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The intensity of interannual variation of spring precipitation over southern China during 1979–2014 and possible reasons for it are investigated in this paper. There is a significant interdecadal change in the intensity of interannual variation of spring precipitation over southern China around 1995/96. The intensity of interannual variation of spring rainfall over South China is stronger during 1979–95 than that during 1996–2014. The possible reason may be the larger amplitude of the sea surface temperature anomaly (SSTA) in the western Pacific Ocean (WP) before 1995/96. The cooler (warmer) SSTA in WP may trigger an abnormal local anticyclone (cyclone) at lower levels. The anomalous southwesterly (northeasterly) flow at the northwestern flank of the WP anticyclone (cyclone) covers South China, transporting more (less) moisture to South China. Meanwhile, the anomalous winds converge (diverge) in South China at lower levels and diverge (converge) at upper levels, which causes the anomalous ascent (descent) to enhance (reduce) the precipitation over there. However, during 1996–2014, the intensity of interannual variation of spring rainfall over the middle and lower reaches of the Yangtze River valley becomes much stronger than that during 1979–95, which is related to the intensified interannual variation of the atmospheric circulation in the middle and high latitudes over Eurasia. The weak (strong) Siberian high and East Asian trough may reduce (enhance) the northerly wind from the middle and high latitudes. As a result, the middle and lower reaches of the Yangtze River valley are subjected to the anomalous southerly wind, favoring more (less) precipitation over there.

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ABSTRACT:In boreal summer, the southern Indian Ocean (SIO) features more evaporation than precipitation and thus it is a moisture source region for the Asian summer monsoon rainfall. In the present study, the interannual variability of moisture source over the SIO is analysed for boreal summer during 1979-2008. The impacts of local sea surface temperature (SST) and the El Niño-Southern Oscillation (ENSO) on the moisture source are compared and the possible mechanisms for the SST influence on the moisture source are investigated. The leading mode of the interannual variation of apparent moisture sink () over the SIO displays an obvious out-of-phase relationship over the western-central SIO and the region to the northwest-north of Australia. The interannual variations of over the above two regions have significant positive correlation with local SST. An increase in local SST enhances convection and ascent, leading to more precipitation. Over the western-central SIO, surface winds weaken corresponding to higher SST in the western-central SIO and lower SST to the northwest-north of Australia, leading to less oceanic evaporation. Thus, the variation of the moisture source over the western-central SIO is contributed by both precipitation and evaporation. However, the variation of the moisture source to the northwest-north of Australia is contributed mainly by precipitation. ENSO amplifies anomalous moisture source induced by local SST anomalies. ENSO and local SST work cooperatively to influence the moisture source over the SIO.