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Zunya Wang and Botao Zhou

Abstract

This article presents a decadal transition from a decreasing trend to an increasing trend in the late 1990s for autumn rainfall in central China. The atmospheric and oceanic background underlying this regime shift is also addressed. Accompanying the above decadal transition, the moisture convergence and ascending motion averaged in central China both switch from a weakening trend to a strengthening trend. Meanwhile, after the late 1990s, the declining of the Asian sea level pressure (SLP) and the deepening of the Lake Balkhash trough may induce more cold air from high latitudes to break out southward. The strengthening of the low-level southerly in East Asia could transport more moisture northward from low latitudes. More cold air and more warm-moist airflow encountering in the targeted region contribute to the increasing trend of local rainfall. The situation before the late 1990s is generally reversed, which accounts for the decreasing trend of autumn rainfall. The movement of the East Asian jet (EAJ) from southward to northward also has a contribution via its influence on dynamic condition. Additionally, changes in the sea surface temperatures (SSTs) in the western Indian and Atlantic Oceans from a cooling trend to a warming trend in the late 1990s play significant roles through their modulations on moisture transport and cold-air activities.

Open access
Zunya Wang, Song Yang, Zongjian Ke, and Xingwen Jiang

Abstract

Based on the observational datasets of rime and glaze from 743 stations in China and the atmospheric circulation data from the NCEP–NCAR reanalysis during 1954–2009, large-scale atmospheric and oceanic conditions for extensive and persistent rime and glaze events were examined with a composite analysis. Results show that rime events mostly occur in northern China while glaze events are mainly observed in southern China. The icing events are accompanied by low temperature and high humidity but not necessarily by above-normal precipitation. The Asian low, blocking highs, strong moisture transport, and an inversion layer related to major abnormal circulation systems contribute to the occurrence and persistence of icing events in China. The Ural blocking high plays a major role in the glaze events, and the Okhotsk blocking high is closely related to the rime events. For glaze events, extratropical circulation anomalies and the southward outbreak of cold air play a dominant role. In contrast, the strong northward transport of warm and moist airflows plays a leading role and the blocking high and the southward outbreak of extratropical cold air take a supporting role for rime events. There is nearly an equal chance for occurrences of rime events under La Niña and El Niño backgrounds. However, glaze events more likely occur under the background of La Niña. Additionally, the sea surface temperatures from the tropical Indian Ocean to the tropical northwestern Pacific Ocean also contribute to the occurrence and maintenance of icing events in China.

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Botao Zhou, Zunya Wang, Ying Shi, Ying Xu, and Zhenyu Han

Abstract

Using station data and Regional Climate Model version 4 (RegCM4) simulations under the representative concentration pathway 4.5 (RCP4.5) scenario, this article addresses historical and future changes of the wintertime snowfall over China. The observational results generally show a decrease in the frequency and an increase in the mean intensity of snowfalls in northwestern China (NWC), northeastern China (NEC), the eastern Tibetan Plateau (ETP), and southeastern China (SEC) since the 1960s. The total amount of wintertime snowfall, however, has increased in NWC, NEC, and ETP but decreased in SEC. The decrease in snow days is primarily due to the reduction of light snowfall events. The increase in the total amount is primarily explained by increases in heavy snowfalls, and the corresponding decrease is the result of decreases in light-to-heavy snowfalls. The RegCM4 ensemble, which can well simulate the observed snowfall climatology, projects that the snow days will be further reduced by the end of the twenty-first century relative to 1986–2005, primarily owing to the decline of light snowfall events. The total amount is projected to increase in NWC but decrease in the other three subregions. The increase in the total amount in NWC is attributed to increases in heavy and large snowfalls. Decreases in light snowfalls play a leading role in the decrease of the total amount in NEC. In ETP and SEC, the decrease in the total amount is the result of overall decreases in light-to-heavy snowfalls. The mechanism for such changes is an interesting topic to study in the future.

Open access
Botao Zhou, Zunya Wang, Bo Sun, and Xin Hao

Abstract

Analyses of observation data from 1961 to 2014 by using the empirical orthogonal function (EOF) method indicate that the primary mode (a monosign pattern) of heavy snowfall over northern China in winter shows evident variations from a negative polarity to a positive polarity in the mid-1990s. Associated with this decadal change, the southward displacement of the polar front jet stream and northward shift of the subtropical jet stream in the upper troposphere are apparent. Accordingly, a negative height anomaly dominates the region from Lake Balkhash to Lake Baikal and a positive height anomaly occupies the midlatitudes of the North Pacific in the middle troposphere. Such anomalous patterns in the middle and high troposphere correspond approximately to the northern mode of the East Asian winter monsoon (EAWM) and may favor the interaction of cold air with moist airflows over northern China, which helps increase local heavy snowfall. Further investigation shows that the shift in the Atlantic multidecadal oscillation (AMO) from a cold phase to a warm phase in the 1990s may also play a role, through its linkage to the above atmospheric circulations with the aid of a downstream propagation of wave train that emanates from the Atlantic Ocean.

Open access
Kaiqiang Deng, Song Yang, Mingfang Ting, Ping Zhao, and Zunya Wang

Abstract

This study applies the maximum temperatures at more than 2000 Chinese stations to investigate the dominant modes of China summer heat waves (HWs). The first empirical orthogonal function (EOF) mode of the HW days reflects an increased frequency of HWs in northern China (NC), while the second and third modes represent two distinct interannual modes, with key regions over the Yangtze River valley (YRV) and southern China (SC), respectively. The NC HWs are possibly associated with the Atlantic–Eurasian teleconnection, showing zonally propagating wave trains over the North Atlantic and Eurasian continent. The YRV HWs are proposed to be linked to the North Atlantic Oscillation, which may trigger a southeastward-propagating wave train over northern Russia and East Asia that results in a high pressure anomaly over the YRV. The SC HWs are obviously dominated by the Indian Ocean and northwest Pacific warm SSTs owing to the transition from the preceding El Niño to La Niña, which excites above-normal highs over SC. The anomalously high pressures over NC, the YRV, and SC are usually accompanied by descending air motions, clear skies, decreased precipitation, and increased solar radiation, which jointly cause a drier and hotter soil condition that favors the emergence of HWs. The GFDL HiRAM experiments are able to reproduce the historical evolution of NC and SC HWs, but fail to capture the YRV HWs. The correlation coefficient between model PC1 (PC2) and observed PC1 (PC3) for the period of 1979–2008 is 0.65 (0.38), which significantly exceeds the 95% (90%) confidence level, indicating that this model has a more faithful representation for the SST-forced HWs.

Open access