Climate Responses to Direct Radiative Forcing of Anthropogenic Aerosols,Tropospheric Ozone,and Long-Lived Greenhouse Gases in Eastern China over 1951–2000

A unified chemistry-aerosol-climate model is applied in this work to compare climate responses to changing concentrations of long-lived greenhouse gases （GHGs, CO2, CH4, N2O）, tropospheric O3, and aerosols during the years 1951-2000. Concentrations of sulfate, nitrate, primary organic carbon （POA）, secondary organic carbon （SOA）, black carbon （BC） aerosols, and tropospheric 03 for the years 1950 and 2000 are obtained a priori by coupled chemistry-aerosol-GCM simulations, and then monthly concentrations are interpolated linearly between 1951 and 2000. The annual concentrations of GHGs are taken from the IPCC Third Assessment Report. BC aerosol is internally mixed with other aerosols. Model results indicate that the sinmlated climate change over 1951-2000 is sensitive to anthropogenic changes in atmospheric components. The predicted year 2000 global mean surface air temperature can differ by 0.8℃ with different forcings. Relative to the climate simulation without changes in GHGs, O3, and aerosols, anthropogenic forcings of SO4^2-, BC, BC＋SO4^2-, BC＋SO4^2- ＋POA, BC＋SO4^2- ＋POA＋SOA＋NO3^-, O3, and GHGs are predicted to change the surface air temperature averaged over 1971-2000 in eastern China, respectively, by -0.40℃, ＋0.62℃, ＋0.18℃, ＋0.15℃, -0.78℃, ＋0.43℃, and ＋0.85℃, and to change the precipitation, respectively, by -0.21, ＋0.07, -0.03, ＋0.02, -0.24, -0.08, and ＋0.10 mm d^-1. The authors conclude that all major aerosols are as important as GHGs in influencing climate change in eastern China, and tropospheric O3 also needs to be included in studies of regional climate change in China.

How the “Best” Models Project the Future Precipitation Change in China

Projected changes in summer precipitation characteristics in China during the 21st century are assessed using the monthly precipitation outputs of the ensemble of three ＂best＂ models under the Special Report on Emissions Scenarios （SRES） A1B, A2, and B1 scenarios. The excellent reproducibility of the models both in spatial and temporal patterns for the precipitation in China makes the projected summer precipitation change more believable for the future 100 years. All the three scenarios experiments indicate a consistent enhancement of summer precipitation in China in the 21st century. However, the projected summer precipitation in China demonstrates large variability between sub-regions. The projected increase in precipitation in South China is significant and persistent, as well as in North China. Meanwhile, in the early period of the 21st century, the region of Northeast China is projected to be much drier than the present. But, this situation changes and the precipitation intensifies later, with a precipitation anomaly increase of 12.4%0-20.4% at the end of the 21st century. The region of the Xinjiang Province probably undergoes a drying trend in the future 100 years, and is projected to decrease by 1.7%-3.6% at the end of the 21st century. There is no significant long-term change of the projected summer precipitation in the lower reaches of the Yangtze River valley. A high level of agreement of the ensemble of the regional precipitation change in some parts of China is found across scenarios but smaller changes are projected for the B1 scenario and slightly larger changes for the A2 scenario.

Variability of Northeast China River Break-up Date

This paper investigates the variability of the break-up dates of the rivers in Northeast China from their icebound states for the period of 1957-2005 and explores some potential explanatory mechanisms. Results show that the break-up of the two major rivers （the Heilongjiang River and Songhuajiang River） was about four days earlier, and their freeze-up was about 4-7 days delayed, during 1989 2005 as compared to 1971 1987. This interdecadal variation is evidently associated with the warming trend over the past 50 years. In addition, the break-up and freeze-up dates have large interannual variability, with a standard deviation of about 10-15 days. The break-up date is primarily determined by the January February-March mean surface air temperature over the Siberian-Northeast China region via changes in the melting rate, ice thickness, and snow cover over the ice cover. The interannual variability of the break-up date is also significantly connected with the Northern Annular Mode （NAM）, with a correlation coefficient of 0.35-0.55 based on the data from four stations along the two rivers. This relationship is attributed to the fact that the NAM can modulate the East Asian winter monsoon circulation and Siberian-Northeast China surface air temperature in January February-March.

Contribution of the Sea Surface Temperature over the Mediterranean-Black Sea to the Decadal Shift of the Summer North Atlantic Oscillation

Recent observational study has shown that the southern center of the summer North Atlantic Oscillation （SNAO） was located farther eastward after the late 1970s compared to before. In this study, the cause for this phenomenon is explored. The result shows that the eastward shift of the SNAO southern center after the late 1970s is related to the variability of the Mediterranean-Black Sea （MBS） SST. A warm MBS SST can heat and moisten its overlying atmosphere, consequently producing a negative sea level pressure （SLP） departure over the MBS region. Because the MBS SST is negatively correlated with the SNAO, the negative SLP departure can enhance the eastern part of the negative-phase of the SNAO southern center, consequently producing an eastward SNAO southern center shift. Similarly, a cold MBS SST produces an eastward positive-phase SNAO southern center shift. The reason for why the MBS SST has an impact on the SNAO after the late 1970s but why it is not the case beforehand is also discussed. It is found that this instable relationship is likely to be attributed to the change of the variability of the MBS SST on the decadal time-scale. In 1951 1975, the variability of the MBS SST is quite weak, but in 1978 2002, it becomes more active. The active SST can enhance the interaction between the sea and its overlying atmosphere, thus strengthening the connection between the MBS SST and the SNAO after the late 1970s. The above observational analysis results are further confirmed by sensitivity experiments.

The Antarctic Oscillation-East Asian Summer Monsoon Connections in NCEP-1 and ERA-40

Connections between the spring Antarctic Oscillation （AAO） and the East Asian summer monsoon （EASM） in two reanalysis datasets-NCEP-1 （NCEP/NCAR Reanalysis 1） and ERA-40 （ECMWF 40- year Reanalysis）-are investigated in this study. Both show significant correlation between AAO and EASM rainfall over the Yangtze River valley, especially after about 1985. Though ERA-40 shows weaker anomalous signals connecting AAO and EASM over southern high latitudes than NCEP-1, both datasets reveal similar connecting patterns between them. A wave-train-like pattern appears in the upper levels, from southern high latitudes through east of Australia and from the Maritime Continent to East Asia. In positive AAO years, the cross equatorial southeasterly flow over the Maritime Continent in the lower levels is strengthened, the specific humidity of the whole atmosphere over East Asia increases, and convective activity is enhanced; thus the summer rainfall over East Asia increases. The spring AAO-EASM connection may be better represented in ERA-40.

The Relationship between the Aleutian Low and the Australian Summer Monsoon at Interannual Time Scales

The relationship between the boreal winter （December, January, February） Aleutian Low （AL） and the simultaneous Australian summer monsoon （ASM） is explored in this study. A significant correlation is found between the North Pacific index （NPI） and ASM index, the bulk of which is attributed to the significant correlation after late 1970s. Significant differences in precipitation and outgoing long-wave radiation between typical negative and positive NPI years appear over the ASM area. A regression analysis of the circulation pattern against the NPI during the three months is performed separately. We propose that the NPI is related with the ASM circulation possibly through the changes in the upper level westerly jet. In a typical negative NPI （strong Aleutian Low） year, the jet is greatly reinforced and the anomalous anticyclonic circulation to the south is thus excited, from which the easterly wind anomalies flowing into the ASM region emanate. Further, strong sinking motion over the northern entrance region of the jet is enhanced, and the local Hadley circulation anomaly between the ASM region and the coast of East Asia is strengthened. In this way, anomalous upward motion over the ASM area can thus be strengthened, and the convective activity intensified. Then the monsoon rainfall over ASM area is increased. An ＂asymmetric＂ connection between AL and the monsoon is found in this study.

Biogenic isoprene emissions over China:sensitivity to the CO2 inhibition effect

Isoprene emissions emitted from vegetation are one of the most important precursors for tropospheric ozone and secondary organic aerosol formation. The authors estimate the biogenic isoprene emissions in China over 2006-2011 using a global chemical transport model （GEOS- Chem） driven by meteorological fields from the assimilated meteorological data from MERRA. The authors incorporate three different parameterizations of isoprene-CO2 interaction into the model, and perform three sensitivity simulations to investigate the effect of CO2 inhibition on isoprene emissions for the period 2006-2011 in China. The annual isoprene emissions rate across China is simulated to be 12.62 Tg C yr-1, averaged over 2006-2011, and decreases by about 2.7%-7.4% when the CO2 inhibition schemes are included. The CO2 inhibition effect might be significant in regions where the CO2 concentration and isoprene emissions are high. Estimates of isoprene emissions can differ depending on the scheme of CO2 inhibition. According to the results obtained from the sensitivity simulations, the authors find that the CO2 inhibition effect leads to 5.6% ±2.3% reductions in annual isoprene emissions over China. The authors also find that inclusion of CO2 inhibition can substantially alter the sensitivity of isoprene emissions to the changes in meteorological conditions during the study period.

Upper-and Lower-tropospheric Circulation Anomalies Associated with Interannual Variation of Pakistan Rainfall during Summer

This study investigated the large-scale circulation anomalies,in both the upper and lower troposphere,associated with the interannual variation of rainfall in Pakistan during summer,using the station observation data in this country and circulation data of the NCEP−NCAR reanalysis from 1981 to 2017.Results showed that the upper-and lower-tropospheric circulation anomalies associated with monthly rainfall variability exhibit similar features from June to August,so analyses were performed on June−August circulation and Pakistan rainfall data.The analyzed results indicated that summer rainfall in Pakistan is enhanced when there is an anticyclonic anomaly to the northwest of Pakistan in the upper troposphere and easterly anomalies along the southern foothills of the Himalayas in the lower troposphere,and vice versa.These upper-and lower-tropospheric circulation anomalies were found to be related,but show unique features.The upper-tropospheric anticyclonic anomaly is closely related to the Silk Road Pattern along the Asian westerly jet,while the lower-tropospheric easterly anomalies are related to the cyclonic anomaly to the south of Pakistan,i.e.,intensified South Asian monsoon trough.The results presented here suggest that the interannual variability of summer rainfall in Pakistan is a combined result of upper-and lower-tropospheric circulation anomalies,and of extratropical and tropical circulation anomalies.

Projections of 2.0°C Warming over the Globe and China under RCP4.5

The outputs of 17 models in the Coupled Model Intercomparison Project Phase 5 (CMIP5) are employed to investigate the temporal and spatial features of 2.0°C warming of the surface temperature over the globe and China under the Representative Concentration Pathways (RCP) 4.5 scenario. The simulations of the period 1860-1899 in the "historical" experiment are chosen as the baseline. The simulations for the 21st century in the RCP4.5 experiment are chosen as the future project. The multi-model ensemble mean (MME) shows that the global mean temperature would cross the 2.0°C warming threshold in 2047. Warming in most of the models would cross the threshold during 2030-2060. For local warming, high-latitude areas in the Northern Hemisphere show the fastest warming over the globe. Land areas warm substantially faster than the oceans. Most of the southern oceans would not exceed the 2.0°C warming threshold within the 21st century. Over China, surface warming is substantially faster than the global mean. The area-averaged warming would cross the 2.0°C threshold in 2034. Locally, Northwest China shows the fastest warming trend, followed by Central North China and Northeast China. Central China, East China, and South China are the last to cross the 2.0°C warming threshold. The diversity of the models is also estimated in this study. Generally, the spread among the models increases with time, and there is smaller spread among the models for the areas with the faster warming.

Model-Simulated Atmospheric Carbon Dioxide: Comparisons with Satellite Retrievals and Ground-Based Observations

Atmospheric CO2 concentrations from January 2010 to December 2010 were simulated using the GEOS-Chem(Goddard Earth Observing System-Chemistry) model and the results were compared to satellite Gases Observing Satellite(GOSAT) and ground-based the Total Carbon Column Observing Network(TCCON) data. It was found that CO2 concentrations based on GOSAT satellite retrievals were generally higher than those simulated by GEOS-Chem. The differences over the land area in January and April ranged from 1 to 2 ppm, and there were major differences in June and August. At high latitudes in the Northern Hemisphere in June, as well as south of the Sahara, the difference was greater than 5 ppm. In the high latitudes of the Northern Hemisphere the model results were higher than the GOSAT retrievals, while in South America the satellite data were higher. The trend of the difference in the high latitudes of the Northern Hemisphere and the Saharan region in August was opposite to June. Maximum correlation coefficients were found in April, reaching 0.72, but were smaller in June and August. In January, the correlation coefficient was only 0.36. The comparisons between GEOS-Chem data and TCCON observations showed better results than the comparison between GEOS and GOSAT. The correlation coefficients ranged between 0.42(Darwin) and 0.92(Izana). Analysis of the results indicated that the inconsistency between satellite observations and model simulations depended on inversion errors caused by data inaccuracies of the model simulation's inputs, as well as the mismatch of satellite retrieval model input parameters.

Projection of the summer northern East Asian low under global warming in CMIP5 models

The East Asian summer climate is modulated by a low-pressure system over northern East Asia（NEAL） and a subtropical high over the western North Pacific. Many studies have focused on the subtropical high, but little is known about NEAL, especially its change in the future under global warming scenarios. This study investigates the projected change in NEAL in the late twenty-first century, using the outputs of 20 models from Phase 5 of the Coupled Model Intercomparison Project — specifically, their historical climate simulations（HIST） and future climate projections under the Representative Concentration Pathway 4.5（RCP4.5） and 8.5（RCP8.5） scenarios. The results show that the models capture the NEAL well in HIST. The NEAL is weakened in the late twenty-first century under the two RCP scenarios, with a stronger weakening under RCP8.5 than under RCP4.5.The weakened NEAL increases the geopotential height zonal gradient in the west and causes a southerly anomaly, which may bring more moisture and rainfall to northern East Asia.

The Asymmetric Connection of SST in the Tasman Sea with Respect to the Opposite Phases of ENSO in Austral Summer

This study uses linear regression and composite analyses to identify a pronounced asymmetric connection of sea surface temperature(SST)in the Tasman Sea with the two opposite phases of El Ni?o-Southern Oscillation(ENSO)during austral summer.In El Ni?o years,the SST anomalies(SSTAs)in the Tasman Sea exhibit a dipolar pattern with weak warmth in the northwest and modest cooling in the southeast,while during La Ni?a years the SSTAs exhibit a basin-scale warmth with greater amplitude.Investigations into the underlying mechanism suggest that this asymmetry arises from a mechanism related to oceanic heat transport,specifically the anomalous Ekman meridional heat transport induced by the zonal wind stress anomalies,rather than the surface heat fluxes on the air-sea interface.Further analysis reveals that the asymmetry of oceanic heat transport between El Ni?o and La Ni?a years is driven by the asymmetric atmospheric circulation over the Tasman Sea stimulated by the asymmetric diabatic heating in the tropical Pacific between the two opposite ENSO phases.

Comparative Analysis of the Mechanisms of Intensified Summer Warming over Europe-West Asia and Northeast Asia since the Mid-1990s through a Process-based Decomposition Method

Previous studies have found amplified warming over Europe-West Asia and Northeast Asia in summer since the mid- 1990s relative to elsewhere on the Eurasian continent, but the cause of the amplification in these two regions remains unclear. In this study, we compared the individual contributions of influential factors for amplified warming over these two regions through a quantitative diagnostic analysis based on CFRAM (climate feedback-response analysis method). The changes in surface air temperature are decomposed into the partial changes due to radiative processes (including CO2 concentration, incident solar radiation at the top of the atmosphere, surface albedo, water vapor content, ozone concentration, and clouds) and non-radiative processes (including surface sensible heat flux, surface latent heat flux, and dynamical processes). Our results suggest that the enhanced warming over these two regions is primarily attributable to changes in the radiative processes, which contributed 0.62 and 0.98 K to the region-averaged warming over Europe-West Asia (1.00 K) and Northeast Asia (1.02 K), respectively. Among the radiative processes, the main drivers were clouds, CO2 concentration, and water vapor content. The cloud term alone contributed to the mean amplitude of warming by 0.40 and 0.85 K in Europe-West Asia and Northeast Asia, respectively. In comparison, the non-radiative processes made a much weaker contribution due to the combined impact of surface sensible heat flux, surface latent heat flux, and dynamical processes, accounting for only 0.38 K for the warming in Europe-West Asia and 0.05 K for the warming in Northeast Asia. The resemblance between the influential factors for the amplified warming in these two separate regions implies a common dynamical origin. Thus, this validates the possibility that they originate from the Silk Road pattern.

The Chinese carbon cycle data-assimilation system(Tan-Tracker)

In this study,the Chinese carbon cyle dataassimilation system Tan-Tracker is developed based on the atmospheric chemical transport model(GEOS-Chem)platform.Tan-Tracker is a dual-pass data-assimilation system in which both CO2concentrations and CO2fluxes are simultaneously assimilated from atmospheric observations.It has several advantages,including its advanced data-assimilation method,its highly efficient computing performance,and its simultaneous assimilation of CO2concentrations and CO2fluxes.Preliminary observing system simulation experiments demonstrate its robust performance with high assimilation precision,making full use of observations.The Tan-Tracker system can only assimilate in situ observations for the moment.In the future,we hope to extend Tan-Tracker with functions for using satellite measurements,which will form the quasioperational Chinese carbon cycle data-assimilation system.

An improved diagnostic stratocumulus scheme based on estimated inversion strength and its performance in GAMIL2

Based on satellite data and the estimated inversion strength(EIS) derived by Wood et al.(2006), a feasible and uncomplicated stratocumulus scheme is proposed, referred to as EIS scheme. It improves simulation of cloud radiative forcing(CRF) in the Grid-point Atmospheric Model of IAP/LASG version 2(GAMIL2.0) model. When compared with the original lower troposphere stability(LTS) scheme, the EIS scheme reproduces more reasonable climatology distributions of clouds and CRF. The parameterization partly corrects CRF underestimation at mid and high latitudes and overestimation in the convective region. Such improvements are achieved by neglecting the effect of free-tropospheric stratification changes that follow a cooler moist adiabat at middle and high latitude, thereby improving simulated cloudiness. The EIS scheme also improves simulation of the CRF interannual variability. The positive net CRF and negative stratiform anomaly in the East Asian and western North Pacific monsoon regions(EAWNPMR) are well simulated. The EIS scheme is more sensitive to sea surface temperature anomalies(SSTA) than the LTS. Therefore, under the effect of a warmer SSTA in the EAWNPMR, the EIS generates a stronger negative stratiform response, which reduces radiative heating in the low and mid troposphere, in turn producing strong subsidence and negative anomalies of both moisture and cloudiness. Consequent decreases in cloud reflection and shading effects ultimately improve simulation of incoming surface shortwave radiative fluxes and CRF. Because of the stronger subsidence, a stronger anomalous anticyclone over the Philippines Sea is simulated by the EIS run, which leads to a better positive precipitation anomaly in eastern China during ENSO winter.

Model Projections of Precipitation Minus Evaporation in China

Changes in precipitation minus evaporation （P-E） are analyzed to investigate the possible impacts of climate change on water resource conditions in China. Simulations of SRES A1B and 20C3M scenarios from the WCRP CMIP3 GCMs are employed in the study. Time slice analysis shows that there would be more annual mean P-E across China in 2040-2055 and 2080-2099, compared to 1980-1999, with the largest percentage change over Northwest China and the Bohai Rim area. Precipitation and evaporation would also increase over entire China during these two periods. Annual mean P-E, precipitation, and evaporation averaged over the whole China and its eight sub-areas all yield generally upward trends during the 21st century. This indicates that on annual mean scale, the global warming related precipitation dominates the hydroclimate conditions in China. On seasonal mean scale, although precipitation is projected to increase over China, P-E exhibits both decreasing and increasing trends over certain regions of China. This suggests that the variation of global warming related evaporation dominates hydroclimate conditions over some parts of China, especially in northern China. Therefore, in hydroclimate condition projections, considering both evaporation and precipitation changes should be more reasonable than considering only precipitation.

Airesea coupling over the Tasman Sea intensifies the ENSO-related South Pacific atmospheric teleconnection

The ENSO-induced PacificeSouth America(PSA)pattern is an important atmospheric bridge in linking the Antarctic climate to the tropical Pacific.The AGCM simulated PSA-like responses to ENSO are evidently weaker than the observed in terms of its intensity due to the lack of airesea coupling processes.The Tasman Sea features active airesea interactions.However,how and to what extent the airesea coupling explains the deficiency of the AGCM responses to ENSO is unclear.In this study,the role of the airesea coupling elsewhere the tropical Pacific in shaping the ENSOeSouth Pacific teleconnection is first estimated by comparing the coupled tropical Pacific pacemaker experiments(PACE)derived from the Community Earth System Model version 1(CESM1)and the parallel Pacific OceaneGlobal Atmosphere experiments(POGA)with the same atmospheric component model of CESM1.Our results suggest that the airesea coupling elsewhere the tropical Pacific greatly intensifies the South Pacific atmospheric response to ENSO.Then the separated impact of airesea coupling over the Tasman Sea is stressed with another set of AGCM experiments forced with the PACE sea surface temperature(SST)outputs in the Tasman Sea.The results show that the atmospheric response to the SST anomalies in the Tasman Sea bears a remarkable resemblance to that due to the airesea coupling elsewhere the tropical Pacific,and explains about 30%of the intensified amplitude.This highlights a substantial contribution of the airesea coupling over the Tasman Sea to intensifying the extratropical South Pacific atmospheric responses to ENSO,and provides a new perspective on the connection between tropical Pacific and Antarctic climate change.