Teleconnected influence of tropical Northwest Pacific sea surface temperature on interannual variability of autumn precipitation in Southwest China

Abstract

The interannual variation of autumn precipitation in Southwest China (SWC) and the possible influence of tropical Northwest Pacific (NWP) sea surface temperature (SST) are investigated. Statistical analysis shows that SWC precipitation is negatively correlated with concurrent NWP SST. The warm NWP SST that promotes dry conditions in SWC is linked to three dynamical processes: (1) Warm NWP SST excites an anomalous cyclone over the South China Sea, which is oriented against the climatological flow, weakening the transportation of moisture from the NWP. (2) Warm NWP SST strengthens the westerlies along the equatorial Indian Ocean. As a result, most of the moisture is transported over the maritime continent and thus the poleward flow carrying moisture from the Indian Ocean into SWC becomes weaker. (3) Warm NWP SST provokes anomalous ascent and upper-level divergence in situ, with one path of the outflow heading northwestward and converging over SWC, which induces compensating subsidence over the SWC region. The results of numerical experiments, forced by SST anomalies in the NWP alone, are found to reproduce the observed atmospheric response, indicating that the impact of NWP SST on SWC precipitation is physical and that the moisture conditions over SWC are triggered primarily by the fluctuation of NWP SST. Due to the persistent SST anomaly over NWP from summer to autumn, the NWP SST in the preceding summer can be considered a predictor for autumn drought in SWC. Furthermore, autumn precipitation in SWC has experienced a significant decrease since 1994, probably maintained by the long-lasting warm NWP SST in recent decades.