| Based on NCEP/NCAR reanalysis data, Climate Prediction Center Merged Analysis of Precipitation (CMAP), sea surface temperature (SST) data provided by NOAA and the daily rainfall from Chinese National Meteorological Information Centre, temporal and spatial characteristics of autumn persistent rainfall (APR) over the western China (WC) and its inter-annual variation are analyzed statistically. The main results are as follows:(1) The seasonal variation of rainfall over WC is characterized by double peaks, one in summer and the other in autumn. The harmonic analysis method has been employed to ensure the onset and withdrawal dates of APR which has been ensured to be in mid-August (pentad45) and mid-October (pentad57), respectively. The onset of APR corresponded with the starting of transition from the east Asian summer monsoon (EASM) to winter monsoon (EAWM), with the landmark circulation change being SW-to-SE wind shift at850-hPa over south of the middle and lower reaches of the Yangtze River (SMLRYR). The negative-to-positive reversal of meridional SLP gradient (SLP over land minus SLP in the sea) in East Asia happened in mid-August (pentad45), causing the850-hPa SW wind over the SMLRYR replaced by SE wind. Such a SE wind will meet over WC with the SW wind from the Bay of Bengal (BOB), thus forming convergence there both in the wind direction and water vapor. As a result, the rainfall over the WC will strengthen again after its summer peak and the APR is thereby established. The ending of APR was associated with the retreat of tropical SW monsoon from the BOB and the complete establishment of EAWM, with the landmark circulation change being SW-to-NE wind shift over the BOB. As the negative-to-positive reversal of zonal SLP gradient in East Asia occurred progressively later southward, the EAWM will advance southward, with NE wind reaching the SMLRYR in early-September (pentad51) and then the South China Sea in mid-October (pentad57). The SW wind from the BOB disappeared, the WC was fully controlled by the continental cold high, and the monsoon meridional circulation changed into winter-type Hadley Circulation in East Asia, a situation marking the complete establishment of EAWM and the withdrawal of APR.(2) The inter-annual and decadal variability characteristics of APR are outstanding. There was a decrease trend during the recent50years which is related to the variation of vapor flux transported by the avenues over the Indian Ocean, the BOB and South China Sea-Pacific, while the westerly avenue does not contribute to this. The four avenues mentioned above have impacts on the inter-annual variability of APR. During the strong vapor flux transportation years, there is more precipitation over the WC. The interaction among the avenues is the key to the drought/flood of APR. There is a positive net vapor flux over the WC during APR period indicating that the WC is a vapor sink. And there is obvious difference in vapor flux at the boundaries of WC between drought and flood years. The intensity variation of the avenues is related to the distribution of synchronous500hPa height,850hPa wind and previous SSTA.(3) The accumulated wind speed has been employed to obtain the annual onset and withdrawal dates of the APR from1961to2010. It is found that the variation of APR onset is related to the time that the EASM transfers to EAWM. When the EASM transfers to EAWM earlier, the APR establishes earlier, and vice versa. While the withdrawal of APR has the relationship with the onset (withdrawal) of EAWM (South Asian Summer Monsoon, SASM), when the EAWM (SASM) establishes (withdraws) earlier, the APR withdraws earlier, and vice versa. The variation of onset-withdrawal of APR is also related to the distribution of SSTA.Therefore, the onset-withdrawal of APR may be associated with the asynchronous summer-to-winter transition between the EASM and SASM. Due to the cooperative contribution of the EASM and SASM, the APR thus becomes the last rainy season of the Asian summer monsoon in mainland China. |
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