In this study we examine the large-scale atmospheric circulation associated with the spring persistent rains (SPR) over Central China in March and April based on the climatological means and we propose a physical explanation of this rainy season.Low-level southwesterlies to the south of the middle and lower reaches of the Yangtze River (southern China) are responsible for SPR. Low-level southwesterlies are identified over southern China on the climatological mean wind field in SPR, and the appearance of the southwesterlies at the end of February is consistent with the onset of SPR. The southerlies, which are limited to southern China, the Indochina Peninsula and the South China Sea, are important for moisture transport to Central China and the moisture convergence there. Seasonal evolutions of low-level temperature, geopotential height and wind fields suggest that the lowlevel southerlies over southern China, the Indochina Peninsula and the South China Sea in SPR are caused by the westward pressure gradient associated with the eastward temperature gradient around the region from the Indochina Peninsula to the western North Pacific to east of the Philippines. The southerlies are the geostrophic winds associated with the westward pressure gradient. The eastward temperature/westward pressure gradients are most evident in March and April, and they are a result of the time-lag in the seasonal warming between the Indochina Peninsula and the western North Pacific to east of the Philippines. In addition, the coincidences of spatial distributions and seasonal evolutions from February through May between the low-level temperature and the surface sensible heat flux (SHF) suggest that the differential heating due to SHF between the two regions is likely responsible for the east-west thermal contrast.Much higher correlations than the 99% significance level among the year to year fluctuations of SPR, the eastward temperature/westward pressure gradients over the region from the Indochina Peninsula to the western North Pacific to east of the Philippines are identified. The close relationship between SPR and the eastward temperature/westward pressure gradients on both the seasonal and interannual bases strongly suggests that the east-west thermal contrast in spring between the Indochina Peninsula and the western North Pacific to the east of the Philippines plays the primary role in SPR formation.

The time and space structure of interannual fluctuations of summer rainfall (May to September) for the period from 1951 to 1990 over China is described. First, a harmonic analysis is applied to the summer rainfall series. The variation with periods of two to four years (hereafter, refered to 2-4 year component) accounts for more than 40 percent of the total variance for all stations, and more than 70 percent of the total variance for 61 percent of stations. The three-year period seems most prevailent among this period band.An EOF (Empirical Orthogonal Function) technique is applied to the 2-4 year period series. The first two EOF modes account for 12.8 and 10.7 percent of the total variance, respectively. EOF mode 1 reveals the seesaw between the Yangtze River valley and the Northern part of China. The spatial pattern of EOF mode 2 is more complicated, but it may be characterized by the oscillation between the Mei-yu region including the Yangtze River valley, and the rest of the country. Although the contribution proportion of the two principle modes is not high, composite maps show that they present the situations (before applying the EOF analysis) well. These two EOF modes exhibit large amplitude modulations. The amplitudes or the squares of the time coefficients tend to become large or small alternatively.Correlation and composite analyses show that EOF mode 1 correlates with the Indian summer monsoon and the SOI (Southern Oscillation Index). This result agrees with that of preceding studies and suggests that the summer rainfall over China is associated with the ENSO (El Nino and Southern Oscillation) events on the time-scale of two to four years. EOF mode 1 seems to precede the anomaly of the SOI. This result supports the previous proposals that the ENSO signals in the eastern equatorial Pacific may originate in the central Asia or the Indian Ocean region. EOF mode 2 seems not to be related to the Indian monsoon and the ENSO.

Relationship between Asian dust frequency observed in Japan (ADF) and dust storm frequency (DSF) in northern China was examined with up-to-date longterm observational data. Possible climatic conditions associated with variations of ADF were also explored. It was found that DSF at stations in Inner Mongolia and its vicinity (IMV) and ADF behaved nearly parallel to each other from the mid 1970s. Significant positive correlation between DSF in IMV and ADF suggests that IMV has a stronger influence on ADF than other dust source regions in northern China. Composite maps of Sea Level Pressure and 700 hPa wind fields for extreme ADF years exhibited a spatial pattern with cyclonic anomalies over the area from Northeast China to north Japan and anticyclonic anomalies around the Gobi desert in high ADF years. This situation corresponds to enhanced cyclonic activities in Northeast China and intensified cold air outbreaks in IMV, which are favorable for formation of dust storms in IMV.

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