Climate Variability Of Summer Rainfalls In China And The Possible Mechanism

Climate variability of summer rainfalls in China on multiple-time-scale, including climatic intra-seasonal oscillation (CISO), interannual variation and inter-decadal variation, is studied in this paper via various statistical analysis methods mainly based on 740 stations daily rainfall datasets in China from 1951 to 2004. Firstly, the staggered and regional advance and retreat of rainy seasons in China are analyzed. Secondly, the periodicity, temporal and spacial distribution, relating circulation and impacting external forcing factors of the CISO, interannual and interdecadal variations of summer rainfalls in China are studied, respectively. Finally, relationships among the aforementioned three variations are further discussed. The major conclusions are as follows:(1) On the basis of the definition of starting and ending dates of rainy seasons in China, it is found that the major rainy season breaks out earliest in middle South China and ends latest in the northern part of Sichuan province and the southern part of Gansu and Shanxi provinces. The duration of rainy seasons span from 4 to 14 pentads, while the amounts of which account for 30%～60% of the annual total amounts. The major rainy season in the eastern part of China, mainly affected by the East Asian monsoon, advances from the south to the north, while in the western part, it is some earlier in the north than in the south, with strong local features due to impacts of the westerlies. The spring and autumn rainy seasons are also prominent in China.(2) The 10~30-day period is much significant in South China and North China, while the 30~60-day period is more obvious in the middle and lower reaches of the Yangtze River valley. Three significant propagations of the 30~60-day oscillation from the south to the north are observed from April to September, with the first one beginning in early April and being related to the start and maintenance of the spring rainfalls in China. The second propagation begins in early June and ends in middle August, with three strong oscillation centers corresponding well to the pro-flooding rainy season in South China, the Meiyu in the middle and lower reaches of the Yangtze River valley and the rainy season in North and Northeast China, respectively. And the last one starts from early August and has close relationship with the autumn rainfalls in areas to the south of the Yangtze River. The CISO modulates significantly the intensity, activity and break of rainy seasons in China. The 30~60-day oscillation also shows obvious westward propagation in areas to the east of 100。E. However, the propagation of the 10~30-day oscillations is not significant. The possible mechanism of the maintenance and propagation of the CISO of summer rainfalls in China lies in that the strong CISO in the low-latitude inspires the EAP (East Asia - Pacific) wave train, whose anomalous cyclones and anti-cyclones form longitudinal anomalous convergence and divergence belts, resulting in the corresponding anomalous rainy belts from East Asian to North Pacific for the convergence causes the ascending airflow, while the divergence causes the descending airflow. Then, the maintenance and propagations of the CISO of the anomalous heating sources in the low-latitude may control the persistence and northward propagation of anomalous rainy belts from East Asia to North Pacific.(3) Significant TBO is observed in summer rainfalls in over 75% of stations in China. And it can account for over 35%, even 55% of the total interannual variance. Regions where summer rainfalls show strongest TBO in China are mainly located in middle Inner Mongolia and from Gansu and Shanxi provinces to the Yangtze and Huaihe River valley. The TBO in summer rainfalls of China is caused possibly by the TBO in the Pacific-Asian and Australian monsoon system through affecting the anomalous thermal situation in the western tropical Pacific. As the TBO of the anomalous heating resource there inspires the EAP and the EU wave trains, impacting the circulation at both the low and mid-high latitude and then causing the anomalies of the warm-wet airflow and the cold-dry airflow simultaneously, summer rainfalls in China oscillate on the biennial period. Moreover, the character, intensity and location of the anomalous heating resource of the warm pool in the tropical Pacific are key factors controlling the major mode of the TBO of summer rainfalls in China.(4) The quasi-10-year, 30~40-year and quasi-80-year periods are observed in summer rainfalls of East China, with the abrupt change in late 1970s. Additionally, other abrupt change points are observed in North China in middle 1960s and in South China in early 1990s. In late 1970s, summer rainfalls in East China changed from the so-called flooding in the south and drought in the north mode to the opposite one. On one hand, the winter and spring snow depth on the Tibet Plateau changed abruptly at late 1970s and began to increase from then on, resulting in the weakening of the heating force of the East Asian continent. On the other hand, the SST of the middle and eastern tropical Pacific in springs and summers shows three abrupt increases in middle 1960s, late 1970s and early 1990s, respectively, causing the strengthening of the heating force of the oceans. Then, the heating contrast between the continent and ocean in East Asia weakened on inter-decadal scale. Accordingly, the Asian summer monsoon weakened. The weakening of the Asian summer monsoon circulation directly results in the southward shift of the rainy belts in East China on inter-decadal scale. And the research further shows that the snow depth on Tibet Plateau has more important impact on the inter-decadal variation of summer rainfalls in East China than the SST in the middle and eastern tropical Pacific wholly, while the abrupt increase of the SST at early 1990s is closely related to the abrupt increase of summer rainfalls in South China during the same period.(5) By discussing the interactions among three aforementioned variations of summer rainfalls in China on multiple-time-scale, it is found that the intensity of the intraseasonal oscillation has significant positive correlation with rainfall amounts. And, in both the flooding year and the flooding decade, the period of intraseasonal oscillation is relatively longer, with the 30~60-day dominating, while it is relatively shorter under a drought climatic background, mainly being 10~30-day. A more significant northward propagation can be observed under a drought background than a flooding one. However, the anomalous rainfalls and circulations on the intraseasonal scale are stronger under the flooding background than the drought background, with both modes of anomalous rainfalls and circulations being similar. As the decrease of summer rainfalls in East China, the intraseasonal oscillation weakens, and vise versa. The integrated interannual variations compose the interdecadal variation, and the interdecadal anomaly is the climatic background of the interannual variation. That is, with the modulation of the interdecadal variation, the interannual variation always shows some trend of being positive or negative anomaly. After late 1970s, the 3~7-year period of summer rainfalls in South China and North China, as well as the 2~3-year period in the middle and lower reaches of the Yangtze River valley, became more significant. But the interdecadal background has few impacts on the temporal and spacial modes of summer rainfalls in China on the interannual scale.