In the background of global warming, rainfall in southern China during boreal summer (June to August) shows distinct interdecadal variability. Using monthly rainfall data for the period of 1920-2014, the leading modes of the interdecadal variability of Southern China Summer Rainfall (SCSR) are revealed. Meanwhile, the interdecadal changes of spatial pattern of SCSR are uncovered. Finally, the underlying possible mechanisms connecting the sea surface temperature in the Indian Ocean, the North Pacific Ocean and the North Atlantic Ocean and the leading modes of SCSR on interdecadal timescale are analyzed. The main results are as follows. (1) The first two leading modes of the interdecadal variability of SCSR, i.e., the monopole pattern and the zonal dipole pattern, are uncovered. The dominance of these two modes experienced an obvious decadal change around 1971/1972. During 1925-1971, the dominant pattern of the interdecadal variability of SCSR is the zonal dipole pattern, while the monopole pattern prevailed during 1972-2009. The key regions of sea surface temperature associated with the first two leading modes also changed around 1971/1972. (2) The atmospheric circulation anomalies associated with the monopole pattern and the zonal dipole pattern of SCSR are uncovered. Corresponding to the positive (negative) phase of the monopole pattern, an anticyclonic (cyclonic) anomaly is located over the South China Sea and northwestern Pacific, which is (not) in favor of moisture transport from the South China Sea to southern China. The lower-level anticyclonic (cyclonic) anomaly over Lake Baikal may lead to more southward transport of cold air, which can converge with the moisture transported from the South China Sea and is favorable for the formation of positive (negative) phase of the monopole pattern. Corresponding to the positive (negative) phase of the zonal dipole pattern, a positive (negative) center is located over the eastern part of southern China, which is favorable for upper-level divergence (convergence) and anomalous ascending (descending) motion. When combined with the lower-level cyclonic (anticyclonic) anomaly, it may result in increased (decreased) rainfall in eastern part of southern China. Meanwhile, the anticyclonic (cyclonic) anomaly over Indo-china peninsula and the Philippines is favorable (unfavorable) for moisture transport from the Bay of Bengal to western part of southern China, which is in favor of generating the positive (negative) phase of the zonal dipole pattern. (3) The combined impacts of sea surface temperature in the Indian Ocean, the North Pacific Ocean and the North Atlantic Ocean on the monopole pattern and the zonal dipole pattern of SCSR are revealed.