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Characteristics of the Asian-Pacific Oscillation in Boreal Summer Simulated by BCC_CSM with Different Horizontal Resolutions


doi: 10.1007/s00376-016-5266-0

  • The summer Asian-Pacific Oscillation (APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models (BCC_CSMs) with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated. The results show that BCC_CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO's spatial pattern, compared to BCC_CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC_CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC_CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC_CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC_CSM1.1.
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  • Chen J. M., P. Zhao, S. Yang, G. Liu, and X. J. Zhou, 2013a: Simulation and dynamical prediction of the summer Asian-Pacific Oscillation and associated climate anomalies by the NCEP CFSv2. J. Climate ,26, 3644-3656, doi:10.1175/ JCLI-D-12-00368.1.10.1175/JCLI-D-12-00368.13955ac92320ada066d5efe19dec69e5ahttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F263890879_Simulation_and_Dynamical_Prediction_of_the_Summer_AsianPacific_Oscillation_and_Associated_Climate_Anomalies_by_the_NCEP_CFSv2http://www.researchgate.net/publication/263890879_Simulation_and_Dynamical_Prediction_of_the_Summer_AsianPacific_Oscillation_and_Associated_Climate_Anomalies_by_the_NCEP_CFSv2Abstract The Asianacific Oscillation (APO) is a dominant teleconnection pattern linking the climate anomalies over Asia, the North Pacific, and other regions including North America. The National Centers for Environmental Prediction (NCEP) Climate Forecast System version 2 (CFSv2) successfully simulates many summer-mean features of the upper-tropospheric temperature, the South Asian high, the westerly and easterly jet streams, and the regional monsoons over Asia and Africa. It also well simulates the interannual variability of the APO and associated anomalies in atmospheric circulation, precipitation, surface air temperature (SAT), and sea surface temperature (SST). Associated with a positive APO are a strengthened South Asian high; a weakened extratropical upper-tropospheric westerly jet stream over North America; strengthened subtropical anticyclones over the Northern Hemisphere oceans; and strengthened monsoons over North Africa, India, and East Asia. Meanwhile, increased precipitation is found over tropical North Africa, South Asia, northern China, and tropical South America; decreased precipitation is seen over subtropical North Africa, the Middle East, central Asia, southern China, Japan, and extratropical North America. Low SAT occurs in North Africa, India, and tropical South America and high SAT appears in extratropical Eurasia and North America. SST increases in the extratropical Pacific and the North Atlantic but decreases in the tropical Pacific. The summer APO and many of the associated climate anomalies can be predicted by the NCEP CFSv2 by up to 5 months in advance. However, the CFSv2 skill of predicting the SAT in the East Asian monsoon region is low.
    Chen X. L., T. J. Zhou, and L. W. Zou, 2013b: Variation characteristics of the Asian-Pacific Oscillation in boreal summer as simulated by the LASG/IAP Climate System Model FGOALS_gl. Acta Meteorologica Sinica, 71( 1), 23- 37. (in Chinese with English abstract)10.11676/qxxb2013.00282e847b3-80d0-4e2d-9aa6-c7a01bc176a95584201312b7d154118379316eabb30d29b146d22chttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-QXXB201301002.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-QXXB201301002.htmThe Asian-Pacific Oscillation(APO) is a phenomenon in which the temperature changes out of phase over the East Asia continent(15°-50°N,60°-120°E) with the North Pacific(15°-50°N,180°-120°W) in the upper troposphere.The APO index reflects the Asian-Pacific zonal thermal contrast.The performance of the fast coupled version of the LASG/IAP climate system model FGOALS_gl in simulating the upper troposphere temperature and the APO index over the 20th century is evaluated. Compared with the ERA-40 reanaiysis data,it is shown that the model performed well in simulating the climatology and the dominant modes of the upper troposphere temperature.However,the results show that the simulated APO index failed to capture the descent trend after 1960s over the East Asia continent as indicated in the ERA-40 data.Based on the power spectrum analysis,the 2 - 3 a variability of the model APO index is equivalent with that in the reanaiysis but the 5-7 a variability is weaker.Despite several regional departures,the large-scale circulation over Asian monsoon section related with the APO index is well reproduced in the model.A comparison among the 20th century simulations shows that external forcing could change the interannual variability of a couple system.The natural forcing causes a spectrum shift to low frequency and the anthropogenic forcing does inversely.Natural forcing and anthropogenic forcing can play different roles in different periods.It seems that anthropogenic forcing could limit the interannual variability of APO and enhance the interdecadal variability.The dominant mode of the upper troposphere temperature in the model is modulated by ENSO and further impacts the interannual variability of APO.The defect of the model in the ENSO simulation may be an important limitation to reproducing the upper troposphere temperature and the variability of APO index.
    Dell'Aquila, A., V. Lucarini, P. M. Ruti, S. Calmanti, 2005: Hayashi spectra of the Northern Hemisphere mid-latitude atmospheric variability in the NCEP-NCAR and ECMWF reanalyses. Climate Dyn., 25, 639- 652.10.1007/s00382-005-0048-xe9e3bae9046613319175c924b919f43dhttp%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-005-0048-xhttp://link.springer.com/10.1007/s00382-005-0048-xWe compare 45 years of the reanalyses of NCEP-NCAR and ECMWF in terms of their representation of the mid-latitude winter atmospheric variability for the overlapping time frame 1957-2002. We adopt the classical approach of computing the Hayashi spectra of the 500 hPa geopotential height fields. Discrepancies are found especially in the first 15 years of the records in the high-frequency-high wavenumber propagating waves and secondly on low frequency-low wavenumber standing waves. This implies that in the first period the two datasets have a different representation of the baroclinic available energy conversion processes. In the period starting from 1973 a positive impact of the aircraft data on the Euro-Atlantic synoptic waves has been highlighted. Since in the first period the assimilated data are scarcer and of lower quality than later on, they provide a weaker constraint to the model dynamics. Therefore, the resulting discrepancies in the reanalysis products may be mainly attributed to differences in the models' behavior.
    Gao F., X. G. Xin, and T. W. Wu, 2012: A study of the prediction of regional and global temperature on decadal time scale with BCC_CSM 1.1 model. Chinese Journal of Atmospheric Sciences, 36( 6), 1165- 1179. (in Chinese with English abstract)10.1007/s11783-011-0280-za2ebf918-bbc8-4096-b758-e7433b82780848253201236652b4ef278af9d7e751f75130fdf54f935http%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK201206009.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK201206009.htmDecadal prediction on 10-30 year time scale is one of the most important contents of the 5th phase of the Coupled Model Inter-comparison Project(CMIP5).According to the experiment requirement of CMIP5,a set of decadal experiments were performed using the Beijing Climate Center Climate System Model(BCC-CSM1.1) which is one of models jointed in CMIP5.This study evaluated the model's prediction capability in regional and global surface temperatures on decadal time scale,and aimed to explore their dependences on the initial observed states of ocean in comparison with the historical experiment in the 20th century using BCC-CSM1.1.The results show as following:(1) BCC_CSM1.1 can simulate the warming trend of 10-year mean global surface temperature not only for oceanic initialization condition but also for without oceanic initialization condition.Nevertheless,the global warming trend simulated by BCC-CSM1.1 can be obviously decreased under the condition of oceanic initialization,which is closer to the observation than that in the historical experiment without oceanic initialization.This feature is much more remarkable in the area between 50N and 50S where there are abundant observation data.(2) The nudging method is used to initialize the model with the SODA temperature data.After a "training" period of 8-12 months,predicted surface temperatures in the first year not only in ocean but also in land between 50S and 50N are close to CRU observations.Due to the warmer SST bias of SODA reanalysis contrast to HadSST2,there is about a period of 2 to 7 years in decadal experiments that adjusts from the observed ocean state to model basic state.The adjustment time for the ocean and land is almost identical in the same decadal experiment.(3) The prediction skill for decadal-mean SST has strong feature.The high correlations with the CRU observations are mainly near the middle-and high-latitude Indian Ocean in the Southern Hemisphere,the western Pacific Ocean,and the Atlantic Ocean.The oceanic initialization does not significantly influence the prediction results.(4) The variation of decadal-mean predicted SST is closely correlated with the surface heat flux.In the tropical and subtropical region,the net long wave radiation and sensible heating flux has larger influence on the decadal mean SST variation than the net short wave radiation and the latent heating flux,but in oceans at higher latitudes,the variation of decadal mean SST is mostly determined by the latent heating flux.
    Gao X. J., M. L. Wang, and F. Giorgi, 2013: Climate change over China in the 21st century as simulated by BCC_CSM1.1-RegCM4.0. Atmospheric and Oceanic Science Letters, 6( 5), 381- 386.10.1080/16742834.2013.11447112b74f6309-acab-4c47-b84c-28dd2e02c4aemag45222013653814616eec0ee0677e774cb320e63ebaa19http%3A%2F%2Fwww.cqvip.com%2FQK%2F89435X%2F201305%2F47350094.htmlhttp://d.wanfangdata.com.cn/Periodical_dqhhykxkb201305028.aspx
    Griffies S. M., M. J. Harrison, R. C. Pacanowski, and A. Rosati, 2004: A technical guide to MOM4. NOAA/Geophysical Fluid Dynamics Laboratory, March 2004.
    [ Available online at http://www.gfdl.noaa.gov/fms.]
    Huang Y. Y., H. J. Wang, and P. Zhao, 2013: Is the interannual variability of the summer Asian-Pacific Oscillation predictable?. J.Climate, 26, 3865- 3876.10.1175/JCLI-D-12-00450.110dec76da494fc881e810b0b1fef87cfhttp%3A%2F%2Fwww.researchgate.net%2Fpublication%2F274437923_Is_the_Interannual_Variability_of_the_Summer_AsianPacific_Oscillation_Predictablehttp://www.researchgate.net/publication/274437923_Is_the_Interannual_Variability_of_the_Summer_AsianPacific_Oscillation_PredictableAbstract The summer (June–August) Asian–Pacific Oscillation (APO) measures the interannual variability of large-scale atmospheric circulation over the Asian–North Pacific Ocean sector. In this study, the authors assess the predictability of the summer APO index interannual variability and the associated atmospheric circulation anomalies using the 1959–2001 hindcast data from the European Centre for Medium-Range Weather Forecasts (ECMWF), Centre National de Recherches Météorologiques (CNRM), and the Met Office (UKMO) general circulation models from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) project. The results show that these models predict the summer APO index interannual variability well and have higher skill for the North Pacific than for the Asian upper-tropospheric temperature. Meanwhile, the observed APO-related atmospheric circulation anomalies in the South Asian high, the tropical easterly wind jet over the Asian monsoon region in the upper troposphere, the subtropical anticyclone over the North Pacific, and the summer southwest monsoon over Asia in the lower troposphere are reasonably well predicted in their spatial patterns and intensities. Compared with the observations, however, these models display low skill in predicting the long-term varying trends of the upper-tropospheric temperature over the Asian–North Pacific sector or the APO index during 1959–2001.
    Ji J. J., M. Huang, and K. R. Li, 2008: Prediction of carbon exchanges between China terrestrial ecosystem and atmosphere in 21st century. Science in China Series D: Earth Science, 51, 885- 898.10.1007/s11430-008-0039-y6d827f55c324378caabfb6131955eec1http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs11430-008-0039-yhttp://www.cnki.com.cn/Article/CJFDTotal-JDXG200806012.htmThe projected changes in carbon exchange between China terrestrial ecosystem and the atmosphere and vegetation and soil carbon storage during the 21st century were investigated using an atmosphere-vegetation interaction model (AVIM2). The results show that in the coming 100 a, for SRES B2 scenario and constant atmospheric CO(2) concentration, the net primary productivity (NPP) of terrestrial ecosystem in China will be decreased slowly, and vegetation and soil carbon storage as well as net ecosystem productivity (NEP) will also be decreased. The carbon sink for China terrestrial ecosystem in the beginning of the 20th century will become totally a carbon source by the year of 2020, while for B2 scenario and changing atmospheric CO(2) concentration, NPP for China will increase continuously from 2.94 GtC center dot a(-1) by the end of the 20th century to 3.99 GtC center dot a(-1) by the end of the 21st century, and vegetation and soil carbon storage will increase to 110.3 GtC. NEP in China will keep rising during the first and middle periods of the 21st century, and reach the peak around 2050s, then will decrease gradually and approach to zero by the end of the 21st century.
    Jiang, J. H., Coauthors, 2012: Evaluation of cloud and water vapor simulations in CMIP5 climate models using NASA "A-Train" satellite observations. J. Geophys. Res., 117,D14105, doi: 10.1029/2011JD017237.10.1029/2011JD01723721854090b9a9c82b24eda4c3fc4d2b1ehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1002%2Fjgrd.50864%2Fcitedbyhttp://onlinelibrary.wiley.com/doi/10.1002/jgrd.50864/citedby[1] Using NASA's A-Train satellite measurements, we evaluate the accuracy of cloud water content (CWC) and water vapor mixing ratio (H
    Jiang Y. M., A. N. Huang, and H. M. Wu, 2015: Evaluation of the performance of Beijing climate center climate system model with different horizontal resolution in simulating the annual surface temperature over Central Asia. Chinese Journal of Atmospheric Sciences, 39( 3), 535- 547. (in Chinese with English abstract)10.1007/BF03178255f607d0b3-9c46-4646-9c06-16c09c1b130c48253201539366fed50172f8a62ce79d060af059f3c7bhttp%3A%2F%2Fen.cnki.com.cn%2FArticle_en%2FCJFDTOTAL-DQXK201503008.htmhttp://en.cnki.com.cn/Article_en/CJFDTOTAL-DQXK201503008.htmThe temporal and spatial distributions of the mean annual surface air temperature and annual precipitation over Central Asia during 1948-2011 have been studied using trend analysis and moving average methods based on the Climatic Research Unit(CRU) dataset and the output of the historical experiments from the Beijing Climate Center Climate System Model version 1.1(BCC_CSM1.1) and the Beijing Climate Center Climate System Model version 1.1 with a Moderate Resolution(BCC_CSM1.1(m)) for the Fifth Assessment Report of the Intergovernmental Panel on Climate Change(IPCC AR5). Heat flux and radiation flux were imported to further assess the capability of the two BCC_CSM versions in simulating the climate over Central Asia. The results show that these two versions effectively simulated the significant upward trend and northouth increasing characteristic of sensible heat flux and radiation flux over Central Asia. The performance of BCC_CSM1.1(m) in simulating the spatial distribution of air temperature, heat flux, and long/short radiation flux improved significantly compared with the results of BCC_CSM1.1. However, the performance of BCC_CSM1.1 in simulating the spatial distribution of the standard deviation of air temperature was better than BCC_CSM1.1(m). The improvement in model resolution more clearly demonstrated the topographic effects and improved the model simulation performance for heat flux and radiation flux. The high-resolution model displayed advantages in simulating the air temperature over Central Asia.
    Kanamitsu M., W. Ebisuzaki, J. Woollen, S. K. Yang, J. J. Hnilo, M. Fiorino, and G. L. Potter, 2002: NCEP-DEO AMIP-II reanalysis (R-2). Bull. Amer. Meteor. Soc., 83, 1631- 1643.4a6f5a5f-a715-4547-bd40-04dbea432fc878182a9c107f7e4a6d116468670fb6a6http%3A%2F%2Fbioscience.oxfordjournals.org%2Fexternal-ref%3Faccess_num%3D10.1175%2FBAMS-83-11-1631%26link_type%3DDOIrefpaperuri:(bee1afafcfe63877b832ff4256555171)http://bioscience.oxfordjournals.org/external-ref?access_num=10.1175/BAMS-83-11-1631&link_type=DOI
    Kidston J., E. P. Gerber, 2010: Intermodel variability of the poleward shift of the austral jet stream in the CMIP3 integrations linked to biases in the 20th century climatology. Geophys. Res. Lett., 37,L09708, doi: 10.1029/2010GL042873.10.1029/2010GL0428736a9bca801944cf47ccd88117893dd7eehttp%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2010GL042873%2Ffullhttp://onlinelibrary.wiley.com/doi/10.1029/2010GL042873/fullFuture climate predictions by global circulation models in the Coupled Model Intercomparison Project Phase 3 (CMIP3) archive indicate that the recent poleward shift of the eddy-driven jet streams will continue throughout the 21st century. Here it is shown that differences in the projected magnitude of the trend in the Southern Hemisphere are well correlated with biases in the latitude of the jet in the simulation of 20th century climate. Furthermore, the latitude of the jet in the models' 20th century climatology is correlated with biases in the internal variability of the jet stream, as quantified by the time scale of the annular mode. Thus an equatorward bias in the position of the jet is associated with both enhanced persistence of the annular mode, and an increased poleward shift of the jet. These relationships appear to be robust throughout the year except in the austral summer, when differences in forcing, particularly stratospheric ozone, make it impossible to compare the response of one model with another. These results suggest that the fidelity of a model's simulation of the 20th century climate may be related to its fitness for climate prediction. The cause of this relationship is discussed, as well as the implications for climate change projections.
    Liu G., P. Zhao, and J. M. Chen, 2011: A 150-year reconstructed summer Asian-Pacific Oscillation index and its association with precipitation over eastern China. Theor. Appl. Climatol., 103, 239- 248.78d86d2028fcd7ac9f2adc5d80ad954dhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00704-010-0294-7http://xueshu.baidu.com/s?wd=paperuri%3A%28c0c15f948074427c88c972cb0024468f%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00704-010-0294-7&ie=utf-8&sc_us=4816599956475115056
    Liu G., P. Zhao, J. M. Chen, and S. Yang, 2015: Preceding factors of summer Asian-Pacific Oscillation and the physical mechanism for their potential influences. J.Climate, 28, 2531- 2543.10.1175/JCLI-D-14-00327.17c67814c0dccf66666ee985231135360http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2015JCli...28.2531Lhttp://adsabs.harvard.edu/abs/2015JCli...28.2531LNot Available
    Man W. M., T. J. Zhou, 2011: Forced response of atmospheric oscillations during the last millennium simulated by a climate system model. Chinese Science Bulletin, 56, 3042- 3052.10.1007/s11434-011-4637-299f589fae9e8e86f88b8c748982b4644http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-JXTW2011Z2009.htmhttp://www.cnki.com.cn/Article/CJFDTotal-JXTW2011Z2009.htm
    Nan S. L., P. Zhao, S. Yang, and J. M. Chen, 2009: Springtime tropospheric temperature over the Tibetan Plateau and evolutions of the tropical Pacific SST. J. Geophys. Res., 114,D10104, doi: 10.1029/2008JD011559.10.1029/2008JD011559acc111f64cbd06bdadef325627ede276http%3A%2F%2Fonlinelibrary.wiley.com%2Fdoi%2F10.1029%2F2008JD011559%2Fpdfhttp://onlinelibrary.wiley.com/doi/10.1029/2008JD011559/pdfUsing monthly mean data from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCAR) reanalysis and HadISST SST data sets, we investigate the relationship between springtime tropospheric temperature over the Tibetan Plateau and sea surface temperature (SST) over the equatorial Pacific and the associated physical processes. When the Tibetan temperature is low (high) in spring, positive (negative) SST anomalies appear over the tropical central-eastern Pacific in spring and summer. The relationship is explained by the Asian-Pacific Oscillation (APO) and the ocean-atmosphere interaction over the tropical Pacific. In the context of the APO, a lower spring Tibetan tropospheric temperature is associated with a higher tropospheric temperature over the subtropical North Pacific, which is accompanied by a weaker subtropical high over the eastern North Pacific. Accordingly, large-scale westerly anomalies appear in the lower troposphere of the equatorial central-eastern Pacific, resulting in an increase in SST over the equatorial central-eastern Pacific. Numerical simulations with both an ocean-atmosphere coupled model (the NCAR Community Climate System Model version 3) and an atmospheric model with a prescribed SST scheme (the NCAR Community Atmospheric Model version 3) demonstrate the impacts of the spring Tibetan thermal condition on the tropospheric temperature and atmospheric circulation over the Asian-Pacific sector and then on the SST over the equatorial eastern Pacific, better explaining the physical processes of the observed Tibetan temperature-Pacific SST relationship.
    Winton M., 2000: A reformulated three-layer sea ice model. J. Atmos. Oceanic Technol., 17, 525- 531.10.1175/1520-0426(2000)0172.0.CO;2e24bcafa223468153c79412dc4c3c29dhttp%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2000JAtOT..17..525Whttp://adsabs.harvard.edu/abs/2000JAtOT..17..525WA model is presented that provides an efficient approximation to sea ice thermodynamics for climate studies. Semtner's three-layer framework is used, but the brine content of the upper ice is represented with a variable heat capacity as is done in more physically based models. A noniterative fully implicit time-stepping scheme is used for calculation of ice temperature. The results of the new model are compared to those of Semtner's original model.
    Wu T. W., 2012: A mass-flux cumulus parameterization scheme for large-scale models: description and test with observations. Climate Dyn.,38, 725-744, doi: 10.1007/s00382-011-0995-3.10.1007/s00382-011-0995-30f12a540a66d6b35ee919ee5bae3cda5http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-011-0995-3http://link.springer.com/10.1007/s00382-011-0995-3A simple mass-flux cumulus parameterization scheme suitable for large-scale atmospheric models is presented. The scheme is based on a bulk-cloud approach and has the following properties: (1) Deep convection is launched at the level of maximum moist static energy above the top of the boundary layer. It is triggered if there is positive convective available potential energy (CAPE) and relative humidity of the air at the lifting level of convection cloud is greater than 75%; (2) Convective updrafts for mass, dry static energy, moisture, cloud liquid water and momentum are parameterized by a one-dimensional entrainment/detrainment bulk-cloud model. The lateral entrainment of the environmental air into the unstable ascending parcel before it rises to the lifting condensation level is considered. The entrainment/detrainment amount for the updraft cloud parcel is separately determined according to the increase/decrease of updraft parcel mass with altitude, and the mass change for the adiabatic ascent cloud parcel with altitude is derived from a total energy conservation equation of the whole adiabatic system in which involves the updraft cloud parcel and the environment; (3) The convective downdraft is assumed saturated and originated from the level of minimum environmental saturated equivalent potential temperature within the updraft cloud; (4) The mass flux at the base of convective cloud is determined by a closure scheme suggested by Zhang (J Geophys Res 107(D14), doi: 10.1029/2001JD001005 , 2002) in which the increase/decrease of CAPE due to changes of the thermodynamic states in the free troposphere resulting from convection approximately balances the decrease/increase resulting from large-scale processes. Evaluation of the proposed convection scheme is performed by using a single column model (SCM) forced by the Atmospheric Radiation Measurement Program's (ARM) summer 1995 and 1997 Intensive Observing Period (IOP) observations, and field observations from the Global Atmospheric Research Program's Atlantic Tropical Experiment (GATE) and the Tropical Ocean and Global Atmosphere Coupled Ocean-Atmosphere Response Experiment (TOGA COARE). The SCM can generally capture the convective events and produce a realistic timing of most events of intense precipitation although there are some biases in the strength of simulated precipitation.
    Wu T. W., R. C. Yu, and F. Zhang, 2008: A modified dynamic framework for the atmospheric spectral model and its application. J. Atmos. Sci., 65( 7), 2235- 2253.10.1175/2007JAS2514.18806da31c6a9dabb5d6065a99424e2fbhttp%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FADS%3Fid%3D2008JAtS...65.2235Whttp://onlinelibrary.wiley.com/resolve/reference/ADS?id=2008JAtS...65.2235WAbstract This paper describes a dynamic framework for an atmospheric general circulation spectral model in which a reference stratified atmospheric temperature and a reference surface pressure are introduced into the governing equations so as to improve the calculation of the pressure gradient force and gradients of surface pressure and temperature. The vertical profile of the reference atmospheric temperature approximately corresponds to that of the U.S. midlatitude standard atmosphere within the troposphere and stratosphere, and the reference surface pressure is a function of surface terrain geopotential and is close to the observed mean surface pressure. Prognostic variables for the temperature and surface pressure are replaced by their perturbations from the prescribed references. The numerical algorithms of the explicit time difference scheme for vorticity and the semi-implicit time difference scheme for divergence, perturbation temperature, and perturbation surface pressure equation are given in det...
    Wu, T. W., Coauthors, 2010: The Beijing Climate Center atmospheric general circulation model: Description and its performance for the present-day climate. Climate Dyn.,34, 123-147, doi: 10.1007/s00382-008-0487-2.10.1007/s00382-008-0487-29600cb2c028bbad11cd4bd3b6dfa2468http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-008-0487-2http://xueshu.baidu.com/s?wd=paperuri%3A%28fa2b07d11e7f035fb2e4ac6679bfc9dd%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1007%2Fs00382-008-0487-2&ie=utf-8&sc_us=2030134516081453740The Beijing Climate Center atmospheric general circulation model version 2.0.1 (BCC_AGCM2.0.1) is described and its performance in simulating the present-day climate is assessed. BCC_AGCM2.0.1 originates from the community atmospheric model version 3 (CAM3) developed by the National Center for Atmospheric Research (NCAR). The dynamics in BCC_AGCM2.0.1 is, however, substantially different from the Eulerian spectral formulation of the dynamical equations in CAM3, and several new physical parameterizations have replaced the corresponding original ones. The major modification of the model physics in BCC_AGCM2.0.1 includes a new convection scheme, a dry adiabatic adjustment scheme in which potential temperature is conserved, a modified scheme to calculate the sensible heat and moisture fluxes over the open ocean which takes into account the effect of ocean waves on the latent and sensible heat fluxes, and an empirical equation to compute the snow cover fraction. Specially, the new convection scheme in BCC_AGCM2.0.1, which is generated from the Zhang and McFarlane scheme but modified, is tested to have significant improvement in tropical maximum but also the subtropical minimum precipitation, and the modified scheme for turbulent fluxes are validated using EPIC2001 in situ observations and show a large improvement than its original scheme in CAM3. BCC_AGCM2.0.1 is forced by observed monthly varying sea surface temperatures and sea ice concentrations during 1949-2000. The model climatology is compiled for the period 1971-2000 and compared with the ERA-40 reanalysis products. The model performance is evaluated in terms of energy budgets, precipitation, sea level pressure, air temperature, geopotential height, and atmospheric circulation, as well as their seasonal variations. Results show that BCC_AGCM2.0.1 reproduces fairly well the present-day climate. The combined effect of the new dynamical core and the updated physical parameterizations in BCC_AGCM2.0.1 leads to an overall improvement, compared to the original CAM3.
    Xin X. G., T. W. Wu, J. L. Li, Z. Z. Wang, W. P. Li, and F. H. Wu, 2013: How well does BCC_CSM1.1 reproduce the 20th century climate change over China?. Atmospheric and Oceanic Science Letters, 6( 1), 21- 26.8c0a2d6e-bb97-4b18-9b70-28f118ee8032mag452220136121d1cbb5cb36fe7603e2aedc3eac55167ahttp%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-AOSL201301005.htmhttp://xueshu.baidu.com/s?wd=paperuri%3A%285a6fa9025606c465abec67db3a8c6552%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqhhykxkb201301004.aspx&ie=utf-8&sc_us=16540022029997389919阶段五联合模型 Intercomparison 工程(CMIP5 ) 实验的历史的模拟由北京气候中心气候系统模型(BCC_CSM1.1 ) 表现了关于时间进化被评估全球并且中国吝啬的表面空气温度(坐) 并且在在最近的十年的中国上的表面气候变化。BCC_CSM1.1 在复制时间进化有更好的能力全球并且比 BCC_CSM1.0 容纳的中国平均数。在一年 2005, BCC_CSM1.1 模型模仿约 1 的一个温暖的振幅
    Xie P. P., P. A. Arkin, 1997: Global precipitation: A 17-year monthly analysis based on gauge observations,satellite estimates, and numerical model outputs. Bull. Amer. Meteor. Soc., 78, 2539-2558, doi: 10.1175/1520-0477(1997)078<2539: GPAYMA>2.0.CO;2.
    Zhao P., Y. N. Zhu, and R. H. Zhang, 2007: An Asia-Pacific teleconnection in summer tropospheric temperature and associated Asian climate variability. Climate Dyn.,29, 293-303, doi: 10.1007/s00382-007-0236-y.10.1007/s00382-007-0236-yd520ac227814cf999598eade7d001a65http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00382-007-0236-yhttp://link.springer.com/article/10.1007/s00382-007-0236-yWe identified the Asianacific Oscillation (APO) and its associated index, a zonal teleconnection pattern over the extratropical Asianacific region. This was done through the correlation and empirical orthogonal function (EOF) analyses on the summer mean tropospheric eddy temperature from the monthly European Center for Medium-Range Weather Forecast reanalysis. The APO reflects an out-of-phase relationship in variability of the eddy temperature between Asia and the North Pacific and is associated with the out-of-phase relationship in atmospheric heating. The APO index shows a decadal variation, tending to a high-index polarity before 1975 and afterward to a low-index polarity. Moreover, the APO index has a quasi-5-year period. With higher APO-index conditions in the upper troposphere, the summer South Asian high and the North Pacific trough are stronger, while the westerly jet stream over Asia and the easterly jet stream over South Asia strengthen. Also, the Asian low and the North Pacific subtropical high are stronger in the lower troposphere. The anomalous southerlies prevail at the midlatitudes of East Asia, accompanied by a more northward Mei-yu front, and the anomalous westerlies prevail over South Asia. Summer rainfall increases in North China, South China, and South Asia, while it decreases from the valley of the Yangtze River to southern Japan, and near the Philippines.
    Zhao P., J. M. Chen, D. Xiao, S. L. Nan, Y. Zou, and B. T. Zhou, 2008: Summer Asian-Pacific Oscillation and its relationship with atmospheric circulation and monsoon rainfall. Acta Meteorologica Sinica, 22, 455- 471.10.1080/00022470.1979.104708712bea9913baf73ccae540db46ba2623adhttp%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-QXXW200804007.htmhttp://d.wanfangdata.com.cn/Periodical/qxxb-e200804006Using the ERA-40 data and numerical simulations, this study investigated the teleconnection over the extratropical Asian-Pacific region and its relationship with the Asian monsoon rainfall and the climatological characteristics of tropical cyclones over the western North Pacific, and analyzed impacts of the Tibetan Plateau (TP) heating and Pacific sea surface temperature (SST) on the teleconnection. The Asian-Pacific oscillation (APO) is defined as a zonal seesaw of the tropospheric temperature in the midlatitudes of the Asian-Pacific region. When the troposphere is cooling in the midlatitudes of the Asian continent, it is warming in the midlatitudes of the central and eastern North Pacific; and vice versa. The APO also appears in the stratosphere, but with a reversed phase. Used as an index of the thermal contrast between Asia and the North Pacific, it provides a new way to explore interactions between the Asian and Pacific atmospheric circulations. The APO index exhibits the interannual and interdecadal variability. It shows a downward trend during 1958-2001, indicating a weakening of the thermal contrast, and shows a 5.5-yr oscillation period. The formation of the APO is associated with the zonal vertical circulation caused by a difference in the solar radiative heating between the Asian continent and the North Pacific. The numerical simulations further reveal that the summer TP heating enhances the local tropospheric temperature and upward motion, and then strengthens downward motion and decreases the tropospheric temperature over the central and eastern North Pacific. This leads to the formation of the APO. The Pacific decadal oscillation and El Nino/La Nina over the tropical eastern Pacific do not exert strong influences on the APO. When there is an anomaly in the summer APO, the South Asian high, the westerly jet over Eurasia, the tropical easterly jet over South Asia, and the subtropical high over the North Pacific change significantly, with anomalous Asian monsoon rainfall and tropical cyclone activities over the western North Pacific. The summer cooling along the upper and middle reaches of the Yangtze River in the past 40 more years is related to the APO, which is possibly. a regional response to the decadal variability of the global atmospheric circulation. An anomalous signal of the APO may propagate to the Arctic and Antarctic. Moreover, the APO also appears in other seasons.
    Zhao P., Z. H. Cao, and J. M. Chen, 2010: A summer teleconnection pattern over the extratropical Northern Hemisphere and associated mechanisms. Climate Dyn., 35, 523- 534.10.1007/s00382-009-0699-057de72b3306c7a72761186de888d2ca2http%3A%2F%2Flink.springer.com%2F10.1007%2Fs00382-009-0699-0http://onlinelibrary.wiley.com/resolve/reference/XREF?id=10.1007/s00382-009-0699-0Using monthly data from the European Center for Medium-Range Weather Forecast 40-year reanalysis (ERA-40), we have revealed a teleconnection pattern over the extratropical Northern Hemisphere through the empirical orthogonal function analysis of summer upper-tropospheric eddy temperature. When temperature is higher (lower) over the Eastern Hemisphere (EH), it is lower (higher) over the Western Hemisphere (WH). The teleconnection manifested by this out-of-phase relationship is referred to as the Asian-Pacific oscillation (APO). The values of an index measuring the teleconnection are high before 1976 and low afterwards, showing a downward trend of the stationary wave at a rate of 4% per year during 1958-2001. The index also exhibits apparent interannual variations. When the APO index is high, anomalous upper-tropospheric highs (lows) appear over EH (WH). The formation of APO is likely associated with a zonal vertical circulation in the troposphere. Unforced control runs of both the NCAR Community Atmospheric Model version 3 and the Community Climate System Model version 3 capture the major characteristics of the teleconnection pattern and its associated vertical structure. The APO variability is closely associated with sea surface temperature (SST) in the Pacific, with a significantly positive correlation between APO and SST in the extratropical North Pacific and a significantly negative correlation in the tropical eastern Pacific. Sensitivity experiments show that the anomalies of SST over these two regions influence the APO intensity, but their effects are opposite to each other. Compared to the observation, the positive and negative anomalous centers of the extratropical tropospheric temperature triggered by the SST anomalies have a smaller spatial scale.
    Zhao P., S. Yang, H. J. Wang, and Q. Zhang, 2011: Interdecadal relationships between the Asian-Pacific Oscillation and summer climate anomalies over Asia, North Pacific, and North America during a recent 100 years. J.Climate, 24, 4793- 4799.10.1175/JCLI-D-11-00054.11eebc06ca756a167598fd9cc62543736http%3A%2F%2Fadsabs.harvard.edu%2Fabs%2F2011JCli...24.4793Zhttp://adsabs.harvard.edu/abs/2011JCli...24.4793ZSummertime relationships between the Asian-Pacific Oscillation (APO) and climate anomalies over Asia, the North Pacific, and North America are examined on an interdecadal time scale. The values of APO were low from the 1880s to the mid-1910s and high from the 1920s to the 1940s. When the APO was higher, tropospheric temperatures were higher over Asia and lower over the Pacific and North America. From the low-APO decades to the high-APO decades, both upper-tropospheric highs and lower-tropospheric low pressure systems strengthened over South Asia and weakened over North America. As a result, anomalous southerly-southwesterly flow prevailed over the Asian monsoon region, meaning stronger moisture transport over Asia. On the contrary, the weakened upper-tropospheric high and lower-tropospheric low over North America caused anomalous sinking motion over the region. As a result, rainfall generally enhanced over the Asian monsoon regions and decreased over North America.
    Zhao P., B. Wang, and X. J. Zhou, 2012: Boreal summer continental monsoon rainfall and hydroclimate anomalies associated with the Asian-Pacific Oscillation. Climate Dyn.,39, 1197-1207, doi: 10.1007/s00382-012-1348-6.10.1007/s00382-012-1348-661f092f3c33559e873971a1a5dce7fa9http%3A%2F%2Fonlinelibrary.wiley.com%2Fresolve%2Freference%2FXREF%3Fid%3D10.1007%2Fs00382-012-1348-6http://onlinelibrary.wiley.com/resolve/reference/XREF?id=10.1007/s00382-012-1348-6With the twentieth century analysis data (1901-2002) for atmospheric circulation, precipitation, Palmer drought severity index, and sea surface temperature (SST), we show that the Asian-Pacific Oscillation (APO) during boreal summer is a major mode of the earth climate variation linking to global atmospheric circulation and hydroclimate anomalies, especially the Northern Hemisphere (NH) summer land monsoon. Associated with a positive APO phase are the warm troposphere over the Eurasian land and the relatively cool troposphere over the North Pacific, the North Atlantic, and the Indian Ocean. Such an amplified land-ocean thermal contrast between the Eurasian land and its adjacent oceans signifies a stronger than normal NH summer monsoon, with the strengthened southerly or southwesterly monsoon prevailing over tropical Africa, South Asia, and East Asia. A positive APO implies an enhanced summer monsoon rainfall over all major NH land monsoon regions: West Africa, South Asia, East Asia, and Mexico. Thus, APO is a sensible measure of the NH land monsoon rainfall intensity. Meanwhile, reduced precipitation appears over the arid and semiarid regions of northern Africa, the Middle East, and West Asia, manifesting the monsoon-desert coupling. On the other hand, surrounded by the cool troposphere over the North Pacific and North Atlantic, the extratropical North America has weakened low-level continental low and upper-level ridge, hence a deficient summer rainfall. Corresponding to a high APO index, the African and South Asian monsoon regions are wet and cool, the East Asian monsoon region is wet and hot, and the extratropical North America is dry and hot. Wet and dry climates correspond to wet and dry soil conditions, respectively. The APO is also associated with significant variations of SST in the entire Pacific and the extratropical North Atlantic during boreal summer, which resembles the Interdecadal Pacific Oscillation in SST. Of note is that the Pacific SST anomalies are not present throughout the year, rather, mainly occur in late spring, peak at late summer, and are nearly absent during boreal winter. The season-dependent APO-SST relationship and the origin of the APO remain elusive.
    Zhou B. T., P. Zhao, 2010: Influence of the Asian-Pacific oscillation on spring precipitation over central eastern China. Adv. Atmos. Sci.,27, 575-582, doi: 10.1007/s00376-009-9058-7.10.1007/s00376-009-9058-70fc407633eae505a3cbefff90857eecdhttp%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs00376-009-9058-7http://d.wanfangdata.com.cn/Periodical/dqkxjz-e201003010The linkage between the Asian-Pacific oscillation (APO) and the precipitation over central eastern China in spring is preliminarily addressed by use of the observed data. Results show that they correlate very well, with the positive (negative) phase of APO tending to increase (decrease) the precipitation over central eastern China. Such a relationship can be explained by the atmospheric circulation changes over Asia and the North Pacific in association with the anomalous APO. A positive phase of APO, characterized by a positive anomaly over Asia and a negative anomaly over the North Pacific in the upper-tropospheric temperature, corresponds to decreased low-level geopotential height (H) and increased high-level H over Asia, and these effects are concurrent with increased low-level H and decreased high-level H over the North Pacific. Meanwhile, an anticyclonic circulation anomaly in the upper troposphere and a cyclonic circulation anomaly in the lower troposphere are introduced in East Asia, and the low-level southerly wind is strengthened over central eastern China. These changes provide advantageous conditions for enhanced precipitation over central eastern China. The situation is reversed in the negative phase of APO, leading to reduced precipitation in this region.
    Zhou B. T., L. Zhang, 2012: A simulation of the upper-tropospheric temperature pattern in BCC_CSM1.1. Atmospheric and Oceanic Science Letters, 5, 478- 482.229ce783bf52f2e4dff997f1ebe8892dhttp%3A%2F%2Fd.wanfangdata.com.cn%2FPeriodical_dqhhykxkb201206007.aspxhttp://d.wanfangdata.com.cn/Periodical_dqhhykxkb201206007.aspx
    Zhou B. T., X. Cui, and P. Zhao, 2008: Relationship between the Asian-Pacific oscillation and the tropical cyclone frequency in the western North Pacific. Science in China D: Earth Sciences, 51, 380- 385.0c9834403f1d27efb471b557da2e88e4http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs11430-008-0014-7http://xueshu.baidu.com/s?wd=paperuri%3A%2893afaade2558aced423032c2ab268931%29&filter=sc_long_sign&tn=SE_xueshusource_2kduw22v&sc_vurl=http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-JDXG200803007.htm&ie=utf-8&sc_us=5834367700748653819
    Zhou B. T., P. Zhao, and X. Cui, 2010: Linkage between the Asian-Pacific Oscillation and the sea surface temperature in the North Pacific. Chinese Science Bulletin,55, 1193-1198, doi: 10.1007/s11434-009-0386-x.10.1007/s11434-009-0386-xfebaf4b9701acbf2bb6c61158b3fef97http%3A%2F%2Flink.springer.com%2Farticle%2F10.1007%2Fs11434-009-0386-xhttp://www.cnki.com.cn/Article/CJFDTotal-JXTW201012012.htmThe linkage between the Asian-Pacific Oscillation (APO) and the sea surface temperature (SST) in the North Pacific during the summertime (June-August) is preliminarily investigated through an analysis of observed data.It is found that APO is significantly and positively correlated to the North Pacific SST,with the correlation coefficient being 0.58 on the interannual timescale during the period 1954-2003,which suggests that a strong (weak) APO corresponds to high (low) SST in the North Pacific.Their in-phase relationship is well supported by the dynamic and thermal conditions in association with the APO anomaly.When APO is in the positive phase,the East Asian westerly jet in the upper troposphere is weakened,and the anomalous anticyclonic circulation prevails in the low-troposphere over the North Pacific.Besides,the negative anomaly of the sensible and latent heat fluxes is predominated in the North Pacific,indicating ocean gets heat flux from the atmosphere.Meanwhile,warm water advection northward is strengthened in the North Pacific.All of these provide beneficial conditions to warm the North Pacific SST,and thus the SST is increased in this region,and vice versa.
    Zhou X. J., P. Zhao, and G. Liu, 2009: Asian-Pacific Oscillation index and variation of East Asian summer monsoon over the past millennium. Chinese Science Bulletin,54, 3768-3771, doi: 10.1007/s11434-009-0619-z.10.1007/s11434-009-0619-z98705d8e22ca4f8e3f192a0d9b1d23e5http%3A%2F%2Fwww.cnki.com.cn%2FArticle%2FCJFDTotal-JXTW200920028.htmhttp://www.cnki.com.cn/Article/CJFDTotal-JXTW200920028.htmTo study the long-term variation of the East Asian summer monsoon(EASM),the Asian-Pacific Oscillation index(IAPO),representing a zonal thermal contrast between Asia and the North Pacific,is recon-structed over the past millennium.During the Little Ice Age(LIA),the variability of the reconstructed IAPO is closely linked to dry-wet anomalies in eastern China on the centennial scale.This correlation pattern is consistent with the observation during the current period,which suggests that the reconstructed IAPO may generally represent the centennialscale variation of the EASM and rainfall anomalies over eastern China during the LIA.
    Zou, Y, P. Zhao, 2011: A study of the relationship between the Asian-Pacific oscillation and tropical cyclone activities over the coastal waters of China during autumn. Acta Meteorologica Sinica, 69( 4), 601- 609. (in Chinese with English abstract)10.11676/qxxb2011.05233f5c79b-c5c4-473a-885e-42ea95f40a355584201147Using the datasets from the JTWC optimal typhoon tracks and the NCEP/NCAR reanalysis data, we investigate the interannual variability of the autumn (September and October) Asian Pacific Oscillation (APO) and its relationships with the atmospheric circulation over the Asian Pacific region and tropical cyclone (TC) activities over the western North Pacific and the coastal waters of China. The results show that the interannual variability of the APO is closely related to the TC activity over the western North Pacific and the coastal waters of China during autumn. Corresponding to stronger (weaker) APO, the TC often appears in a more westward (eastward) position and there is a more (less) TC number in the coastal waters of China. The APO may affect the vertical shear of the zonal wind between high and low levels of the troposphere, the lower tropospheric convergence, and the mid tropospheric steering current over the coastal waters of China and thus the TC activity over the western North Pacific and the coastal waters of China. When the APO is stronger (weaker), the long wave trough over the extratropics of East Asia is weaker (stronger) and the East Asian winter monsoon is weaker (stronger), accompanying the invasion of weaker (stronger) cold masses into the tropical western North Pacific and the coastal waters of China, favoring (not favoring) the occurrence and development of the TC over these seas. Moreover, corresponding to stronger APO, the subtropical ridge over the western North Pacific is located in a more westward position and the easterly current south of the ridge is stronger, which favors the TC to move westwards along the stronger easterly steering current or most likely causes the TC to turn its moving direction in a more westward position. Corresponding to weaker APO, the ridge is weaker and located in a more eastward position, the easterly steering current is weaker, both of which does not favor the TC to move westwards or favors the TC to change the direction in a more eastern position.
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Manuscript received: 04 February 2016
Manuscript revised: 30 May 2016
Manuscript accepted: 23 June 2016
通讯作者: 陈斌, bchen63@163.com
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    沈阳化工大学材料科学与工程学院 沈阳 110142

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Characteristics of the Asian-Pacific Oscillation in Boreal Summer Simulated by BCC_CSM with Different Horizontal Resolutions

  • 1. School of Atmospheric Sciences, Nanjing University, Nanjing 210023
  • 2. PLA 61936 Troop, Haikou 571100

Abstract: The summer Asian-Pacific Oscillation (APO) is a major teleconnection pattern that reflects the zonal thermal contrast between East Asia and the North Pacific in the upper troposphere. The performance of Beijing Climate Center Climate System Models (BCC_CSMs) with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), in reproducing APO interannual variability, APO-related precipitation anomalies, and associated atmospheric circulation anomalies, is evaluated. The results show that BCC_CSM1.1(m) can successfully capture the interannual variability of the summer APO index. It is also more capable in reproducing the APO's spatial pattern, compared to BCC_CSM1.1, due to its higher horizontal resolution. Associated with a positive APO index, the northward-shifted and intensified South Asian high, strengthened extratropical westerly jet, and tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over North Africa and the Indian Ocean in the lower troposphere, are realistically represented by BCC_CSM1.1(m), leading to an improvement in reproducing the increased precipitation over tropical North Africa, South Asia, and East Asia, as well as the decreased precipitation over subtropical North Africa, Japan, and North America. In contrast, these features are less consistent with observations when simulated by BCC_CSM1.1. Regression analysis further indicates that surface temperature anomalies over the North Pacific and the southern and western flanks of the Tibetan Plateau are reasonably reproduced by BCC_CSM1.1(m), which contributes to the substantial improvement in the simulation of the characteristics of summer APO compared to that of BCC_CSM1.1.

1. Introduction
  • A large-scale extratropical teleconnection pattern in boreal summer, named the Asian-Pacific Oscillation (APO), has recently been identified. It is characterized by a zonal seesaw of midlatitudinal upper-tropospheric temperature between Asia and the Pacific. The high (low) tropospheric eddy temperature over Eurasia is usually accompanied by low (high) eddy temperature over the North Pacific (Zhao et al., 2007). The APO exhibits notable interannual and interdecadal variability, and appears in the non-summer seasons as well (Zhao et al., 2008).

    Previous studies have revealed that the APO is closely linked to the Asian summer monsoon and monsoonal precipitation (Zhao et al., 2007). Corresponding to a higher APO index, low-level anomalous southerly wind prevails over the midlatitudes of East Asia, and anomalous westerly wind occupies South Asia and the South China Sea region, which indicates a strengthened Asian summer monsoon. As a result, precipitation increases over South Asia and the northern and southern sides of the Yangtze River, but decreases around the Yangtze River and the Philippines. Therefore, the APO index can be used to indicate the variability of the Asian monsoon and rainfall (Zhao et al., 2007, 2008). From a decade with low APO to one with high APO, rainfall generally enhances over the Asian monsoon region and decreases over North America (Zhao et al., 2011). In addition, the relationship between the APO index and Asian monsoonal precipitation has also been investigated on the interdecadal time scale (Zhou et al., 2009; Liu et al., 2011).

    The APO is positively correlated with tropical cyclone frequency in the western North Pacific. When the APO is above (below) normal in summer, more (fewer) tropical cyclones tend to form in the western North Pacific (Zhou et al., 2008; Zou and Zhao, 2011). The APO index has a significant negative correlation with the western Pacific subtropical high (Huang et al., 2013). Associated with the variation of the summer APO, significant anomalous circulation signals can even be observed over the Asia-Pacific-America sector (Zhou and Zhao, 2010). Furthermore, the APO's variability is closely linked with SST in the Pacific, with a significant positive (negative) correlation between the APO index and SST over the extratropical North Pacific (tropical eastern Pacific) on the interannual timescale (Zhou et al., 2010; Zhao et al., 2010).

    Therefore, exploring the physical mechanism responsible for the formation and maintenance of the APO is necessary for predicting the variation in the climate of the Northern Hemisphere. (Zhao et al., 2008) suggested that the formation of the APO is related to the difference in solar radiation between the Asian continent and the North Pacific. The thermal effect of the Tibetan Plateau (TP) intensifies the temperature of the local troposphere and decreases the tropospheric temperature over the North Pacific through zonal and vertical circulations, leading to the formation of the APO. A number of attempts have been made to understand the mechanism responsible for the formation of the APO and its associated climate anomalies by using global climate models. As a result, it has been proven that the major characteristics and dynamical structures of the APO in summer can be captured by some coupled climate system models (CSMs; Zhao et al., 2010; Man and Zhou, 2011; Chen et al., 2013b), and the relationship between the APO and Pacific SST can be successfully reflected in CCSM3 simulations (Nan et al., 2009; Zhao et al., 2010). Recently, (Huang et al., 2013) assessed the predictability of the summer APO index using the European Multi-model Ensemble System and found that these models can predict the interannual variability of the summer APO well. (Chen et al., 2013a) further indicated that the summer APO and its associated climate anomalies can be predicted by NCEP CFSv2 by up to 5 months in advance.

    The above studies illustrate the importance of evaluating the capability of various climate models in simulating the characteristics of the APO when attempting to predict circulation anomalies associated with it. The capabilities of CSMs to realistically reproduce the current state of regional and global climate is vitally important for reliable projections of climate change in the future (Kidston and Gerber, 2010). However, few efforts have been made to investigate the impact of model resolution on the simulation of the APO. Since the complexity of the topography and underlying surface state can be better described in climate models with relatively higher resolution, high-resolution model simulations provide us with an opportunity to analyze the subsequent influence of such high horizontal resolutions on the simulation of the APO.

    The present study evaluates the ability of two versions of BCC-CSM, with different resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), in simulating the variability of the large-scale APO pattern and associated atmospheric circulation anomalies. The aim is to address whether the observed characteristics of the summer APO can be reproduced in the two models, and, if so, what the impact is of the higher horizontal resolution in BCC_CSM1.1(m) on the simulation of the characteristics of the summer APO. Reasons for any identified improvements in the simulation of the summer APO by BCC_CSM1.1(m) will then be identified.

    Following this introduction, the models, datasets and method applied in the study are described in section 2. Section 3 presents the observed characteristics of the APO and associated precipitation, as well as the model results. The possible reasons for any identified improvements in the simulation of the APO by BCC_CSM1.1(m) are discussed in section 4. Finally, conclusions and a discussion are provided in section 5.

2. Models, data and methods
  • BCC_CSM is a coupled climate system model, including atmosphere, ocean, land surface and sea ice components. There are two versions of the model system with different horizontal resolutions, i.e., BCC_CMS1.1 and BCC_ CSM1.1(m). Both models have been involved in CMIP5 (Jiang et al., 2012). A comprehensive atmospheric general circulation model (BCC_AGCM2.1) derived from NCAR CAM3 and modified by (Wu et al., 2008), version 4 of the GFDL's MOM (Griffies et al., 2004), GFDL's Sea Ice Simulator (Winton, 2000), and version 1.0 of the BCC's Atmosphere and Vegetation Interaction Model (Ji et al., 2008), are interactively coupled in BCC_CSM1.1 using version 5 of the NCAR's coupler (Wu, 2012). No flux adjustment is implemented in BCC_CSM1.1. The OGCM, MOM, has 40 vertical layers and the nominal horizontal resolution is 1° × 1°, with equatorial refinement to 0.33° between 30°S and 30°N. The horizontal resolution and the sea-land distribution in the sea ice model are the same as that in MOM. BCC_CSM1.1(m) is an advanced version of BCC_CSM1.1, with a moderate atmospheric resolution. Compared to BCC_AGCM2.1, which is used in BCC_CSM1.1 and runs at a T42 spectral resolution (approximately 2.8°× 2.8°), the atmospheric component of BCC_CSM1.1(m) is BCC_AGCM2.2, which runs at a T106 horizontal resolution (approximately 1.125°× 1.125°). Both models use a terrain-following vertical hybrid sigma-pressure coordinate, with 26 levels and a rigid lid at 2.914 hPa. The dynamical framework and physical processes are the same in the two models and are fully introduced in Wu et al. (2008, 2010). A fair number of studies have evaluated the performance of the two models in climate simulation and the projection of future climate change, especially regarding the simulation of precipitation and temperature fields (Gao et al., 2012; Gao et al., 2013; Xin et al., 2013). Nevertheless, many previous studies have focused mainly on changes in surface air temperature. Few researchers have paid attention to the models' abilities in reproducing the variation in upper-tropospheric eddy temperature (Zhou and Zhang, 2012).

    Monthly model outputs of the CMIP5 historical simulation experiments of the two models for 31 summers (June-July-August) from 1979 to 2009 are used in this study. The external forcing of the historical simulations changes with time, including mixed greenhouse gases (CO2, N2O, CH4, CFC11, and CFC12), aerosols, ozone, volcanoes and solar radiation. All the forcing data are provided by CMIP5, except for volcanoes. The temporal resolution of CO2 emissions and solar radiation is 1 year, while the time interval of the aerosol data is 10 years. For the purpose of comparison with other model results, the present study uses the observational data from NCEP-DOE Reanalysis-2 (Kanamitsu et al., 2002) and CMAP (Xie and Arkin, 1997) to validate the simulations and discuss the biases. The data after 1979 are chosen because satellite observations become available since then and the reanalysis data are more reliable and homogeneous than during the pre-satellite period (Dell'Aquila et al., 2005).

    EOF analysis is applied to the eddy temperature to identify the APO teleconnection pattern over the Northern Hemisphere. A weighting by latitude is applied to the EOF results. Regression and correlation analyses are conducted to explore relationships between pairs of variables, while the statistical significance of correlation coefficients, regression values and long-term variation trends are assessed using the Student's t-test. Besides, in order to compare the simulations with observations, the model results are interpolated to the observational grids using the bilinear interpolation method.

3. APO simulation results
  • Since the APO is defined by the upper-tropospheric (500-200 hPa) eddy temperature (T'; Zhao et al., 2007), which is obtained by removing the zonal mean temperature \((\overline{T})\) from the total air temperature (T), i.e., \(T'=T-\overline{T}\), we first investigate the distributions of observed and simulated summer mean total air temperature and eddy temperature over the upper troposphere (Fig. 1). The observed T gradually decreases from the south to the north, and the main temperature band is oriented from west to east, with a maximum temperature center greater than -20°C located over the southern flank of the TP (Fig. 1a). Compared with the observation, both models capture the distributional feature of T, which decreases from low to high latitudes. However, BCC_CSM1.1 systematically underestimates the intensity of T. The range of the simulated main temperature band is much smaller than the observed result, and the temperature center is mainly located in the TP region (Fig. 1b). Compared with BCC_CSM1.1, BCC_CSM1.1(m) realistically simulates not only the temperature band that is oriented from west to east, but also the maximum temperature center. This result indicates a remarkable improvement in reproducing the upper-tropospheric total air temperature by BCC_CSM1.1(m), due to its higher horizontal resolution (Fig. 1c).

    The features of observed and simulated eddy temperature are further examined in Fig. 1. A prominent out-of-phase variational pattern of eddy temperature exists in the midlatitudes between Asia and the Pacific in boreal summer. Positive values occupy the lower and middle latitudes of Asia, with a high temperature center of 4°C located over the TP region. Negative values appear over the central-eastern Pacific, with a minimum value of -3°C (Fig. 1a). Additionally, another center of negative eddy temperature occurs over the Atlantic region. These features are realistically captured by the two models, except that the value of the positive eddy temperature center simulated by BCC_CSM1.1 is about 6°C, which is approximately 2°C higher than observed; plus, the simulated negative eddy temperature center is about -2°C, which is lower than observed by approximately 1°C. Compared with the simulation of BCC_CSM1.1, the simulated positive center over the TP region by BCC_CSM_1.1(m) is closer to the observation, but the simulated negative center over the central-eastern Pacific is located further eastward than observed.

    Figure 1.  The climatology of the summer mean upper-tropospheric (500-200 hPa) $T$ (color-shaded; units: $^\circ$C) and $T'$ (contours; units: $^\circ$C) during 1979-2009 for (a) NCEP, (b) BCC_CSM1.1, and (c) BCC_CSM1.1(m), respectively. The thick black line denotes the orographic isocline of the main body of the TP. And the green boxes represent the key regions selected to construct the APO index.

    Figure 2.  Differences in climatological $T$ (color-shaded; units: $^\circ$C) and $T'$ (contours; units: $^\circ$C) between (a) BCC_CSM1.1 and NCEP, (b) BCC_CSM1.1(m) and NCEP, and (c) BCC_CSM1.1(m) and BCC_CSM1.1. The thick green line indicates the orographic isocline of the main body of the TP. The thick green line indicates the topographic contour of 3000 m.

    The difference between the model results and the reanalysis data shows that the climatological total air temperature is notably underestimated throughout the Northern Hemisphere in the results of BCC_CSM1.1. The simulated warm biases of eddy temperature appear along the northern flank of the TP, while cold biases mainly occur over South America (Fig. 2a). Compared with that in BCC_CSM1.1, the intensity of simulated total air temperature is effectively enhanced in BCC_CSM1.1(m) (Fig. 2c), especially over midlatitude areas. Warm biases of the simulated total air temperature in BCC_CSM1.1(m) mainly occur over the central-western Pacific, while cold biases mainly appear over South America. The distribution of the simulated eddy temperature bias in BCC_CSM_1.1(m) is similar to that of the total air temperature. The positive anomaly center is situated over the central Pacific, with a maximum value exceeding 2.5°C; and the negative anomaly center lies over South America, with a minimum value lower than -2°C (Fig. 2b).

    The Taylor diagram between outputs of the two models and observations shown in Fig. 3 further illustrates that, although BCC_CSM1.1(m) is better able to reproduce the upper-tropospheric total air temperature than BCC_CSM1.1, the eddy temperature simulated by BCC_CSM1.1 is overall more agreeable with the observation, not only in the Northern Hemisphere but also in regions over East Asia and the North Pacific. Meanwhile, it is noteworthy that both models exhibit better capacity for simulating the eddy temperature over East Asia than over the North Pacific, which is contrary to the results of previous model studies (Huang et al., 2013; Chen et al., 2013a). Possible reasons will be discussed in section 5.

    Figure 3.  Taylor diagram comparing the spatial statistics between the simulations of the two BCC models and observations for the summer mean $T'$ over the Northern hemisphere (NH), East Asia (EA) and the North Pacific (NP) regions during 1979-2009. "REF" denotes the NCEP-DOE reanalysis; the azimuth angle indicates the spatial correlation coefficient between observations and model outputs; the radial distance indicates the standard deviation between observations and model outputs; and the distance from "REF" represents the centralized RMSE.

  • Because the APO is identified through the contrast in upper-tropospheric eddy temperature between Asia and the North Pacific, one may speculate that the reasonable simulation of climatological eddy temperature in BCC_CSM1.1 should lead to a better simulation of the characteristics of the APO. To examine this assertion, the simulated spatial pattern and interannual variability of the APO are examined in this subsection.

    Following Zhao et al. (2010, 2012), an EOF analysis of the observed and simulated eddy temperature is performed for the period 1979-2009 to reveal the teleconnection pattern over the Northern Hemisphere in summer. The regression map of the vertically integrated eddy temperature from observations and simulations between 500 and 200 hPa with respect to the normalized PC1 is shown in Fig. 4. The first EOF mode of observations accounts for 20.3% of the total variance, manifesting a prominent out-of-phase relationship of temperature between Africa-Eurasia and the North Pacific region. Two positive centers are located over North Africa and East Asia, respectively, with the maximum value exceeding 0.04, while a negative center is situated over the North Pacific, with a minimum value below -0.04 (Fig. 4a). Compared with observations, it is found that BCC_CSM1.1 can reproduce the negative center over the North Pacific, but fails to reproduce the two positive centers over North Africa and East Asia. Instead, it produces a false positive center over South America (Fig. 4b). The EOF1 pattern simulated by BCC_CSM1.1(m) agrees well with the observation, and the out-of-phase relationship of temperature between Africa-Eurasia and the North Pacific region in the Northern Hemisphere is reflected well. The spatial correlation coefficients between observations and outputs of BCC_CSM1.1 and BCC_CSM1.1(m) are 0.40 and 0.77, respectively, indicating that the simulated spatial pattern of the APO is greatly improved in BCC_CSM1.1(m).

    Figure 4.  Observed and simulated regression maps of the vertically integrated summer mean eddy temperature between 500 and 200 hPa with respect to the normalized PC1 over the Northern Hemisphere during 1979-2009, based on (a) NCEP-DOE, (b) BCC_CSM1.1, and (c) BCC_CSM1.1(m).

    Figure 5.  Normalized summer (a-c) AI, (d-f) PI, and (g-i) APO indices during 1979-2009, based on NCEP-DOE (upper panels), BCC_CSM1.1 (middle panels), and BCC_CSM1.1(m) (lower panels). The detrended correlation coefficient (CC-I) between observations and model results is marked in the top-right corner of each panel, and the solid line indicates the linear trend of the index.

    To investigate the interannual variability of the APO, we define the APO index as the arithmetic difference between the Asian tropospheric eddy temperature index (AI) and the North Pacific tropospheric eddy temperature index (PI), where the AI and PI are computed by the regionally averaged upper-tropospheric (500-200 hPa) T' over (15°-45°N, 70°-110°E) and (15°-45°N, 170°-110°W), respectively (Zhao et al., 2007; Huang et al., 2013). The above selected key regions for AI and PI are marked in Fig. 1. The capability of the models in reproducing the APO's interannual variability is measured by the detrended correlation coefficients (CC-I) between the observed and simulated AI, PI and APO indices. Figure 5g shows that the observed APO index exhibits significant interannual variability in the last 30 years, with a linear descending trend of -0.041°C yr-1 (exceeding the 95% confidence level), which is consistent with previous findings (Huang et al., 2013). The variational trend of the AI is consistent with that of the APO index (Fig. 5g), while the PI displays a weak ascending trend that is not significant (Fig. 5d). Therefore, the weakening trend of the APO index can be mainly attributed to the decreasing trend of the upper-tropospheric eddy temperature over land, implying an enhanced thermal contrast between Asia and the North Pacific in recent decades. Since the APO index has a distinct linear trend, all the linear trends of the indices are removed to investigate the interannual variability of the APO.

    Figure 6.  Regression maps of summer precipitation (units: mm d$^-1$) with reference to the normalized APO index for (a) CMAP, (b) BCC_CSM1.1, and (c) BCC_CSM1.1(m). The blue (brown) color indicates the negative (positive) precipitation anomalies. Areas of light (dark) shading are values at/above the 90% (95%) confidence level. The thick green line indicates the topographic contour of 3000 m.

    The CC-I values of the AI, PI and APO index are -0.17, 0.02 and -0.09, respectively, in BCC_CSM1.1, which are very low and not significant. Compared with the observation, BCC_CSM1.1 fails to reproduce the interannual variability of the APO index. Further analysis indicates that this failure can be attributed to the unreasonable simulation of the AI and PI, suggesting that BCC_CSM1.1 performs poorly in reproducing the variation of the APO. Although a linear trend can be detected in BCC_CSM1.1, the result is opposite to that derived from the observation (Fig. 5; middle panels). Figure 5 (lower panels) displays the evolution of the summer AI, PI and APO index from 1979 to 2009 in BCC_CSM1.1(m). The CC-I values of the AI, PI and APO index are 0.35, 0.33 and 0.40, respectively, and all exceed the 95% confidence level. These results demonstrate that BCC_CSM1.1(m) is more capable than BCC_CSM1.1 when it comes to reproducing the interannual variability of the APO. However, the linear decreasing trend of the APO index is not significant in the results of BCC_CSM1.1(m), suggesting that it lacks skill in reproducing the long-term variability of the APO pattern. Note that this phenomenon is also found in some other climate system models (Huang et al., 2013).

  • Figure 6 displays regression maps of precipitation against the APO index from the model outputs and observations. The observed result (Fig. 6a) shows that, corresponding to the APO's positive phase, positive precipitation anomalies mainly occur over the summer monsoon regions of the Northern Hemisphere, including Mexico, East Asia, South Asia and West Africa. Positive precipitation anomalies also occur in the lower-latitude region of the central Pacific. On the contrary, negative precipitation anomalies appear over the western and northern parts of the monsoon regions, such as extratropical North America, central-western Asia, the Middle East, and North Africa, where the monsoon-desert coupling phenomenon is distinct (Zhao et al., 2007). The above results derived from observations are consistent with previous findings (Zhao et al., 2012).

    Figure 6c presents a regression map of summer precipitation with respect to the APO index in BCC_CSM1.1(m). In general, BCC_CSM1.1(m) captures the characteristics of the rainfall distribution associated with the APO index well, such as the positive precipitation anomaly band extending from 30°E to 150°W, as well as the negative precipitation anomaly band along 40°N that extends from western Africa to the extratropical Pacific and North America. Positive precipitation anomalies are mainly found over the tropics of South America, Mexico's monsoon region, the subtropics of central-western Pacific, the Indochina Peninsula, India, and tropical North Africa. However, the simulated positive precipitation anomalies in Mexico and tropical North Africa are less significant than observed. The simulated relationship between precipitation and the APO index in BCC_CSM1.1(m) agrees qualitatively with the observation.

    Figure 7.  Regression maps of (a-c) 200 hPa (left panels) and (d-f) 850 hPa (right panels) winds (units: m s$^-1$) against the normalized APO index in summer for (a, d) NCEP-DOE reanalysis, (b, e) BCC_CSM1.1, and (c, f) BCC_CSM1.1(m), respectively. Shaded areas are values exceeding the 95% confidence level; the prevailing wind is indicated by the red arrow, and the "C" ("A") denotes the anomalous cyclone (anticyclone) center. The thick dashed line denotes the orographic isoline of 1500 m in right panels.

    Compared with the observation, BCC_CSM1.1 fails to reproduce the relationship between the simulated precipitation anomalies and the APO index (Fig. 6b). The positive precipitation anomalies over the summer monsoon regions of the Northern Hemisphere are largely underestimated, especially over India, the Indochina Peninsula, and Mexico. The simulated negative rainfall anomaly band along 40°N is also not consistent with observations, especially over central-western Asia and North America. Moreover, the simulated precipitation anomalies over the Pacific are much less significant than observed, indicating a lower ability of BCC_CSM1.1 in reproducing the APO-related precipitation in the Northern Hemisphere, as compared with BCC_CSM1.1(m).

4. Possible reasons for the simulation improvement
  • Previous studies have revealed that the APO's variability is closely linked to large-scale atmospheric circulation anomalies that directly influence APO-related precipitation, such as the extratropical westerly jet over Eurasia, the western Pacific subtropical high, the South Asian high, and the Asian summer monsoon (Zhao et al., 2010, 2012; Huang et al., 2013). Thus, we further examine the APO-related atmospheric circulation anomalies simulated by the models.

    Figure 7 separately shows regression maps of winds at 200 hPa and 850 hPa with respect to the summer APO index from 1979 to 2009, based on observations and the simulations of the two models. As shown in Fig. 7a, a positive APO index is associated with the large-scale anomalous anticyclonic circulation that covers the midlatitude region from 60°E to 150°W at 200 hPa, with three centers near western Asia, northeastern Asia and the central-northern Pacific, respectively. This anticyclonic anomaly actually corresponds to the northward-extended and intensified South Asian high. Meanwhile, anomalous easterly winds prevail from the central-eastern Pacific to Eurasian regions, and westerly anomalies appear along the northern flank of the aforementioned anomalous anticyclone centers over Eurasia and the North Pacific, indicating an enhanced extratropical upper-level westerly jet stream and a strengthened summer monsoon over South Asia and the South China Sea. The observed anomalous atmospheric circulations in the upper troposphere related to the APO are consistent with previous findings (Zhao et al., 2007; Huang et al., 2013). Figure 7b shows that BCC_CSM1.1 fails to reproduce the anomalous anticyclone center over Eurasia, and the simulated easterly wind anomalies are much weaker and less significant than observed, especially over the North Pacific. Compared with BCC_CSM1.1, BCC_CSM1.1(m) generally reproduces the circulation anomalies associated with the APO; namely, the strengthened and northward-extended South Asian high, the intensified extratropical westerly jet, and the tropical easterly jet. This result suggests that the variation of the Asian summer monsoon associated with a positive APO index can be reasonably reproduced in BCC_CSM1.1(m). However, the simulated anomalous anticyclone center over North Pacific is located further eastward than observed (Fig. 7c). Figure 7d further shows that when the APO index is above normal, two anomalous anticyclone centers emerge in the central-northern Pacific and Japan, respectively, at 850 hPa. Easterly wind anomalies extend from the central Pacific to southern Japan, and anomalous southerly winds are observed over northeastern China, indicating that the East Asian summer monsoon intensifies. Meanwhile, westerly wind anomalies prevail from the western Indian Ocean to the South China Sea, corresponding to the strengthened southwesterly monsoonal flow over these areas when the APO index is positive. The above atmospheric circulation anomalies in the lower troposphere are similar to results published in earlier studies (Zhao et al., 2007, 2012). Compared with the observation, the anomalous anticyclone center situated over Japan is not reproduced in the simulation of BCC_CSM1.1, and the simulated easterly wind anomalies over the central Pacific and the southwesterly wind anomalies over the Indian Ocean are not as significant as they are in the observations, implying an underestimation of the anomalous circulations in the lower troposphere associated with the variation of the Asian summer monsoon (Fig. 7e). Figure 7f illustrates that, not only the anomalous anticyclone centers, but also the prevailing wind anomalies associated with the APO index, are well represented in BCC_CSM1.1(m). Apparently, BCC_CSM1.1(m) has significantly improved the simulation of the atmospheric circulation anomalies over the lower troposphere, as compared with BCC_CSM1.1.

    Overall, the variational features of the Asian summer monsoon are better captured by BCC_CSM1.1(m) than BCC_CSM1.1. This is the reason why the simulation of monsoonal precipitation is improved in BCC_CSM1.1(m), as shown in Fig. 6. Precipitation anomalies around the TP can induce soil moisture variation, and lead to increases in the tropospheric temperature and intensification of the APO pattern (Liu et al., 2015). Therefore, compared with BCC_CSM1.1, the realistic simulation of the Asian summer monsoon's variation in BCC_CSM1.1(m) could be a possible contributor to reproducing a more reasonable APO, since the interannual variability of the APO is closely linked to the variation of the Asian summer monsoon and monsoonal precipitation (Zhao et al., 2007)

  • The above analysis demonstrates that BCC_CSM1.1(m) exhibits an encouraging ability to reproduce not only the interannual variation of the APO, but also the precipitation and atmospheric circulation anomalies related to the APO. Compared to BCC_CSM1.1, BCC_CSM1.1(m) has remarkably improved the simulation of the characteristics of the APO in summer. To better understand the reason why the finer horizontal resolution of BCC_CSM1.1(m) leads to a more reasonable simulation of the APO, we further investigate the observed and simulated APO-related surface temperature, which plays a crucial role in the formation and maintenance of the APO (Zhao et al., 2008). Figure 8 presents regression maps of surface temperature against the APO index in summer, from observations and model results. Both the surface air temperature over land and SST are used for computation.

    As shown in Fig. 8a, the APO's variability is closely linked to SST anomalies over the Pacific. When the APO index is above normal, large-scale significant and positive SST anomalies appear over the extratropical North Pacific from 120° E to 180°E. Meanwhile, significantly negative SST anomalies prevail over the tropical eastern Pacific. Such a spatial pattern of SST anomalies suggests a potential linkage between the APO and ENSO, which is consistent with the findings of (Zhou et al., 2010). However, sensitivity experiments have demonstrated that the SST anomalies over the two regions have opposite impacts on the intensity of the APO (Zhao et al., 2010), and the simultaneous SST variation over the tropical eastern Pacific alone cannot trigger a large-scale teleconnection pattern like the APO (Zhao et al., 2008). Thus, the tropical eastern Pacific SST anomalies captured by both models are not the key factor in determining the APO characteristics in summer. Figure 8b shows that, although BCC_CSM1.1 can reproduce the negative SST anomalies over the tropical eastern Pacific, it fails to capture the positive SST anomalies in the midlatitudes of the North Pacific. (Zhou et al., 2009) proposed that a significantly positive correlation exists between the APO and North Pacific SST on the interannual timescale. When the APO index is above normal, an anomalous anticyclone dominates the lower troposphere over the North Pacific, which is favorable for a warming of SST in the North Pacific. Meanwhile, negative heat fluxes appear in the North Pacific, accompanied by an intensification of northward warm water advection. All these factors are favorable for a warming of SST in the North Pacific. Compared with that in BCC_CSM1.1 (Fig. 8b), the regression map of positive SST anomalies over the North Pacific in BCC_CSM1.1(m) (Fig. 8c) is more consistent with the observation (Fig. 8a), which contributes to the improvement in the BCC_CSM1.1(m) simulation of the characteristics of the APO. This may also explain why the simulated PI is more realistic in BCC_CSM1.1(m) than in BCC_CSM1.1, since the North Pacific is a key region for defining the PI (Fig. 5).

    Figure 8.  Regression maps of surface air temperature (units: $^\circ$C) over land and SST (units: $^\circ$C) over ocean, with respect to the summer APO index, based on (a) NCEP-DOE reanalysis, (b) BCC_CSM1.1, and (c) BCC_CSM1.1(m). Shaded areas are values exceeding the 95% confidence level. The thick green line indicates the topographic contour of 3000 m.

    On the other hand, the formation of the APO is closely correlated with an elevation in the heating effect of the TP (Zhao et al., 2008). In a recent study, (Liu et al., 2015) proposed a new physical mechanism linking winter Pacific SST to the subsequent summer's APO. They pointed out that the previous winter's Pacific SST anomalies can persist until spring to cause an SLP anomaly over the North Indian Ocean in the subsequent spring and summer. The latter induces anomalous vertical motion that modulates the surface air temperature over the southern and western TP, maintaining the summer APO. The regression map of observed surface temperature shows that, associated with a positive APO index, prominent negative surface air temperature anomalies appear over the southern TP, while positive surface air temperature anomalies occur over the western TP region. Meanwhile, negative SST anomalies emerge in the northern Indian Ocean (Fig. 8a). The observed variation in surface temperature related to the APO index is highly consistent with the results of Liu et al. (2015, Fig. 5a). Compared with the observation, the variational features of the surface temperature associated with the APO are successfully reproduced in BCC_CSM1.1(m), i.e., both the negative (positive) surface air temperature anomalies in the southern (western) TP, and the SST anomalies over the northern Indian Ocean associated with the thermal effect of the TP, are successfully captured by BCC_CSM1.1(m) (Fig. 8c). In contrast, the surface air temperature anomalies over the southern and western TP, and the SST anomalies over the northern Indian Ocean, are not reproduced well by BCC_CSM1.1 (Fig. 8b). This result indicates that the physical processes responsible for maintaining the summer APO can be realistically reflected in BCC_CSM1.1(m) but not in BCC_CSM1.1, which explains why BCC_CSM1.1(m) can produce a more reasonable AI (Fig. 5) and is more capable of reproducing the characteristics of the summer APO compared with BCC_CSM1.1.

5. Conclusions and discussion
  • The APO is an important teleconnection pattern that is closely associated with climate variations over the subtropics in summer, especially the Asian-Pacific sector. The APO index is a useful index for assessing large-scale circulation anomalies (Zhao et al., 2007, 2012). In this paper, we use the monthly historical simulations of two versions of BCC_CSM, with different horizontal resolutions, i.e., BCC_CSM1.1 and BCC_CSM1.1(m), along with NCEP-DOE reanalysis data, to evaluate the models' performances in reproducing the interannual variability of the APO. APO-related precipitation and associated atmospheric circulation anomalies are also investigated. The reason for the better simulation of the APO by BCC_CSM1.1(m) is examined. The main results can be summarized as follows:

    (1) Both models can capture the spatial distribution of the upper-tropospheric total air temperature, which decreases from low to high latitudes in summer. Compared with BCC_CSM1.1, BCC_CSM1.1(m) effectively increases the simulated intensity of the total air temperature, and the results are closer to observations. However, analysis of the Taylor diagram shows that the simulated eddy temperature in BCC_CSM1.1 is more consistent with the observation, not only over East Asia and the North Pacific, but also over the entire Northern Hemisphere, as compared to that simulated by BCC_CSM1.1(m).

    (2) Compared with BCC_CSM1.1, the spatial pattern of EOF1 simulated by BCC_CSM1.1(m) is highly consistent with that from the observation. The spatial correlation coefficients between observations and the outputs of BCC_CSM1.1 and BCC_CSM1.1(m) are 0.40 and 0.77, respectively. Meanwhile, the correlation coefficients between the observed and simulated AI, PI and APO index in BCC_CSM1.1(m) are 0.35, 0.33 and 0.40, respectively, which are much higher than those between observations and the simulations of BCC_CSM1.1. BCC_CSM1.1(m) shows an encouraging capacity to reproduce not only the spatial pattern of the APO, but also the APO's interannual variability, due to its higher horizontal resolution. In particular, BCC_CSM1.1(m) exhibits greater skill in simulating the interannual variability of the eddy temperature index in Asia than it does over the North Pacific.

    (3) Based on comparisons between model results and observations, it is found that BCC_CSM1.1(m) can successfully reproduce the APO-related atmospheric circulation anomalies, such as the northward-shifted and intensified South Asian high, the strengthened extratropical westerly jet, and the tropical easterly jet in the upper troposphere, as well as the southwesterly monsoonal flow over the Indian Ocean and the intensified subtropical anticyclone over the North Pacific and Japan in the lower troposphere. As a result, the increased precipitation over tropical North Africa, South Asia and East Asia, and the decreased precipitation over subtropical North Africa, Japan and North America, simulated by BCC_CSM1.1(m), agree qualitatively with observations. In contrast, the circulation anomalies associated with a positive APO index in the simulation of BCC_CSM1.1 are less consistent with observations, which indicates a poor performance of BCC_CSM1.1 in simulating APO-related precipitation.

    (4) Regression analysis further indicates that BCC_ CSM1.1(m) can realistically capture SST anomalies over the North Pacific and northern Indian Ocean, as well as the anomalous surface air temperature along the southwestern flank of the TP. These temperature anomalies are closely linked to the maintenance of the APO. However, these relationships are missed by BCC_CSM1.1, suggesting that a higher horizontal resolution is crucial for BCC_CSM to reasonably simulate the physical processes involved in the formation and maintenance of the APO in summer. This may explain why BCC_CSM1.1(m) can reproduce the APO's interannual variability and accompanying circulation anomalies more reasonably than BCC_CSM1.1, and presents a substantial improvement in simulating the characteristics of the APO and APO-related precipitation anomalies.

    Although BCC_CSM1.1(m) is capable of simulating the APO teleconnection and its interannual variability in summer, it fails to reproduce the observed long-term variational trend of the APO index and the AI and PI. In fact, this phenomenon is also found in some other CSMs. Since the temporal resolution of aerosols used in most CSMs is 10 years, (Huang et al., 2013) argued that it is hard for models to realistically simulate decadal changes in winter/spring snow depth over the TP under a constant aerosol concentration. This directly influences the simulation of the long-term variation in tropospheric temperature over land areas of Asia.

    Besides, several previous studies have revealed that CSMs always demonstrate a higher predictive skill over the North Pacific, as compared to land areas of Asia. The lack of predictability over land areas is possibly associated with the complicated land-atmosphere interaction and feedback processes at play, which may not be represented well in models (Chen et al., 2013a; Huang et al., 2013). Our results support this argument, since the increased horizontal resolution in BCC_CSM1.1(m) helps to improve the description of this complicated land-atmosphere interaction, and subsequently the surface temperature over land (Jiang et al., 2015).

    It is worth noting that the results of the present study were obtained based on the historical simulation experiments of the two models. Since the APO is closely linked to the variation of weather and climate, it is also necessary to assess the models' abilities in predicting the APO's variation using results from hindcast experiments. Moreover, the teleconnection pattern over the upper troposphere also exists in other, non-summer, seasons. Further studies that evaluate the performance of the models in simulating and predicting the APO and its associated climate variations in non-summer seasons are needed.

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