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Xiaojun Yuan, Dake Chen, Cuihua Li, Lei Wang, and Wanqiu Wang

Abstract

A linear Markov model has been developed to predict sea ice concentration (SIC) in the pan-Arctic region at intraseasonal to seasonal time scales, which represents an original effort to use a reduced-dimension statistical model in forecasting Arctic sea ice year-round. The model was built to capture covariabilities in the atmosphere–ocean–sea ice system defined by SIC, sea surface temperature, and surface air temperature. Multivariate empirical orthogonal functions of these variables served as building blocks of the model. A series of model experiments were carried out to determine the model’s dimension. The predictive skill of the model was evaluated by anomaly correlation and root-mean-square errors in a cross-validated fashion . On average, the model is superior to the predictions by anomaly persistence, damped anomaly persistence, and climatology. The model shows good skill in predicting SIC anomalies within the Arctic basin during summer and fall. Long-term trends partially contribute to the model skill. However, the model still beats the anomaly persistence for all targeted seasons after linear trends are removed. In winter and spring, the predictability is found only in the seasonal ice zone. The model has higher anomaly correlation in the Atlantic sector than in the Pacific sector. The model predicts well the interannual variability of sea ice extent (SIE) but underestimates its accelerated long-term decline, resulting in a systematic model bias. This model bias can be reduced by the constant or linear regression bias corrections, leading to an improved correlation skill of 0.92 by the regression bias correction for the 2-month-lead September SIE prediction.

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Lei Wang, Xiaojun Yuan, Mingfang Ting, and Cuihua Li

Abstract

Recent Arctic sea ice changes have important societal and economic impacts and may lead to adverse effects on the Arctic ecosystem, weather, and climate. Understanding the predictability of Arctic sea ice melting is thus an important task. A vector autoregressive (VAR) model is evaluated for predicting the summertime (May–September) daily Arctic sea ice concentration on the intraseasonal time scale, using only the daily sea ice data and without direct information of the atmosphere and ocean. The intraseasonal forecast skill of Arctic sea ice is assessed using the 1979–2012 satellite data. The cross-validated forecast skill of the VAR model is found to be superior to both the anomaly persistence and damped anomaly persistence at lead times of ~20–60 days, especially over northern Eurasian marginal seas and the Beaufort Sea. The daily forecast of ice concentration also leads to predictions of ice-free dates and September mean sea ice extent. In addition to capturing the general seasonal melt of sea ice, the model is also able to capture the interannual variability of the melting, from partial melt of the marginal sea ice in the beginning of the period to almost a complete melt in the later years. While the detailed mechanism leading to the high predictability of intraseasonal sea ice concentration needs to be further examined, the study reveals for the first time that Arctic sea ice can be predicted statistically with reasonable skill at the intraseasonal time scales given the small signal-to-noise ratio of daily data.

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Zhiwei Heng, Yunfei Fu, Guosheng Liu, Renjun Zhou, Yu Wang, Renmin Yuan, Jingchao Guo, and Xue Dong

Abstract

In this paper, the global distribution of cloud water based on International Satellite Cloud Climatology Project (ISCCP), Moderate Resolution Imaging Spectroradiometer (MODIS), CloudSat Cloud Profiling Radar (CPR), European Center for Medium-Range Weather Forecasts Interim Re-Analysis (ERA-Interim), and Climate Forecast System Reanalysis (CFSR) datasets is presented, and the variability of cloud water from ISCCP, the Special Sensor Microwave Imager (SSM/I), ERA-Interim, and CFSR data over the time period of 1995 through 2009 is discussed. The results show noticeable differences in cloud water over land and over ocean, as well as latitudinal variations. Large values of cloud water are mainly distributed over the North Pacific and Atlantic Oceans, eastern ITCZ, regions off the west coast of the continents as well as tropical rain forest. Cloud water path (CWP), liquid water path (LWP), and ice water path (IWP) from these datasets show a relatively good agreement in distributions and zonal means. The results of trend analyzing show an increasing trend in CWP, and also a significant increasing trend of LWP can be found in the dataset of ISCCP, ERA-Interim, and CFSR over the ocean. Besides the long-term variation trend, rises of cloud water are found when temperature and water vapor exhibit a positive anomaly. EOF analyses are also applied to the anomalies of cloud water, the first dominate mode of CWP and IWP are similar, and a phase change can be found in the LWP time coefficient around 1999 in ISCCP and CFSR and around 2002 in ERA-Interim.

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Rui Wang, Yunfei Fu, Tao Xian, Fengjiao Chen, Renmin Yuan, Rui Li, and Guosheng Liu

Abstract

Variations and trends of atmospheric precipitable water (APW) are examined using radiosonde data from Integrated Global Radiosonde Archive (IGRA) and China Meteorological Administration (CMA) from 1995 to 2012 in mainland China. The spatial distribution of the climatological mean APW shows that APW gradually decreases from the southern to the northern regions of mainland China. The seasonal mean pattern of APW shows clear regional difference, except for higher APW in summer (June–August) and lower APW in winter (December–February). Four regions show significantly downward trends in APW. Moreover, the trends of APW calculated using reanalysis datasets are consistent with the results of radiosonde data. Furthermore, the relationship between APW and the general circulation is investigated. The summer East Asian monsoon intensity and El Niño events show positive correlations with APW, whereas the North Atlantic Oscillation shows negative correlation with APW. The downward trend of APW is in accordance with the downward trend of mean column temperature (1000–300 hPa) at most stations, which suggests that decreasing mean column temperature results in decreasing APW in mainland China. Additionally, statistical analysis has revealed the regional trends in APW are not consistent with the regional trends in precipitation, implying that not all the variation of precipitation can be explained by APW.

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Dan-Qing Huang, Jian Zhu, Yao-Cun Zhang, Jun Wang, and Xue-Yuan Kuang

Abstract

Spring persistent rainfall (SPR) over southern China has great impact on its society and economics. A remarkable feature of the SPR is high frequency. However, SPR frequency obviously decreases over the period of 1997–2011. In this study, the possible causes have been investigated from the perspective of the individual and concurrent effects of the East Asian subtropical jet (EASJ) and East Asian polar front jet (EAPJ). A close relationship is detected between SPR frequency and EASJ intensity (but not EAPJ intensity). Associated with strong EASJ, abundant water vapor is transported to southern China by the southwesterly flow, which may trigger the SPR. Additionally, frequencies of both strong EASJ and weak EAPJ events are positively correlated with SPR frequency. Further investigation of the concurrent effect indicates a significant positive correlation between the frequencies of SPR and the strong EASJ–weak EAPJ configuration. Associated with this configuration, southwesterly flow strengthens in the lower troposphere, while northerly wind weakens in the upper troposphere. This provides a dynamic and moist condition, as enhanced ascending motion and intensified convergence of abundant water vapor over southern China, which favors the SPR. All analyses suggest that the EASJ may play a dominant role in the SPR occurrence and that the EAPJ may play a modulation role. Finally, a possible mechanism maintaining the strong EASJ–weak EAPJ configuration is proposed. Significant cooling over the northeastern Tibetan Plateau may induce a cyclone anomaly in the upper troposphere, which could result in an accelerating EASJ and a decelerating EAPJ.

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Hua Li, Ke Fan, Shengping He, Yong Liu, Xing Yuan, and Huijun Wang

Abstract

The reversal of surface air temperature anomalies (SATA) in winter brings a great challenge for short-term climate prediction, and the mechanisms are not well understood. This study found that the reversal of SATA between December and January over China could be demonstrated by the second leading mode of multivariate empirical orthogonal function analysis on the December–January SATA. It further reveals that the central Pacific El Niño–Southern Oscillation (CP ENSO) has contributed more influence on such a reversal of SATA since 1997. CP ENSO shows positive but weak correlations with SATA over China in both December and January during the pre-1996 period, whereas it shows significant negative and positive correlations with the SATA in December and January, respectively, during the post-1997 period. The CP ENSO–related circulations suggest that the change of the Siberian high has played an essential role in the reversal of SATA since 1997. The pattern of sea surface temperature anomalies associated with the CP ENSO leads to a westward-replaced Walker circulation that alters the local meridional circulation and, further, has impacted the Siberian high and SATA over China since 1997. Moreover, the seasonal northward march of the convergence zone from December to January causes a northward-replaced west branch of the Walker circulation in January compared with that in December. The west branch of the Walker circulation in December and January directly modulates local Hadley and Ferrel circulations and then causes contrasting Siberian high anomalies by inducing opposite vertical motion anomalies over Siberia. The reversal of SATA between December and January, therefore, has been more frequently observed over China since 1997. The abovementioned mechanisms are validated by the analysis at pentad time scales and confirmed by numerical simulations.

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Dongliang Yuan, Jing Wang, Tengfei Xu, Peng Xu, Zhou Hui, Xia Zhao, Yihua Luan, Weipeng Zheng, and Yongqiang Yu

Abstract

Controlled numerical experiments using ocean-only and ocean–atmosphere coupled general circulation models show that interannual sea level depression in the eastern Indian Ocean during the Indian Ocean dipole (IOD) events forces enhanced Indonesian Throughflow (ITF) to transport warm water from the upper-equatorial Pacific Ocean to the Indian Ocean. The enhanced transport produces elevation of the thermocline and cold subsurface temperature anomalies in the western equatorial Pacific Ocean, which propagate to the eastern equatorial Pacific to induce significant coupled evolution of the tropical Pacific oceanic and atmospheric circulation. Analyses suggest that the IOD-forced ITF transport anomalies are about the same amplitudes as those induced by the Pacific ENSO. Results of the coupled model experiments suggest that the anomalies induced by the IOD persist in the equatorial Pacific until the year following the IOD event, suggesting the importance of the oceanic channel in modulating the interannual climate variations of the tropical Pacific Ocean at the time lag beyond one year.

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Yuan Wang, Jonathan H. Jiang, Hui Su, Yong-Sang Choi, Lei Huang, Jianping Guo, and Yuk L. Yung

Abstract

Observations show that the Arctic sea ice cover has been shrinking at an unprecedented rate since the 1970s. Even though the accumulation of greenhouse gases in the atmosphere has been closely linked with the loss of Arctic sea ice, the role of atmospheric aerosols in past and future Arctic climate change remains elusive. Using a state-of-the-art fully coupled climate model, the authors assess the equilibrium responses of the Arctic sea ice to the different aerosol emission scenarios and investigate the pathways by which aerosols impose their influence in the Arctic. These sensitivity experiments show that the impacts of aerosol perturbations on the pace of sea ice melt effectively modulate the ocean circulation and atmospheric feedbacks. Because of the contrasting evolutions of particulate pollution in the developed and developing countries since the 1970s, the opposite aerosol forcings from different midlatitude regions are nearly canceled out in the Arctic during the boreal summer, resulting in a muted aerosol effect on the recent sea ice changes. Consequently, the greenhouse forcing alone can largely explain the observed Arctic sea ice loss up to the present. In the next few decades, the projected alleviation of particulate pollution in the Northern Hemisphere can contribute up to 20% of the total Arctic sea ice loss and 0.7°C surface warming over the Arctic. The authors’ model simulations further show that aerosol microphysical effects on the Arctic clouds are the major component in the total aerosol radiative forcing over the Arctic. Compared to the aerosol-induced energy imbalance in lower latitudes outside the Arctic, the local radiative forcing by aerosol variations within the Arctic, due to either local emissions or long-range transports, is more efficient in determining the sea ice changes and Arctic climate change.

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Bowen Pan, Yuan Wang, Jiaxi Hu, Yun Lin, Jen-Shan Hsieh, Timothy Logan, Xidan Feng, Jonathan H. Jiang, Yuk L. Yung, and Renyi Zhang

Abstract

The radiative and microphysical properties of Saharan dust are believed to impact the Atlantic regional climate and tropical cyclones (TCs), but the detailed mechanism remains uncertain. In this study, atmosphere-only simulations are performed from 2002 to 2006 using the Community Atmospheric Model, version 5.1, with and without dust emission from the Sahara Desert. The Saharan dust exhibits noticeable impacts on the regional longwave and shortwave radiation, cloud formation, and the convective systems over West Africa and the tropical Atlantic. The African easterly jet and West African monsoon are modulated by dust, leading to northward shifts of the intertropical convergence zone and the TC genesis region. The dust events induce positive midlevel moisture and entropy deficit anomalies, enhancing the TC genesis. On the other hand, the increased vertical wind shear and decreased low-level vorticity and potential intensity by dust inhibit TC formation in the genesis region. The ventilation index shows a decrease in the intensification region and an increase in the genesis region by dust, corresponding to favorable and unfavorable TC activities, respectively. The comparison of nondust scenarios in 2005 and 2006 shows more favorable TC conditions in 2005 characterized by higher specific humidity and potential intensity, but lower ventilation index, wind shear, and entropy deficit. Those are attributable to the observed warmer sea surface temperature (SST) in 2005, in which dust effects can be embedded. Our results imply significant dust perturbations on the radiative budget, hydrological cycle, and large-scale environments relevant to TC activity over the Atlantic.

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