of EHTs in Central Asia (CA) are decreasing over the last four decades when the effects of climate warming are removed. From 1979 to 2020, an interdecadal shift in CA's EHTs occurred around the mid‐1990s, preceded by high values (1983–1997) and followed by low values (1998–2016). The observational analysis and numerical simulations using National Center for Atmospheric Research Community Atmosphere Model version 5.3 both demonstrate that the phase transition of Atlantic multidecadal oscillation (AMO) has an important influence on this interdecadal shift. During 1998–2016, the AMO was in a positive phase (+AMO), meaning warmer sea surface temperature anomalies (SSTA) in the North Atlantic, which could lift the geopotential height and cause anticyclonic circulations over the basin. Through the cyclonic–anticyclonic circulation alternating teleconnection wave train over Eurasia, the atmospheric system over the North Atlantic could induce cyclonic circulations and reduce the geopotential heights over CA, that is, weaken the high pressure at the upper level, which is not conducive to warming and EHTs in CA. For the −AMO period (1983–1997), due to the similar atmospheric response mechanism, the negative SSTA in the North Atlantic and the wave train with reversed anomaly centres can be found, causing anticyclonic circulations and rising geopotential heights over CA. The high pressure at the upper level in CA would be strengthened, leading to favourable EHT conditions. Overall, we conclude that the current AMO (+AMO) has a mitigating role on the EHTs of CA, and the findings also emphasize that if the future AMO shifts to a negative phase, the EHTs of CA may worsen than at present.

Impact of the uplift of the Central Asian Orogenic Belt and NE Tibetan Plateau on the East Asian climate since the late Miocene

The uplift of the Central Asian Orogenic Belt (CAOB) and the northeastern (NE) Tibetan Plateau has exerted an important effect on the evolution of the East Asian climate since the late Miocene. However, previous modeling studies were generally based on modern boundary conditions, a direct comparison between the impacts of the uplift of the CAOB and NE Tibetan Plateau was still absent, and the climatic effects of the uplift of these mountains since the late Miocene remain ambiguous. Here, we use the Community Atmosphere Model version 4 (CAM4) with a high horizontal resolution (∼0.63° × 0.47°) combined with late Miocene boundary conditions to investigate the effects of the CAOB and NE Tibetan Plateau uplift on the East Asian climate since the late Miocene. The results indicate that the combined uplift of these landmasses increased the annual precipitation in the uplifted regions and eastern Asia, decreased the annual precipitation in central Asia, strengthened summer monsoon winds over northern East Asia, weakened winter monsoon winds over southern East Asia, and strengthened summer and winter high tropospheric westerly wind at northern high latitudes. Of the two uplifting landmasses, the uplift of the CAOB increased the summer precipitation and strengthened the summer high tropospheric westerly wind at higher latitudes and had a greater impact on the changes in winter monsoon winds and winter high tropospheric westerly wind. By comparison, the uplift of the NE Tibetan Plateau was also important in some aspects, such as the increased annual precipitation in eastern Asia. These results emphasized the important climate effects of the CAOB and NE Tibetan Plateau on the East Asian climate and improved our understanding of the dynamic link between orogenic evolution and paleoclimatic change since the late Miocene.

Summer Arctic Atmospheric Circulation and Its Association With the Ensuing East Asian Winter Monsoon Variability

AbstractThe impact of autumn Arctic sea ice reduction on the winter climate in Eurasia has been widely studied, but the role of the atmospheric circulation of the preceding summer has been mostly neglected. This study investigates a linkage between the summer Arctic circulation anomaly and the ensuing East Asian Winter Monsoon (EAWM) variability. The Arctic circulation anomaly, characterized by an anomalous anti‐cyclone occupying the Arctic Ocean and its marginal seas, can strongly melt Arctic sea ice in the Kara Sea‐Laptev Sea‐East Siberian Sea‐Chukchi Sea‐Beaufort Sea through dynamic and thermodynamic processes. The combination of the summer anti‐cyclone (cyclone) anomaly and light (heavy) autumn sea ice leads to an enhanced (weakened) EAWM. By introducing the coupling relationship between summer circulation and autumn sea ice into numerical experiments, the Community Atmospheric Model version 4 model has a good capability for capturing EAWM variability. Nevertheless, the dominant features of EAWM variability cannot be reproduced when the autumn sea ice mismatches the preceding summer circulation in the model. Therefore, the preceding summer circulation may modulate the impact of sea ice on Eurasia to some extent, which may increase uncertainties in the impact of Arctic sea ice loss on winter atmospheric circulation variability.

Climate Variations in the Past 250 Million Years and Contributing Factors

AbstractWe simulate climate variations in the past 250 million years (Myr), using the fully coupled Community Earth System Model version 1.2.2 (CESM1.2.2) with the Community Atmosphere Model version 4 (CAM4). Three groups of simulations are performed, each including 26 simulations, with a 10‐million‐year interval. The Control group is constrained by paleogeography, increasing solar radiation, and reconstructed global mean surface temperatures (GMSTs) by tuning CO2 concentrations. No ice sheets are prescribed for all simulations except for the pre‐industrial (PI) simulation in which modern geography, ice sheets and vegetation are used. Simulated zonal mean surface temperatures are always higher than those of proxy reconstructions in the tropics, but lower than those of proxy reconstructions at middle latitudes. The relative importance of individual contributing factors for surface temperature variations in the past 250 Myr is diagnosed, using the energy‐balance analysis. Results show that greenhouse gases are the major driver in regulating GMST variations, with a maximum contribution of 12.2°C. Varying surface albedo contributes to GMST variations by 3.3°C. Increasing solar radiation leads to GMST increases by 1.5°C. Cloud radiative effects have relatively weak impacts on GMST variations, less than ±0.8°C. For comparison, two groups of sensitivity simulations are performed. One group has the CO2 concentration fixed at 10 times the PI value, and the other group has fixed CO2 concentration of 10 times the PI value and fixed solar radiation at the present‐day value, showing that varying both paleogeography and solar constant and varying paleogeography alone result in GMST changes by 7.3°C and 5.6°C, respectively.

Evaluating BC Aging Processes in the Community Atmosphere Model Version 6 (CAM6)

AbstractThe uncertainty of the climatic effect of Black carbon (BC) remains large. One critical uncertainty source that needs to be captured is BC aging. Here we use the Community Atmosphere Model version 6 (CAM6) configured with the four‐mode version of the Modal Aerosol Module (MAM4) to evaluate the modeled BC aging process with recent laboratory and in‐situ measurements over China. As revealed by the comparison of BC aging timescale and number fraction of aged BC against recent measurements, the modeled condensation aging timescale is estimated to be about 0.8 hr (17%) faster than the chamber measurement, and the diurnal variations of modeled BC aging degree are typically higher than observations mainly due to the fast increase in modeled BC aging degree during daytime. Further analysis shows that the condensation aging dominates (>70%) BC aging across China. More specifically, the condensation of secondary organic aerosol (SOA) vapor contributes most to BC aging over China. Slowing down BC aging increases the modeled surface BC concentration over remote Western China and BC burden, but hardly changes surface BC concentration over Eastern China. Our results suggest that BC aging representation in the MAM4 needs to be further improved toward slowing down the BC aging rate, especially the condensation aging by SOA, to improve the BC simulation over remote areas and its impact on BC transport in MAM4.