A precursory signal for June precipitation anomalies over the middle and lower reaches of the Yangtze River—the Asian Pacific Oscillation

Using the precipitation data from the 160 stations of China and the NCEP reanalysis data and the CCSM3 simulations, influencing factors and precursory signals for anomalous precipitation over the middle and lower reaches of the Yangtze River are analyzed in terms of tropospheric temperature. The results show that the anomalous Asian-Pacific Oscillation (APO), reflecting the anomalous zonal land-sea thermal contrast at the middle upper troposphere, is an important factor for anomalous precipitation over the middle and lower reaches of the Yangtze River. Statistical observational and simulation results suggest that there is a significantly negative correlation between the May APO index and the following June precipitation over the middle and lower reaches of the Yangtze River. The relationship is closer than that between previous winter El NiNo （La NiNa） and June precipitation, and has a relatively higher stability. Further analyses indicate that APO has a good continuity from May to June, therefore May APO can modulate the intensity and location of the June western Pacific subtropical high, anomalous southwesterly winds over the southeastern coasts of China and anomalous northerly winds at the middle latitudes of eastern China, and eventually leads to anomalous June precipitation over the middle and lower reaches of the Yangtze River. In addition, a clearly weaker May APO is also a critical reason for the above normal June precipitation over the middle and lower reaches of the Yangtze River in 2011. These results suggest that besides SST forcing, the middle-upper tropospheric anomalies should be considered as an important precursory influencing factor, and it seems worth trying to forecast precipitation using previous APO anomalies.