The changes in summer rainfall over the Tarim Basin, China, and the underlying mechanisms have been investigated using the observed rainfall data at 34 stations and the NCEP–NCAR reanalysis data during the period of 1961–2007. Results show that the summer rainfall over the Tarim Basin, which exhibits a significant increasing trend during the last half century, is closely related to the summer middle and upper tropospheric cooling over central Asia. Mechanism analysis indicates that the middle and upper tropospheric cooling over central Asia results in a location farther south of the subtropical westerly jet over western and central Asia with anomalous southerly wind at lower levels and ascending motion prevailing over the Tarim Basin. Such anomalies in the atmospheric circulations provide favorable conditions for the enhanced summer rainfall over the Tarim Basin. Further analysis suggests that the weakened South Asian summer monsoon (SASM) could be potentially responsible for the middle and upper tropospheric cooling over central Asia. This is largely through the atmospheric responses to the diabatic heating effect of the SASM. A weakened SASM can result in an anomalous cyclone in the middle and upper troposphere over central Asia. The western part of the anomalous cyclone produces more cold air advection, which leads to the cooling. This study suggests indirect but important effects of the SASM on the summer rainfall over the Tarim Basin.

Annual precipitation anomalies over eastern China are characterized by a north–south dipole pattern, referred to as the “southern flooding and northern drought” pattern (SF/ND), fluctuating on decadal time scales. Previous research has suggested possible links with oceanic forcing, but the underlying physical mechanisms by which sea surface temperature (SST) variability impacts the dipole pattern remains unclear. Idealized atmospheric general circulation model experiments conducted by the U.S. CLIVAR Drought Working Group are used to investigate the role of historical SST anomalies associated with Pacific El Niño–Southern Oscillation (ENSO)-like and the Atlantic multidecadal oscillation (AMO) patterns in this dipole pattern. The results show that the Pacific SST pattern plays a dominant role in driving the decadal variability of this dipole pattern and the associated atmospheric circulation anomalies, whereas the Atlantic SST pattern contributes to a much lesser degree. The direct atmospheric response to the Pacific SST pattern is a large-scale cyclonic or anticyclonic circulation anomaly in the lower troposphere occupying the entire northern North Pacific. During the warm phase of the Pacific SST pattern, it is cyclonic with northwesterly wind anomalies over northern China pushing the monsoon front to the south and consequently SF/ND. During the cold phase of the Pacific SST pattern, the circulation anomaly reverses with southeasterly winds over northern China allowing the monsoon front and the associated rainband to migrate northward, resulting in southern drought and northern flooding. The Atlantic SST pattern plays a supplementary role, enhancing the dipole pattern when the Pacific SST and Atlantic SST patterns are in opposite phases and weakening it when the phases are the same.

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