资源环境科技发展态势分析平台

The interdecadal variability of early summer South China (SC) monsoon rainfall (SCMR) is investigated based on several long-term rainfall data sets from 1926 to 2013. Both correlation and composite analyses suggest a significant out-of-phase relationship between interdecadal fluctuations of SCMR and the preceding wintertime Pacific Decadal Oscillation (PDO), with negative (positive) PDO epochs favouring above-normal (below-normal) SCMR. The physical mechanism relating the wintertime PDO to the interdecadal variability of SCMR appears to take place via atmospheric and oceanic teleconnections. The below-normal SCMR is characterized by a meridionally arranged vortex (MAV) pattern across the East Asia-western North Pacific (WNP) sector, with an anomalous cyclone, an anomalous anticyclone and an anomalous cyclone occurring alternately over the South China Sea (SCS)-WNP, Yangtze Basin and Lake Baikal in the early summer. This may respond to the PDO-related positive sea surface temperature (SST) anomalies locally over the SCS-WNP and remotely over the tropical eastern Pacific, with the latter forcing anomalous Hadley and Walker circulations. SC is located directly under the divergent moisture environment of anomalous northeasterlies between the anomalous cyclone over the SCS-WNP and the anticyclone over the Yangtze Basin. Again, the below-normal SCMR is preceded by negative (positive) SST anomalies in the midlatitude North Pacific (tropical eastern Pacific) associated with positive PDO in wintertime. Such an anomalous SST pattern subsequently induces a strengthened and northward-shifted upper-tropospheric jet in early summer, leading to significant upper-tropospheric convergence over SC. The out-of-phase relationship between SCMR and wintertime PDO as well as related physical processes are validated by coupled model simulations.