Relationship between Interannual Changes of Summer Rainfall over the Yangtze River Valley and South China Sea–Philippine Sea: Possible Impacts of the Tropical Zonal Sea Surface Temperature Gradient

This study investigates the interannual variability of rainfall over the middle and lower reaches of Yangtze River Valley (MLYRV) and that over the South China Sea and Philippine Sea (SCS-PS) during boreal summer (June-August) from 1979 to 2012. Results exhibit out-of-phase rainfall variations between the MLYRV and the SCS-PS, which is closely related to the tropical zonal sea surface temperature gradient (ZSG) between the northern Indian Ocean (NIO; 5°-25°N, 60°-100°E) and the equatorial central and eastern Pacific (CEP; 5°S-5°N, 180°-130°W). The ZSG can explain as much as 40% of the total variance of the summer rainfall over the MLYRV and the SCS-PS in the past 40 years. This is much higher than that explained by the NIO SST (24%), CEP SST (14%) and Niño-3.4 index (16%) alone. A positive ZSG between the warm NIO and the cold CEP tends to increase rainfall over the MLYRV and decrease rainfall over the SCS-PS, whereas a negative ZSG between the cold NIO and the warm CEP is generally favorable for rainfall over the SCS-PS and unfavorable for rainfall over MLYRV.

The close connection between the ZSG and the out-of-phase interannual rainfall variation over the two regions can be explained by influences of the ZSG on atmospheric circulation over East Asia. A positive ZSG induces anomalous easterlies and Walker-like circulation in the tropics, which results in an anomalous subsidence and boundary layer divergence over the SCS-PS. As a result, summer rainfall decreases in this region. Meanwhile, moisture transport increases due to the anomalously strong southwesterlies along the northwestern flank of the intensified western Pacific subtropical high, providing more precipitable water to the MLYRV region. In contrast, a negative ZSG induces surface westerlies and favorable environmental condition for rainfall over the SCS-PS. Dry descending flow induced by local anomalous Hadley circulation develops over the MLYRV around 30°N, which is unfavorable for rainfall over the MLYRV. The mechanism further examined using numerical experiments. These thermal-dynamical processes induced by the ZSG work together to cause the out-of-phase interannual changes of rainfall between the MLYRV and the SCS-PS, suggesting that the ZSG is highly indicative of the interannual change of summer rainfall in the two regions.